Introducing the Pellucid compressor - a new original design stompbox compressor for guitar and bass. Unlike the Reckless diode compressor, which was a feed-forward design, this is a more traditional feedback compressor design with a two transistor peak detecting sidechain that is derived from the DOD 280, as used in just about every other guitar compressor out there including the Dynacomp, the Ross compressor, the DOD 280 optical compressor, the Diamond optical compressor, the Mooer yellow comp and lots of boutique clones.
The Pellucid uses an LM13700 in the feedback loop of a NE5532 low noise op amp for its VCA. This keeps background noise down to an ultra low level. This is the same basic technique for low noise that is used in the Engineer's Thumb compressor. I've paralleled up both halves of the LM13700 for a further noise reduction, and drive the OTA with a balanced differential signal to minimize control signal breakthrough.
The design goals were simplicity, transparency and low noise. It has a small component count and is made of standard parts all cheaply available from Tayda electronics. It fits into a 1590B enclosure. There are just 2 controls: sustain and volume. Attack and release time are fixed, and both pretty fast, suitable for a wide range of electric guitar styles. The amount of compression available goes from very little (essentially a clean boost) to up to 40dB of gain reduction, which is a massive squish even with single coil pickups. It remains quiet as a mouse even at extreme compression settings due to the architecture and use of a low noise op amp. You could substitute an LM833 or LM4562 in place of the NE5532 if you wish, although the improvement would probably be negligible. I'd avoid the TL072, RC4558 etc as they are generally too noisy for this application.
Hear it here:
I have designed a PCB for this compressor. It's a 2 layer board, carefully laid out and optimized for low noise operation. It is 50mm x 55m in size. If you want one, I have a few spares and can post you one for $5 plus postage.
Here is a link to my Google drive folder, which contains the schematic, bill of materials, several photographs of the unit, and the PCB Gerber and NC drill files should you wish to etch a PCB yourself:
https://drive.google.com/drive/folders/1p9QCJDEhjd2qE1EpuAr-Ndzg3K435dOq?usp=sharing (https://drive.google.com/drive/folders/1p9QCJDEhjd2qE1EpuAr-Ndzg3K435dOq?usp=sharing)
I'm very happy with this compressor design. It's simple, cheap, easy to build and sounds great.
Jonny
(https://i.postimg.cc/JGTvssXq/Capture.png) (https://postimg.cc/JGTvssXq)
(https://i.postimg.cc/DWs07KXF/pellucid-00.jpg) (https://postimg.cc/DWs07KXF)
Jonny, you're killing me! I just finished your diode compressor (v2), and now this. It sounds incredible, although maybe the difference is not that great that I have to make it. The diode comp is also plenty quiet for my ears, and is everything I expected it to be as a clean front end. How would you compare the two sound-wise?
This compressor seems really nice. I would like to try it. I'd need a veroboard layout, because I build using that stuff, maybe I'll try to make my own layout. :D
I'm not a fan of compressor pedals, one of my first pedal I built was the Orange Squeeze, just because it was a simple one, but for some reason it was enough volume, I need to get mexed. After some years I tried to set the internal trimmer, but I can get enough volume without to cause distortion. (I added a tone pot in the end of the circuit, but I'm not sure this casued the volume drop.) Maybe I'll built it again, just to not lose the box drilled and labeled. And, I liked that squish sound, it was kind of funny to play with it.
Now, after some months, I finally boxed up the Engineer's Thumb, but, though I built two boards, I had some issue about a louder attack of the first note at certain setting of the Release and Threshold control, that causes even a loud click noise when I act on the footswitch. I partially fix this, but is not perfect, especially at high compression settings.
So, I would like to try another compressor. :)
Quote from: Ben N on June 16, 2019, 04:28:02 AM
How would you compare the two sound-wise?
The diode compressor is more transparent due to its feed-forward side-chain. It's more like a studio compressor in terms of its dynamics response. The pellucid is more like a dynacomp in terms of its feel, albeit much lower noise and with faster release. It has that classic quack, and you really feel the compression when playing, since it is acting like a limiter rather than a traditional ratio compressor. Both have their uses. The engineer's thumb, diode compressor and this each have their own character, and they complement one another quite nicely IMO. You can probably assemble a pellucid in a couple of hours; it's a simple circuit. You have to be careful building this on veroboard, because the high loop gain around the 5532 / 13700 means it tends to oscillate unless it is well grounded and parasitics are kept to a minimum. The PCB took me a couple of iterations to get working really cleanly.
Quote from: jonny.reckless on June 16, 2019, 02:08:34 PM
The diode compressor is more transparent due to its feed-forward side-chain. It's more like a studio compressor in terms of its dynamics response. The pellucid is more like a dynacomp in terms of its feel, albeit much lower noise and with faster release. It has that classic quack, and you really feel the compression when playing, since it is acting like a limiter rather than a traditional ratio compressor.
A servo AGC loop should give you more stable behaviour ...
In looking at your schematic
You’re driving two low-z current inputs (OTA biases) thru 10k scaling resistors
from a high-z cap and resistor combo
Yielding a very short release time in the process
I suspect your circuit is operating on the verge of acceptable
in that respect
If wanted you could buffer that control line with a rail-rail opamp,
where voltage turns to current,
and have a longer (and variable if it matters to anyone) release time
your impression of feed-fwd and feed-back might change once you do that
—-
again a very original and interesting design effort
the only limiter I’ve seen add current back into an opamp (remotely) this way
is the SWR circuit
Quote from: Eb7+9 on June 17, 2019, 11:45:25 PM
I suspect your circuit is operating on the verge of acceptable in that respect.
I think everything is OK? I've tested it on the bench with input signals from 10mVpp to 3Vpp, from 80Hz to 8kHz, with continuous tones, bursts, pink noise, and step amplitude changes, and it seems to be working acceptably. Attack time is level (and frequency) dependent but nominally measured at 3ms, release time around 200ms.
As configured, each OTA requires approximately 400uA into its bias pin to hard limit the amplitude at maximum compression with a maximum signal level of 3Vpp. This causes the collectors of TR1 and TR2 to rise to about 5V at maximum gain reduction, due to the 10k current sharing resistors, well short of saturation. Release time is set by C1, with 47uF giving a release time of around 200ms measured on the bench. I quite like fast attack and release for guitar compression, it makes it feel more 'alive' when you are playing, especially with single coils. Most dynacomp style compressors have way too slow release for my taste. You might want to increase C1 to 100uF for a bass compressor though.
Quote from: jonny.reckless on June 18, 2019, 03:29:08 AM
I've tested it on the bench with input signals from 10mVpp to 3Vpp, from 80Hz to 8kHz, with continuous tones, bursts, pink noise, and step amplitude changes, and it seems to be working acceptably.
I'd be interested to see the gain curves?
Quote from: merlinb on June 18, 2019, 03:35:12 AM
I'd be interested to see the gain curves?
Do you mean input dBV vs output dBV steady state? Or dynamic response to a step change in amplitude? I'll have to figure out how to do a screen capture on my oscilloscope for the latter :)
I'll see if I can do that at the weekend.
Quote from: jonny.reckless on June 18, 2019, 03:40:00 AM
Do you mean input dBV vs output dBV steady state?
Yes steady state. Not necessarily in dBV, ordinary V will do.
Ha. Fight of the giants has started!
Heh heh. Are we witnessing the beginning of a "pellucid thumb" compressor?
While I doff my cap and bow deeply and humbly at a thoughtful and generous design/designer, and appreciate every diligent attempt to reduce noise WITHIN a circuit, too many observers and novices misattribute the "noisiness" of a compressor to the compressor itself. In reality, the compressor is simply doing what you asked it to do: amplifying low-level signals and trimming back high-level ones. Realistically, one of the best things one can do to minimize noise in a compressor is simply feed it the lowest-noise signal you can. Those with any experience will likely respond to this with "Well DUH!". But every day there are thousands more newcomers who don't understand how compression works, and need to be reminded/informed of such things.
Quote from: Mark Hammer on June 18, 2019, 07:59:44 AM
But every day there are thousands more newcomers who don't understand how compression works, and need to be reminded/informed of such things.
My tame guitarist still thinks that "sustain" is a thing (other than the physics of his guitar). I've explained to him how a compressor actually works, but no, the world is still flat...
Seeing as everyone else is here, I'll join in to.
Nice idea. A few thoughts:
1. low noise and a NE5532 biased via a 1M resistor are not entirely consistent.
2. you don't need separate resistors (R9,10) for the LM13700 bias pins. Buried in the app notes is mention that these are balanced and see e.g. Fig 32 in the application note (below).
3. you're not using the diode bias. I know that Merlin isn't convinced these are worth the bother, but I'm not sure I entirely agree.
4. R11-13. How well balanced are the amp inputs? The use of trimmers in most of the app notes suggest you may not be getting very good balance/cancellation with this setup and/or the ORIGINAL engineers were overzealous (again).
http://www.ti.com/lit/ds/symlink/lm13700.pdf
Edit - +1 on seeing the static gain curves please.
Quote from: samhay on June 18, 2019, 08:53:56 AM
1. low noise and a NE5532 biased via a 1M resistor are not entirely consistent.
Thanks for your comments. I'm thrilled people are paying such detailed attention!
From a noise perspective, the 1M input resistors are in parallel with the guitar pickups at audio frequencies, typically about 5k - 15k, which reduces the noise considerably. I agree 1M alone would be noisy as hell, as you could hear if you ran the device flat out with the input open circuit. The compressor is really very quiet; quieter than any of my other compressors (I've got about a dozen) with the exception of the Empress which is about the same. Noise in compressors is tricky to quantify because it depends on gain reduction, output volume, input impedance, bandwidth of measurement etc but it really is "low noise" :) I can run this into my amp with the gain cranked and I don't hear any obvious increase in hiss when I kick the compressor on, unlike the Dynacomp which sounds like a steam kettle under the same circumstances. Bear in mind that with little or no signal, the LM13700 is off, and contributing no noise to the circuit, so it's just the noise gain of the opamp based on the sustain setting. NE5532 is a quiet opamp when driven from low impedance sources. The voltage noise and current noise equal out about 22k ohms input impedance. With my strat pickups the voltage noise dominates. If you switch the opamp for a TL072 you can clearly hear the hiss level increase compared with the NE5532. I really like the NE5532 and NE5534 opamps for audio - they're cheap, quiet, and can drive quite low impedance loads with low distortion. I only use TL072s when super low input bias current is absolutely necessary. LM4562s are nice but not as robust as NE5532 - I've blown a lot of them up while prototyping, which almost never happens with a 5532.
It's interesting you don't need to current sharing resistors according to the datasheet, because the voltage on those pins differs by about 10-15mV on the bench, enough that one of them would hog most of the current if I hard wired them in parallel (~60mV per decade re Ebers Moll). I guess they even out as the current increases? Generally I use current sharing resistors anytime I have bipolar transistors in parallel, as in this case (the bias pin looks like 2 diodes to ground). I'd certainly want current sharing resistors if I were going to build that state variable filter from figure 32, you need close matching of the transconductance between the two halves to get the poles coincident. For the cost of one extra resistor (a penny or so) it's worth it to me anyway :)
I've never really seen much benefit in the linearizing diodes. It makes biasing harder, and the few times I've tried it, I couldn't get the quoted 10dB in noise reduction at the same distortion level.
You're absolutely right re R11-R13, trimmer balance here would be better, with 1% resistors I am limited to 40dB CMRR best case. I just decided to run a balanced signal into the OTA because I already had an inverted signal to drive the transistor peak detector, and it seemed sensible to use it. Whilst not perfectly balanced, the common mode input signal will be much smaller than if I didn't drive it this way, so you don't really lose anything. I find LM13700 don't behave that well at 9V or below (the output impedance is asymmetrical +ve and -ve going), they really like 15V or more, so anything you can do to help them out is probably a good idea. For a guitar effect the asymmetry generates even order harmonics which is not necessarily a bad thing.
Quote from: bool on June 18, 2019, 06:44:31 AM
Ha. Fight of the giants has started!
No fight here. I have the utmost respect for Merlin and others on this forum (his valve amp book is great BTW). The Engineer's Thumb compressor is a great design; I have one on my pedalboard right now. The pellucid and the diode compressor fill a niche which I thought was missing.
I'm using the "OTA in the feedback loop" approach from the ET, and the "2 transistor peak detector" sidechain approach from the DOD280, Dynacomp etc. I just married them up to make an original circuit from bits of other designs. But it really is very quiet I promise :)
Here is a link to the static gain curves of the Pellucid compressor with the sustain set to minimum, 12 o'clock and maximum:
https://docs.google.com/spreadsheets/d/1g14rXTT5gZJL6CVnEW8s0eTMZVhcRcZsx16u4_0d22k/edit?usp=sharing (https://docs.google.com/spreadsheets/d/1g14rXTT5gZJL6CVnEW8s0eTMZVhcRcZsx16u4_0d22k/edit?usp=sharing)
As you can see, it's basically acting like a limiter (high ratio) with variable threshold, rather like the Dynacomp, Ross, Boss compressors. The two transistor peak detecting sidechain clamps the output pretty hard. I added a 10k resistor in series with each transistor base, which softens the knee and allows a little bit of slope in the limiting region.
Just noticed something on the schem: R16/17 aren't really 1k are they? (Consuming 4.5mA!)
Also, I'm thinking you could use one of the on-board Darlington transistors to buffer your side chain from R9/10, which would give you access to longer release times.
Quote from: jonny.reckless on June 18, 2019, 05:14:11 PM
Quote from: bool on June 18, 2019, 06:44:31 AM
Ha. Fight of the giants has started!
No fight here.
BS. ...
We want fight!
We want fight!
We want fight!
heh heh
OT. wrt darlington; yes of course; but if you aim for the uber-gains etc it may be better (layoutwise) to use a separate discrete darl. to split the control path from the signal path powerwise. (theoretically maybe perhaps)
And a darl. will drop how much from the control electrode (b-to-e)? 1V or so... a small mosfet could be even better if it works but will drop g-to-s 2-or so volts. Can you live with that?
Quote from: jonny.reckless on June 18, 2019, 05:06:05 PM
Thanks for your comments. I'm thrilled people are paying such detailed attention!
You're welcome.
>From a noise perspective, the 1M input resistors are in parallel with the guitar pickups at audio frequencies,...
Indeed, that's a valid point if you put the compressor at the start of your signal chain. Not quite so good when it goes after a Fuzz Face. It's a compromise, and I've certainly designed things with an NE5532 in a very similar input stage.
The other issue though is how much DC offset you end up with. Via the inverting stage, you could have quite a difference in DC levels going into R11 and R13.
>The compressor is really very quiet; quieter than any of my other compressors (I've got about a dozen) with the exception of the Empress which is about the same....
Perhaps the fairest comparision would be against Merlin's ET, which I would guess is pretty similar.
>It's interesting you don't need to current sharing resistors according to the datasheet, because the voltage on those pins differs by about 10-15mV on the bench,...
Thanks for the real world information. Much more useful than most data sheets.
> I've never really seen much benefit in the linearizing diodes. It makes biasing harder, and the few times I've tried it, I couldn't get the quoted 10dB in noise reduction at the same distortion level.
Fair enough.
>You're absolutely right re R11-R13,...
And also fair enough. Interesting observation RE 9V supply for LM13700. I've noticed that too, but wasn't sure how representative it was.
Personally I quite like the short release time. :) It's a gripe of mine that most compressors have too slow a release time for my taste. I think 100ms to 200ms is great for guitar, maybe twice that for bass or vocals? The compromise is getting fast release without too much amplitude modulation at low frequencies. I experimented with using a second order filter in the sidechain but it didn't really sound any better to my ears. But it would be cool to see how a longer release worked with the Darlington buffers. It would however mean that the collectors of TR1 and TR2 would now have to rise to 4 diode drops above ground before any gain reduction occurred which would slow down the attack time somewhat.
Re noise performance: the ET uses a TL07x and one half of the LM13700 in its VCA. The Pellucid uses NE5532 and both halves of the LM13700 paralleled. Also the base resistors feeding the OTA are 10x smaller in the Pellucid (this actually makes an appreciable difference to noise since the signals there are so small). Overall this contributes to making the Pellucid about 11dB quieter than the stock ET at no input with a 5k source impedance (typical strat pickup) and compression on maximum. I was being honest when I said it's the quietest guitar compressor I have (except for the Empress which I believe uses a THAT corp Blackmer VCA). The engineer's thumb redux mods I did, take the ET to the same noise performance (NE5534 with external compensation, both halves of the LM13700, and 1k / 22 ohm base attenuator resistors) and are totally worth doing IMHO since they add little or zero cost and complexity but significantly improve low level noise performance.
Quote from: merlinb on June 19, 2019, 03:21:23 AM
Just noticed something on the schem: R16/17 aren't really 1k are they? (Consuming 4.5mA!)
Haha, yes! I run everything from a Voodoo labs pedal power 2+ so I don't typically worry about that, but they could quite happily be 10k if you were concerned about running from a PP3 battery :)
Good demo. Despite any small points I think you achieved you goal!
Quote1. low noise and a NE5532 biased via a 1M resistor are not entirely consistent.
It usually boils down to the size of the input cap. Maybe increasing the 22n to 47n would improve things a small amount.
Quoteyou're not using the diode bias. I know that Merlin isn't convinced these are worth the bother, but I'm not sure I entirely agree.
In practice it's hard to tell the difference. Puzzling from a theoretical point of view but true.
QuoteQuote from: merlinb on June 19, 2019, 03:21:23 AM
Just noticed something on the schem: R16/17 aren't really 1k are they? (Consuming 4.5mA!)
Haha, yes! I run everything from a Voodoo labs pedal power 2+ so I don't typically worry about that, but they could quite happily be 10k if you were concerned about running from a PP3 battery
That caught my eye as well.
Perhaps some pre-emphasis/de-emphasis would help noise.
Quote from: Rob Strand on June 21, 2019, 02:49:59 AM
Perhaps some pre-emphasis/de-emphasis would help noise.
Indeed it would. I've played around with this idea: basically putting a treble booster in front of the compressor and a complementary treble cut after it. Not only does it reduce noise (assuming your treble booster is super quiet that is), but it affects the way the side-chain works so it becomes less sensitive to bass frequencies. In some ways this a better way to do compression for guitars depending on what you place after it in the signal chain. It's more subtle and natural sounding and sounds less squished even at high gain reduction, especially with humbucking pickups which are bass heavy to begin with.
In other circuits where I use an LM13700, the Reckless Vibe and the Auto de FET guitar amp, I use pre-emphasis to reduce the noise. OTAs are pretty noisy devices unless you put them in the feedback path of an opamp or use emphasis to manage hiss. It's also absolutely essential to keep the base impedance as low as possible; I find 22 ohms is the sweet spot, anything above 100 ohms and the noise floor really starts to rise especially as the bias current approaches maximum. Also, beware cheap LM13700s from China on eBay. I've been stung with some absolute crap. Noisy as hell and high distortion compared with legit TI parts :( In my experience the TI LM13700s are quieter than the NSC versions. Mouser sells the DIP16 TI version at $1.76 each for 10 off which is not bad. Tayda only stocks the NSC one.
I guess we should really all be using THAT corp Blackmer VCAs which are super quiet and high fidelity 8)
QuoteNot only does it reduce noise (assuming your treble booster is super quiet that is), but it affects the way the side-chain works so it becomes less sensitive to bass frequencies. In some ways this a better way to do compression for guitars depending on what you place after it in the signal chain. It's more subtle and natural sounding and sounds less squished even at high gain reduction, especially with humbucking pickups which are bass heavy to begin with.
It affects the side-chain if the detector is detecting to signal after the pre-emphasis and before the de-emphasis. The feed-forward compressors don't change but the feedback compressors depend on where the circuit blocks are placed.
QuoteI find 22 ohms is the sweet spot, anything above 100 ohms and the noise floor really starts to rise especially as the bias current approaches maximum.
You did a good job hunting that down. I've have seen that type of thing before but I don't remember going down that far. Maybe the transistors in the LM13700's these days have lower "rbb" parameters making it worthwhile. The NE5532 lets you drop the resistors to anything you want, so it all worked out nicely.
Iirc TL Audio channel strips also used a ota-around-opamp compressor topology. So if you can hunt down some TLA schemos, you could also look there how they did it.
Quote from: jonny.reckless on June 21, 2019, 03:59:48 PM
I guess we should really all be using THAT corp Blackmer VCAs which are super quiet and high fidelity 8)
Once you go Blackmer, you never go back(mer).
After reading the many very informed comments here, I'm must say that I'm a little hesitant to post this. I built the Reckless Diode comp and liked it, so thought I might try try the Pellucid. :icon_lol:
So here's my vero layout - I've built it, and it works just fine. So consider this verified.
Thanks for your design work Jonny - much appreciated.
(https://i.postimg.cc/LYdGHfqz/Pellucid-Compressor.png) (https://postimg.cc/LYdGHfqz)
Quote from: jonny.reckless on June 18, 2019, 03:29:08 AM
I think everything is OK? I've tested it on the bench with input signals from 10mVpp to 3Vpp, from 80Hz to 8kHz, with continuous tones, bursts, pink noise, and step amplitude changes, and it seems to be working acceptably. Attack time is level (and frequency) dependent but nominally measured at 3ms, release time around 200ms.
As configured, each OTA requires approximately 400uA into its bias pin to hard limit the amplitude at maximum compression with a maximum signal level of 3Vpp. This causes the collectors of TR1 and TR2 to rise to about 5V at maximum gain reduction, due to the 10k current sharing resistors, well short of saturation. Release time is set by C1, with 47uF giving a release time of around 200ms measured on the bench. I quite like fast attack and release for guitar compression, it makes it feel more 'alive' when you are playing, especially with single coils. Most dynacomp style compressors have way too slow release for my taste. You might want to increase C1 to 100uF for a bass compressor though.
Hi Jonny,
How might one go about adjusting the attack, if this is at all possible? I put a 100uf cap on a switch with the 47uf for the release rate, I wouldn't mind to have the option to play with the attack a little. If not, I might just add a blend pot.
Thanks for your work, it's a very nice little compressor (along with your diode comp).
Andy
Quote from: andy-h-h on June 29, 2019, 12:59:45 AM
How might one go about adjusting the attack, if this is at all possible? I put a 100uf cap on a switch with the 47uf for the release rate, I wouldn't mind to have the option to play with the attack a little. If not, I might just add a blend pot.
Thanks for your work, it's a very nice little compressor (along with your diode comp).
Thanks, I am glad you like it.
I don't think you could easily adjust the attack using this two transistor peak detecting sidechain. Maybe increase the 10k resistors (R2, R4) feeding the transistor bases? Or add a resistor (say 1k) between the collectors of TR1 and TR2 and the 47uF release cap C1. It would have some effect, but also probably make the knee softer.
Jonny
> adjust the attack
(https://i.postimg.cc/CBWDj6zd/Pellucid-attack.gif) (https://postimg.cc/CBWDj6zd)
??
Yes, like that :) although 10k might be a bit too long? I guess try it and see how it sounds. You might get overcompression with slow attack in a feedback limiter like this.
Quote from: PRR on July 02, 2019, 06:11:04 PM
> adjust the attack
(https://i.postimg.cc/CBWDj6zd/Pellucid-attack.gif) (https://postimg.cc/CBWDj6zd)
??
I'll try it on the weekend. I like the idea of having the option to soften the knee too, so if that's an unintended result of this, I'll be happy with that.
Thanks both.
Still think I might try a blend pot as well, just to see how it sounds. I have a feeling that I'm going to play with things, and then just decide that it works just fine as is.... :icon_lol:
> 10k might be a bit too long?
I would counter that 47uFd (against 100k release) is "long".
However a 10k LIN will easily set to 1k, 50ms, which leaves only a small blip of un-limited signal. A 10k AUD will trim to a few hundred ohms for few-mS attack. Further timing tweaks should be obvious.
The 100k is just there to pull the collectors down the last volt or so to ground when there is no signal. The release time is defined by the 47uF cap and the 2 x 10k resistors into the bias pins, which look like 2 diodes to ground. Theoretically 235ms but measured to be less than this, somewhere around 150ms on my unit by eye on the scope, which is about where I like release time for guitar compression. It's a subjective thing but most commercial compressors have too slow release for my taste. The guitar feels more alive and responsive to me when you play with a shorter release, especially with a strat.
Quote from: jonny.reckless on July 05, 2019, 04:48:41 AM
The release time is defined by the 47uF cap and the 2 x 10k resistors into the bias pins, which look like 2 diodes to ground.
So could one replace the two 10Ks with a dual gang pot and have more adjustment on the release? I also play bass, which is why I'm interested in both attack and release settings.
I'm enjoying this and your diode comp. :icon_smile:
Thanks Jonny, this is quite interesting! Will try to build one once I finish current projects (15+) :)
> release time is defined by the 47uF cap and the 2 x 10k resistors
Yes; I was blind.
> replace the two 10Ks with a dual gang pot
No. If you turn pots to zero the OTA burns up. You could add series resistance to both. But this is coming down to not having much spare gain in the sidechain. Dinking with resistors is likely to just make the action softer without so much control of timing.
As a general thing, to change *both* attack and release the same way, change the 47uFd.
If you really want to be able to control both release and attack, you probably want to implement Merlin's suggestion of using one of the spare buffers in the LM13700 to buffer the control voltage from R9 and R10, then implement Paul's suggestion of a 10k pot in series with the collectors of TR1 and TR2. When you have done this, at this point, R3 defines the release time (in conjunction with C1) and the 10k series pot defines the attack time. I would keep R9 and R10 both as 10k.
This will definitely change the character of the compressor quite a lot, and will probably sound a bit weird with slow attack times and short release due to the sidechain architecture and values involved, but I'd be interested to see how (if) it works out for you.
Jonny
(https://i.postimg.cc/R6Z3fpL2/pellucid.jpg) (https://postimg.cc/R6Z3fpL2)
QuoteJonny
[pic]
In that type of configation, sometimes moving the release cap before the attack is better since it avoids forming a voltage divider when attack is long (large Rattack) and release short (small Rrelease).
> avoids forming a voltage divider when attack is long
(https://i.postimg.cc/21c4x8m8/Pellucid-attack-2.gif) (https://postimg.cc/21c4x8m8)
Quote from: PRR on July 05, 2019, 09:08:24 PM
> avoids forming a voltage divider when attack is long
(https://i.postimg.cc/21c4x8m8/Pellucid-attack-2.gif) (https://postimg.cc/21c4x8m8)
I tried the other version, and I was getting quite a bit of distortion on some settings. I did use the buffer on 9 & 10, but that shouldn't matter. Still have some distortion on some settings with this version, but it's not as bad. :icon_confused:
When A B testing the original vs modded, I'm preferring the original. Might just be a case of it ain't broke, so don't fix it. I'm sticking with the original. :icon_biggrin:
Hallo, It s very interesting what you are posting, i was thinking about building a dual compressor, with a crossover like the one trace elliot used on their comp, and using a lows compression with an attack of about 20ms anda release of 120 and the Highs being compressed with attack of few ms, 2-5 and a 70ms release. Being fixed, could these times be implemented in that design? Thanks a lot for your efforts and research
Quote from: sergiomr706 on July 06, 2019, 04:25:39 AM
Hallo, It s very interesting what you are posting, i was thinking about building a dual compressor, with a crossover like the one trace elliot used on their comp, and using a lows compression with an attack of about 20ms anda release of 120 and the Highs being compressed with attack of few ms, 2-5 and a 70ms release. Being fixed, could these times be implemented in that design? Thanks a lot for your efforts and research
You could build 2 of these and have different values of C1 to control the time constants, or try the attack / release mods as described above. Also consider putting a filter in the sidechain rather than requiring dual compressors. A treble boosted sidechain can sound great for clean guitar and bass.
You do need to be careful with the crossover design if you want the end result to sound coherent. I've dabbled with this sort of idea. Generally 4th order Linkwitz-Riley filters are considered to be the best approach since you get constant relative phase between the 2 halves.
https://en.wikipedia.org/wiki/Linkwitz%E2%80%93Riley_filter (https://en.wikipedia.org/wiki/Linkwitz%E2%80%93Riley_filter)
BTW I was head of R&D at Trace Elliot for a while back in the early 90s, fond memories :)
Quote from: jonny.reckless on July 07, 2019, 03:28:50 AM
BTW I was head of R&D at Trace Elliot for a while back in the early 90s, fond memories :)
So can I blame you for why my oh-so-tiny-and-cute BLX-80 weighs so freakin' much??! ;D
Amazing! You were involved in the gp7sm /12 or V series? I played the 12 bands for a little time, It was not mine, but we shared rehersal place with another bassplayer who owned one, and It was such a great tone, (at least to me). Lately I started reading about the comp part and It seems that lows and Highs share a wide space btwn 250 and 900 Hz.. maybe parte of his charm? I first had thought of using a 3rd order linkwritz and the try different crossover points, 300 /400/800... So what can i day? Double thank you for your efforts and designs. This all helped shape a lot of great bass sounds!
Quote from: jonny.reckless on July 07, 2019, 03:28:50 AM
...
BTW I was head of R&D at Trace Elliot for a while back in the early 90s, fond memories :)
...
How cool is that! TE's were my no.1 go-to amps in 90's and 00's. I still have a small "7-band eq" combo from that era somewhere in the closet (with burned mosfets and bridge I think). And everywhere I played I required a TE 400 or 500-watter (usually a combo) as my backline amp. The only other amps I tolerated were swr redheads. ("like no ampeg for me, please").
Btw, whoever did it, the mid-shape1 was a little better than mid-shape2, a sentiment that was common to other TE players around here ... but sound eng's hated it!
QuoteBTW I was head of R&D at Trace Elliot for a while back in the early 90s, fond memories
Hey, that's pretty cool. You must be proud of all the big names using that stuff. The bass amps were everywhere at that time.
In the back of my mind I seem to remember the crossover on the high and low compressor bands didn't overlap symmetrically like speaker crossover. Do you remember the reason for that?
> I'm preferring the original. Might just be a case of it ain't broke, so don't fix it.
I have a feeling that, for the number of parts used, the original does about as good as can be. Tacking on "a couple more parts" asks too much of its simplicity.
A "real" compressor with full control of timing will have 2 to 12 more opamps in the sidechain. Many dozens of such things have been posted to the internet, plagiarize shamelessly.
Truly simple schemes, like the Orange Squeeze (within limits) and this Pellucid, are harder to find. It is good to have another easy-build alternative. It is hardly "a pig", but still I doubt lipstick is going to make it "prettier" than it already is.
Quote from: jonny.reckless on July 07, 2019, 03:28:50 AMAlso consider putting a filter in the sidechain rather than requiring dual compressors.
As I am given to understand, basically the approach of the emphasis control Marshall's ED-1 - a Big Muff tone control in the sidechain. Very subtle effect there.
Quote from: Rob Strand on July 07, 2019, 06:19:59 PM
In the back of my mind I seem to remember the crossover on the high and low compressor bands didn't overlap symmetrically like speaker crossover. Do you remember the reason for that?
That was one of Clive Button's designs. He designed a lot of the early stuff, he was a one man R&D department for Trace for a long time before I joined. There was a lot of empirical tweaking involved in those products, probably it was done that way just because he liked the sound of it :) The graphic EQ design on the 12 band series was also "wrong" in that the filter Qs were all over the place, but it sounded good and people liked it.
I had prototyped another dual compressor with 4th order LR crossovers around the same time, I seem to recall that one went into production in 1997 just before we went bust and then got taken over by Gibson for a while, at which point I and many others were laid off.
At that time I worked with Andy Ewen (now head of R&D at Victory amps) and Paul Stevens (now senior designer at Blackstar amps) on the Triumph valve guitar amplifier designs (Bonneville, Speed Twin, Trident etc), the V type bass amps, the Tramp tube guitar hybrids (very early use of JFETs in a guitar amp circuit), Trace Acoustic PA, lots of pedals (the quad chorus was one of mine), power amps, and many others. It was a fun ride. I spent a few years as Engineering director at ARCAM (the British HiFi company) then I switched to predominantly software engineering shortly after that, which I have done ever since, but I do keep my hand in with hardware these days. I've spent the last 11 years in the San Francisco bay area.
QuoteThat was one of Clive Button's designs. He designed a lot of the early stuff, he was a one man R&D department for Trace for a long time before I joined. There was a lot of empirical tweaking involved in those products, probably it was done that way just because he liked the sound of it :) The graphic EQ design on the 12 band series was also "wrong" in that the filter Qs were all over the place, but it sounded good and people liked it.
I guess that's the difference between "studio perfect" designs and purpose-built instrument preamps/amps. I've seen the funky Q thing on a few products.
QuoteI had prototyped another dual compressor with 4th order LR crossovers around the same time, I seem to recall that one went into production in 1997 just before we went bust and then got taken over by Gibson for a while, at which point I and many others were laid off.
I don't think I've seen that one. There was one design, probably around that era, which used only a first-order crossover. I vaguely remember the design was feed-forward instead of feedback like the older designs.
Quotebefore we went bust and then got taken over by Gibson for a while, at which point I and many others were laid off.
I'm not sure what it is with the music instrument/amp business but there's a pattern like that in many companies. Ernie Ball seems to have a done a good job with Music-Man.
QuoteAt that time I worked with Andy Ewen (now head of R&D at Victory amps) and Paul Stevens (now senior designer at Blackstar amps) on the Triumph valve guitar amplifier designs (Bonneville, Speed Twin, Trident etc), the V type bass amps, the Tramp tube guitar hybrids (very early use of JFETs in a guitar amp circuit), Trace Acoustic PA, lots of pedals (the quad chorus was one of mine), power amps, and many others. It was a fun ride. I spent a few years as Engineering director at ARCAM (the British HiFi company) then I switched to predominantly software engineering shortly after that, which I have done ever since, but I do keep my hand in with hardware these days. I've spent the last 11 years in the San Francisco bay area.
I see analog electronics as a dying art. Software engineering is pretty stable, although there's quite a gap between web-design and real-time processing.
Cool post, interesting stuff.
QuoteI had prototyped another dual compressor with 4th order LR crossovers around the same time, I seem to recall that one went into production in 1997
I had a poke around in my schematics and I can't find that one. My collection is by no means complete though.
I don't have a copy of any of them any more so I can't help there, sorry.
QuoteI don't have a copy of any of them any more so I can't help there, sorry.
No worries. I was interested to see the evolution.
I don't actually keep any circuits from any place I've worked at. It's just a policy I've adopted.
Does it sound good with humbuckers?
Hi there,
I tried to analyze the current noise on this thing because my experience with very similar input stages (though non-compressing) is also that they are very quiet compared to, say, a TL072. But with the relatively high loop impedance and the 10k series resistor on the input, I would seem to think that there is room for improvement, no? Unfortunately, I am not an EE and was unable to dig up a useful explanation of a rigorous noise analysis. All I did find, and I found a lot, exhausts itself in oversimplified guesstimations and narrow examples, which doesn't really help me understand how to do the analysis properly. Would someone be so kind to point me to a good source? Especially for how to treat a guitar pickup in a current noise calculation and why the input cap seems to make such a big noise difference.
Thanks,
Andy
Anyone?
> exhausts itself in oversimplified guesstimations and narrow examples
OK, I'm sure you know what you are asking, but I don't know the question or even the references.
> very similar input stages
Similar to which one? Jonny's first post? Or some later proposal?
The first post in this thread IS a BJT input, in a hiss-critical application (idle compressors hiss), and Jonny is no fool in these matters. He just puts a 1Meg on a '5532, and lives with the DC offset. He can't have been panicked about current hiss in guitar context because he has that 10k in front. Also 10k in the NFB loop.
Hi Paul,
I was referring to the first schematic. By similar I mean NE 5532 as a non inverting first boost stage with a 1M bias resistor and a few kiloohms of parallel resistance in the NFB loop.
Thing is, Jonny certainly does know what he's doing and when he claims that this thing is not noisy I'll take his word for it. But then there is this 10k input resistor and almost 10k parallel resistance in the NFB loop at the highest setting and these should contribute rather substantial noise in an opamp stage with substantial input bias current and current noise, like the 5532. The Johnson noise of the input resistor is amplified by the gain of the stage, which is 40db when maxed.
My question is: does any of this matter or is it all being swamped by the inherent noise of the pickups? To evaluate this, I would need to know how to calculate all the noise contributions. There are formulas for that and they are easy enough to find. However, all of the ones I found only consider resistance. But I know from personal experimentation that in a stage like this, the input cap makes a very clearly audible difference. Anything below 220n before a 1M bias resistor adds audible noise. How does Jonny get away with 22n here? Why does the cap even matter for the noise? What about the inductance of the pickups, does that interact with the input current of the opamp and if so how?
It's not a simple question with a simple answer. Hence my asking for a source to teach me the full rigorous treatment of these matters instead of the simplified versions you find in many app notes and manufacturer reports.
Thanks,
Andy
> Anything below 220n before a 1M bias resistor adds audible noise.
I've seen subsonic rumble from small input caps, but you seem to be describing something else. I don't know.
For most purposes we can get away with calling a guitar "50k"; the volume pot dominates all else except full-up.
That plus 10k in input and 10k in NFB makes 70k, which is 1.4dB higher than just 50k.
OSI for '5532 is like 4k (but quite broad) so 70k is well off optimum. It's too late here to figure what that comes out to.
TL072 has "no" hiss current but is way off from ideal hiss voltage. There are better choices today. Including, for many cases, the old fashioned '5532, if only because prime parts won't have excess hiss.
Sorry for the delay I've been out of the country for a while.
A full noise analysis is quite complex due to the feedback nature of the way the variable gain stage works. The NE5532 is pretty quiet, you have to consider both current and voltage noise. Typically for a 5532 the point at which they are equal is around 22k. This is where you theoretically get optimum noise figure. Treat the guitar pickup (grossly simplified) as a 10k resistor in series with 1H inductor. The 10k (R20) on the input is a "grid stopper" and there to prevent parasitic input oscillations due to pickup inductance and cable capacitance. So you get a source impedance around 20k at the input of the opamp. Not perfect, but pretty close to as good as you can get with such a cheap and simple circuit. The feedback noise gain is higher, and varies with gain reduction so you have to consider the effect of that at high compression settings and no gain reduction. Although R14 (10k) dominates that node from a noise perspective. There's not much I could do about that without making all the impedances so low that the currents would be too high to use an LM13700 in the feedback path.
Oh - I wudda thought R20 were there to soften the D1/2 clipping knee of "too hot" transient attacks.
Quote from: Fancy Lime on November 07, 2021, 03:31:19 PM
Thing is, Jonny certainly does know what he's doing and when he claims that this thing is not noisy I'll take his word for it. But then there is ...
... when the unit is idling and you crank the SUSTAIN/COMP/RATIO control (1Meg) what happens to background noise ?!
Quote from: Eb7+9 on April 22, 2022, 02:38:43 PM
... when the unit is idling and you crank the SUSTAIN/COMP/RATIO control (1Meg) what happens to background noise ?!
It's at its maximum, due to the gain being at full, as with all compressors. However in this case you can consider the LM13700 to be out of circuit and not contributing to noise, and compute the output noise based on the NE5532 and surrounding electronics. It's important to consider the impedance of the source when doing this.
Quote from: jonny.reckless on April 27, 2022, 10:59:08 AM
It's at its maximum ... as with all compressors ...
well, not really ...
only the two-birds-for-a-stone designs such as the Dynacomp and, similar to yours, the THUMBS compressor
notice the NE570/571 current-feedback circuits (similar to yours and THUMBS) do not use variable gain around op-amp
theirs is fixed, for a good reason
---
I get the simplicity of your overall design // definitely an elegant solution
but if you decouple signal path from side-chain
then you can operate your signal path at minumum noise at all settings instead ...
for example, this is how the LA-2A and 1176 do it ... noise floor is not affected by side-chain settings
maybe worth a try!
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I made use of the idea in tacking on an optical limiter stage to my Acoustic 360 preamp clone project
https://viva-analog.com/360-kit-ver2-0-add-on-limiter-board/ (https://viva-analog.com/360-kit-ver2-0-add-on-limiter-board/)
which was derived from my "Envelope Lock" design ...
consider the noise-less/transparency aspect of this one-stage opto design mentioned in this review:
http://www.ovnilab.com/reviews/envelope.shtml (http://www.ovnilab.com/reviews/envelope.shtml)
you can go an analogous way with yours, and not change dynamic specs any
simply by adding an extra op-amp gain stage to the works ...
eg., opa134 for the signal path and TL072 for sidechain
---
Quote
However in this case you can consider the LM13700 to be out of circuit and not contributing to noise
yes, I see that - and really like your side-chain mixed current-voltage drive circuit
---
two components in your circuit play a big role in making this potential transition happen
seeing that R9/R10 current dividers are also clamping OTA's at around 800uA each (1.6mA total)
I think you can drop those safely to 4k7 if you needed to ... or even a little further
and then be able to live with a fixed 10k resistor in the feedback path of the signal amplifier
(yielding a total max gain of x2 which is acceptable in a limiter)
---
again, I very much like your design Jonny ...
just saying that wrecking that gestalt by one op-amp should give you pro-level noise specs without changing anything else
... an idea to try out if ye be interested
is this circuit a worthy alternative to VCA compressors. It is known that studio professional compressors use VCA chips (of course we are not talking about Tube and Fet compressors now), but I have never seen OTA compressors among studio models. OTAs are only found in vintage pedals like the dynacomp and its clones. This circuit is different from the dynacomp and clearly better, but can it be as good as real VCAs?
Quote from: POTL on May 01, 2022, 08:40:39 PM
is this circuit a worthy alternative to VCA compressors. It is known that studio professional compressors use VCA chips (of course we are not talking about Tube and Fet compressors now), but I have never seen OTA compressors among studio models. OTAs are only found in vintage pedals like the dynacomp and its clones. This circuit is different from the dynacomp and clearly better, but can it be as good as real VCAs?
No it's not as quiet or as low distortion as a THAT corp VCA, but it doesn't cost as much either. It's OK as a guitar compressor, which is its intended purpose, and much quieter than a dynacomp or similar, but would need a much more sophisticated circuit to make it suitable for studio use, at which point you'd be better off using a more conventional circuit from one of the excellent THAT application notes.
point is that you "can" make it nearly as quiet as a THAT VCA, for those who would like a somewhat cleaner effect ... not to crap on the holy Dynacomp, but I did re-designed it the same way - by separating side-chain from signal-path, and get the OTA to operate in its better S/N zone (ie., with a much lower output resistor than the stock 150k) ... same thing happens, noise stays low'ish when cranking the COMP control ... that's with the side-chain simply preceded by a variable gain stage that goes down to no output for a minus infinity RATIO, real simple step to make any coupled comp design a little more noise friendly without changing dynamic characteristics (timing, etc ...)