Hey Friends,
I was expecting this circuit to be as quiet or quieter than a IC, but not so. It's not very noisy by any stretch, but it hisses just enough to be noticeable, even when plugged into a quiet, low gain amp. With apologies for the artwork, here's the schemo:
D I S C R E T E O P A M P
9V
|
+------+-----+
| |
Z Z
5.6M Z Z 100ohm
Z Z
| +------------------+
| | |
| +---- | -----------+ |
| c| |c | |e
1uF | b|/ \|b | b|/
IN >---||---+----|\ /|---+ +---|\
| e| |e | |c 1uF 36K
| +--+--+ | +----||---N N N-+---> OUT
| | | | |
Z Z | Z Z
560K Z 100K Z | Z 3.3K Z 12K
Z Z | Z Z
| | | | |
+---------+--------+-----+------+---------------+
|
(2SC1583) G (2N5087)
The resistor divider at the input gives about half the supply voltage on the collector of the 2N5087. The resistor divider at the output knocks the gain down to about unity. The dual 2SC1583 and the 2N5087 are reputed to be low noise components, and all resistors are 1/2w metal film. On the actual build, I decoupled the supply with a 33uF tant and a .1uF ceramic. Also, there's a 470pf from the base (input) of Q1 to ground. Anyone have any ideas why this is not as quiet as it might be? All help much appreciated!
Regards,
Joe
Hi.
I'm no expert, but I wouldn't usually like to put a 5.6M resistor to the base of a gain-stage transistor because of the amount of thermal noise the transistor puts in. Check out "noiseless biasing" for info on simple setups that don't inject all that noise.
(A simple approach would be to connect the 5.6M and 560k away from the base. From their junction, connect (1) to the base with a moderate-sized resistor (100 to 470k?), and (2) to ground with a 22uF cap.
I'm sure that the gurus will have other ideas, too.
have fun
Have you tried asking on this forum?
http://prodigy-pro.com/forum/
Not to shoo you away from the great minds on this forum, but these prodigy-people babble about discrete opamps all day long.... :icon_eek:
There are a handful of people over there that seem to have a lot of hands on experience with this, so maybe woth a try.
C
http://aronnelson.com/gallery/KHE/Boss_ROD10_Schematic?full=1
Are you running it open loop? If the gain is real high, that might cause noise. Might also try different transistors to select for the most quiet.
Here are some Boss discrete op amps.
Well I can see a couple of things.
First off, a previous poster was right about that 5.6Meg resistor, it will inject a ton of thermal noise, not to mention limit the amount of current into the input transistor. For noiseless biasing and control over the bias, hook up a 50k trim pot with the outside legs to V+ and ground and connect the wiper to the input through a 100k-220k resistor, also decouple this bias supply with a 10-100uf cap from the wiper to ground. This way you can dial in the bias, and supply enough current to the input.
Secondly, the 100k on the emitters of the diff. pair is too big and limits the amount of current through the diff. pair. I'd drop it down to 22k.
Third, you need to connect the emitter of the output transistor to V+ independently, not in conjunction with the 100ohm resistor on the diff. pair.
And last, and probably most importantly, you ARE running this open loop which means max gain, which also means max noise.
The collector of the output transistor is the output and the base of the transistor in the diff. pair that DOESN'T receive the input signal is the negative input for the opamp. Your bias is so screwy with the 5.6Meg because you have the neg. input attached directly to ground when the opamp wants it to be at the same voltage as the pos. input.
You need a feedback path from the output to the negative input, just like in a regular opamp, for this to work properly.
Discrete opamps are quiter than regular opamps, you are just missing a couple of things that make your circuit an opamp.
Good luck,
Jay Doyle
http://www.diystompboxes.com/pedals/schems/DiscreteBoost.pdf
And then you have this.
Good God. Has it really been almost five years since I designed that? :icon_redface:
Wow.
C5 is redundant and not needed and the layout is horribly big but in essence, that is it.
I hope it helps.
Jay Doyle and Brett are right - you're injecting a lot of noise in your biasing. Change those biasing resistor to 1/10 of what they are now and put a 1M from their junction the transistor base. Decouple the biasing junction at the bias string with a 1uF cap.
Noise should go down some.
It's called "noiseless biasing".
Okay, guys, thanks! I had feeling it was in the biasing (really I did!). Ordinarily, I'd do as everybody's said and use a 50K trimmer as the divider from supply to ground, and then a 1M or so from wiper to the base, not forgetting the decoupling, of course. I didn't do that here because I was trying to save space. This op amp is part of a larger circuit (an analog delay) and it's really crammed into a small piece of perf board. I appreciate the more involved corrections from Jay, but I'm probably not going to do a lot of reworking on this one, mostly because of the elbow-room factor. Besides, as unlinear as the circuit probably is, it does sound very good (aside from the noise).
Thanks so much for the help Brett, Coriolus, WGTB, Jay, and RG!
Regards,
Joe
PS: I'll try to get back here with a lab report after I make the bias changes. . . .
QuotePS: I'll try to get back here with a lab report after I make the bias changes. . . .
Please do! I'd very much like to do some experimenting of my own with discrete opamps, since they're low parts count, and - so I hear - leaps and bounds ahead of monolithics if done right.
C
Other bits, I prefer using a transistor instead of a resistor for the current source, helps make for a longer tail pair.
There are some cool discrete op-amp designs out there, Dean Jensen has one, there are some in many old mixing boards. The biggest problem is getting those input devices to match. Best technique is to use ICs that contain matched transistors, relatively cheap and you can do an entire discrete op-amp with one. Don't listen when people say that there is an IC there, it is discrete transistors.
Quote from: Coriolis on April 18, 2006, 03:29:42 PMPlease do! I'd very much like to do some experimenting of my own with discrete opamps, since they're low parts count, and - so I hear - leaps and bounds ahead of monolithics if done right.
It really depends on what you are going for. For our purposes, most of the time, just use an IC. Unless you are looking for something specific and interesting, like using a Ge transistor as the output device, there isn't much advantage to a discrete unit. In fact, they take up a lot of space.
The do offer an interesting overdrive, it seems to be smoother to my ears. And you can specifically control certain elements, like slew rate, that in an IC are most of the time immovable.
BUT, they don't have anywhere near the gain of an IC and to start to get closer to that ideal, you start adding so many parts that the work gets a bit overwhelming. This makes building things like a discrete Tube Screamer kind of frustrating as the gain isn't sufficient to get the same amount of distortion.
One easy way to improve a discrete opamp is to add a Class A or push-pull follower output stage so that it can drive things better, including it's own feedback loop.
I would recommend Joe D.'s Diode Compression Opamp if you are looking for distortion.
Quote from: Coriolis on April 18, 2006, 03:29:42 PMPlease do! I'd very much like to do some experimenting of my own with discrete opamps, since they're low parts count, and - so I hear - leaps and bounds ahead of monolithics if done right.
It really depends on what you are going for. For our purposes, most of the time, just use an IC. Unless you are looking for something specific and interesting, like using a Ge transistor as the output device, there isn't much advantage to a discrete unit. In fact, they take up a lot of space.
The do offer an interesting overdrive, it seems to be smoother to my ears. And you can specifically control certain elements, like slew rate, that in an IC are most of the time immovable.
BUT, they don't have anywhere near the gain of an IC and to start to get closer to that ideal, you start adding so many parts that the work gets a bit overwhelming. This makes building things like a discrete Tube Screamer kind of frustrating as the gain isn't sufficient to get the same amount of distortion.
One easy way to improve a discrete opamp is to add a Class A or push-pull follower output stage so that it can drive things better, including it's own feedback loop.
I would recommend Joe D.'s Diode Compression Opamp if you are looking for distortion.
Quote from: WGTP on April 18, 2006, 11:08:38 AM
http://aronnelson.com/gallery/KHE/Boss_ROD10_Schematic?full=1
Here are some Boss discrete op amps.
Hey WGTP: Thanks for posting this Boss thing--cool and worth a long look at. Just the fact they went with discretes says something. ICs don't squash the same way, and even in a clean context, they aren't (to my ear) as immediately musical-sounding. The one I posted is very forgiving when overdriven with a line-level signal. I like to think of it as a pseudo analog-tape sound. Sounds even more "tape-ish" when built with Ge xstrs.
Hey Jay: I've seen some push-pull followers designs, but I'd like to try Class A. Is there something you could point to as an example?
Joe
I thought it very interesting. Jay, any speculation as to why Boss is using discrete op amp in a number of their distortions? Notice that the OD III and Distortion use 2 op amps in series for more gain.
It is cool to look at the circuit variations and then the variations in waveform. Dragonfly has them posted in the Layouts Gallery.
http://aronnelson.com/gallery/KHE/Boss_ROD10_Notes_Corrections?full=1
Joe -
Unfortunately, I don't have a specific example but I can describe it, hopefully adequately:
Essentially, you are tacking on a follower stage to the output of your discrete opamp. I'll use a JFET as an example, but it would work for any type of transistor. Connect the gate of the JFET to the junction of the collector and resistor on the output transistor. Drain goes to V+ and the source goes to ground through a resistor 1k-10k (the value matters less when you are using feedback but in your case, because you are running open loop, it directly affects the output impedence). The junction of that resistor and the source now becomes the output of your opamp.
Does that make sense?
WGTP -
You know, I can only make guesses. First, I definitely think that it is a different tone when overdriven, not as harsh as an IC, because the gain is limited and it tends to squash, or round of the corners instead of just slamming into the power rails like an IC. NOTE THAT THIS DOES NOT MAKE IT SOUND LIKE A TUBE (that wasn't for you WGTP ;) ) Second, it is more subtle because of the gain. Note that they are only used in the 'overdrives' not in the megablaster-type distortions. The Blues Drives has two discrete opamps in series. Third, and this is speculation, but it could be cost. Transistors, to a company like Boss, are probably a penny a dozen, which is four discrete opamps (without resistors), while ICs would probably still be a penny or two a piece. And lastly, those waveforms are interesting but I think that what you are seeing in the differences has more to do with the filtering involved than the distortion.
Jay Doyle
Yes, I agree the waveform differences are mostly from the filtering. I guess the low noise might also be an issues. The first OD1 appears to be set up (diode wise) as the SD-1. The ODII is more like a Tube Screamer. The ODIII us 2 op amps in series with no clipping diodes, just Jfets distoring. The Distortion is similar with diodes to ground at the end, like a DS-1. The Fuzz does not use Jfets. I need to find one of those cheep. :icon_cool:
I bought the Boss OD-3 when it came out because I prefer overdrives to distortion types of pedals, and because I liked the sound of the OD-3. And they use that kind of discrete opamp in it.
When Boss refers to that kind of discrete circuits, they seem to call it Natural Overdrive, which is more realistic than calling it Tube Sound.
I may be wrong, but I think they use the discrete part for the sound, and the ICs part for sustain, and the more extreme overdrive setting. They call it a dual-stage design.
At that time, I wanted to work on a circuit of mine with these specs, and wanted to know how a real pedal would sound, and use it as a guide for mine. I finally never built mine...
It's not my fault, I like discrete circuits... So, it's been 5 years since I built Jay Doyle's Discrete Opamp? Hummm, time really flies.
Gilles
QuoteIt really depends on what you are going for
Actually I was thinking more of micpre's and such. This is also where my post about Prodigy-Pro forum comes in. :icon_mrgreen:
Ch
Quote from: Coriolis on April 19, 2006, 02:43:40 PMActually I was thinking more of micpre's and such. This is also where my post about Prodigy-Pro forum comes in. :icon_mrgreen:
Oh man. I'm not even going to attempt to try to get into that. :) I don't have the dicipline or ability to get into studio-type stuff. Maybe Gus will jump in here, that
This is what you are looking for:
http://anklab2.pirit.info/Schemes/Other/je-990.pdf
Not exactly a simple build. It is basically and IC opamp without the IC.
QuoteEssentially, you are tacking on a follower stage to the output of your discrete opamp. I'll use a JFET as an example, but it would work for any type of transistor. Connect the gate of the JFET to the junction of the collector and resistor on the output transistor. Drain goes to V+ and the source goes to ground through a resistor 1k-10k (the value matters less when you are using feedback but in your case, because you are running open loop, it directly affects the output impedence). The junction of that resistor and the source now becomes the output of your opamp.
Thanks Jay--very clear. Actually I'm thinking along the lines of . . .
QuoteActually I was thinking more of micpre's and such. This is also where my post about Prodigy-Pro forum comes in. icon_mrgreen
We're on the same page Coriolis! Probably shouldn't annoy the folks here with such meanderings, but here's an idea I'd like to pursue: Discrete op amp with Ge xstrs, phantom power, transformer in/out. Hence the inquiry about followers, because the need to drive a fairly heavy load. But my goals for linearity wouldn't be anywhere near what a Jensen 990 could do (thanks Jay!). I'd be more interested in "character" over precision. Still the 990 is supposed to sound quite good, and doesn't look like a totally impossible build. Would that output stage be considered push pull?
Joe
I would suggest to check this first, related to the JE-990
http://1176neve.tripod.com/id9.html
More designs here http://www.prodigy-pro.com/forum/viewtopic.php?t=281
Gilles
Hey Friends,
Haven't had hands-on time to do this mod yet, but I'd like to hit the bull's-eye when I do, hence some questions. I'm not going to have room on my board for a trimmer so I'm going to do as RG says and knock down my bias resistors by a 10th, then run a 470K ohm from the divider junction to the base of Q1. But when I do this, will the ratio I've established still hold? That is, if I go from 5.6M/560K to 560K/56K, will the circuit still see essentially the same bias point? And how about if I drop it another decimal point, and use 56K/5.6K, same thing, right? Thanks for the help!
Joe
Quote from: Joe Kramer on April 21, 2006, 02:04:40 PM
Hey Friends,
Haven't had hands-on time to do this mod yet, but I'd like to hit the bull's-eye when I do, hence some questions. I'm not going to have room on my board for a trimmer so I'm going to do as RG says and knock down my bias resistors by a 10th, then run a 470K ohm from the divider junction to the base of Q1. But when I do this, will the ratio I've established still hold? That is, if I go from 5.6M/560K to 560K/56K, will the circuit still see essentially the same bias point? And how about if I drop it another decimal point, and use 56K/5.6K, same thing, right? Thanks for the help!
Joe
Yup, for a voltage divider, it is the RATIO that matters not the absolute values.
I think there is a post missing of yours that I never saw.
Quote from: Jay Doyle on April 21, 2006, 02:06:45 PM
Yup, for a voltage divider, it is the RATIO that matters not the absolute values.
I think there is a post missing of yours that I never saw.
Hey Jay,
Many thanks! Bench time is minimal for me lately, and that's going to help me immensely to be able to jump in and have that change be right on target. I'm not sure what you mean about the missing post though. . . .
Joe
Just found this, another Discrete Op Amp again by Jay Doyle. I think it is in Schematics II. Note Mu-Amp input and Jfet clippers. Cool stuff. :icon_cool:
http://www.diystompboxes.com/pedals/schems/Shaka%20Discrete.pdf
Quote from: Jay Doyle on April 21, 2006, 02:06:45 PM
Yup, for a voltage divider, it is the RATIO that matters not the absolute values.
is this correct? RG sez
Quote from: R.G.The intent of providing a bias voltage is to keep the reference at some middle voltage. If you do this with resistors, any current you suck out of the middle point (or push into it!) will move the reference voltage around. How much depends on the resistor values.
the whole article is here: Designing Bias Supply (Vbias or Vb) Networks for Effects (http://www.geofex.com/circuits/Biasnet.htm)
Somebody needs to post either the link to the schem or the schem itself for the ROD-10. That little table-top half-rack is a compendium of discrete op-amp circuits and circuit tweaks. Takes the sort of stuff you've seen in the BD-2 and other Boss pedals and then goes farther with it.
Not exactly the same but check this out:
http://www.diystompboxes.com/analogalchemy/misc/diodeopamp.html
I built a MXR+ and Red Fuzz with it and it sounds very good.
mac
http://aronnelson.com/gallery/KHE/Boss_ROD10_Schematic
Interesting variations. :icon_cool:
Thank you sir. :icon_biggrin:
For the uninitiated, I do believe that every one of those 3-transistor combinations where you have a pair of FET "facing" each other, followed by a bipolar with a path between collector and the gate of one of those FETs is a discrete op-amp.
QuoteI do believe that every one of those 3-transistor combinations where you have a pair of FET "facing" each other, followed by a bipolar with a path between collector and the gate of one of those FETs is a discrete op-amp.
I took a look. You're correct, all of the two-FET diffamps and a bipolar following is a discrete opamp. Or rather, a discrete feedback amp. They're all identical.
Why use those instead of integrated opamps? Good question.
I think it is because there is no frequency compensation needed, so they have full output through the whole audio range. A diffamp followed by a bipolar gain stage is a two stage amplifier. If it's done properly, a two stage amplifier is inherently stable in feedback. There are only two time constants to add phase shift (at the frequencies that matter) before the gain gets below unity, so no compensation is needed. The whole genre of two-bipolar feedback pairs used this dodge as well.
QuoteThat little table-top half-rack is a compendium of discrete op-amp circuits and circuit tweaks.
They did throw in everything but the kitchen sink.
OD I is a tube screamer-ish distortion with asymmetrical clipping. OD II is the same, but with symmetrical diode clipping. OD III seems to be banging two series discrete opamps against a reduced (5.6V) power supply, and DIST is very similar to OD III but with the bigger 8V power supply and then further limited with diodes to ground. FUZZ is actually our old friend the Univox Super Fuzz octave section, but modified for not-symmetrical rectification. I think this would FWR very imperfectly, which seems to be the point. I'm not surey why the didn't also go ahead and toss in a more perfect FWR section for a more prominent octave.
The tone control section is a modified feedback Baxendall, but with a gyrator-inductor midrange control following it (opamps 1a, 1b, 2a, and 2b). Lots of miscellaneous low pass filters and JFET switching sprinkled around, too.
This is the sort of circuit/unit that could be very instructive if accompanied by illustrative sound clips. Being able to see the circuit differences, and hear them as well has always been tremendously instructive for me.
Quote from: Mark Hammer on May 30, 2006, 01:07:40 PM
Thank you sir. :icon_biggrin:
For the uninitiated, I do believe that every one of those 3-transistor combinations where you have a pair of FET "facing" each other, followed by a bipolar with a path between collector and the gate of one of those FETs is a discrete op-amp.
it depends what definition you go with - if you go back before the 90's you still see people confusing op-amps for OTA's in papers (and at work !) ... what you're missing to really call this a bone-fide op-amp is : (i) greater open-loop gain, and (ii) a voltage buffer at the output ... a traditional Miller-integrating op-amp can be seen simply as a voltage buffered OTA - that's the part people missed in the past ... so in this case you've got a low-drive op-amp/OTA if you will - where the output drive is provided by a 2k collector load - very marginal drive compared to if you followed the 2k load with, say, an emitter follower and took the output at the emitter ...
OD makers realized at some point that using a high-drive op-amp to clip diodes made for sharp "clip" corners on the waveform - and therefore producing more "fizz" in the sound ... using these circuits in conjunction with diodes in the FBK path gives "soft" clipping ... that's the deal here ...
ps. one obvious problem with the Jensen op-amp clone is in the way the output devices are biased ... in my extimation using 1n4148's for turn-on diodes will likely produce an idling current that's too high in the output pair ...
http://aronnelson.com/gallery/KHE/Boss_ROD10_Notes_Corrections?full=1
Here is the waveform visual part. i can't help with the soundclips.
Thanks for the explainations of the circuit. :icon_cool:
Quotewhat you're missing to really call this a bone-fide op-amp is : (i) greater open-loop gain, and (ii) a voltage buffer at the output ...
I guess that we all pay homage to the immortal William Jefferson Clinton's "it all depends on what your definition of "is" is."
The best working definition of an operational amplifier is one where the operation is set by the elements around the opamp, not the opamp itself. Clearly that is what is happening here. The dual JFET stage could be considered an OTA if you modulated the gain with source current - but then all diffamps are like that. The bipolar transistor is running as a transimpedance opamp - you feed it a current, it puts out a voltage. It's analogous to the voltage gain stage in the stock Linn-style power amp and the voltage amplifier stage in the monolithic opamp.
While the definition of the ideal opamp requires nearly infinite gain and near voltage-source output drive, these qualities are relative. The circuit as shown has an open loop gain of 44.5db - that's a gain of 167. So it's "ideal" within engineering tolerances up to gains of 16 or so when the non-infinite gain starts peeking through. The -3db point of the circuit is heavily dependent on the actual devices, but it's somewhere over 190kHz for reasonable guesstimates of parts. The output loading plays heavily into this, but for all loads over about 22K, the loading can be ignored, because there is enough current available to let it look "ideal-ish".
It's worth noting that the same circuit with a current source load instead of the 2.2K collector resistor has an open loop gain of 58db, clearly within the opamp range.
So it's low-ish gain for an opamp, but works without an output voltage buffer for many loads.
Probably they are using the opamp structure for easy setting of bias conditions - which it does very well indeed - and then letting the
What is probably going on is that they're using the oapmp type structure to
Quotea traditional Miller-integrating op-amp can be seen simply as a voltage buffered OTA - that's the part people missed in the past ... so in this case you've got a low-drive op-amp/OTA if you will - where the output drive is provided by a 2k collector load - very marginal drive compared to if you followed the 2k load with, say, an emitter follower and took the output at the emitter ...
Actuall, the input diff stage is a voltage to current stage, a transconductance amp. It's not an
operational transconductance amp unless you arrange to modulate it by varying the current, the idea being to let the external parts set the conditions, and part of what RCA defined OTA as meaning. The voltage gain stage takes in a current, puts out a voltage, so it's a transimpedance stage. The combination of the two is a voltage-> current ->voltage, or a pure voltage gain. The output buffer in the normal opamp is just that - a current buffer. I grant you that it's tricky as most people have never heard of transimpedance stages, but it's not an OTA. It is a low drive, low open loop gain operational amplifier.
If you followed the output with an emitter follower, you would have to put in dominant pole compensation to keep it from oscillating in all likelihood. The phase shifts from the diffamp/voltage stage/ and follower stage would add up, the follower would let it run to much higher frequencies, and you'd get classic gain-phase oscillation unless you were very careful. Three time constants, gain and feedback give you an oscillator unless you try hard not to.
QuoteOD makers realized at some point that using a high-drive op-amp to clip diodes made for sharp "clip" corners on the waveform
Almost. It's the low open loop gain making the corners softer, not the current drive.
...I wonder if we need all those extras components between the diff pair and the output transistor/s in a real opamp, ie, how they affect the tone?
mac
Quote...I wonder if we need all those extras components between the diff pair and the output transistor/s in a real opamp, ie, how they affect the tone?
To a very large extent, the whole opamp game is to make what's in the opamp circuit NOT matter. That's the whole point of operational amplifiers - you can (mostly) ignore the innards and just have the external components set how it operates. To that end, the makers of the ICs put in all the parts that they need to to make the opamp perform as a good opamp for their typical markets. They don't put in stuff they don't need to make it a good opamp, as they see opamps.
But that's not what you asked.
There are issues with mainstream opamps. The high gain and more than two stages forces the use of frequency compensation, and that starts rolling that high gain off at quite low frequencies. The high gain also makes the entry into the nonlinear regions of operation very abrupt. Abrupt changes in audio signals are always associated with high frequency content.
As long as the opamps in effects never run into clipping or nonlinearities, they are very, very neutral and clear sounding. Let the opamp bang against the power supplies and you hear it as buzzy distortion. But we mostly don't do that - we make the opamp drive diodes and transistors into nonlinearities, and listen to those distortions.
In the case of these discrete opamps, it may be good that the open loop gain is low. Low open loop gain lets the underlying amplifier show through, and softens up the edges of nonlinearity by there not being enough gain to cover up the nonlinearities.
Is that good?
Who knows???
I think that sometimes it is, sometimes it isn't. And most likely, like every other situation in musical electronics, it depends on the circuit conditions you have around it and what you're trying to do. Some musical circuits need good DC accuracy. None of that in low-gain discretes. Some need soft, reliable transition into overload. That point goes to low gain discretes. Some need good high frequency gain, as in filters. Point to IC opamps. Some need... well, you see where this is going. You can't really say "Ahah! This opamp has lots of junk between the diffamp and the output buffer, therefore it sounds worse than (whatever else)." with any degree of accuracy if the circuits are not also stated.
Even among IC opamps there is a big variation in the innards, and therefore in the performance: open loop gain, input bias current, input offset voltage, open loop output impedance, dominant pole location, large signal high frequency response, output current, current limiting (the earliest ones went up in a puff of smoke if you touched the wrong pin), input pin common mode voltage range, input differential voltage range, input and output voltage compliance, power supply rejection, and all the other things on datasheets.
The issue is not simple at all. And there isn't a simple answer to your question, as far as I know. It depends - on everything else.
Sorry.
great info, R.G. i have been wondering about the transistor vs. op amp choice. i have read people's preferences for discretes, observing that ICs sound harsh to them, but not seen any explanation of why that might be.
and it's nice to hear that there are lots of variables left to fool around with. :icon_cool: sometimes it seems like there isn't much left to discover--like it's all been done. "it depends" suggests to me lots of opportunity for musicality: tweakable tones and dynamics produced with different circuits.
Quote
The issue is not simple at all. And there isn't a simple answer to your question, as far as I know. It depends - on everything else.
Sorry.
Thanks, RG. You answered my question completely.
mac
Anybody here know of a schematic for using a quad op-amp to emulate a germanium transistor? :icon_wink:
Hi Friends,
Glad to see this thread enjoying some longevity. Finally got some time to remedy the bias/noise problem and here's what the circuit looks like now:
D I S C R E T E O P A M P
9V
5.6K 56K |
+- N N N-+-N N N---+--+
| | |
G Z Z
Z 560K Z 100ohm
Z Z
| +------------------+
| | |
| +---- | -----------+ |
| c| |c | |e
1uF | b|/ \|b | b|/
IN >---||---+----|\ /|---+ +---|\
e| |e | |c 1uF 36K
+--+--+ | +----||---N N N-+---> OUT
| | | |
Z | Z Z
100K Z | Z 3.3K Z 12K
Z | Z Z
| | | |
+--------+-----+------+---------------+
|
(2SC1583) G (2N5087)
The bias network on the input xstr is the same ratio as before, only lowered by several decimal points. There is also a 10uF decoupling cap from the divider junction to ground, not shown. The bias point slipped up a small amount, as per the formula RG writes about, and gauss-markov brought to my attention--thanks! The whole thing is over-biased by about one volt, but this is not a problem, since the circuit is very forgiving in that respect. FWIW, I've often found myself over-biasing many xstr circuits anyway, because they sound a little sweeter to my ears that way.
As promised, this fix greatly lowered the noise, and now it's virtually inaudible. This also lowered the overall noise of the analog delay, of which the above circuit forms the input amp. Because it feeds a compressor going into the BBD, it's noise tended to be magnified even more, and on long delays, there was a sort of "noise halo" around the repeats. That problem is also solved now.
BTW, if you replace the output resistor divider on this circuit with 50K pot, it makes a very sweet booster. I'd recommend a trimmer arrangement for the bias, to accommodate whatever xstrs you happen to have on hand. I don't have a circuit for emulating a Ge xstr with a quad op amp (Ge Whiz), but you could build four of these and emulate a quad op amp with Ge xstrs! Just substitute a 47K for the 100K input emitter resistor, and a 2.2k for the 3.3K output collector resistor.
Thanks everybody for your help in optimizing this circuit--sounds great now!
Regards,
Joe
Is anyone interested in ROD-10 clips - I can make some if anyone is interested - let me know.
The ROD-10 is a decent (above average) sounding OD, fairly average distortion in distortion mode, and really nasty fuzz. What I really love about this unit is the EQ - to me is always seemed to be the most intuitive to have a mid-freq and mid-level control.