Do any current preamp/boost pedals on the market use a dual op-amp circuit?

[circuit_learner_noob]

I am considering building my own clone of the Korg SDD-3000 preamp (schematic here). It looks like - after the buffer - it has two op amps in series (TL072 and 4558D).

I was wondering if any overdrives or boost or preamp pedals currently on the market have similar circuit topology.

I stumbled on this page and it looks like to my noob eyes that the Boss overdrives also have two op-amps in series. Obviously the rest of the circuitry is quite different.

Could someone explain the big differences in the SDD-3000 preamp clone's circuit and these other overdrive circuits? Also, if anyone knows of other pedals which more closely resemble the SDD-3000 preamp circuit, I would really appreciate the insight.

Thanks!

[tootsMcgee]

That's a pretty open ended question with lots of answers, but I'll dump some thoughts. Lots and lots of pedals will have something similar! The IC can be used for multiple purposes. The shape/topology of the circuit around it dictates what it does. DOD overdrives have a similar setup, although they usually use both halves of one chip as a buffer, one half of the other chip as a boost, and the last half of the other chip as part of the active tone stack. https://www.diystompboxes.com/smfforum/index.php?topic=131556.msg1283713#msg1283713 So the signal goes chip 1 buffer 1 -> gain -> tone -> chip 1 buffer 2

Not to toot my own horn too much (heh) but the ENGL Fuzzyhead has kind of a similar idea, where it has one gain stage (adjustable) followed by another gain stage (fixed). https://www.diystompboxes.com/smfforum/index.php?topic=131981.0

I haven't posted the other half of the schematic yet, but it looks like half of the other IC goes to a baxandall tone stack and the last half goes to a volume boost or output buffer.

Guessing based on the SDD-3000:

The first half of TL072 (IC1A) is a high impedance buffer to sample the signal and make it available to the rest of the circuit at low impedance. Most pedals are like this except for things like the Fuzz Face which were low impedence and as such were very greedy with the signal, interacting in weird ways with the guitar pickups. Anyway, you can tell it's a buffer because the feedback to -IN is just a straight shot from the output.

The second half of the TL072 is your main adjustable boost and I'll be honest I have no idea why it's wired the way that it is, with diodes to ground *before* it. I don't think the buffer can drive the signal that high to make them useful. Never seen that before and I hope I learn what it does in this thread. But the pot in the feedback loop is what tells me it's an adjustable boost.

The first half of the next JRC4558D IC is a fixed boost. No idea why it's a different type of op-amp. The specs seem inferior to the TL072 (lower slew rate == lower ability to change amplitude rapidly, higher current draw...?) But what gives it away for me as a fixed boost is that there's a fixed resistor in the feedback loop.

You see some pedals that have a fixed gain stage later in them to make up for volume loss in passive tone stacks...I don't think that's happening here.

I'm spitballing here...hoping to see some interesting discussion. I too want to know why it is the way it is.

p.s. it only has one op amp but it uses both stages in series for gain, my favorite pedal to date: https://aionfx.com/app/files/docs/cerulean_documentation.pdf

[ElectricDruid]

If you want the SDD-3000 preamp circuit, build the SDD-300 preamp circuit. The others are different. Whether they're different enough to make difference dpends on exactly what you do and what you build, but the obvious solution if you've got a specific end-point in mind is to simply build that circuit.

My bet for why they used a JRC4558 for the other stages and a TL072 for the input stages is because the TL072 had better noise figures at the time, but the '4558 was cheaper for the stages where it mattered less because the signal level had already been boosted.

There's a good clone already done here, so I wouldn't bother doing your own:

https://aionfx.com/app/files/docs/eclipse_legacy_documentation.pdf

HTH,
Tom

[tootsMcgee]

My bet for why they used a JRC4558 for the other stages and a TL072 for the input stages is because the TL072 had better noise figures at the time, but the '4558 was cheaper for the stages where it mattered less because the signal level had already been boosted.

That was what came to mind, ironically now the TL072 is so ubiquitous, the JRC4558 is more expensive now. TL072s are like the Camry of op-amps. But that makes perfect sense for the time period.

[ElectricDruid]

That was what came to mind, ironically now the TL072 is so ubiquitous, the JRC4558 is more expensive now. TL072s are like the Camry of op-amps. But that makes perfect sense for the time period.

Yeah, funny how that happens...the better op-amp became the "standard" (and god, it really did!) but the "lesser" op-amp is now probably much more lusted after for its perceived imperfecions. Whether it *really* makes much of a different in a given sitution is a question of measurements (and not listening tests by people who've already decided what they think!) but it's never going to be a huge deal, outside of a few extreme situations.

[GGBB]

If you ignore the input buffer and the op-amp input (inverting vs. non), it's a common op-amp based two-stage design. A variable gain stage followed by a small fixed boost/make-up stage. A large majority of boosts and overdrives use this topology. I think the use of the inverting input in both gain stages is possibly the only thing unusual. An input buffer is not uncommon either, but we might be more used to seeing a JFET for more mojo instead of an op-amp stage. It is weird that they could have just made the first gain stage non-inverting to avoid the low impedance input that necessitated the input buffer, and therefore would have been able to save the cost of a second IC altogether. Maybe that inverting design is what makes it sound the way it does. And/or maybe the mojo of the 4558.

[ElectricDruid]

I agree with Gord. It's entirely unremarkable as a design.

The input buffer is absolutely standard. I would put something virtually identical to this on pretty much every pedal design I do. I don't generally bother with the prtoection diodes, but whatever - go for your life and include them. Unless someone whacks it with massive inputs, it makes no odds anyway.

The second and third stages are where it gets marginally more interesting. Like Gord said, the choice of inverting rather than non-inverting stages is a bit weird. It would be pretty easy to combine the function of the two inverting stages into *one* non-inverting stage and thereby remove the need for the second op-amp chip. And yeah, sure, do the whole thing on one 4558 if you want to add "mojo", whatever, I don't care...

D3 and D4 (on the Aion Schematic) are one part I don't understand. Again, they look like protection diodes, but given the earlier protection diodes, and given that they're connected to a virtual ground point, I don't see how they'd *ever* actually do anything. Perhaps the original ciruit had something else that made them required in some circumstance or other? Dunno. I certainly don't see them doing anything in the Aion schematic, or many other similar clones.

If it was me, I'd look at the gains and the frequency response (there's *no* big frequency shaping going on) and wrap the whole thing up onto one JRC4558 chip (just for vibes) as a buffer and a non-inverting gain stage. And I'll bet you 50 earth dollars you can't tell the difference in a blind test, too

[PRR]

WELCOME!

explain the big differences

It may be like my chili: not deliberate decisions but random ingredient tossing. Or cleaning-out the parts bins. Or re-visiting old themes in new combination.

[circuit_learner_noob]

If you ignore the input buffer and the op-amp input (inverting vs. non), it's a common op-amp based two-stage design. A variable gain stage followed by a small fixed boost/make-up stage. A large majority of boosts and overdrives use this topology. I think the use of the inverting input in both gain stages is possibly the only thing unusual. An input buffer is not uncommon either, but we might be more used to seeing a JFET for more mojo instead of an op-amp stage. It is weird that they could have just made the first gain stage non-inverting to avoid the low impedance input that necessitated the input buffer, and therefore would have been able to save the cost of a second IC altogether. Maybe that inverting design is what makes it sound the way it does. And/or maybe the mojo of the 4558.

Thanks everyone for the discussion! Very interesting to read.


So... am I just looking for a boost that uses opamps and not diodes? @EletricDruid says that the two variable+fixed gain op-amps can be combined into a single non-inverting op amp.


Does that mean that an MXR microamp should sound virtually similar? Maybe with some eq'ing to mess with the fine details?

[circuit_learner_noob]

I probably should have said earlier - but I am trying to recreate the famous Edge/U2 tone but would rather not spend $400-500 on an SDD3000. I have a strat+AC15, just need the boost he gets from the SD3000... If I could just do an MXR microamp or similar boost into a delay, that would be perfect.

[bluelagoon]

There are a whole range of FA-1 Preamp pedals over on Reverb
And if its value for money you want, then you cant go past the clones of the original Boss FA-1, that are reproduced accurately component for component same as originals. Check the following Reverb Store, there are plenty Box variations and some tweaks to FA-1 circuit variations, at reasonable prices
Audio Blend
The best unit is the Edge EQ, which is the FA-1 original circuit followed in series with a Parametric EQ for a whole lot of Edgyness Tonal variation with a whole lot of Boost.
Edge EQ
Then there is the Demo Vid on Youtube for the same -
edge EQ Youtube Demo
The Boss FA-1 is same pedal as used by the Edge throughout his musical career, as part the arsenal to get his sound across.

[bluelagoon]

Does that mean that an MXR microamp should sound virtually similar? Maybe with some eq'ing to mess with the fine details?

As for the MXR MicroAmp, compared to the Boss FA-1, they are both preamp boosts, but pretty much distinctively different
sounding to each other, and the FA-1 does have the extra niceness of the Tone controls

[circuit_learner_noob]

There are a whole range of FA-1 Preamp pedals over on Reverb
And if its value for money you want, then you cant go past the clones of the original Boss FA-1, that are reproduced accurately component for component same as originals. Check the following Reverb Store, there are plenty Box variations and some tweaks to FA-1 circuit variations, at reasonable prices
Audio Blend
The best unit is the Edge EQ, which is the FA-1 original circuit followed in series with a Parametric EQ for a whole lot of Edgyness Tonal variation with a whole lot of Boost.
Edge EQ
Then there is the Demo Vid on Youtube for the same -
edge EQ Youtube Demo
The Boss FA-1 is same pedal as used by the Edge throughout his musical career, as part the arsenal to get his sound across.

I have found multiple sources which show that The Edge used the SDD3000 as a key part of his sound:

https://youtu.be/7HnWpSaZXu0?si=VGz2n7CL_Kc1fxQA

https://youtu.be/9tHsp2BX5IM?si=nyLe0kkPl9DLCSpt

https://youtu.be/b0bkPZHHVrA?si=O2oPSn6GURJbw3nz

He does use the FA-1 on Pride and Unforgettable Fire, but the SDD is more representative of Where The Streets Have no name etc.

If, according, to the comments, its preamp can be folded into a single non-inverting opamp, then that would be really cool!

[bluelagoon]

Probably likely true, he may have favored the SDD3000 more for a range of songs and less so the FA-1 for less songs. However the FA-1 was an integral part of the guitar sound of the Edge in U2, for his really big distortion, he used the small Boss FA-1 fet amp to boost the lower output pickups on some of his Stratocasters and his Rickenbacker 330/12 guitar.

You said you wanted a preamp to get something of the U2 sound, so I believe that the FA-1 being part of that sound would be a reasonable start point, and if you were wanting to build it yourself, is an easy circuit build for a beginner, likely easier than the SD3000, although with Aion FX having the PCB's pre fabbed for sale, both would likely be an easy build.

You have to realize also from those 3 video links on the SDD3000, they are mostly adds advocating the benefits of the SDD3000, particularly the last one direct from Korg itself, So doubtless plenty of favor shown to the pedal. Also realize the Edge used the SDD3000 a lot for its Delay facility, Which from what I took of your initial post was your interest in the Preamp Boost effects used from U2. Which were both the SDD3000 and the FA-1 both were used as preamps by the Edge, so you could just place a delay after the FA-1 which would get you near enough.

here are couple of links describing the Rig Rundown of the Edge, He used a lot more than just preamps -

https://forum.u2guitartutorials.com/content.php?56-Article-with-Video

https://www.uberproaudio.com/who-plays-what/158-u2-the-edge-guitar-gear-rig-and-equipment

[StephenGiles]

As I have said for the last 30-40 years, in a band situation playing at gig levels, nobody is going to tell the difference between any opamp, transistor or valve used - human ears just hear LOUD!!!

[ElectricDruid]

I've had a bit of a play with this circuit this morning.

Here's the circuit, copied from Aion's schematic:


Here's the response:


As you can see, it's basically a flat-across-the-audio-band response, which is what you'd expect (it's from a bit of studio equipmwnt after all, so it should be hi-fi). There's a little bit of low-end droop because C5 on my schematic (C3 on Aion's) feeds a 3K3 resistor, which makes for a highpass you might theoretically notice on bass.

What I've done in my circuit is copy the original circuit across the bottom, with the output at "Original". I've then tapped the signal from the Pre pot and experimented combining the two inverting stages into one non-inverting stage. This is the "Simplified" part at the top. This works pretty well. Having the two versions of the circuit on one schematic makes it easy to compare them and tweak it.

There's a few things to notice:

1) I had to add R19/3K3 to ground to copy the low-end droop of the original. If you want an improvement, remove it.

2) The original has a little bit of high-end droop above 20KHz. I haven't copied that. You can increase the value of C2 to 22p to match the droop at maximum gain, but it doesn't capture the effect at lower gain. The simplest way would probably to add a small lowpass cap to the output network.

3) The original circuit allows the gain control to actually turn the signal *down*. This happens because when the value of the gain/level pot goes below 3K3, the gain goes below one. You can't do this with a non-inverting amp arrangement, so there's a tiny region at the far end of the pot where the two responses differ.

Otherwise, I think it's pretty good. Certainly it's close enough you won't hear any difference. The minor differences on the graph are in subsonic/ultrasonic regions. With a bit more fiddling about, you can probably get it closer still, but I've got other things I need to do this Sunday!

[circuit_learner_noob]

I've had a bit of a play with this circuit this morning.

Here's the circuit, copied from Aion's schematic:


Here's the response:


As you can see, it's basically a flat-across-the-audio-band response, which is what you'd expect (it's from a bit of studio equipmwnt after all, so it should be hi-fi). There's a little bit of low-end droop because C5 on my schematic (C3 on Aion's) feeds a 3K3 resistor, which makes for a highpass you might theoretically notice on bass.

What I've done in my circuit is copy the original circuit across the bottom, with the output at "Original". I've then tapped the signal from the Pre pot and experimented combining the two inverting stages into one non-inverting stage. This is the "Simplified" part at the top. This works pretty well. Having the two versions of the circuit on one schematic makes it easy to compare them and tweak it.

There's a few things to notice:

1) I had to add R19/3K3 to ground to copy the low-end droop of the original. If you want an improvement, remove it.

2) The original has a little bit of high-end droop above 20KHz. I haven't copied that. You can increase the value of C2 to 22p to match the droop at maximum gain, but it doesn't capture the effect at lower gain. The simplest way would probably to add a small lowpass cap to the output network.

3) The original circuit allows the gain control to actually turn the signal *down*. This happens because when the value of the gain/level pot goes below 3K3, the gain goes below one. You can't do this with a non-inverting amp arrangement, so there's a tiny region at the far end of the pot where the two responses differ.

Otherwise, I think it's pretty good. Certainly it's close enough you won't hear any difference. The minor differences on the graph are in subsonic/ultrasonic regions. With a bit more fiddling about, you can probably get it closer still, but I've got other things I need to do this Sunday!

Really interesting. Thanks for your analysis, this answers a lot of questions so I appreciate your time. Pretty clearly shows that shelling out however many hundreds of dollars just for the "magic preamp sound" is totally unnecessary, and that a MXR Micro-amp can probably recreate enough of the boost.

For my own knowledge - if the frequency responses are identical, can there still be differences if you push the opamp hard enough to make it clip? Could it be possible that below the clipping level the circuits have identical behavior, but once they're overdriven they start to react differently? Though if this isn't the case, I assume I can simply get a hold of a Micro-amp type pedal that has input and output knobs.

I read that people ascribe the Edge's tone from the SDD3k preamp to be from him clipping the preamp circuit (the Aion schematic appears to have an input level control before the TL072+4558 circuit). He would also apparently select the "-30dB" setting for the input attenuator, but probably that makes it easier to clip - not sure if the attenuator circuit itself imparts any tone?

[ElectricDruid]

Really interesting. Thanks for your analysis, this answers a lot of questions so I appreciate your time.

You're welcome. It was intersting to play about with.

Pretty clearly shows that shelling out however many hundreds of dollars just for the "magic preamp sound" is totally unnecessary, and that a MXR Micro-amp can probably recreate enough of the boost.

Certainly as far as "clean boost" goes, one is the same as another, by definition.

For my own knowledge - if the frequency responses are identical, can there still be differences if you push the opamp hard enough to make it clip? Could it be possible that below the clipping level the circuits have identical behavior, but once they're overdriven they start to react differently?

Yes, absolutely. Not all op-amps clip identically, so if you push stages that weren'T necessarily designed to do that into clipping, you can get various results.

I read that people ascribe the Edge's tone from the SDD3k preamp to be from him clipping the preamp circuit (the Aion schematic appears to have an input level control before the TL072+4558 circuit). He would also apparently select the "-30dB" setting for the input attenuator, but probably that makes it easier to clip - not sure if the attenuator circuit itself imparts any tone?

No, doesn't look like the attenuator adds anything much. The Aion schematic follows the original circuit switched into the -30dB setting, and that's what I copied above, so the frequency response is what you see.
The first of the two gain stages is an inverting op-amp with 100K/3K3 (in the -30dB position), which is x30 gain, so yes it might be possible to make it clip. If that's the case, it'd be the TL072 that was clipping. However, it's also possible that that stage stays just the right side of the headroom, and no clipping occurs until the next stage, which gives a 330K/100K = x3.3 boost. If you really whacked it, perhaps you could get a clipped signal from the first one, in which case the second one would definitely clip as well. At this point, you'd be into classic "gain-staging", but whether the Edge drives this preamp that hard, I have no idea. It seems unlikely. It's a hard clip with *no* tone shaping whatsoever (flat frequency response), so it wouldn't be most people's first choice of drive pedal! A bit of clipping from the second gain stage seems more likely (4558 distortion is better behaved than the TL072, for a start) so I'd guess that's what's going on.

HTH

[circuit_learner_noob]

No, doesn't look like the attenuator adds anything much. The Aion schematic follows the original circuit switched into the -30dB setting, and that's what I copied above, so the frequency response is what you see.
The first of the two gain stages is an inverting op-amp with 100K/3K3 (in the -30dB position), which is x30 gain, so yes it might be possible to make it clip. If that's the case, it'd be the TL072 that was clipping. However, it's also possible that that stage stays just the right side of the headroom, and no clipping occurs until the next stage, which gives a 330K/100K = x3.3 boost. If you really whacked it, perhaps you could get a clipped signal from the first one, in which case the second one would definitely clip as well. At this point, you'd be into classic "gain-staging", but whether the Edge drives this preamp that hard, I have no idea. It seems unlikely. It's a hard clip with *no* tone shaping whatsoever (flat frequency response), so it wouldn't be most people's first choice of drive pedal! A bit of clipping from the second gain stage seems more likely (4558 distortion is better behaved than the TL072, for a start) so I'd guess that's what's going on.

HTH

I found a number of sources online that confirm the settings he used on the SDD 3000, and they apparently cause clipping in the preamp (the SDD has an level readout meter, and the Edge's settings put the meter in the "red").


So it sounds like the "magical" tone is using the TL072 (or anything) to clip the 4558 opamp.


Or in other words a distortion/overdrive pedal where the clipping stage is a 4558 and not a diode? Dang, those probably aren't made so I'll have to buy/make the clone if I really want it that badly.

[ElectricDruid]

Or in other words a distortion/overdrive pedal where the clipping stage is a 4558 and not a diode? Dang, those probably aren't made so I'll have to buy/make the clone if I really want it that badly.

There are plenty of pedals where the op-amp clips. A lot of the MXR Distortion+/DOD250 style stuff, for example. They tend to be followed by *more* clipping diodes, so you don't hear the op-amp clipping until after it's been diode-clipped as well, but it's in there. Getting something like that and putting a 4558 in it and taking the diodes out would be a good way to start!

Compare what we've been looking at with this. They're not so different:

https://buildyourownclone.com/products/lil-gray-od-kit