A FET compressor

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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mictester
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Post by mictester »

Back in the 80s, we played around with ways of avoiding putting those nasty Transconductance Amplifiers in the audio path. The problems of the Dynacomp (and its derivatives) are well known, and many musicians didn't like the "grab and release" operation of that type of compressor. The limitations of optical types are well known, too. A guy called "Wa" at Maxon came up with a good idea - feed the audio through a low gain transistor or FET stage, and introduce variable negative feedback using a 3080. The the attenuation increases, the negative feedback becomes more prevalent, so the distortion actually reduces.... this is the obverse of the usual results with a voltage controlled attenuator. The limitations of the approach soon become apparent. The range of control is only about 12 -15 dB, but in practice this has the benefit that very low level signals (and hum and noise) don't get amplified, and it's enough range to significantly sustain a guitar.

The circuit is taken from the lab notes (pencil scribbles) from about 30 years ago. I breadboarded and tried this out earlier, and it's as good as I remember. It makes the Dynacomp sound crude. The net effect of this device is to make the guitar behaviour "livelier", and to make it seem that the pick-ups are more sensitive. All the parameters can be adjusted. There's a threshold preset - it was found that there was no point in making it a front panel control - the levels that come from a guitar will usually be fairly similar and the object of the compressor (partially) is to reduce the range of guitar levels....

The prototype had a tantalum 10µ for the timing capacitor, but I didn't find any benefit in using one (except for size!). My prototypes didn't use anti-log pots - I used 500k for the sustain pot with a couple of padding resistors and a 250k for the attack, again with padding resistors. The audio path capacitors were just cheap green polyester ones, and they worked fine. I used a TL062 for the sidechain (I just had one) and an LM833 for the audio. I've made this now with a 3080, half a 13600 and a BA662 - all worked flawlessly. You may have to select the series resistors for your LED - these values worked well to get equal subjective brightness on the clear bi-colour ones I used.

The circuit draws about 11mA as I've built it (BA662), so it's not a battery-killer. It is possible to run the thing from 15 or 18V for greater headroom, but I didn't hear any improvement when I tried it. This really is a guitar effect - bass sounds really "thin" through it, though it would be possible to modify the values if someone really wanted it.
Board layouts to follow.
Board layouts to follow.
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Post by snofla1900 »

This sounds very exciting Mictester ! I bought one of the original dynacomps back in the old days and was reasonably satisfied with, I used it a lot for my leadsounds , the Dynacomp after the distortion to smooth it out , worked great. But the main drawback was always the distortion it introduced when used in a clean way. I switched to a Yamaha then and this was better distorionwise but the feel wasn't as good.

Years later I built another dynacomp with the Ross mods and still use it now and again.

But if yours is as good as you're saying I would be very glad to finally get a really good compressor.
Look forward to building it . Will you make a pcb board design that would be great, I don't build vero.

Regards , Alf

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Post by mictester »

Hi Alf

The first layout will be Vero (that's what I've built my two prototypes on), but I'll also do a PCB / perfboard layout. I'll put them up here when I've double-checked them later on.
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Post by tube-exorcist »

What I don´t understand:
The 3080 is part of the feedback, NOT in the audio path,....
"feed back" - where does it "feed back" to when not into the audio path ?
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Post by mictester »

tube-exorcist wrote:What I don´t understand:
The 3080 is part of the feedback, NOT in the audio path,....
"feed back" - where does it "feed back" to when not into the audio path ?
The audio is not directly fed through the 3080 - it's used as part of the feedback around the FET buffer stage. You're hearing the FET, not the 3080
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Post by earthtonesaudio »

mictester wrote:The audio is not directly fed through the 3080 - it's used as part of the feedback around the FET buffer stage. You're hearing the FET, not the 3080
Don't miss a teachable moment, Mic. :wink:

The transfer function from gate to source of the JFET is approximately something like:

gainOfJFET/(1 + gainOfJFET*gainOf3080)

And since the gain of the JFET is approximately 1, we can simplify to:

1/(1+gainOf3080)

Which tell us that when the gain of the 3080 is large, the output is small, and vice versa. So the net effect is that you hear very little of the 3080 in the final sound.
But that's not the same as it not being in the audio path - that would be similar to saying the clipping diodes are not in a Tube Screamer's audio path. They clearly are, but because of negative feedback their behavior is radically different compared to when they are used without feedback.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by tube-exorcist »

:scratch: ... and I thought that the output of the FET-buffer is fed to the inverting input of the OTA.....
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Post by mictester »

earthtonesaudio wrote: Which tell us that when the gain of the 3080 is large, the output is small, and vice versa. So the net effect is that you hear very little of the 3080 in the final sound.
That's the point. There's a lot less distortion through this circuit than through a Dynacomp.

Other pedals use this kind of approach too - the Morley compressor uses the 3080 in the feedback loop of an op-amp. Their circuit needs about 12V supply for reasonable results, which makes it unsuitable for the traditional 9V battery operation. Electro-Harmonix also had a 3080 feedback gain cell in one version of their "Black Finger" compressor.

In each case. the 3080 distortion is reduced by the feedback action of the associated amplifier stage.

The other advantage of this circuit is that everything can be adjusted - input gain, attack, ratio, threshold, decay, slope etc. This is because the sidechain is entirely separate from the audio path. The values shown are ones that worked well with guitars. Frequency-dependent feedback (through the 3080) can make this into a really "jangly" effect, or it can be configured as a (sort of) noise reduction circuit.
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Post by perfector »

Mictester, you genius!
mictester wrote:bass sounds really "thin" through it, though it would be possible to modify the values if someone really wanted it.
Yes please for the bass too :)

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Post by merlinb »

So... how is this a FET compressor? The OTA does the compressing, the FET is just a buffer. Any old buffer would do the same job.

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Post by roseblood11 »

The Boss FA-1 was sold as a "FET Amplifier" - and became a classic...

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Post by friedtransistor »

i like the idea, but can the effect be modified for those of us who don't have transconductance op amps? i would like to make this thing... unless it doesn't do much squash. i like the punchy, plunky type sound, and am always looking for new compressor designs. any help is appreciated.
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Post by earthtonesaudio »

merlinb wrote:So... how is this a FET compressor? The OTA does the compressing, the FET is just a buffer. Any old buffer would do the same job.
Two reasons:
1. it's common practice to name a device after the coolest part of it (i.e. "tube" reverb units)
2. (better reason) because "any old buffer compressor" doesn't have the same ring to it

:block:
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by earthtonesaudio »

friedtransistor wrote:i like the idea, but can the effect be modified for those of us who don't have transconductance op amps? i would like to make this thing... unless it doesn't do much squash. i like the punchy, plunky type sound, and am always looking for new compressor designs. any help is appreciated.
Not easily. You could make an OTA out of discrete transistors and diodes, but it would suck.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by coldcraft »

very nice mictestor. I haven't looked at it too hard yet but would this design be capable of downward expansion below a certain threshold with the right addition to the circuit?
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Post by earthtonesaudio »

coldcraft wrote:very nice mictestor. I haven't looked at it too hard yet but would this design be capable of downward expansion below a certain threshold with the right addition to the circuit?
Not with the OTA in the feedback loop.

You could take the OTA out of the feedback loop and make it more of a true side chain by taking the input signal from the first op-amp (or even the input buffer BJT). That would free you up to tweak the envelope however you want.
This will completely change the response of the effect though, so other things may need significant tweaking.
rocklander wrote:hairsplitting and semantics aren't exactly the same thing though.. we may need two contests for that.

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Post by coldcraft »

earthtonesaudio wrote:Not with the OTA in the feedback loop.

You could take the OTA out of the feedback loop and make it more of a true side chain by taking the input signal from the first op-amp (or even the input buffer BJT). That would free you up to tweak the envelope however you want.
This will completely change the response of the effect though, so other things may need significant tweaking.
ah, good point. in order to act like a gate, it must be feed-forward. I guess you could create a second envelope detector tapped from the same post-FET point, and use the other OTA from a 13600 to Gate the signal closer to the output.
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Post by DrNomis »

I'm interested in building one of these as I happen to have a couple of LM3080 ICs spare, so looking forward to the Vero layout, one question, will an MPF102 work in this circuit, or do I need to use the type specified?..... :thumbsup

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Post by peterc »

Hi all

This looks like an amazing compressor, did anyone do a PCB?

I would prefer to use the 13600, how can I sub it into the circuit?

I personally love smooth compression and this machine really piques my interest!

Thx Peter

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Post by Lani »

peterc wrote:Hi all

This looks like an amazing compressor, did anyone do a PCB?

Thx Peter
Here ya go....just whipped it up yesterday so I have not had the chance to test it yet. I will in the next couple of days though. I'm out of toner for my printer at the moment. Good luck and sorry for all the jumpers :wink:
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