THE ENGINEER'S THUMB... At last, a better compressor!

[jtn191]

The only difference is 3 more pots, 3 more knobs, what seems like much more wiring

[samhay]

Hopefully this is not too far off topic. I have a hypothetical question about the precision current source in the ET.

Say I wanted to drive 2 OTAs (i.e. both sides of an LM13700) with the ET's envelope detector. Can I:
- halve the 1k resistor to Vref and then split signal from the BJT's collector with a couple of low value resistors.
- parallel two BJTs, each with their own 1k and 1M resistors?

Are these reasonable, and/or is there a better way?

[merlinb]

Say I wanted to drive 2 OTAs (i.e. both sides of an LM13700) with the ET's envelope detector. Can I:
- halve the 1k resistor to Vref and then split signal from the BJT's collector with a couple of low value resistors.
- parallel two BJTs, each with their own 1k and 1M resistors?

The first way is a bit dodgy because current probably won't divide equally between the two OTAs, and you don't want to overdrive one or the other. It might work with decent sized collector resistors, but unfortunately there isn't enough voltage headroom available for large resistors.

The second method has the same problem with current sharing, but this time between the two transistors.

A better way is to connect only the base of the second transistor to the base of the first one, but no other parallel connections. Both transistors have their own 1k/1M. Use matched transistors.

What is it you're trying to do? Why do you want to control two OTAs with the same control voltage?

[samhay]

OK, that makes sense. I didn't like option 1, and I guess option 2 should keep the op-amp happier - Thanks Merlin.
Something like this (sorry about the resolution)?


What do I want it for? Mostly a thought experiment at this stage - a few ideas:
A stereo ET, if the signals were summed before going into the envelope detector
A compressor with a noise gate on the front end and a common envelope detector...

[merlinb]

Something like this (sorry about the resolution)?

Yes that's what I meant. It might be nice to add a protection diode on the other transisitor too, from base to emitter, although I may be being over cautious...

[samhay]

Yeah, I noticed some interesting spikes in the simulations. Diode added, and will let you know if I ever get to the breadboard stage.

[jtn191]

In studying the attack/release times of more compressor pedals, I thought I'd suggest lower value pots for attack/release...possibly log taper for attack. With 1M, the ET gives 4ms-160ms attack time but most of the usable-for-electric-guitar values are scrunched up in the low values. Dynacomps have 5ms, my Orange Squeezer is sitting at ~22ms (which is what I'd consider near the longer end of what a guitar player would want), Demeter Optocomp is spec'd at 1ms. I may try 1MA, 500KB/A, or 100K/A

[samhay]

Playing with bits of this on the breadboard - it is a very nice circuit. What is the 1k in the feedback path of U1A doing and/or why is it not directly at the output of the OTA?

[merlinb]

Playing with bits of this on the breadboard - it is a very nice circuit. What is the 1k in the feedback path of U1A doing and/or why is it not directly at the output of the OTA?

TBH, I doubt it is needed for anything. But something about connecting the output of the OTA directly to the input of the opamp gave me the willies, so I put in the resistor for a bit of 'compliance' as it were. But if it works the same for you, feel free to leave it out.

[samhay]

^something about connecting the output of the OTA directly to the input of the opamp gave me the willies
You and me both. It works fine without it, but I will be leaving it there.

The thing that threw me was that you placed it in between the (-) input and the resistor to (virtual) ground. I'm guessing there is no magic in placing it there rather than straight off the output of the OTA?

[PRR]

I almost posted a thought about the 1K working against the 100pFd to force a tame phase-curve around two cascaded gain-chips in a loop.

However the OTA output is 9,999 or 4,999 Ohms, so another K didn't seem to make much difference.

Then I realized that, for DIY one-off, it is NOT worth fretting over a 12 cent resistor.

[samhay]

^Then I realized that, for DIY one-off, it is NOT worth fretting over a 12 cent resistor.
Not fretting, just trying to work out how it works and whether something unexpected will happen if I poke it with a stick...

[tombaker]

Just built this vero version up and replaced it in my fuzz face DI. The fuzz face wasn't that useful but I've just run drums, bass, guitar and vocals through this and it's really useful.
I've currently only got ratio and volume hooked up but I might add a release knob just for some versatility with shaping.
I like using guitar pedals during a mix and have been using the boss blue comp with monte allums mod which is great and this is just a whole new colour.
So thanks Merlin, much appreciated.

[seedlings]

Time to build a compressor.

There was one post in this thread that said the LM13600 works in place of the LM13700.  Would there be any considerations in the substitution - anything to change from the page one 2 knob layout?

Mucho Gratis!

CHAD

[jtn191]

http://www.ovnilab.com/reviews/thumb.shtml A review by a bass player/compressor fan

[garcho]

There was one post in this thread that said the LM13600 works in place of the LM13700.  Would there be any considerations in the substitution - anything to change from the page one 2 knob layout?

No difference, use either.

LM13600 vs LM13700 Revisited

[seedlings]

There was one post in this thread that said the LM13600 works in place of the LM13700.  Would there be any considerations in the substitution - anything to change from the page one 2 knob layout?

No difference, use either.

LM13600 vs LM13700 Revisited

Thank you!  The off-board pots are all I have left before first testing.

CHAD

[seedlings]

AAAAAaaaaaaand... will a germanium PNP work OK?  It's a GT108V.  I ask because there is clean signal, louder with ratio rotation, but no compression.

CHAD

[orangepit]

For anyone who wants to use the pcb by batelfixx from page 4...
It's too low resolution in the pdf and pads close to each other seem to haze together, so I photoshopped the components out of the high res layout image. It etches beautifully.
It is 300dpi and the dimensions are written.


I don't have the two film 1uF caps so it isn't completely populated yet. I'll probably stick my only 1uF in one slot and a nonpolar alum cap in the other for the time being. the bare space on the top left is where the millennium [won't] go.


Don't use this PCB! It's wrong. I should have checked it carefully considering it was the guy's first pcb. Nothing but heavy distortion. The part after the two diodes leading to the two 1uF caps is screwed up. I haven't checked anything else.

[tombaker]

Will a charge pump do any good?
A max1044 voltage doubler specifically.
I searched the thread and can only find a mention of it from midwayfair early on but no follow up.
The IC and transistor datasheets will allow it but what other considerations should I be taking into account?
Thanks