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

[merlinb]

For a long time I've felt guilty that my pedal board didn't include a compressor. Finally, after about a year of breadboarding, I have produced a circuit that can take on a Dynacomp and win, boasting:

   
• Up to five times more headroom than a Dynacomp
• Less noise than a Dynacomp
• Same available sustain as a Dynacomp
• Fewer parts than a Dynacomp
• Cheaper than a Dynacomp
• Feedforward side chain
• Possibility for all five controls
• Fits in 1590B
• Millennium bypass

Despite its final simplicity, I went though just about every possible configuration of OTA while chasing acceptable performance. As you can see from the schem, the OTA is working as a current-controlled resistor in the feedback loop of an opamp. This reduces the contribution of OTA noise.

The input signal is also coupled directly to the side chain, which consists of a precision rectifier (U1b) and precision current source (U2a) that dumps more current into the OTA control pin as the audio signal gets larger.

The ratio control blends compressed and uncompressed signals, so its variable from zero to infinite ratio (limiting)! I actually built two versions of this, one with threshold the other with ratio, but I found that basically the same sounds obtain with either, depending on how you use them, so I will probably sell one of them.

More info, sound clip and PCB layouts can be found here: http://valvewizard.co.uk/engineersthumb.html
I'm now working on a five-knob version with VU meter...

[merlinb]

[frequencycentral]

Clever bugger!

[alfafalfa]

This is really great !!

Will there also be a pcb version ?  I always etch my boards.

Thanks

[merlinb]

Will there also be a pcb version ?  I always etch my boards.

Yes, there is a link in the first post.

[Slade]

merlin, your compressor looks awesome, very cool design.
Did you tried release and attack pots? How do they behave?

Thanks a lot for your work, can't wait to try it!!

[sugonidamaso]

Wow! Great work merlin! Cool sound! Let me check if there's 13700's here in my area.built a dynacomp couple of years back (CA3080 costly ) Thank you!

[Earthscum]

Nice design.

So, what are you doing with the other half of the OTA? Funkee idea: use other half to build an "expander" of some sort using the same idea as the comp. "Expand In/Out" and "Comp In/Out", 2-in-1. I know there's simpler chips that do just this out there, but if they were that great, you wouldn't have had a reason to design something "better", obviously.

[garcho]

Nice one Wiz! Top of the to-do list.

[Steve Mavronis]

Nice original design and sound clip demo good! I'm not familiar with a symbol on your schematic. What is the double circle thing?

[merlinb]

Did you tried release and attack pots? How do they behave?

Yes, in the 5-knob version I am using 1M log pots for both (with a 220k resistor in series with the release pot), and they behave perfectly. Very long attack times are possible!

Nice original design and sound clip demo good! I'm not familiar with a symbol on your schematic. What is the double circle thing?

The double circle is the IEC symbol for a current source, so if you bolt that onto a generic opamp symbol you get the usual symbol for an operational transcondutance amplifier. It takes the voltage difference between its inputs (like an opamp) and converts into into a current output.

[kaycee]

Thanks for sharing, cool project indeed!

[BubbaFet]

Brilliantly elegant !

[Steve Mavronis]

The double circle is the IEC symbol for a current source, so if you bolt that onto a generic opamp symbol you get the usual symbol for an operational transcondutance amplifier. It takes the voltage difference between its inputs (like an opamp) and converts into into a current output.

Ah, on the MXR Dyna Comp factory schematic they didn't do that for the CA3080. Learn something new every day.

[head_spaz]

VERY nice Merlin!
You really do design the BEST toys !!!
Thank you!

[Earthscum]

I'm not sure if it's universally correct, but whenever I draw any OTA, I use the circles, and include (in hand drawings) a dash for the diode bias pin between the in and out for the ones that use it. That lets me thumb through and know the item is an OTA (of some sort), and the dash usually lets me know it's something like a LM13x00.

[Cliff Schecht]

Why not cut down on op amp count and use the OTA for the envelope follower or expo current source? A FET could buffer the 4.5V reference just as well as the op amp circuit and you could maybe shrink the overall footprint of the design a bit. Just a thought..

[jonasx26]

Cool design! The sidechain and overall response is very linear. I really like it. Thanks a lot for sharing!

[Mike Burgundy]

There's also the two on-board darlington buffers that you could still put to some use. The fact that there's two of everything on a chip begs a multi-band
Nice work, brilliant workaround for OTA noise. Gotta build me one of these - thanks!

[bonaventura]

pls enlighten me,

Vref is coming out fm an opamp,in this case what is the significance of U2B?

thanks.