Author Topic: THE ENGINEER'S THUMB... At last, a better compressor! (Read 292502 times)

merlinb · « **on:** April 21, 2012, 10:17:37 AM »

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 · « **Reply #1 on:** April 21, 2012, 10:18:09 AM »

frequencycentral · « **Reply #2 on:** April 21, 2012, 11:03:40 AM »

Clever bugger!

alfafalfa · « **Reply #3 on:** April 21, 2012, 11:04:19 AM »

This is really great !!

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

Thanks

merlinb · « **Reply #4 on:** April 21, 2012, 11:05:22 AM »

Quote from: alfafalfa on April 21, 2012, 11:04:19 AM

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

Yes, there is a link in the first post.

Slade · « **Reply #5 on:** April 21, 2012, 11:14:43 AM »

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 · « **Reply #6 on:** April 21, 2012, 11:19:26 AM »

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 · « **Reply #7 on:** April 21, 2012, 11:51:56 AM »

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 · « **Reply #8 on:** April 21, 2012, 12:08:00 PM »

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

Steve Mavronis · « **Reply #9 on:** April 21, 2012, 12:19:11 PM »

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 · « **Reply #10 on:** April 21, 2012, 01:10:04 PM »

Quote

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!

Quote from: Steve Mavronis on April 21, 2012, 12:19:11 PM

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 · « **Reply #11 on:** April 21, 2012, 01:21:36 PM »

Thanks for sharing, cool project indeed!

BubbaFet · « **Reply #12 on:** April 21, 2012, 01:31:52 PM »

Brilliantly elegant !

Steve Mavronis · « **Reply #13 on:** April 21, 2012, 01:39:35 PM »

Quote from: merlinb on April 21, 2012, 01:10:04 PM

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 · « **Reply #14 on:** April 21, 2012, 01:45:24 PM »

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

Earthscum · « **Reply #15 on:** April 21, 2012, 01:47:05 PM »

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 · « **Reply #16 on:** April 21, 2012, 02:14:37 PM »

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 · « **Reply #17 on:** April 21, 2012, 02:23:45 PM »

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

Mike Burgundy · « **Reply #18 on:** April 21, 2012, 02:24:56 PM »

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 · « **Reply #19 on:** April 21, 2012, 02:32:40 PM »

pls enlighten me,

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

thanks.

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Author Topic: THE ENGINEER'S THUMB... At last, a better compressor! (Read 292502 times)