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Pete Cornish - OC-1 Optical Compressor  [traced]

PostPosted: 15 Nov 2019, 22:21
by aionios
All right, the legendary Mystery Pedalâ„¢ is finally done!

I traced the Cornish OC-1 Optical Compressor over the summer and today I'm ready to share it.

Pete claims this is an all-original circuit design, and fortunately I can confirm that it's not quite like anything I've seen before. The audio path is pretty simple, but the sidechain has a few unfamiliar tricks, perhaps taken from an obscure op-amp application note someplace.

In any case, though, it sounds great!

Schematic:
cornish-oc1-trace-schematic.png


The unit traced was from 2014. There are no known circuit revisions since that time.

Full tracing journal here:
https://aionelectronics.com/blog/tracin ... ompressor/

Imgur gallery with 50+ photos including X-rays:
https://imgur.com/a/sjtezCB

PCB available here:
https://aionelectronics.com/project/oce ... ompressor/

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 16 Nov 2019, 11:03
by guiddruid
IC3A/IC3B looks like the "precision full wave rectifier" circuit from Horowitz & Hill (don't have my copy to hand, but IIRC this is it; https://en.wikipedia.org/wiki/Precision_rectifier), but with D2 connected on a separate op-amp. Seems to me this should be functionally equivalent. Not sure what (if any) advantages this would bring. [edit] Sorry, D2 is also backwards compared to that circuit, and the signal is going into inverting input of IC3B. Not sure if that's then achieving the same result. [edit yet again] Nope. I don't understand how IC3B functions at all with pin 5 connected to GND. If it were connected to Vb, I think it would work as a "perfect" diode, preventing that node from dropping below Vb & would also bias that node in the circuit.

IC4B/Q2/Z1/R37 looks like an opamp current source, converting the voltage at IC4B + to a current through the LED. Again, I'm pretty sure this is in Horowitz & Hill; https://en.wikipedia.org/wiki/Current_s ... nt_sources

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 18 Nov 2019, 21:25
by bajaman
Don't forget - the LM358 is a single supply optimised dual op amp and does not require a half bias voltage to function in this application :wink:
cheers
bajaman

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 18 Nov 2019, 21:58
by guiddruid
But the only way to forward-bias D2 is for pin 6 to go below GND. Even if that were possible, with the supply rails on IC3 being +Vc/GND, I don't see how the input circuitry in the opamp can work.

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 21 Nov 2019, 10:33
by Ben N
How about a roll-your-own vactrol--will that work?

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 22 Nov 2019, 22:35
by Agent86

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 28 Nov 2019, 10:24
by andy-h-h
Finished a vero layout for this, will give it a run on the weekend and post if it goes well. I'm pretty happy with my bajaman optical limiter, so see how it compares.

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 09 Dec 2019, 04:39
by tubegeek
andy-h-h wrote:Finished a vero layout for this, will ... post if it goes well.


Didn't go well? Curious about this one. Thanks!

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 09 Dec 2019, 08:09
by andy-h-h
tubegeek wrote:
andy-h-h wrote:Finished a vero layout for this, will ... post if it goes well.


Didn't go well? Curious about this one. Thanks!


Yes it did go well - just waiting on an NSL-32 to confirm 100% (should arrive this week). It sounds good, I was using a different vactrol, which was working, but not quite right (compressing, but not enough).

This is the layout I used.

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 27 Jan 2020, 14:20
by luix
Hi guys,

Based on this schematic I've drawn a PCB for this compressor, it's still unverified and unfortunately I'll not be able to build it untill april, any attempt to check it will be reward with many thanks and infinite love :D

I know it's not really tidy but I've done my best, red sign are top wire bridge, the only thing not included is the buffer.

Let me know if it works.

EDIT: I forget dimensions are 52x84mm (pdf are 1:1 and don't need it).

regards
luix

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 27 Jan 2020, 17:31
by stefanogueli92
luix wrote:Hi guys,

Based on this schematic I've drawn a PCB for this compressor, it's still unverified and unfortunately I'll not be able to build it untill april, any attempt to check it will be reward with many thanks and infinite love :D

I know it's not really tidy but I've done my best, red sign are top wire bridge, the only thing not included is the buffer.

Let me know if it works.

EDIT: I forget dimensions are 52x84mm (pdf are 1:1 and don't need it).

regards
luix


I have an order on jlcpcb of my drawn of tube driver tou find in the last page of apposite thread... So if you can upload the gerber file, I'll order it and I'll verify within 1 week

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 27 Jan 2020, 20:41
by luix
You can try but I think I've not respected the DRC rules, I've designed it for photoetching process.

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 27 Jan 2020, 22:22
by stefanogueli92
luix wrote:You can try but I think I've not respected the DRC rules, I've designed it for photoetching process.

Maybe i can modify it... Thank you very much bro!!!! So let's try to do it together

Re: Pete Cornish - OC-1 Optical Compressor

PostPosted: 28 Jan 2020, 10:39
by luix
STOP! I found an error, the anode of the zener diode is connected to V+ instead of GND, I'll correct quickly.

The top layer is essentially the same, I've updated gerber and bottom.