Pete Cornish - OC-1 Optical Compressor  [traced]

General documentation, gut shot, schematic links, ongoing circuit tracing, deep thoughts ... all about boutique stompboxes.
Post Reply
aionios
Breadboard Brother
Information
Posts: 134
Joined: 22 Mar 2012, 00:12
Location: Des Moines, IA
Has thanked: 6 times
Been thanked: 203 times
Contact:

Post by aionios »

All right, the legendary Mystery Pedal™ 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 OC-1 Schematic
Cornish OC-1 Schematic
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/
Attachments
cornish-oc1-trace-schematic.pdf
Cornish OC-1 Schematic (PDF)
(311.59 KiB) Downloaded 150 times
Aion Electronics - DIY PCB projects for classic and obscure circuits

guiddruid
Breadboard Brother
Information
Posts: 90
Joined: 14 Aug 2009, 12:18
Has thanked: 9 times
Been thanked: 4 times

Post 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

User avatar
bajaman
Old Solderhand
Information
Posts: 4414
Joined: 26 Jun 2007, 21:18
Location: New Brighton, Christchurch, NZ
Has thanked: 473 times
Been thanked: 1718 times

Post 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
be kind to all animals - especially human beings

guiddruid
Breadboard Brother
Information
Posts: 90
Joined: 14 Aug 2009, 12:18
Has thanked: 9 times
Been thanked: 4 times

Post 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.

Ben N
Resistor Ronker
Information
Posts: 399
Joined: 12 Dec 2008, 03:34
my favorite amplifier: Ampeg J12D Jet
Location: Israel
Has thanked: 47 times
Been thanked: 18 times

Post by Ben N »

How about a roll-your-own vactrol--will that work?

Agent86
Information
Posts: 10
Joined: 22 Dec 2008, 13:37
Been thanked: 2 times

Post by Agent86 »


User avatar
andy-h-h
Breadboard Brother
Information
Posts: 91
Joined: 22 Jul 2017, 03:54
Location: Brisbane, Australia
Has thanked: 15 times
Been thanked: 29 times
Contact:

Post 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.

tubegeek
Information
Posts: 1
Joined: 15 Dec 2011, 03:37
Has thanked: 2 times

Post 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!

User avatar
andy-h-h
Breadboard Brother
Information
Posts: 91
Joined: 22 Jul 2017, 03:54
Location: Brisbane, Australia
Has thanked: 15 times
Been thanked: 29 times
Contact:

Post 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.
Attachments
PETE CORNISH OC-1 copy.png

luix
Information
Posts: 15
Joined: 17 Mar 2009, 01:56
Been thanked: 5 times

Post 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
Attachments
OC1-BOTTOM.pdf
Bottom view ready to be printed, 1:1
(92.34 KiB) Downloaded 22 times
OC1-BOTTOM_MIRROR.pdf
Bottom view mirrored and ready to be printed, 1:1
(92.64 KiB) Downloaded 24 times
OC1-TOP.pdf
PDF with transparent background.
(147.53 KiB) Downloaded 26 times
Image of the top of the PCB with black background
Image of the top of the PCB with black background

stefanogueli92
Information
Posts: 17
Joined: 28 Jan 2019, 13:20
Has thanked: 9 times
Been thanked: 2 times

Post 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

luix
Information
Posts: 15
Joined: 17 Mar 2009, 01:56
Been thanked: 5 times

Post by luix »

You can try but I think I've not respected the DRC rules, I've designed it for photoetching process.
Attachments
Cornish OC-1.rar
gerber files
(172.96 KiB) Downloaded 38 times

stefanogueli92
Information
Posts: 17
Joined: 28 Jan 2019, 13:20
Has thanked: 9 times
Been thanked: 2 times

Post 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

luix
Information
Posts: 15
Joined: 17 Mar 2009, 01:56
Been thanked: 5 times

Post 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.
Attachments
OC1-BOTTOM_MIRROR.pdf
pcb bottom mirrored
(92.81 KiB) Downloaded 49 times
OC1-BOTTOM.pdf
pcb bottom non mirrored
(92.46 KiB) Downloaded 45 times
OC-1_gerber_V1.rar
Gerber file with mod
(86.33 KiB) Downloaded 44 times

Post Reply