Cloning 2 Knob Compressor

[Steve Mavronis]

I'm interested in another DIY project, building a vintage era 2 knob compressor into a 1590B size enclosure. I'm thinking of cloning the old Ross Compressor if possible, or the '76 MXR Dyna Comp as second choice. I'm not sure if I want a hybrid of the two. I also prefer to reverse engineer my own PCB layout basing it on reference pictures of the original. So far I've only found a low rez component side shot of the Ross. I have more info on the MXR front and back. I've read the two circuits are basically the same except Ross added extra filtering? Is there a schematic drawn from the actual Ross or MXR pedals that I won't have any restrictions making my own PCB layout from, or clear front and back images taken of the PCB's so I can develop my own schematic and cloned pedal? Thanks.

[jkokura]

Steve, I got my boards from the Tonepad projects. Have you looked at the schem there? What's cool about that project is that Francisco showed the differences between the Dyna and the Ross right there in the project file. You can theoretically build both of them using a SPDT switch to change between Dyna and Ross type.

Jacob

[Steve Mavronis]

Thanks but it must be my Vista computer or something. I've never been able to get to Tonepad with my PC because all I get is this error message in my web browser:

Microsoft JET Database Engine error '80040e21'

The field is too small to accept the amount of data you attempted to add. Try inserting or pasting less data.

/includes/tracking.asp, line 9

I guess whichever brand I get more info about (it's MXR so far) then that's what I'll have to clone. I need something to give me more sustain and I thought one of these classic compressors would do the job without getting too complex with added features. I'll only be adding an LED, 3PDT true bypass, and DC jack. For everything else I want to be 'spec' part values.

P.S. Does anyone here have an original Ross Compressor? That could provide a lot of fresh reference pics and info, or I have a tan Ross Distortion for trade.

[CynicalMan]

I think RG has the info you are looking for:
www.geofex.com/PCB_layouts/Layouts/d&rpub.pdf

[Steve Mavronis]

I was confused when I saw those schematics. What's with that 500K Sustain variable resistor (pot lug) going to nowhere on the right hand side? Is it just left hanging or to ground?

[CynicalMan]

The third lug is not connected because the pot is being used as a 2-terminal variable resistor. It's common practice to connect the second lug to the one that's not connected, and you can see that in the layout.

[Steve Mavronis]

common practice to connect the second lug to the one that's not connected, and you can see that in the layout.

I didn't know that. I saw it connected in the layout graphic with the pots wired. You'd think that would be reflected in the schematic however it's wired up to the pot. I'm still very much a novice working with schematics. That's why I'm backwards working from the physical original PCB itself to create my own custom layout. I only like schematics for showing parts values and documenting off board wiring.

[CynicalMan]

Often the lugs' connection is reflected in the schematic. I do it with all of my schematics. However, it sometimes isn't shown, as the potentiometer acts more or less the same electrically whether or not those lugs are connected. They are connected often because the floating lug can sometimes act as an antenna. Also, when pots fail, the middle lug often disconnects, but if the pots lugs are connected, instead of acting as an open circuit, the pot goes to its maximum resistance.

Schematics are way better if you want to understand the design of an effect, but I can see how you would prefer layouts if you were more interested in just building stuff or replicating commercial effects.

[Steve Mavronis]

Well then since I'll probably incorporate PCB mounted pots again to support the board, there should be no harm in connecting those 'lugs' (pins) on the PCB layout design I'll develop. My dad said that would probably be better since with it left unconencted if the pot goes bad the pedal would appear dead. With the pins connected the effect should still work in that event but the sustain control wouldn't be able to be adjusted. But at least for troubleshooting you wouldn't have to waste time wondering which part on the PCB went bad.

[Steve Mavronis]

I've started working on a PCB layout today with ExpressPCB. My current project codename is "3080 Sustainer" with a final name to be decided later:



I'm just trying to get a general parts fit at the moment so everything will probably get shifted around while I'm trying to build the circuit on the PCB. I'm not even sure at this stage if it's possible to fit all the parts on my standard 2.0" x1.7" PCB shape. I've also flipped my pot placement so the level output is on the same side as the signal out jack. This will fix an annoyance that I ran into wiring up my last 741 Overdrive pedal with the input and output wires criss-crossing to opposite sides of the PCB. In retrospect I probably should have made this change then. Also I've moved the pots inward a 1/10th an inch so the 1" diameter MXR or DOD old school knobs I'll use fit better on a Taiwan B-size box without overhanging the more rounded edges than the Hammond 1590B has that I used in my last project. It's a purely cosmetic thing I know.

[Steve Mavronis]

I just found these images from a Japanese site about the Dyna Comp. The PCB looks like a good clone copy of the real thing:





The description says:

Sequence of a typical leg of the transistor transistor CBE BCE and made a copy made in Japan to change its point of difference sequence, so the leg. Was used for 2SC2603, 2SC1815 around okay I think.

[Brymus]

Thanks for posting that ,it makes a nice edition to my Ross folder.

[Steve Mavronis]

Thanks for posting that ,it makes a nice edition to my Ross folder.

You might want to put those graphics it into your MXR folder instead!

I'm back to my board template with the 1.2" pot spacing for a Hammond 1590B. I think when adding my aluminum etched panel graphic on top it will look better at the sharper edges instead of the rounded ones of an MXR style box. But it's easy to adjust my layout if I change my mind again which box to use. Man, it's rough getting all these parts crammed into a 2.0"x1.7" new PCB layout but I think it will be doable. I might end up with a late 70's MXR script clone because the Ross has several extra parts to deal with! Still a classic pedal though. Hopefully this will work out, or else I'll be plopping down some $$$ for a new Dunlop custom shop '76 reissue or snag up one of Chris' unpopulated factory boards (http://www.diystompboxes.com/smfforum/index.php?topic=86957.0) and just build myself an exact replica.