Trace Elliot SMX Dual Compressor

[lars-musik]

Hello diystompers,
I am trying to built a bass compressor and found the schematic of a Trace Elliot SMX Dual Compressor at the other forum. I redraw the schematic in Diptrace as far as I could. Due to a lack of readability in the original combined with a lack of electronic knowledge on my part I am stuck. Some months ago I asked the arisen questions over at the other forum (because the thread was started there) but didn't get any answers.

So I am porting my questions and my willingness to learn over here.
My main problem is the power supply of the circuit. How is it supposed to work? What symbols represent what (GRD, +/-)?
Is that right

??

What is the construction in the lower left Corner? What is the 3V0 Zener doing there??



On the schematic there's a hint about a max. current consumption of 10 mA. The amp schematic Shows +/-15V. In my limited knowledge that is something I would try to solve with a LT1054.

I'll attach the original schematic my re-drawn one and a schematic of a Trace Elliot GT12 SMX amp that also includes the circuit (save image as... for full Resolution)

Original:


Re-Draw:



GT12 SMX:

[samhay]

It's single supply:



from top to bottom: V+ (18V, it runs on 2 9V batteries), ground, half supply (~9V).

[lars-musik]

Thanks for your answer, Sam. I have updated the schematic, but I am still uncertain.
Isn't an output pot usually connected to ground? This one here is connected to V/2? And the ground pin of the SSM2024 would connect to V/2, too?
Could you – or somebody else - have a look at the updated schematic?
Thanks!

[teemuk]

What is the construction in the lower left Corner?

Google search "single supply opamps". It's an arrangement to derive required "bipolar" power supply voltages from an unipolar supply. (Opamps call for Vcc and Vee power supply "rails" and a reference voltage from between those values).

What is the 3V0 Zener doing there??

If you look at both SMX Dual Compressor and similar SM GP12 schematics more carefully you'll see that VCA's control signal's magnitude is controlled by different means for "low" and "high" sections. One solely controls the amplitude of the envelope follower signal, the other does this as well but also injects DC offset to the VCA control line.

Since GP12 has a bipolar power supply you can see an additional resistive divider used for deriving this offset. (It's not the same as half supply "bias"). In Dual Compressor, which has an unipolar power supply, the offset is generated across a steady voltage reference ... a zener diode. The other end, when wired like that, is already conveniently hooked up to "half supply" reference point.

Isn't an output pot usually connected to ground? This one here is connected to V/2?

Not always. Since output of the preceding opamp is biased to DC offset of "V/2" and the other terminal of the potentiometer referenced to "V/2" this - aside from DC voltage offset - does not really differ from arrangement where we would use ground instead of "V/2". Subsequently you see capacitive coupling isolating the DC offset (C22) and the actual -output- of the device being referenced to ground (TP9). This is very typical arrangement.

[lars-musik]

Thanks for your explanations, teemuk. I see that what seems uncommon to the amateur is not necessarily so to the expert.

However, I am afraid that circuit isn't exactly my cup of tea at the moment. I guess I won't be able to handle the debugging when I finally got that circuit on hand with my limited knowledge.

If somebody's interested in the diptrace file (and willing to go on with that project) I'll happily share it.

Best, Lars

[SwamiRob]

Sorry to revive a pretty old thread, however it has a decent copy of the schematic of this pedal which I could find a decent link to anywhere, just really blurred ones you can't read.

Anyway, I was looking to see what the max ampage this thing could take safely? Need to get a new 18v adaptor and wanna be getting as much headroom as possible. I don't really know that much about electronics so don't know if the ampage will help toward that end, or whether it's just voltage that controls signal levels in that regard.

Also, I was wanting to change the crossover setup, so it was focused a fair bit lower, and also if possible reduce the overlap between the two compressor circuits. The 'composite crossover' (is that when the signals are mixed back together?) is at 350hz at the moment, and the low compressor filters above 225hz with the high compressor filtering below 900hz, both with a 12db per octave roll off. Is there any way of changing any of this without ripping the circuits apart too much? Or is there a couple of values that could be changed to at least make the crossover or filter frequencies lower?

Also the circuit is known for rolling off lows and highs a little, is there a part of the circuit that anyone can see that would cause this? It would be nice if I could get a little bit of a wider sound out of it too.

As you might have already deduced from something I've said above, I've not got much knowledge about electronics, and havent really done anything other then solder a few dry or broken connections back together since I did a design and tech GCSE at school about 15 years ago. However I'm willing to put a bit of time into learning some so I can mod this pedal, as it's made a big difference in cleaning up the low end on my bass and letting me have it really big without being really boomy, just could do with a few tweaks so it's not affecting anything higher then it needs to. Plus the FEA labs dual comp that I really want is a little more expensive then I can cope with and also is so popular that it has a 9 to 12 month waiting list to get your hands on one.