Technical questions about Ross Comp (and guitar comps in general)

[xorophone]

Hello! I'm currently building two Ross compressor clones (the Tonepad layout: http://tonepad.com/project.asp?id=9 ). One for me and one for my friend. I'm very familiar with classic line level compressors (those with controllable threshold, ratio, envelope controls etc), but I've never used a compressor pedal before.

What does the sustain knob actually do? Does it change the threshold, the ratio, the input gain or is it some kind of combination? Also, will the output volume be lowered when the sustain gets pulled up or does it have some kind of built in gain makeup? The level pot only seems to cut, not add anything, but maybe the output is louder with the level pot all the way up than with the pedal turned off.

Of course these are things I can try out for myself when I'm done with the builds, but I just want to make sure I get it right, because I like to be aware of what I'm controlling.

Thanks!


Edit: I also think I read somewhere that the Ross Compressor separates the frequency spectrum and compresses different frequencies separately (like a multiband compressor but without any control over the separate bands). Is that true?

[Mark Hammer]

Easy question first:  No.  The Ross, like the Dynacomp it is largely a copy of, applies compression to the entire signal in monolithic fashion.  No splitting.  No favoritism towards one part of another of the spectrum.

The circuit applies a control current to pin 5 of the 3080 chip that controls the gain.  The Sustain pot limits how much bias current is fed to that pin.  More bias current = more gain.

The 27k resistor sets a limit on the maximum bias current, and the 500k sustain pot simply subtracts further from that.

Normally, that last transistor before the sustain pot feeds enough current to bring the chip up to whatever maximum gain it is set at.  The rate at which it reaches that maximum current is dictated by the 10uf cap just before that last transistor and the 150k resistor charging up the cap.  NOTE: That resistor, or its functional equivalent, is generally what is varied when you see any compressors having an "attack" control.  If the resistance is smaller, the 10uf cap charges up and reaches max current more rapidly.

The rectifier circuit that culminates in the two transistors, whose collectors are tied to the junction of the 150k and 10uf, essentially subtracts from that control current, momentarily dropping the gain of the 3080 in response to a sudden peak.  The 10uf cap starts charging up again, at a rate dictated by the 150k resistor, until max gain is once again reached.

So, in the big picture, what the Sustain pot does is adjust how much contrast there can be between the max gain, and what the gain gets reduced to, before it comes back up again.  This is why one needs to adjust the volume, whenever you change the Sustain setting.

make sense?

[ashcat_lt]

So the sustain pot is basically threshold and makeup gain at the same time while the parameters are essentially fixed?

[amz-fx]

regards, Jack

[xorophone]

Easy question first:  No.  The Ross, like the Dynacomp it is largely a copy of, applies compression to the entire signal in monolithic fashion.  No splitting.  No favoritism towards one part of another of the spectrum.

The circuit applies a control current to pin 5 of the 3080 chip that controls the gain.  The Sustain pot limits how much bias current is fed to that pin.  More bias current = more gain.

The 27k resistor sets a limit on the maximum bias current, and the 500k sustain pot simply subtracts further from that.

Normally, that last transistor before the sustain pot feeds enough current to bring the chip up to whatever maximum gain it is set at.  The rate at which it reaches that maximum current is dictated by the 10uf cap just before that last transistor and the 150k resistor charging up the cap.  NOTE: That resistor, or its functional equivalent, is generally what is varied when you see any compressors having an "attack" control.  If the resistance is smaller, the 10uf cap charges up and reaches max current more rapidly.

The rectifier circuit that culminates in the two transistors, whose collectors are tied to the junction of the 150k and 10uf, essentially subtracts from that control current, momentarily dropping the gain of the 3080 in response to a sudden peak.  The 10uf cap starts charging up again, at a rate dictated by the 150k resistor, until max gain is once again reached.

So, in the big picture, what the Sustain pot does is adjust how much contrast there can be between the max gain, and what the gain gets reduced to, before it comes back up again.  This is why one needs to adjust the volume, whenever you change the Sustain setting.

make sense?

That's a great explanation! Thank you so much, Mark! I love the fact that you explained the attack too. That will be very handy!

It does make sense, but I'll have to read it a few more times while looking at the schematic to understand it fully.

[xorophone]

regards, Jack

Ah nice! That image will be very handy too.

Thank you so much, Jack!