Rockman Compressor feeding Distortion

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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VivMeLol
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Post by VivMeLol »

Compressor before Distortion vs Piecewise function generator before Distortion

In some patents, Tom Scholz extols the virtues of compression before distortion

In fact, almost every SRD product that has distortion stage has also a compressor before it

The compressor evens out the initial high energy attack and effectively boosts the sustain at the end of a note, feeding the distortion stage with a more even signal from start to finish than if it was fed directly with a guitar. This makes the sound more uniform in strength and in harmonic content as compared to guitar directly feeding the distortion stage.


I got thinking that the compression does two things. One in the amplitude domain and one in the time domain.

But a piecewise function generator can do something similar in the amplitude domain. It can act as a compressor with zero attack time and zero release time.

But the biggest difference is that the Piecewise function generator will create high levels of distortion.

However if the next stage is going to create huge amounts of distortion anyway, would the added distortion of the Piece-wise function generator make a difference ?

There could be some benefits of replacing the compressor with a Piecewise function generator in a distortion generator : Lower cost, no hard to find FET, no set up procedures, easier for the DIYer

So I decided to do a SPICE FFT and Transient analysis of

Compressor feeding a hard clipping Distortion stage

versus

Piecewise Function Generator set to same amplitude graph as above compressor, feeding same hard clipping Distortion stage


I found that the final wave shape, wave amplitude, harmonic content of the above two schematics were fairly similar.


It is interesting to note that very few commercial distortion pedals have a compressor built into them, and almost none have a piecemeal function generator before the hard clippers.


Sample function generated by a Piece-wise function generator:
Image

Sample response of two settings of a compressor:
Image


Compressor curves from Patent 4627094
Image

Keywords : X100, Ultimatum, Sustainor, Distortion Generator, SRD, Rockman, Rockmodule

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mr coffee
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Post by mr coffee »

Interesting idea. Have you listened to just the piecewise linear amp output without hard clipping after it?

You didn't show a schematic of how you did the piecewise linear process (assuming you didn't just contruct it mathematically in Spice). Some real world implementations are less mathematically precise than others and the turn on characteristics of the diodes used actually smooth the piecewise approximation into more of a curve. That can sound more like a moderately-overdriven tube sound stage, and doesn't generate the buzzy, high frequency distortion products to the same degree.

The advantage of the compressor before heavy clipping distortion for the Scholtz-Boston distortion sound is the smoothness of the initial note attack and the added sustain (which you noted), but that is a minor effect on the heavily clipped Boston distortion sound.

I think Scholtz uses the compressor more for it's effect on the "semi-clean" tones which are a mild overdrive\clipping sound intended to sound like an amp being pushed. The compressor makes a huge difference in that sound setting.

If the distortion stage being driven is typical Scholtz back to back diode clipping, I'm doubtful that there would be much difference between having his compressor or the piecewise approximation clipper in front, or for that matter, just more gain from another plain distortion stage.

Adjusting the bias for the Scholtz JFET compressor stage can be done by ear quite successfully, although you may want to socket the jfet in case the first one you pick has a Vpinchoff threshold that isn't well suited. JFET parameters vary a lot due to inherent processing quirks. Many JFETs with different numbers are off the same wafer and simply selected after packaging (see 2N5457, 2N5458, 2N5459 data sheet).

Breadboard some of this stuff and try tweaking it. Maybe you'll be the next SRD genius!

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