# Proposition Distortion Mockup Pedal - Arriving Soon



## music6000 (Jul 23, 2022)

Proposition Distortion - PedalPCB.com
					

Compare to Rockman Ultimatum Distortion Generator




					www.pedalpcb.com


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## VivekMEHTA (Jul 30, 2022)

Hello everyone, I'm Vivek Mehta.

About 3 years ago, I got interested in Rockman gear, especially the X100. I decided to study it with the goal to understand it, learn from it, make improvements and mods.

It took me about 200 hours on LTSPICE studying actual measured data from 16 X100s before I cracked the code on how the X100 compressor was set and how the "factory selected parts" were chosen.

One beautiful moment around March 2020 or so was when a X100 refurbisher / repairer reached out to me and said he had 3 X100 units that he could not repair since the compressor was not working properly. Every FET and resistors he tried lead to an extremely squashed sound.

I calculated the "factory selected resistors" he needed for his new compressor FETs. He reported that those values made his X100s work exactly as new ones would !


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## VivekMEHTA (Jul 30, 2022)

*Tom Scholz's Compressor with two release time constants*

In Patent 4627094 of December 2. 1986, Tom Scholz described a guitar compressor.

He also highlighted an issue with the release times of the compressor.

Tom described the ADSR curve of a guitar signal as follows: 

_"On a guitar, the first sound or pulse that comes out can be a huge peak which is almost always much stronger than the signal that follows after a few milliseconds."_

Then he described the problem with his earlier versions of the compressor:

_"The operation has not been found to be optimised....When a loud note is followed by a soft note, the transition does not follow (correct compression) curve exactly due to the time constants. The (compressor) tends to act as a fixed gain amplifier when a hard note is quickly followed by a weaker note; there is a tendency for the weak note to start quietly and ride up to the previous note volume"_


If the compressor has one slow release time
Lots of hard notes will raise the voltage on the integrator cap
Then if the next note is soft, but the integrator cap is fully charged and discharging very slowly, it will keep the gain of the compressor low even though the current note is low volume. 
As the integrator cap slowly discharges, the compressor gain starts to increase and the weak note starts to get louder.

Basically the issue was that a fast release is needed initially, followed by a slow release. Ie a compressor with two release time constants. This would track the amplitude changes in guitar signals in a better way.

Which obviously means two discharge paths for the integrator cap.

Tom solved that problem very elegantly in the Sustainor and Ultimatum by adding various diodes in parallel to the integration capacitor.





Here the output signal of an Opamp goes to R1 (which sets the attack time)

D3 rectifies the signal and charges integration Capacitor C1.

C1 normally discharges via R3

Tom placed diodes D4-D7 as the additional discharge path. He took advantage of the nonlinear VI curve of diodes to basically make a variable resistor, the value of which depends on the imposed voltage.

When C1 has a higher voltage, the diodes D4-D7 appear as lower resistors and discharge C1 at a fast pace. 

When C1's voltage drops, the diodes appear to be higher value resistors, and C1 continues to discharge via R3 as usual.

This leads to 2 release time constants, a fast one initially and a slow one later on.






The green curve is a sine wave input signal that starts at t=0.5s and stops at t=1.5s

The red curve shows the exponential discharge of C1 in case there were no D4-D7. It is basically a discharge via R3 with time constant R3.C1

The blue curve shows the discharge of C1 with D4-D7. We can see around t=1.5 till 3.5s, the blue curve shows faster discharge than the red curve. That is because initially C1 is discharging via D4-D7 since they appear to be low value resistors. 

However after about t=3.5s, the time constants of the blue and red curves appear to be similar. That is the zone when the diodes D4-D7 appear to be high value resistors.

*Suggested Mod: *if you add a switch to bypass D4, the initial release will be very fast, aiding lead work on the guitar. This the famous "Lead Leveler" mod.


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## conleycd (Aug 2, 2022)

VivekMEHTA said:


> *Tom Scholz's Compressor with two release time constants*
> 
> In Patent 4627094 of December 2. 1986, Tom Scholz described a guitar compressor.
> 
> ...


Very cool.  Thanks for posting.  My guess is Bobby Cedro worked this stuff out more than Tom.  Bob was the head engineer at SR&D and has been at MXR when SR&D was sold to Dunlop.  I’ve wondered if he’s incorporated these old designs into new MXR and related products.


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