MXR Super Comp vs. Dyna Comp Attack mod

[Fearless Bob]

Hi all,
while looking through two MXR super comps I noticed that the attack pot is connected differently than the common mod for the dynacomp.

In the following I will relate to the part numbers as given in Jack Ormanns schematic.

Typically the Attack mod replaces the 150 k restrictor (connecting collectors of Q3 and Q4 as well as the base of Q5 to V+) with a variable restrictor.

The Super Comps in contrast have the pot connected in a way that leaves the resistance between Q3/Q4 and V+ at a constant 150 K while its wiper is connected to the base of Q5. With a little resistor network the sweep is restricted to something around 50 - 70 K depending on the model.

While I understand the effect of the std. attack mod (decrease the 150 k and increase the charge time for the 10 µ cap attached to it), I am struggling to understand in how far the super comp version is different from the sensitivity pot following right after Q3. We are no longer influencing the time constant of the R/C combination, so how does it work

Anybody? Thank you very much in advance!

Cheers
Bob

[Fearless Bob]

err.. typo It should be like this: "decrease the 150 k and decrease the charge time for the 10 µ cap attached to it"

[strato56]

Can you draw the actual circuit that follows the wiper of the 150k pot?
Since the 150k and 10uF remains constant, the R-network that you mention must be important.

[mister riff]

hi, i'm also looking to super comp attack mod that is very good about me, do you have schematic?
bye

[jones12ax7]

Hello Guys,

This is an outdated post, but I would like to put some fire on it.

If I understood it right, the Supercomp version changes the attack using another principle.
The function of Q5 is to generate the bias current to OTA, that way controlling the gain. This control current is proportional to its base voltage. So, Q5 is working as transconductance amplifier.

In standard mod, they change RC time, changing the 150k resistor, so OTA gain reacts faster or slower, as the base voltage of Q5 is changing accordingly.

In supercomp they are not changing RC time, but the sensivity of Q5 to the RC voltage change.

I'll bellow some fictitious values just to make an example.
The 10uF C is not always going to go fully charged or discharged, so only a part of the "ramp" will be used. Let's say, it's charging and growing from 1V to 2V in 1 second ( not real values).

In standard mod, you make it sweeps in only 0.5 second for example to increase the attack. It would change the current from, say, 1uA do 2uA.

In supercomp, they still go in the original 1 second, but the current changes from 1uA do 2uA when the voltage goes only from 1V to 1.5V, so taking only half of constant RC time to delivers the same amount of current.

These are only my speculations, based on my electronics experiencing after reading the circuit. I'm building a MXR dynacomp these days and will check it out for sure, making the 2 mods and doing some tests.

I can't imagine how different the two mods would sounds like, but let's hear!

I hope it had bring some light on the subject. Please correct me if I'm wrong.

See you
Jonathan

[matt239]

I would like to know about this also.