MXR Carbon Copy: How to Increase "Modulation" Effect

[juniomarbles]

What's special about MXR's Carbon Copy is the modulation button and its pitch-changing effect.
Though modulation width was increased with the second edition (substituting to a 100kΩ in position R14), the effect is still too subtle.

What can I do to increase the volume of the modulated part of the delay? It's particularly subtle with short delay times.

I experimented with reducing R14 to ca. 60kΩ, but that did not add modulation width or intensity. Besides, R14 may not determine the intensity anyway, but just the width of the swing.

Lacking a schematic, I do not know what components determine the intensity of the modulation effect, and to what degree I could increase it.

Has anyone experimented with this?

[thermionix]

I had a Carbon Copy for a while, it had two trim pots inside to customize the mod effect.  I never messed with them myself, but doesn't one of them increase the intensity?

[Mark Hammer]

The manual does indeed identify the two internal trimmers that often show up as Rate and Depth panel-mount controls on other pedals in larger enclosures (or with much smaller knobs).

The impact of the Depth (sweep width or intensity) control will depend on how it is implemented in the pedal.  So, if the delay time is determined by the summed input of an LFO and second source, then one would expect that pitch wobble would be as strong, if not perceptibly stronger, at shorter delay times.  So, if whatever the LFO contributes has the net effect of adding and subtracting 20msec from final delay, then you will notice it less at 500msec delay time than you will at 50msec.

If the manner in which the LFO influences delay time is via the parallel time-setting resistance, it will work the opposite way.  So, if the LFO drives the base of a transistor placed in parallel with a delay-time pot to mimic a parallel voltage-controlled resistor (and many 2399-based delays use such a trick), the impact of changes to that collector-emitter resistance on the combined parallel resistance will be smaller when the delay pot is set to short delays versus long ones.  Put another way, if one were to stick a 10k-1meg photocell in parallel with a 100k fixed resistor, variations in light would have more impact on the combined parallel resistance than if the same photocell were placed in parallel with a 4k7 resistor.

I don't know how MXR/Jeorge (Tripps, because it was his design) implemented the modulation sub-circuit.

[juniomarbles]

Thanks for the theoretical explanation of how things may work in my unit.
If that clears up your thinking: on the Carbon Copy, the pitch-shift decreases in modulation setting with decreased delay times to almost imperceptible at the shortest delay times.

So.... back to my original question: how can I increase the volume of the pitch wobble? Since I posted, I have now completely shorted R14, and the maximum pitch shift volume is still very modest, even in maximum trim-pot setting.

[Mark Hammer]

How much wobble is there when the trimmer is maxed and delay time is long?  That should provide some idea about whether wobble can be increased.

[juniomarbles]

After shorting of R14, the modulation effect is clearly more noticeable in maximum trimmer and delay settings. But as it's mixed in with the un-modulated delay portion of the signal, the effect on long delays is more like a classic phase shifter from the 1970s rather than a pitch shifting wobble.

But at short delay settings, which I prefer for my tone, the wobble effect is barely audible, even after shorting R14 and at max trimpot setting.

Maybe what I want - a more distinct wobble at short delay settings- is technically impossible because the delayed portion of the signal is so close in time to the dry signal?

[Mark Hammer]

Another possibility is that the LFO modulates the delay time linearly, but what you want/need is logarithmic.

That's the thing with human perception. https://en.m.wikipedia.org/wiki/Stevens%27_power_law

[juniomarbles]

So do I give up?