EHX Deluxe memory man headroom/overload modification

[pnadora]

Hi everyone,

I have pretty bad overload problems with my vintage DMM (pointy knobs, external power supply), even though my hottest pickups are PAFs.
The common mods offered by Howard Davis and Analogman to solve this seem to be the impedance mod and the "high headroom/hot pickups" mod. There is almost no information regarding this hot pickup mod and many of the old DIY/modder sites from the early days of pedal-making that are quoted regarding this are down.

I found two forum posts on mod-wiggler and thegearpage that are somewhat conflicting (if I understand them correctly).

One says to modify the second opamp stage right before the compressor part. I think that would be IC1B in the schematic below. I am not sure what should be done here though, since this one sits before all the parts of the circuit that might overload.

The other post that I found suggests modding the last opamp stage (IC4B) by lowering R32. I think this would only affect the wet part and would not solve the problem that the dry of the signal also overloads.

I have written to Mike at Analogman and he pointed me here since shipping the pedal to him from the EU is not really an option.

I would be very happy if someone could point me in the right direction regarding this. I think the goal of the mod is to increase the level of the pedal after the overloading stages in order to be able to decrease the input level via the level knob.

Any help is greatly appreciated!
BR

[antonis]

Hi & Welcome..

The impedance mod should be a bit tricky 'cause inverting configuration limits input impedance, which is 100k (R2)..
(ignoring R1 in parallel..)

You can raise R2 value up to 1M say (risking noise issues) while getting U1A output from a voltage divider formed with an output series resistor and 47k (R28) to maintain the same amount of U1A gain..

[pnadora]

Thank you for your answer!
I think the gain reduction in the first stage is not a big problem, at least for me. In theory, you can use the DMM as a booster, but I just want relatively clean unity gain for the dry path.

Additionally, someone suggested in another forum to reduce the gain of U1B by lowering R6 and maybe compensate with an increase of the gain of U4B. This should prevent overloading in the wet path.

I still don't fully understand why the dry path is overloading (I have the non-true bypass version). The first gain-stage should be the only one relevant when the Footswitch is in off and that to me seems like a pretty standard gain stage that should not really clip at unity gain..

[antonis]

I still don't fully understand why the dry path is overloading (I have the non-true bypass version). The first gain-stage should be the only one relevant when the Footswitch is in off and that to me seems like a pretty standard gain stage that should not really clip at unity gain..

U1A unity gain is obtained with (an ideal !!) VR1 set at 7.8% of its rotation [ (78k + 22k) / 100k ]..
(the above resistance can be taken even with the slightest shaft rotation 'cause many pots can't travel all down to zero Ohms..)

[pnadora]

Makes sense. I wonder if there is a bigger problem because on my unit unity gain is at like 30% of the rotation at that point it is overloading a lot. Reading many forum posts regarding this issue, it seems to be a common problem.
I will change IC1 and see what happens. Seems like the newer versions of the pedal have solved this issue, so I wonder if anyone knows a reliable schematic of the new version. That way I could compare it to the old one to see what EHX themselves have done to improve this.