# EHX DMM Volume Drop. How Do I Fix This???



## hollowbody (Jan 15, 2008)

Ok, I love my DMM, and I don't want to get rid of it, but the volume drop when the effect is engaged is starting to really annoy me. What can I do to fix it? It's a newer model one with the wall-wart.


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## Voxguy76 (Aug 23, 2006)

Yep i'm familiar with that your talking about. The reason is because of it being true bypass. I sold my true bypass EHDMM in favour of one of the older reissue versions that werent true bypass. 

If your noticing a drop in signal when the effect is engaged, then you will have to turn up the level to where its unity gain with the effect off. I know alot of people prefer the true bypass version of this pedal so i guess i'm in the minority, but i really prefer the non true bypass version. I have no volume issues, and i like how i can run the preamp level up to boost the signal.


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## hollowbody (Jan 15, 2008)

riffboy76 said:


> Yep i'm familiar with that your talking about. The reason is because of it being true bypass. I sold my true bypass EHDMM in favour of one of the older reissue versions that werent true bypass.
> 
> If your noticing a drop in signal when the effect is engaged, then you will have to turn up the level to where its unity gain with the effect off. I know alot of people prefer the true bypass version of this pedal so i guess i'm in the minority, but i really prefer the non true bypass version. I have no volume issues, and i like how i can run the preamp level up to boost the signal.


But mine isn't true bypass. The level knob turns up the gain whether the pedal is on or not. If it didn't, I would be able to turn it up to match the bypass signal.


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## Voxguy76 (Aug 23, 2006)

Ok thats weird. Sounds like it needs to be serviced. Mine does not do that.


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## hollowbody (Jan 15, 2008)

riffboy76 said:


> Ok thats weird. Sounds like it needs to be serviced. Mine does not do that.


Yeah, I'm going to play around with the volume and blend knobs when I get home. It's always been doing this (I got it used), but I finally started to get annoyed enough to want to correct it.


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## Voxguy76 (Aug 23, 2006)

what a coincidence. Plugged mine in tonight and i noticed the same damn thing. Never had this problem before. I did notice that cranking my level to about 12 o'clock that the signal seemed to be at unity gain. Not sure why this is happening though. I love these pedals (i've had 3 different ones) and despite being a fantastic sounding pedal, they've been prone to reliability issues. Not a pedal i'd feel confident gigging with.


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## mhammer (Nov 30, 2007)

Okay. Pulled up a schematic of the reissue DMM. Turns out (if the factory schematic is to be believed...and sometimes they aren't) the unit is wired such that the "bypass" switch is a single-pole/single-throw that simply selects from the mixing point on the blend pot or the dry side of the blend pot. All gain/level adjustments applied hold for both effect mode and "bypass" mode. While the input stage of the DMM provides reasonable buffering (meaning that it helps accomplish something that TB will not do for you), it unfortunately means that there is no simple way to disable the gain setting or have separate gain for effect and bypass.

On the other hand, since the problem is signal drop in effect mode, there IS a way to reduce level in bypass mode such that effect and bypass can be at the same level. I have to bring my kid to the orthodontist now, but I'll get back to you with one or two ideas.


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## mhammer (Nov 30, 2007)

Okay, assuming yours is the model with the 24v wallwart supply, here's what you *might* do.

The input buffer/gain stage is a 4558 marked U1. The output is pin 1, which goes both to the delay line and to the final output and blend control.

That signal at pin 1 goes to a 47uf cap. The 47uf cap has a 47k resistor to ground, but is also connected to the 10k blend pot.

If one was to insert a small-value resistor between the 47uf cap and 47k resistor, the 47k unit and added resistor would form a voltage divider; basically the same thing as a fixed volume control set for a drop. Presently, it behaves like a 47k volume pot set to max. If one was to insert a 4k7 resistor between the cap and the 47k unit, the signal taken at the point where the existing and new resistor meet would be like a 47k+4k7=51k7 pot turned down about 1/10 of the way.

The upshot is that you would be altering the level of the clean signal in both bypass and effect mode. One of the implications of this is that you'd need to set your blend control for more clean signal than before, because you'd have less of it to blend. The gain pot would still work as before, though. The one niggling detail is how much attenuation you'd not to produce with that added resistor. My guess is, not much.

In general, what I've described is a case of lowering the river instead of raising the bridge.


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## hollowbody (Jan 15, 2008)

mhammer said:


> Okay, assuming yours is the model with the 24v wallwart supply, here's what you *might* do.
> 
> The input buffer/gain stage is a 4558 marked U1. The output is pin 1, which goes both to the delay line and to the final output and blend control.
> 
> ...


That sounds relatively simple. I might just give that a go. I'll have to take a look at the board to see how cramped it is, because I'm still so-so when it comes to PCB jobs, but it might do the trick.


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## mhammer (Nov 30, 2007)

My schematic shows a SPST switch for "bypass". Knowing Mike Matthews penchant for saving on manufacturing costs by adopting whatever inventory redundancies he can, I'm wondering if the bypass switch is actually something more complex than a SPST and they're only using two lugs of what might be a more complex switch.

If there ARE one or more sets of contacts "available for duty", then it *may* be possible to do the level adjustment via changes to the gain of the input stage, rather than altering the output level.

If you could look undetr the hood and tell me a bit more about the stompswitch, I might be a little more helpful.


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## hollowbody (Jan 15, 2008)

mhammer said:


> My schematic shows a SPST switch for "bypass". Knowing Mike Matthews penchant for saving on manufacturing costs by adopting whatever inventory redundancies he can, I'm wondering if the bypass switch is actually something more complex than a SPST and they're only using two lugs of what might be a more complex switch.
> 
> If there ARE one or more sets of contacts "available for duty", then it *may* be possible to do the level adjustment via changes to the gain of the input stage, rather than altering the output level.
> 
> If you could look undetr the hood and tell me a bit more about the stompswitch, I might be a little more helpful.


I'm at work now, but the person I bought it from did mention he replaced the switch, so whatever is in there, it's not the original part. I think it's a 3PDT, but I'll have to look.


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