Adding a blend to a Promethium (HM-2)

Danbieranowski

Danbieranowski

Well-known member
Hey all,

I've tried a few different things with this, and all have failed for one reason or another.

I just want to add a blend to a Promethium.

If I use the Promethium board alone (no blend mod), it sounds and works great.

I've tried the GuitarPCB Buff n' Blend, but it oscillates. No oscillation if I don't run it through the send and return of the Buff N Blend, so it's definitely the BNB causing the issue.

I've tried a Mini-Mixer (little parallel mixer), with the idea of giving him one volume knob that's his clean signal, and one volume knob that's the HM-2 so he can mix to taste. The problem that I have with this is:
A) I get a little bit of volume or tone loss with the Mini-Mixer (found here: http://www.generalguitargadgets.com/effects-projects/boosters/mini-mixer/).
B) I can't think of a way to wire it up where, when switched off, the pedal is just standard true-bypass, but when on, I can feed the clean input signal to one pot of the Mini-Mixer, and feed the output signal of the circuit to another pot of the Mini-Mixer.

I'm not sure what else I'm not thinking of here, but I feel like there's a glaringly obvious way to do this, and I just need a sanity check from the PedalPCB folks. Any help would be appreciated!
 
Dan M said:
I keep checking here to see if one of the resident circuit wizards has the answer...

I was curious about this too, mainly after watching the Fuzzlord HM-6 release video. I found this: http://peperspedals.blogspot.com/2017/08/boss-hm-2-moddingthis-time-blend-control.html?m=1

In the hm-6 video, Jason mentions he boosted the clean path to get the volume closer to the distorted path. So maybe the mod in the above link with a clean boost added?
Click to expand...
This is great info, fellow Dan. Thank you for sharing. I'm trying to work out where the blend would actually go in the path, and how this would be wired to a 3PDT. Same with this one: http://tagboardeffects.blogspot.com/2012/02/split-n-blend.html

Just don't totally understand how you wire them to a 3PDT so that when the effect is off it's True Bypass, but when it's on, then the blend knob and HM2 circuit take effect.
 
Try this:
1. get a B100K pot. I think that will provide enough isolation. If you get squealing, then use a buffer in place of the 1uF cap in step 2.
2. Connect a 1uF film cap from pin 1 of the pot to Q1-S.
3. Remove the wire that goes from the OUT pad on the board to the stomp switch.
4. Connect pin 3 of the pot to the OUT pad on the board.
5. Connect pin 2 of the pot to the stomp switch.

Using a blend pot on a distortion pedal does not always yield the desired results. The tone controls introduce phase-shift, which can lead to cancelling at some frequencies. When the note decays, the dirty/clean blend changes because the dirty signal is compressed and the clean signal isn't.

Good luck!
 
I don’t need another project in the queue, I don’t need another project in the queue, I don’t need another project in the queue, ...

Well I guess I’ll go order the pcb and build a mouser bom.
 
Chuck D. Bones said:
Remember, this mod is untested. It might work right from the git-go, and it might need a little tweaking...
Click to expand...
Hey Chuck, Tried both the 1uF Film cap as well as a Tillman buffer/preamp (I read that was a good basic buffer), and I still get the oscillation at the highest volume setting with both. If I roll back the volume like 10% the issue goes away. Again, this isn't an issue if I don't try to do a blend. It works just fine without the mod. Now I'm starting to think a better solution would be to somehow limit the volume pot to a lower level somehow. Is this possible?
 
I tried the Peperspedals mod ( on real HM-2) yesterday , so I'm still experimenting, but it's working really well. No distortion bleeds in when mixed clean, which remains at bypass volume level. It should be great for bass

Like you, I mostly play guitar, and would want the clean to be as loud as the cranked pedal sometimes. The HM-2 has it's own "rules".
 
Mir9 said:
I tried the Peperspedals mod ( on real HM-2) yesterday , so I'm still experimenting, but it's working really well. No distortion bleeds in when mixed clean, which remains at bypass volume level. It should be great for bass

Like you, I mostly play guitar, and would want the clean to be as loud as the cranked pedal sometimes. The HM-2 has it's own "rules".
Click to expand...
Rad! Did you do the mod on the Promethium board? And if so, where did you dip into and out of the circuit?
 
I find it weird that the blender would oscillate. Are you sure you’re not accidentally wiring so that the output is fed back into the input (like a feedbacker)? Also I was wondering what you meant when you mentioned wiring it with a 3PDT board?
 
giovanni said:
I find it weird that the blender would oscillate. Are you sure you’re not accidentally wiring so that the output is fed back into the input (like a feedbacker)? Also I was wondering what you meant when you mentioned wiring it with a 3PDT board?
Click to expand...
The current version I have is wired exactly as Chuck mentioned above. So with a 3PDT board it works as follows:
Input Jack to Input Jack pad on 3PDT board.
In on 3PDT board to In on Promethium PCB.
Out on Promethium PCB to leg 3 on blend pot.
Leg 2 on blend pot to Out on 3PDT board.
Output Jack pad on 3PDT board to output Jack.

3PDT board in question is the standard PedalPCB 3PDT board.

Ive also tried using the GuitarPCB Buff N Blend and get the same issue.

If I don’t try to utilize a blend circuit, I do not encounter the issue.

I have no other wiring that would feed the output into the input.

The issue only occurs at the last bit of volume, which is louder than the clean signal regardless, so if I could find some way to limit the volume pot’s output I’d be set I think. I imagine I could do this with additional or less resistance somewhere. I’m just not sure where.
 
Well, the output is wired to the input via some resistor, but I can’t tell if that would explain the positive feedback and thus oscillation.
One thing to try could be a blender pedal like the Wetter Box or the Parallel Mixer to see if it’s just the distortion in question that’s causing the problem or something about the wiring. How did you wire the BuffNBlend?
 
giovanni said:
Well, the output is wired to the input via some resistor, but I can’t tell if that would explain the positive feedback and thus oscillation.
One thing to try could be a blender pedal like the Wetter Box or the Parallel Mixer to see if it’s just the distortion in question that’s causing the problem or something about the wiring. How did you wire the BuffNBlend?
Click to expand...
Could the “resistor” in question be the blend knob itself? That’s wired between input and output to pass the clean signal directly to the output.
The Buff N Blend was wired:
Input Jack to Input on BNB.
Send on BNB to input of Promethium.
Out on Promethium to Return on BNB.
BNB out to output Jack.
 
Yeah that’s what I meant. But if you get the same effect with the BNB my guess is that it’s something to do with the distortion circuit itself. But I don’t know what that could be.
 
giovanni said:
Yeah that’s what I meant. But if you get the same effect with the BNB my guess is that it’s something to do with the distortion circuit itself. But I don’t know what that could be.
Click to expand...
All good. Thank you for trying to help! @Mir9 has a potential solution for me if he did his with the Promethium.
 
Another positive note is that this forced me to do my first ever stripboard/perfboard build of that Tillman preamp and it worked! So that's cool.
 
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