Electro Harmonix Bass Big Muff  [schematic]

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dylan159
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Post by dylan159 »

I haven't seen this one here already, so I gave it a trace both for personal fun and curiosity:
bassmuff.png
Some values aren't there because they weren't marked on the silkscreen like the others, and I'm not going to pull them out to check. You get the gist of it though: they basically doubled up most caps with another one in parallel. The tone stack is a flatter variant. The "bass boost" literally mixes some lowpassed signal with the tone stack output. The parallel dry blend is done in a way that doesn't affect the original circuit and has a negligible loading. The direct output is just disgusting though :lol:
Opinions aside, there are some interesting ideas and I hope it can be useful for troubleshooting.
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Post by JiM »

dylan159 wrote: 26 Apr 2021, 14:18they basically doubled up most caps with another one in parallel.
Good BOM optimisation, as they for sure have a huge stock of components for the standard Big Muff.
I only give negative feedback.

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Post by dylan159 »

JiM wrote: 28 Apr 2021, 15:33
dylan159 wrote: 26 Apr 2021, 14:18they basically doubled up most caps with another one in parallel.
Good BOM optimisation, as they for sure have a huge stock of components for the standard Big Muff.
Yeah, easy and effective, even if they didn't think for long whether the doubling is desired or is doing anything.
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Post by Whoismarykelly »

Some of the doubling is perplexing though. They parallel 100n throughout to achieve 200n coupling caps but then parallel 47n when they could have just used a single 100n they already had in the BOM.

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Post by Cub »

Hello Dylan, thank you for doing this ! I've been wondering about the differences between the original one and the Nano version.
Would you happen to know the PCB number and revision of the unit you traced ?
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Post by dylan159 »

Cub wrote: 05 May 2021, 17:24 Hello Dylan, thank you for doing this ! I've been wondering about the differences between the original one and the Nano version.
Would you happen to know the PCB number and revision of the unit you traced ?
Just a moment... And it says EC-D40 Rev.A 2008
I guess I'll take the chance to make the diode switch on off on :)
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Post by G.Bisson »

I was preparing to mod my bass big muff and found this schematic very helpful. Component numbers on Dylan's schematic didn't match labels printed on the pcb I had. I went over the schematic and marked it up to match the pcb I had. My PCB version is EC-D40 Rev.A, identical to Dylans . There is a number printed in the top right corner of the pcb written in LCD alarm clock font that is likely a production code. My pcb had 0943, built 43rd week of 2009.

Notes and comments:
1. Updated the schematic with missing labels.
2. Capacitance values printed on the pcb are the parallel value of both capacitors. These match typical Russian big muff values.
3. C1 & C30 are underneath the board
4. There are extra diode pads underneath the board labeled D1A + D1B and D2A + D2B.
5. Blank C8 pad is parallel to R32.
6. Blank R42 pad is parallel to the entire Bass Boost section. Soldering a jumper here would convert Bass Boost mode into Tone Bypass mode.
7. C37 isn't shown on the schematic. It goes from Vcc to Gnd.
8. C35 isn't shown on the schematic. I believe one end is connected to Vr and the other end trace is hidden underneath U1.
BassBigMuff_schem_REV_GBisson.png

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Post by G.Bisson »

Attached photos
BassBigMuff_pcb_top.jpg
BassBigMuff_pcb_bottom.jpg

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Post by G.Bisson »

Added Tone Bypass and Mid Shift mods to my pedal. These mods are easy to make; pcb doesn't need to be removed from the box and there's no need for desoldering or touching any SMD.
Toggle switch on the left is Mid Shift. It's a SPDT On/Off/On. Pin 1 goes to 5.6nF cap. Pin 3 goes to 15nF cap. The other ends of both caps are twisted together and wired to TONE_3. Pin 2 of Mid Shift switch is wired to Q3 collector and a jumper also goes to pin 2 of the Tone Bypass switch. Switch in down position is flat response, switch up is mid boost and center position is stock mid scoop.
Toggle switch on the right is Tone Bypass. It's a SPDT On/On type. Pin 1 is wired to TONE_2. Pin 2 is jumped and wired to Q3 collector. Switch in up position is normal, switch in down position is tone bypass. This shorts out the tone section and bass boost section. Dry mode is still functional with tone bypass on.
Also swapped in a green LED and replaced the footswitch with a soft-click type with dress nut.
BassBigMuff_mods_top.jpg
BassBigMuff_mods_bottom.jpg

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Post by ppluis0 »

Hi pals,

Something that intrigues me is why EH continues to use the 4 transistors in TO92 format, even though the other components have been migrated to SMD :scratch:

Why not also use the SMD version of the BC547 ?

Cheers,
Jose

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Post by dylan159 »

G.Bisson wrote: 03 Aug 2022, 16:11 I was preparing to mod my bass big muff and found this schematic very helpful. Component numbers on Dylan's schematic didn't match labels printed on the pcb I had. I went over the schematic and marked it up to match the pcb I had. My PCB version is EC-D40 Rev.A, identical to Dylans . There is a number printed in the top right corner of the pcb written in LCD alarm clock font that is likely a production code. My pcb had 0943, built 43rd week of 2009...
Thanks for the work you put into perfecting this. At the time I made no effort to match the pcb labels, or desoldering every single smt capacitor to be sure of its value. I guess that's not necessary anymore. Interesting that the differences are even less from a standard muff if those capacitors add up to the same value.

In addition to the pcb number and revision, the alarm clock code says 0909, so it's from the beginning of 2009. I'm attaching a picture if you find other details of your interest.

About my unit, I got it used in not the best aestethic conditions, and replacing the footswitch is one of the first things I had to do. Congrats for keeping the original pcb daughterboard. Personally, I have no use for those things, so after confirming it's the traditional true bypass I rewired it by hand. I also have some mods, inspired by zorgeffects and made before I knew how to do things my own way.
One three-way switch changes between stock, led and lift clipping for both stages; the other adds 33n or 10n in parallel with C9 on the board (analogous to your mid shift), which we now know means adding even more mids to the already flat tone stack. The third switch is a silly feedback switch. The pot on the side is a bias mod, which might actually have been not just copied from somewhere: a 1M pot in parallel with R3 on the board, which goes from standard to completely cut off, unlike the zorg one that pulls the base up to 9V. The final pot on the front is a "master volume", which is a simple fix I had found online to counteract the two other modes making the pedal louder and the dry level being stuck at unity. I'd probably do half of these differently now, but it's been like this for a while since the bigger muff has covered most of my recent muff needs.
One thing I wanted to say is that yours isn't the only way to "defeat" the tone stack: a popular one I'm a fan of lifts the capacitor and resistor from ground. In practice, one could twist their ends off the pcb and ground them with a switch, or otherwise scrape two traces. Not only you keep the bass boost functional, but you avoid creating a positive feedback loop around that op-amp.
Cheers, it's been fun revisiting this one.
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Post by kiltro »

G.Bisson wrote: 03 Aug 2022, 16:11 I was preparing to mod my bass big muff and found this schematic very helpful. Component numbers on Dylan's schematic didn't match labels printed on the pcb I had. I went over the schematic and marked it up to match the pcb I had. My PCB version is EC-D40 Rev.A, identical to Dylans . There is a number printed in the top right corner of the pcb written in LCD alarm clock font that is likely a production code. My pcb had 0943, built 43rd week of 2009.

Notes and comments:
1. Updated the schematic with missing labels.
2. Capacitance values printed on the pcb are the parallel value of both capacitors. These match typical Russian big muff values.
3. C1 & C30 are underneath the board
4. There are extra diode pads underneath the board labeled D1A + D1B and D2A + D2B.
5. Blank C8 pad is parallel to R32.
6. Blank R42 pad is parallel to the entire Bass Boost section. Soldering a jumper here would convert Bass Boost mode into Tone Bypass mode.
7. C37 isn't shown on the schematic. It goes from Vcc to Gnd.
8. C35 isn't shown on the schematic. I believe one end is connected to Vr and the other end trace is hidden underneath U1.

BassBigMuff_schem_REV_GBisson.png
Isn't SW1A flipped?

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Post by G.Bisson »

Looks like SW1A is drawn backwards on the schematic. I didn't verify the footswitch pinouts during my modding session.

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Post by dylan159 »

I guess so! Since I had to trace it to wire a new switch in, I'm confident it's as usual: pin 3 goes to ground, the OUT label is on pin 1.
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Post by Guroots »

G.Bisson wrote: 03 Aug 2022, 16:11 I was preparing to mod my bass big muff and found this schematic very helpful. Component numbers on Dylan's schematic didn't match labels printed on the pcb I had. I went over the schematic and marked it up to match the pcb I had. My PCB version is EC-D40 Rev.A, identical to Dylans . There is a number printed in the top right corner of the pcb written in LCD alarm clock font that is likely a production code. My pcb had 0943, built 43rd week of 2009.

Notes and comments:
1. Updated the schematic with missing labels.
2. Capacitance values printed on the pcb are the parallel value of both capacitors. These match typical Russian big muff values.
3. C1 & C30 are underneath the board
4. There are extra diode pads underneath the board labeled D1A + D1B and D2A + D2B.
5. Blank C8 pad is parallel to R32.
6. Blank R42 pad is parallel to the entire Bass Boost section. Soldering a jumper here would convert Bass Boost mode into Tone Bypass mode.
7. C37 isn't shown on the schematic. It goes from Vcc to Gnd.
8. C35 isn't shown on the schematic. I believe one end is connected to Vr and the other end trace is hidden underneath U1.

BassBigMuff_schem_REV_GBisson.png
Hey Guys,

Thanks a lot for the job done, I will give you some data..

I already have the same PCB than you. Whereas modify my BMBass I plan to build a new. (with more change change 2 // capa to single, add a blending pot betwen effect and dry... I will post my scheme soon)

According to what I can find on electrosmash or http://www.bigmuffpage.com/
I am 100% sure C10=C19=470pF

Regarding Bass boost section R30 and C31 are low pass filter (I my mind it does not change sound a lot) but 3.9 nF or 5nF for C31 can set the LP filter frequency to 400 or 300 hz
I do not find yet C38 value

On the final output C32 and R37 are hight pass filter 100nF can set the frequency to 15hz


See you

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Post by mictester »

ppluis0 wrote: 04 Aug 2022, 13:56 Hi pals,

Something that intrigues me is why EH continues to use the 4 transistors in TO92 format, even though the other components have been migrated to SMD :scratch:

Why not also use the SMD version of the BC547 ?

Cheers,
Jose
That will be because EHX bought a few million BC547 for less than $0.001 each! I have a few boxes of transistors bought in bulk like that, and they are insanely cheap - cheap enough to use them as diodes as well!
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