Big Muff Opamp

LordSven · July 08, 2018, 04:42:54 AM

Hi everyone this is my first post regarding a build. I've started to build this Big Muff Op Amp with a great PCB bought here to a very kind forum member (bloxstompboxes), everything is soldered except the footswitch. I've tried the pedal which works but for me there's a problem because there's not that much gain. it was intended to work with a A10K for sustain, B10K for Tone and B50K for volume, I've done it first this way than changed to a B10k for sustain and B100k for volume : no real changes...Maybe my solder joints are not so well made or could it be an IC problem ? I've got some other I can try. here are some pictures if some members have some clues. This isn't my first try actually the 14th build (that works) I know I have to get still better with soldering

teemuk · July 08, 2018, 05:03:43 AM

There's one obvious problem in mods you performed: None of them increases or effects gain.

My suggestion is that you modify resistor values of OpAmp feedback loops. Unfortunately I don't know what your schematic reference is so I can't pinpoint what resistor(s) you should change and into what value(s). But they are basic opamp stages, no rocket science to increase gain.

LordSven · July 08, 2018, 05:15:55 AM

thanks for your reply but I've followed the parts list provided with the PCB and it is supposed to have plenty of gain. I suppose I haven't done something properly and I'm just trying to debug this build I wasn't to try to mod the circuit which I wanted to leave stock.
I had to just tried to change the pots because I had seen some Op amps BM fitted like this.

ElectricDruid · July 08, 2018, 06:34:19 AM

I don't think your soldering looks that bad. There's the odd joint where you've got a bit too much solder or a bit too little, or maybe not heated it quite long enough, but in general I'd say that's pretty good.

When you say there's not enough gain, do you mean it doesn't go crunchy enough, or it doesn't go loud enough?

It's possible with diode clipping circuits like the muff to have a signal which is very heavily clipped and of a fairly low volume (so a "high gain" sound but at low volume). This is more likely to be noticed by people who have a fairly hot output from their guitar, since then the clipped level is less than the dry signal.

Given that I don't think your soldering is suspiciously awful, I'd be double- and triple-checking component values to see if I'd got something in the wrong place.

HTH,
Tom

LordSven · July 08, 2018, 08:29:07 AM

thanks Tom for the advice, I'm going to check all the components values... I thought there wasn't enough gain compared to other Big Muff I've got (EHX BM pi RI, Ram's Head Clone using Coda's Effects PCB (great sound and lots of gain) and another BM Opamp using an Aliexpress PCB which sounds quite good and full of distortion too... That's why I was expecting more gain from this one but thanks to this forum I'm sure we'll find out why.

chuckd666 · July 08, 2018, 09:35:39 AM

Yeah your soldering looks fine. When things like this happen it can sometimes be a component with the wrong multiplier value - ie they put in a 1k instead of a 100k or... something like that.

duck_arse · July 08, 2018, 10:42:55 AM

on te very bottom of your photo, near the white wire, is a resistor. can you put your meter across that and give us the resistance, please? also, without build docs or your voltage measures, we can only guess.

LordSven · July 08, 2018, 11:19:51 AM

Quote from: duck_arse on July 08, 2018, 10:42:55 AM
on te very bottom of your photo, near the white wire, is a resistor. can you put your meter across that and give us the resistance, please? also, without build docs or your voltage measures, we can only guess.

the one you asked me to test should be a 100k and i only read 1k

could that be the problem ?

duck_arse · July 08, 2018, 11:35:50 AM

it would certainly put a crimp on your volume, yes. try lifting one leg of the resistor, see if volume comes good. remeasure the resistor while it's disconnected [I wasn't sure of the colour bands due to your lighting].

LordSven · July 08, 2018, 12:05:43 PM

Quote from: duck_arse on July 08, 2018, 11:35:50 AM
it would certainly put a crimp on your volume, yes. try lifting one leg of the resistor, see if volume comes good. remeasure the resistor while it's disconnected [I wasn't sure of the colour bands due to your lighting].

i lifted one leg of the resistor and got 100k as it should be. but when soldered on the board i only read about 14k. Must i do the measures with power on ? I've changed the resistor and the sound is the same. There are also about 5 resistances that i tested and were not the good values. Do you want the two ICs voltages ?

ElectricDruid · July 08, 2018, 12:24:09 PM

IC voltages and the schematic would be a great help, yes please.

LordSven · July 08, 2018, 12:53:32 PM

JRC4558
1 :4,10
2 :4,10
3 :4,05
4 :0
5 :4,08
6 : 4,10
7 : 4,11
8 : 8,24

LM741
1 : 0
2 : 4,51
3 : 4,32
4 : 0
5 : 0
6 : 4,52
7 : 8,41
8 : 0

i'll scan the schematic

bloxstompboxes · July 08, 2018, 01:44:05 PM

This is my PCB design. I will try to get voltages today if I can. Pretty busy day.

LordSven · July 08, 2018, 03:14:41 PM

thanks Eric

Keppy · July 08, 2018, 03:40:09 PM

You still haven't said whether you mean "gain" as in volume, or "gain" as in distortion. Your posts make it sound like you mean distortion, but some of the replies seem to be taking it as volume. Please clarify so we can help you better.

Your resistors look like the correct values (though there's one hidden by a capacitor in the photo). Resistors are responsible for most of the gain/volume/distortion levels, so that's where I looked first. Your voltages seem reasonable, too.

If you have enough volume but not enough clipping, maybe one of the diodes has a problem. If one of the diodes failed open or had a bad solder joint, then you would get plenty of volume, but less distortion because the diodes are intended to clip the signal. Try metering the 741 from pin 6 to pin 2 with your meter in diode mode. You should read 1.5-2.0v (3 diode drops added together) regardless of which lead you place on which pin. If you get a much different reading in one direction than the other, then you have a bad diode or a bad solder joint on a diode.

Note that C6 and R10 are in parallel on this same path, so they may throw your reading off a little bit. Don't worry if it reads a little different than what I said above, so long as it reads the same in both directions. Good luck!

bloxstompboxes · July 08, 2018, 04:25:41 PM

I think you have a UA741 which should work. However, something looks a little funny about the way it's in the socket. Might just be the angle. Can you reseat it? Still gonna get to voltages sometime today, hopefully.

LordSven · July 08, 2018, 04:38:29 PM

Quote from: Keppy on July 08, 2018, 03:40:09 PM
You still haven't said whether you mean "gain" as in volume, or "gain" as in distortion. Your posts make it sound like you mean distortion, but some of the replies seem to be taking it as volume. Please clarify so we can help you better.

Your resistors look like the correct values (though there's one hidden by a capacitor in the photo). Resistors are responsible for most of the gain/volume/distortion levels, so that's where I looked first. Your voltages seem reasonable, too.

If you have enough volume but not enough clipping, maybe one of the diodes has a problem. If one of the diodes failed open or had a bad solder joint, then you would get plenty of volume, but less distortion because the diodes are intended to clip the signal. Try metering the 741 from pin 6 to pin 2 with your meter in diode mode. You should read 1.5-2.0v (3 diode drops added together) regardless of which lead you place on which pin. If you get a much different reading in one direction than the other, then you have a bad diode or a bad solder joint on a diode.

Note that C6 and R10 are in parallel on this same path, so they may throw your reading off a little bit. Don't worry if it reads a little different than what I said above, so long as it reads the same in both directions. Good luck!

I was meaning the distortion, the volume is okay, thanks for the information i'm going to check this tomorrow after work

LordSven · July 08, 2018, 04:43:51 PM

Quote from: bloxstompboxes on July 08, 2018, 04:25:41 PM
I think you have a UA741 which should work. However, something looks a little funny about the way it's in the socket. Might just be the angle. Can you reseat it? Still gonna get to voltages sometime today, hopefully.

Yes you're right the 741 was a bit strange in the socket, i've put it back properly, I'll try all this tomorrow because my girlfriend won't like me plugin the guitar at 22:45

thanks very much everyone for your support I really enjoy doing this and playing with DIY Muffs

bloxstompboxes · July 08, 2018, 05:38:51 PM

Your voltages match mine, minus the Vbatt difference. However, I assume that pin 3 on the lm741 is a typo and should read around 4.3V. That would make more sense. Otherwise, it looks good. Is your audible testing being done in or out of the box. The distortion should be through the roof on this one. Can we hear it?

DIY Bass · July 09, 2018, 03:58:48 AM

Sounds like your next project should be a headphone amp so you can play at night.

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