SOLVED PPCB Cobbler Preamp Volume

[Locrian99]

I built the cobbler tonight sounds pretty good need to mess around with it some more but I was rather surprised but what I perceived as being low volume from pedal.

If I had the gain turned down so I was getting a non clipped signal I was able to max the volume pot and still not hit unity. I didn’t do a bunch of other experiments. And it would hit unity at higher gain settings but still just wasn’t giving much of a volume boost far lower than any other overdrive type pedals. Is this normal for this does anyone know? Should I expect this from a preamp type pedal in general? If it’s not normal anyone got any tips in which resistors to look at? All of my voltages are what I would expect on the tl072s. Just over 17 on pin 8 and about half that on all the others 8.62 if I remember correctly. There’s schematics for the John and kraken clone but not this one.

Thanks

 

[music6000]

Is there a 47 ohm resistor just before the Out pad on the PCB that may be incorrect.
47 ohm = Yellow, Purple, Black, Gold, Brown.

 

[Locrian99]

Is there a 47 ohm resistor just before the Out pad on the PCB that may be incorrect.
47 ohm = Yellow, Purple, Black, Gold, Brown.

The resistor in the same spot as r17 on the John board is a 10k on the cobbler. And r17 is a 47k, which is between the 1u cap and the output pad from what I can see.

 

[temol]

Turn your guitar cable into an audio probe and check the signal path - you should be able to find a place where level drops.
I've marked a few points on John's schematic that you can check. Put VOL to max and compare the signal at the output with the marked points. Start from the OUT towards the IN.

 

[Locrian99]

Turn your guitar cable into an audio probe and check the signal path - you should be able to find a place where level drops.
I've marked a few points on John's schematic that you can check. Put VOL to max and compare the signal at the output with the marked points. Start from the OUT towards the IN.

View attachment 40633

Ah yes audio probe makes sense, I’ll have to try that tomorrow. I do think based on values in the positions the output section for the cobbler is different unless that 47k should be the 47r… but the board says 47k….

 

[harmaes]

The other 2 preamps of which the documentation is available use 47r so I reckon the 47k is wrong? @Robert?

 

[Locrian99]

The other 2 preamps of which the documentation is available use 47r so I reckon the 47k is wrong? @Robert?

I think the 220r might actually be wrong. Idk. The John has one more resistor than the cobbler. The 47k looks to be r18 which is 10k in the John and go to ground following the output cap. The John has a 220k in the feedback loop of the last op amp right after the volume there’s a 220r in the cobbler that from the spot on the board looks like it might be doing the same. Which I think controls the gain of that final op amp. An actual cobbler schematic would help as I’m just kind of guessing going to try the audio probe in a little bit once the house wakes up.

 

[harmaes]

Interested in this as it’s an upcoming build for me.

 

[Robert]

Nope, the 220R and 47K resistors are both correct.

I'll get the doc for this one posted ASAP.

 

[Locrian99]

Nope, the 220R and 47K resistors are both correct.

I'll get the doc for this one posted ASAP.

I’m a previous post you mentioned specific part numbers for the zeners. Looking those up I see they are 500mw zeners. The ones I used are 1w, would this effect volume ?

 

[Locrian99]

I’m a previous post you mentioned specific part numbers for the zeners. Looking those up I see they are 500mw zeners. The ones I used are 1w, would this affect volume ?

Also notice from the silk screen of the others it appears as though the silk screen on the board has the value of the volume and gain pots swapped. I might be looking at it wrong though.

 

[Locrian99]

@Robert can you confirm those pot values are correct please?

Makes sense to my novice self that is where the problem lies here.

Pic from a Cetus build I snagged from the forum



And the pic from the board I have cobbler



I’m really thinking the silk screen is wrong here rather not try removing those pots until I get confirmation though

 

[Robert]

They're correct.

I don't know if it was intentional or not, but that's how they were installed in the originals.

 

[Locrian99]

Well this is interesting. The 10k right above the A1M was measuring 0 ohms, so I removed it tested 10k. Tested continuity between the pads and it’s 0 ohms…

 

[Robert]

Try turning down the Bass pot and measuring again.

If it's all the way up it'll shunt that 10K to ground.

 

[Locrian99]

Try turning down the Bass pot and measuring again.

If it's all the way up it'll shunt that 10K to ground.

And you’re right thx. I’ll start the audio probe adventure as I have time.

Of course this is all I can seem to think about ag the moment but the wife and kids have other things they want to do lol.

 

[Locrian99]

first time problems are always fun. Robert was nice being to supply the schematic which was helpful in tracing the audio signal. I had already figured out from probing the output section was working and boosting volume. No good at the input of the op amp before that. Good at the one before the treble/bass pots. Good at pin 1 and 3 of the treble pot bad at lug 2. Huh. So I went to lift it see if somehow it was grounding or something and….



I think I found the problem!

Looks like that was my last b1m pcb leg though but I had a lug one so I wired it up problem solved plenty of volume.

 

[music6000]

first time problems are always fun. Robert was nice being to supply the schematic which was helpful in tracing the audio signal. I had already figured out from probing the output section was working and boosting volume. No good at the input of the op amp before that. Good at the one before the treble/bass pots. Good at pin 1 and 3 of the treble pot bad at lug 2. Huh. So I went to lift it see if somehow it was grounding or something and….

View attachment 40698

I think I found the problem!

Looks like that was my last b1m pcb leg though but I had a lug one so I wired it up problem solved plenty of volume.

Yep, that would look to have been the culprit.
Great that you got it sorted!