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Got this together, tested that it worked, it did, so boxed it up…
I didn’t verify the volume, just the function.
I like the sound of it, but it’s quiet. With the boost switch on and the volume all the way up, it’s about at unity.
Reflowed all the resistor connections, the pot connections, and the capacitor connections I could reach.
Wiggled the socketed components to no difference.
Swapped the TL072 to another, to no difference.
Best I can think is that it’s a joint I haven’t reflowed yet–but I figure that would be intermittent or off, not consistent quiet volume–or a value’s off that I missed.
All values were tested before installation, but if I swapped a spot…
All the values that I can see look to be correct. To help any further we need clear photos of the board underside and all off board wiring, accompanied by voltages readings of input voltage and from pins of the mosfets and the IC. Should be standard for any support post. Check out Barry’s Crash Course on the Guides page.
Alright, some poking and prodding.
Can’t get good pictures of the underside with the pots in place, but there we go.
Measurements, all knobs full:
Input: 9.26v
Q1: 4.06, 4.51, 4.51
Q2: 4.51, 4.09, 4.11
IC: 1:4.52 8: 9.06
2: 4.52 7: 4.43
3: 4.27 6: 4.43
4: 0 5: 4.15Gain pot is also showing 4.54 on all legs, all other pots and legs 0. Just poked it out of curiosity.
Those seem fine to me. The circuit should also sound good with Q1, Q2 and D1 removed and the boost switch open. Q1, Q2 and D1 just provide clipping for the overdriven sound. BTW which mosfets did you use and did you verify the pinout (can’t see them)? What are you using for D1 diode? I would measure voltages again at the op amp with all knobs fully CCW, as is normally done, and again with the IC removed, just for reference.
Since I see some ‘slag’ here and there on your boards my guess tends toward signal loss from solder bridge. I would take a tooth pick and rub it between all solder pads that are close and shouldn’t touch and clean off anything that shouldn’t be there. Check carefully under the pots that are in place. Your soldering itself looks mint. I would reflow off- board connections, though, as these fail with cold solder joints far more often than others. For example I’ve pulled wires right from jacks that looked great but weren’t. Maybe I was in a hurry that soldering day or it wasn’t Barry’s wire. And pay extra attention to on-board ground pads because they usually take extra heat to melt the solder due to the size of the copper plane within the circuit board design taking longer to heat up fully. You can see what I mean when you try to shine light through Barry’s boards. Some of the invisible connections inside are large and take an extra second or two of soldering iron. A cold joint can have a partial connection resulting in weird outputs. I give a lesson because I’m hoping there are more eyes reading this than just yours and mine.
Beyond that and more guessing an audio probe is your best tool. It will reveal the bad area when expected signal isn’t there. Mine is kept handy in the bin marked test tools.
Let us know what you find out.
Transistors are 2n7000, from Tayda IIRC, so should be source-gate-drain. Not sure how to verify that outside of datasheets.
D1 is a 1n60p. Or at least labelled as such. I’m getting suspicious of any diodes meant to replace older ones now.
I’ll be doing some more poking and prodding in a bit and will get comparison readings.I’m not blaming jacks on this one; bypassed is fine. And so far everything sounds good, it’s just quiet. So I expect a faulty pot (I’ll be testing those more) or you’re right and it’s a bridge to something, somewhere.
I’ll be giving it another rundown and testing out my audio probe later.
I agree all of the voltages are correct but the transistor voltages would be similar if the transistors were reversed so suspecting a orientation issue is a good thought. I would also suspect anything else we cannot see (since voltages and resistor values were verified) whether it is capacitor values, cold solder joints, intermittent connections, etc.
That silicon diode while labeled a 1N60P is not a Germanium Diode as in the 1N60P Germanium we normally think of.
I am also not clear on how Quiet you mean. Like just a little below unity gain quiet or barely audible quiet with the Volume and Drive turned up.
I’ll test with one of the old germanium 1n60p’s I have. Wasn’t particularly worried about that yet because, like mentioned, it’s mostly for clipping. They’re more Tayda parts, so I figured the same thing–or at least a reliable recreation–in a different package.
I’ll get to reflowing the rest of the connections and testing a bunch of stuff once I can get some time at it.
By ‘quiet’, I meant just under unity volume with the boost switch on and all knobs full-on. Maybe about 5% quieter than bypass, and that’s the most I can get from it.
It gets quieter from there, and is extremely quiet or inaudible under about half, from memory.Alright, some poking and prodding later…
At no point does it raise above input volume. Working backward, I believe I lose signal at the left leg of C5.
Contact at neither leg of R8 passes sound through, lower leg of R9 is quiet, upper leg seems OK.
Will figure out how to best deal with the pot in the way and start there.Update: Poking, prodding, everything’s reflowed, no change for better or worse.
Will get the probe at it yet again later, and start replacing bits.
Also a note: this ‘no clean’ solder sure leaves a lot of gunk behind.
If you follow along in the schematic your audio path should be input >C1 >R2 to pin 3 of IC1A. Pin 1 >R6 >R7 to pin 5 of IC1B. Pin 7 >C7 >R10 to Vol tab 3. The other components affect either the voltage or frequencies within the overall audio signal. Mostly the unwanted ones.
Pin 1 (an output) should be louder than pin 3 (the input) because of the resistance of the gain control combined with R5, and in relation with R4, causing the amplification to happen. Same with pins 5 (input) and pin 7 (output) because of R8 and R9. Pin 7 should be loudest spot if you had a way to measure that. R10 quiets it. R11 reduces that quieting and the Volume makes that signal quieter. Hopefully all that makes sense.
I want to retract what I said about the circuit working with the diodes removed. That is in fact the path that VB voltage takes to pin 5, so they will need to be installed properly.
Further poking and prodding while I had a few moments.
Replaced the Tayda 1n60p with a germanium I got here. Now at or slightly above unity volume with the volume pot full-on and the boost switch off. Tone thicker and slightly darker.
Audio probe reveals good signal when applied to C1 in (bottom leg), but nothing but slight hum at C1 out. Continues as such until IC Pin 7. Then signal returns, reduced, and remains until full volume at upper lug of R10.To me, this says something between C1 and IC Pin5 burnt out. Likely R7.
R7 can be tested properly by lifting one side to remove it from the circuit.
A quote from Cybercow “The 2N7000 devices are just for hard clipping. The circuit will work with or without them. It will simply be louder and cleaner without the 2N7000s. D1 is just part of the hard clipping configuration to provide asymmetric clipping. Just make sure it’s oriented properly. If it’s not, it just won’t clip right”
With the mosfets in place you are hearing the result of the voltage interference between pins one and five and C1 and pin three. Sorry. But have you measured the gain resistance? And does the gain function work properly? Less resistance – less gain? Not every pot is a good one. With the mosfets removed what are your results?
Gain pot measures at 990k max. It does seem to be working properly.
Removing both the mosfets brings the volume back up to what I’d expect. Slightly less volume with both than with 1, completely subjectively.
Not noticing much of a tonal difference on my tiny tabletop test amp. I’ll have to drag over my proper amp to tell, if it’s a relatively subtle one. They could just be overpowering the thing.
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