SOLVED Tayda Purple Plexi 800 Noise

[yazooligan]

EDIT: I tried putting a 47R in place of the 10R in R3, and the squeal at max gain is gone, though I think it loses just a bit of attack/gets a little sponge-y.

Any other values you guys suggest?

I’d also love to have the Level pot have more range with unity gain being near the middle instead of the first 10% of the throw. Not sure how to make that happen.

I found through more googling that changing the Gain pot to a C1K can make the gain range wider and more usable. If anyone has one I can use to try it I’d appreciate it. Tayda doesn’t have any!

OP: I wired this thing up following their diagram and got a cacophony of noise no matter how the knobs were set, except for Level all the way down. Turning the knobs made some filtering noises too. My guitar signal could still be heard in the noise too, but again, completely unusable. The LED also varied in brightness as I handled the footswitch. I used a 2k2 resistor per the build docs for the LED.

I double checked the wiring diagram with mine. It looked good, but I decided to wire another footswitch in just in case I’d burned up the first one. The LED came on solid but the noise issue persisted.

Triple checked my wiring, jacks etc. Still correct.

So I put another LM386 in the socket. No change.

I re-flowed all my solder joints. No change.

The only difference in my build is that I didn’t use a battery snap, which shouldn’t matter, right?

 

[yazooligan]

 

[PJS]

I can't fully follow all the wiring, but you seem to be missing the connection from power ground to the input/output jack grounds.

 

[yazooligan]

I can't fully follow all the wiring, but you seem to be missing the connection from power ground to the input/output jack grounds.

Good eye! I added that cable and I’m getting effected signal now, BUT, the last 5% of the Gain knob still causes a high pitched squeal.

Any ideas? Also, I owe you a beer!

 

[yazooligan]

Good eye! I added that cable and I’m getting effected signal now, BUT, the last 5% of the Gain knob still causes a high pitched squeal.

Any ideas? Also, I owe you a beer!

I’ve verified every cap and resistor, so unless one of the pots is bad, I’m going to blame this one on Tayda and not buy any more PCB’s from them.

 

[PJS]

some circuits can get a squeal if they are not well laid out, but that usually only happens with very high gain stuff. My impression is that this is not that gainy. I would try with an audio probe so you can at least see which part of the circuit the squeal is coming from. Also, if it is just because of gain, then there are some things to try and minimise it - twist the power supply leads together - use shielded wire on the input and output jacks and ground the shield. Also, make sure the case is making good contact with the ground. I had one pedal that was very noisy all the time, and it turned out that the footswitch barrel was just not grounded - I had used the plastic washer to isolate it from the case and that was a bad plan - when I removed that washer and made sure the actual stomp switch was grounded through the case then the noise stopped.

 

[BuddytheReow]

Do you have an audio probe?

 

[yazooligan]

Do you have an audio probe?

I do indeed.

 

[yazooligan]

some circuits can get a squeal if they are not well laid out, but that usually only happens with very high gain stuff. My impression is that this is not that gainy. I would try with an audio probe so you can at least see which part of the circuit the squeal is coming from. Also, if it is just because of gain, then there are some things to try and minimise it - twist the power supply leads together - use shielded wire on the input and output jacks and ground the shield. Also, make sure the case is making good contact with the ground. I had one pedal that was very noisy all the time, and it turned out that the footswitch barrel was just not grounded - I had used the plastic washer to isolate it from the case and that was a bad plan - when I removed that washer and made sure the actual stomp switch was grounded through the case then the noise stopped.

Copy that. I appreciate the reply. I just drilled an enclosure for it today so I’ll keep that all in mind.

 

[yazooligan]

Got it all mounted in the enclosure and the problem persists. I’m wondering if the Gain pot could be the culprit. It has almost no change in gain no matter how the pot is turned, except for the squeal at the end of the dial. Do bad pots happen? I’ve never come across one.

 

[yazooligan]

My impression is that this is not that gainy.

It’s actually fairly high gain, at least mine is. I’m in Guns N’Roses territory over here and turning the Gain down doesn’t really do anything.

 

[PJS]

Pots cab be bad for sure - for a variety of reasons. You can test basic function with a multimeter - test between the 2 outer lugs and you should get close to the value of the pot (it may vary a bit depending on the circuit and also your meter's input impedance. Measure between the central lug and one end and then turn the pot and the value should change from 0 to the value of the pot. Do the other side and you should get the reverse.

 

[Robert]

I have no experience with this specific PCB, but this is a fairly common problem with the Purple Plexi circuit.

Off the top of my head:

There are different versions of the LM386 - Yours appears to be LM386-N1, which I believe is correct.

Lead dress - Keep the input/output wires as short and as far apart as possible

I believe some folks have remedied this problem by bumping up the value of R3.

 

[yazooligan]

WHOA!

This guy’s build sounds and functions just like mine! Pretty sure there’s nothing wrong with mine now.

 

[yazooligan]

I believe some folks have remedied this problem by bumping up the value of R3.

EDIT: I tried a 47R in place of the 10R in R3, and the squeal at max gain is gone, though I think it loses just a bit of attack/gets a little sponge-y.

Any other values you guys suggest?

I’d also love to have the Level pot have more range with unity gain being near the middle instead of the first 10% of the throw. Not sure how to make that happen.

I found through more googling that changing the Gain pot to a C1K can make the gain range wider and more usable. If anyone has one I can use to try it I’d appreciate it. Tayda doesn’t have any!