Sound with MV at zero

[jokn]

Hi everybody, first post here (aside from introduction). I'm finishing my project, which is a preamp of my own concoction (firmly based on existing high-gain designs) and an AX84 PP50 power amp/power supply with some changes. I'm a little perplexed (I'm easily perplexed by this stuff , being a neophyte ) because on channel 2 I'm getting sound out of the speaker even with the gain about half-way up and channel volume at zero. This only occurs on channel 2, not on channel 1. I also tried grounding the  wiper of the CH 2 volume pot, same result. How is the signal getting to the PI and onward?

[sluckey]

Need to see schematics of your preamp and power amp.

[jokn]

D'oh! I thought I'd managed to attach those to the original post, but guess not. Let's try again.

[PRR]

FULL sound? Or a little tinny leak?

That is a HIGH gain side-path. Would not take much leakage to be audible. IF the pot is well grounded, I would expect mostly capacitive sneakage, which will be mostly highs.

[jokn]

It's a very low volume leak, and there's definitely no low end whatsoever. It doesn't really bother me that much, I just wish to understand what the mechanism is.

[kagliostro]

May be also a defect on the pot, I've seen some (old) pot that didn't correctly go to zero ohms

Franco

[jokn]

I measured the pot before installing it and it does go to zero. Also, shorting the signal to ground from the top of the pot should take the pot totally out of the picture, right?

[jokn]

Progress: I unsoldered the wire from the NFB to the presence pot, and presto, no more sound audible with the volume on zero. Now my question is, is there anything I could do to keep the presence control and alleviate the problem? Would a shielded NFB wire from the speaker jack to the board be a good idea?

[sluckey]

Progress: I unsoldered the wire from the NFB to the presence pot, and presto, no more sound audible with the volume on zero. Now my question is, is there anything I could do to keep the presence control and alleviate the problem? Would a shielded NFB wire from the speaker jack to the board be a good idea?

Swap the OT primary plate leads.

[jokn]

When I originally finished the build and powered on, thre was a terrible howl. A bout of googling led to the solution of swapping the plate leads. I had already shortened the plate leads so I swapped the secondary leads instead. That fixed the howling problem, and now I'm afraid I'll have the same problem if I swap them back.

[kagliostro]

Give a try to this

disconnect Relay 2 and use at his place a pair of mix resistors (220K-330K-470K at your taste)

to connect the CH1 & CH2 MV pot wipers to the following circuit

What happen ?

Franco

[jokn]

Ok, I'll give that a try as soon as I get a chance.

[Tony Bones]

Seems to me that signal from V3 is sneaking into your NFB wire. Check the location of associated leads. See if moving them helps.

Where is the NFB resistor (R49)? It should be closer to the junction of R48, R36, C18 and the presence pot than the OT. In other words, if you need to have a long wire going past other stuff, you want it on the OT side of R49, not the PI side. That side will be less susceptible to picking up other signals and noise.

[jokn]

Franco: I removed relay 2 from the circuit and connected the two channel volumes to V4 pin 2 via 220k resistors. No effect on the bleed-through, but a great improvement as it lowers part count and improves reliability. I knew there had to be a simpler way than having two relays, i just wasn't smart enough to find the answer. Mille grazie!

Tony: tried moving those leads around, but no joy. In the pictures you can see the NFB circuit. It is pretty much as you say it should be, with the long wire on the speaker jack side of R49, and everything else close together. Thanks for the suggestions, guys. I really appreciate you taking the time to help!

[kagliostro]

Glad about what you say, but I was hoping in a different result

I was thinking that the AC signal was jumping from one contact of the relay to the other due to the capacitance between electrodes

but that isn't the case

OK, if the arrangement with mix resistor will be your final solution, remember you can adopt a larger mix resistor value for the channel

you like to tame to have a level similar to the other channel when you swap from one channel to the other

(220k + 330k or 220K + 470K as an example)

Franco

[jokn]

that's a good point about the different resistors, Ch2 could use some padding relative to Ch1. In a further development, I connected the two volume pots (via the mixing R's) to C17, bypassing the FX loop- and that solves the bleed-through entirely. So now it's on to figure out what in the FX loop was causing the trouble.

[kagliostro]

Using an highlighter control the schematic with your layout, sometime happens to be wrong and not able to see where

http://el34world.com/Forum/index.php?topic=17701.0

Ciao

Franco