PRS Sonzera 20. Broke

[capndenny1]

I bought a broken one of these amps on ebay. The descriptionsaid that the guitar could still be heard even though the amp volume was on zero.

I verified that, but there is also a lot of hiss on the clean channel with the volume at zero. As I turn the volume up the noise and leakage signal stay low, and then the ormal signal comes on. So as long as you have a guitar signal it’s ok. But when you stop the hiss is still there.

I tried swapping out V1: and that d no effect.

It doesn’t seem to do it on the gain channel, but I’m not sure on that.

I tried plugging my guitar into the Return input of the effects loop, and that was silent with the guitar volume on zero. Of course there was no guitar signal to bleed over, but there also wasn’t any hiss. Interestingly the reverb was present in the signal with the guitar in to the Return input?

The V1 tube does input gain for both channels. The V2 tube does additional gain for the gain channel. The V3 tube is for reverb. And the final 12ax7 the V4 is the pi circuit. Then the two 6l6 power tubes.

So this test clears the V3, V4, and the power tubes, and the associated circuits. It Also clears the B+ supply to those tubes.

There are no schematics. This very problem was reported by several people who bought the Sonzera when it first came out. They either returned it or got it repaired under warranty. PRS hasn’t said anything about it, and someone on their forum asked about it, and they were warned against talking about factory defects on the PRS forum. It is against the rules.

This is a really great sounding amp. I think it sings on par with my Twin Reverb, which to me is about as goodasit gets. So I really want to ix it.

It is interesting that theis no manufacture date or serial number on the back of the amp. There is a spot for it, but it is blank? The ebay seller seems to sll a lot of shipping damaged amps and instruments. Perhaps they remove the S/N when they declare it scrap?

I haven’t asd PRS about it. I thought I would take a stab at it first.

So I have read that the thng to ook for is perhaps a power supply filter cap on the V1 B+ that is bad, or missing. Then the sigl from the 1st half of V1 couples thrugh he power supply to the 2nd half of V1. Maybe a shared cathod bypass cap on those two halves is bad, so it cople through the cathode instead of the anode. Then when they do the gain they switch in a different cap which doesn’t allow the coupling.

Perhaps the clean channel bypasses V2 when the clean is active, and that is where the noise gets injected. A ungrounded signal line would pick up radiated noise.

But it also lets hiss or white noise to couple into the signal. So that makes more sense to be the anode filtering.

Anyone else seen this problem? I will get the scope and inject atone and try to trace it.

[g1]

The V1 plate resistor can give the proper B+ to the tube but still be noisy. That is a possible suspect.
Seems others have run in to the problem, but none stated a solution:
https://forums.prsguitars.com/thread...s.24304/page-2

[capndenny1]

Noisy yes, but how does that couple to the guitar signal past the volume control?

[g1]

Have you verified that the volume control goes right down to zero ohms when turned all the way down? You could be dealing with more than 1 fault which could confuse things, especially when working with no schematic available.
Same with the assumption that the volume control is after V1, yes that is how it is usually done, but with no schematic you must verify.
If you thought the problem could have possibly been caused by V1, then V1 plate resistor should also be a valid suspect.

[capndenny1]

I finally started on the amp. I was tracing the circuit, but I couldn’t follow it with it still mounted. I onlt got the first half of V1 sketched up.

The signal goes to the 1st half of V1, then they run it to the gain pot in the gain circuit, and the volume pot in the clean circuit. The clean signal goes from V1 to about 5 inches away where the volume pot is. It goes through the pot, and then thru a 100k pot and and through a relay and then to the control grid on the other side of V1.

The signal going to the vol pot and the signal from the pot, after the 100k resistor, run parallel for 4 inches or so. That leaves the amplified signal next to a high impedance input to the 2nd amplifier stage.

I will continue to trace out the circuit. And I will try an experiment to see if that is the source of the cross talk.

[capndenny1]

You know what, it may just be the volume pot. I kind of rejeted that because, I didn’t understand why it would get noisy at zero. If the pot just won’t go to zero, but has some resistance left between the wiper and gnd, then it would let signal thru. Ok that makes sense. But what about the noise. Well suppose that instead of havinga few ohms left, the wiper goes to an open circuit? Then you would have a high impedance input left open. I could see that picking up noise and the signal from the 1st amp stage. The other thng peculiar with the pot is that it stays low until at least going from 0 till 1, then it starts getting louder.

Here is my circuit tracing progress. I used a photo of the back side to add the traces to the picture on the front.

[capndenny1]

Here is the reply I got from PRS, and my response to them. Now I am more confused than ever.

Me--
OK, so it was left noisy on purpose?

That doesn't explain the crosstalk of the guitar at zero on the vol pot?

Why are some amps noisy and some quiet? And why does it change?

And why does it not have the noise in the gain channel?

When I plugged into the return input with my guitar with the guitar pot on zero there was no noise, so the reverb didn't seem to be adding to it.

Can I bypass the effects loop? I never use it anyway.

I get it. The amp is wide open after the vol on the clean. The gain channel has gain early, and probably the master or vol for that channel is later on, so it turns down the noise too.

I sure can't argue with how sweet the clean channel sounds. It's the pure sound that makes me want more. I have amps that I love, but the hum is much louder than the hiss in this amp.

Interesting stuff. Thanks for the explanation. I may stop drawing the schematic, and just go back to enjoying the amp.

Dennis Kelley




----- Original Message -----
From: "Paul Reed Smith Customer Service" <custserv@prsguitars.com>
To: "capndenny1" <capndenny1@centurylink.net>
Sent: Wednesday, April 10, 2019 10:26:42 AM
Subject: RE: Sonzera 20 crosstalk and noise.

Hi Dennis,


The MOSFET-driven effects loop and the reverb are both after the volume control, so the RF820 MOSFETs and the Reverb tube/circuit both introduce a little bit of background noise, and there is no controls left after those elements to be able to turn the signal down to silence. This was a design choice to get the tones that we were looking for with the Sonzera.

Thanks,
Sean


Sean Littleton
Paul Reed Smith Guitars, Ltd.
Customer and Technical Support
380 Log Canoe Circle
Stevensville, Md. 21666-2166
(410)643-9970


-----Original Message-----
From: Dennis CentLink [mailto:capndenny1@centurylink.net]
Sent: Tuesday, April 09, 2019 3:03 PM
To: Paul Reed Smith Customer Service <custserv@prsguitars.com>
Subject: Re: Sonzera 20 crosstalk and noise.

Hello Sean,

I bought this off ebay, but it was originally from Sweetwater, because it was shipped in the original PRS Sonzera packaging, and there was a Sweetwater card congratulating me on my new amo. The serial number and model fields on the back are blank? There is a cooyright on the nain pcb that says

Copyright 2016
Sonzera 20 Rev G.3
Production PCB1

It was sold by a company in NJ who sells damaged amps.

I’m not expecting it to be under warranty. I do amp repairs, as a hobby, but I have 35 years as a EE and 5 years as a Tech before that.

It appears there was an issue with the early releases. I was just hoping to save myself some time and effort.

Glorious sounding amp by the way!

Thanks,

Dennis Kelley

PS Even a hint would be appreciated. I have it narrowed down between the Vol pot on the clean channel, and the Return input. And seems to only be the clean channel. I am thinking the pcb layout needed to change.

Last time I traced a noise issue on an Ampeg Reverberrocket RI I found a high impedance signal trace with a heater trace going under a resistor in the signal path. I looked at a later production pcb and they had changed the layout to fix that problem.

Thanks,

Dennis


Sent from my iPhone

> On Apr 9, 2019, at 1:33 PM, Paul Reed Smith Customer Service <custserv@prsguitars.com> wrote:
>
> Hi Dennis,
>
> Thanks for contacting PRS Guitars.
> Can I get the serial number of your Sonzera 20 amp and the name of the dealer where it was purchased ?
>
> Thanks,
> Sean
>
>
> Sean Littleton
> Paul Reed Smith Guitars, Ltd.
> Customer and Technical Support
> 380 Log Canoe Circle
> Stevensville, Md. 21666-2166
> (410)643-9970
>
>
> -----Original Message-----
> From: Dennis [mailto:capndenny1@centurylink.net]
> Sent: Tuesday, April 09, 2019 2:26 AM
> To: Paul Reed Smith Customer Service <custserv@prsguitars.com>
> Subject: Sonzera 20 crosstalk and noise.
>
> It seems pretty clear that the Sonzera 20 amps had an issue with noise and bleed-thru of the signal past a zeroed volume control.
>
> I recently acquired one of these amps, and I love the sound of the amp, but it has a noticeable hiss level at zero volume 100k series resistor kepps the pot from eliminating the signal
>
> The problem is I can guess a hundred possible issues, none of which are going to help.
>
> So any help you can give would very much appreciated.
>
> Thank you,
>
> Dennis Kelley
>
>

[capndenny1]

I managed to get rid of the guitar crosstalk problem. I used a piece of shielded cable to re-direct the signal from the volume pot center lug away from the signal from the 1st stage. They run next to each other for about 4 or 5 inches. While it fixed the bleed thru, the clean channel hiss is still there though. I will keep looking. I may try a effects loop master volume. Since the hiss is gone if I plug into the return input directly with the guitar.

The tone controlshave no effect. So it must be the Mosfet that drives the Send signal.

[g1]

the clean channel hiss is still there though. I will keep looking. I may try a effects loop master volume. Since the hiss is gone if I plug into the return input directly with the guitar.

The tone controlshave no effect. So it must be the Mosfet that drives the Send signal.

If the noise has to do with the loop, then it must be on both channels.
If the noise is only on the clean channel, then it must be something that is not common to both channels. Make sure the noise is not there when dirty channel level is up full.
What the PRS rep said does not seem to make sense as far as this particular fault goes.

[capndenny1]

I asked him for more info, but he is done with me.

interesting point about it being in the gain channel too?

I haven't figured out yet how it switches between channels. The V1 tube is used by both channels, but with relay switching different signal paths.

The V2 tube is only used by the gain channel. I don't have that mapped out yet. It looks like it goes through the gain tube, then the tone stack, then the Level (Master) pot. So I suspect the noise is getting in just before the effects loop. Perhaps after the tone stack on the clean channel.

A351E984-FFA1-488E-AAD9-01EFD73D4B4B by Dennis Kelley, on Flickr

The SPDT relay at the left, I think goes to the effects loop. The pot marked Level is the Master vol for the gain channel. So for it to be the same as the clean channel I would need that full on. That's what he is saying, since the clean channel has no master, any noise that is there gets amplified by the whole power amp full out.

That's what I was going to do with a volume pot inserted in the effects loop. I can just turn up the clean volume, and turn down the effects volume. That will make the guitar quieter, but it will also make the noise quieter.

I think that's why the gain channel sounds quieter, because that Level or master volume is turning down all that gain, and with it, the noise level. But the clean channel doesn't have any such way to lower the noise.

The gain channel has tons of noise when you turn the Level (master) up to full. As soon as the Gain starts to let signal through it is LOUD. But even with the master from the gain channel on zero, there is still some hiss on the gain channel.

I will get the rest mapped out. I have about 3 or 4 other amps I need to fix, and a couple more headed my way. So i can't keep playing with this one as much as I'd like. I also love playing through it.

If you are trying to crank it and get the clean channel to get a little pushed, then you won't really care about the little bit of hiss. It is not amplified by the volume control.

If you are in your quiet room like a bedroom, or a basement where I play most often, then every little noise drives you nuts. This ain't hifi! I need to remind myself of that.

The amp sounds incredible, not doubt. If getting rid of the hiss makes the amp stop sounding as incredible as it does, then I will take the hiss!

[g1]

The gain channel has tons of noise when you turn the Level (master) up to full.

Ok this is what I was getting at.
Earlier, you mentioned a hiss that was only on the clean channel. Can I rephrase as follows? 'A hiss that is there on both channels but goes away on the lead channel if you turn down the level pot' ?
Is it the same hiss? If so, it must be something common to both channels, but before the loop, as the level pot kills the noise.

[capndenny1]

Or there is noise in both channels, not necessarily the same source.

It’s not that bad. I just kind of expected more from PRS. But this is basically an SE version of a PRS amp.

[g1]

Or there is noise in both channels, not necessarily the same source.

That's what I was hoping to determine. Does it sound like the same hiss or not?
Not sure what you mean by SE? Normally I would think you mean 'single ended' but according to the specs, this is a push-pull amp.
Here is what they claim to be the tube setup. Pulling various tubes might help isolate the noise:

V1 12AX7, Common Input Gain Stage
V2 12AX7AC5, Gain Channel
V3 12AX7AC5, Reverb
V4 12AX7AC5, Phase Inverter

[capndenny1]

Ok. I have replaced V1 and it had no effect. Forget about the gain channel for now.

If V2 is only for the gain channel as PRS says, hen that’s not it.

I plug my guitar into the return input on the effects loop, and set the guitar to zero, and it is much quieter. Not objectionable. So it’s not power tubes nor the phase inverter.

That leaves the Send amplifier or the circuit between the clean channel tone stack and the Send amp circuit.

The reverb is after the Return input, so while it may add some noise it’s not bad.

The clean tone stack does not have any effect on it.

So that leaves the circuit aftr the tone stack, up to and including the circuit in the Send output.

I will probably return the circuit to stock. It was cool fixing the zero volme bleed through, but it really wasn’t a problem. I was hoping that issue and the hiss were the same problem, but it doesn’t appear to be so.

Thanks for your input. It helps discussing it.

[g1]

I'm following with all that except the send. The send should be after the point where the two channels join back up. So if it were in the send, it should be noisy when on the gain channel with the level down. Don't 'forget about the gain channel' because you know the problem can not be after the point where the level pot feeds the remainder of the circuit (pre loop). You need to use that info for narrowing down the fault.
It might help to try and draw up a block diagram.