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PRS Sonzera 20 noise
PRS Sonzera 20 noise
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Old 11th April 2019, 01:15 PM   #1
CapnDenny1 is offline CapnDenny1  United States
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Default PRS Sonzera 20 noise

I bought a PRS Sonzera 20 combo on ebay for about $300. They sell new for $700 or $800 so I figured I would take a chance. The reported problem was that it passes signal with vol on zero. It does that, but it also has a level of hiss that seems high, also with the volume on zero.

I am in process of drawing the schematic from the pcb. A bit tedious.

I found several references about people having these issues. Some got the fixed by PRS, or had PRS tell them there’s nothing wrong. But I decided to send an email to PRS Tech Repair. They replied pretty quickly, but it easlly surprised me what they said? Here is their response, see if you guys agree with this. It smells like bs to me. But it a fabulous sounding amp, so maybe?

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

Last edited by CapnDenny1; 11th April 2019 at 03:08 PM.
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Old 11th April 2019, 01:27 PM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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The hiss at zero volume is caused by cheap components or bad design, as is the signal being passed at zero volume.
As it is after the volume control, pay no attention to anything other than the power amp section.
MosFets can, if over driven/stressed, cause hiss, as can poor layout as the amplifier breaks into HF oscillation.
Check on your Oscilloscope for spurious oscillation before digging in too deep!
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Old 11th April 2019, 01:37 PM   #3
CapnDenny1 is offline CapnDenny1  United States
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I did a test where I plugged my guitar into the Return input of the effects loop, and there is no bleed thru on zero volume, and there is no hiss. There is reverb, so that comes after the effecs loop.

The vol is after the 1st stage in the V1 tube. Then it goes thru the2nd stage in the V1 tube. It does this for the gain and clean chnnel, but only the clean has the bleed thru or crosstalk issue. I can’t say about the hiss on the gain channel, because it’s just noisy anyway, and it has the gain in V1, but then later here is a vol for the gain channnel, so I may just be turning it down. The clean only has the vol on the signal between the two amps inside the V1 12ax7.

I also need to verify the pot doesn’t go to an open on the wiper when you set it low, because that would leave it open going into the second stage.
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Old 11th April 2019, 01:42 PM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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In that case, check for a noisy valve.
Can't suggest anymore without schematics. Good luck.
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Old 11th April 2019, 03:07 PM   #5
CapnDenny1 is offline CapnDenny1  United States
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I tried swapping out V1. No change.

V2 is for the gain channel which shouldn't affect the clean channel.

V3 is the reverb , and V4 is the PI, which are both good based on my test going into the return input.

Working on the schematic.

From PRS:

PREAMP TUBE POSITIONS & FUNCTIONS
V1 12AX7, Common Input Gain Stage
V2 12AX7AC5, Gain Channel
V3 12AX7AC5, Reverb
V4 12AX7AC5, Phase Inverter

dlk
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Old 14th April 2019, 05:21 PM   #6
CapnDenny1 is offline CapnDenny1  United States
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Partial schematic

Click the image to open in full size.A351E984-FFA1-488E-AAD9-01EFD73D4B4B by Dennis Kelley, on Flickr

The area circled is the two traces in schematic marked HERE. The out put of 1st stage and input of second stage.

On the circuit board these are circled in white.

Click the image to open in full size.99F5575E-EC29-40E0-9B0B-6B8EB4E38568 by Dennis Kelley, on Flickr

The two long traces circled are where I think the guitar signal may be bypassing the volume control.

Last edited by CapnDenny1; 14th April 2019 at 05:23 PM.
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Old 19th April 2019, 02:57 PM   #7
CapnDenny1 is offline CapnDenny1  United States
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So, a bit of a victory here. I got the amp back together. With my mod seen here. And the the fixes to the broken wires from looking at the front and back of the pcb a hundred times.

It bypasses the offending pcb trace of the output of the 1st stage lying next to the input of the 2nd stage.

The crosstalk is gone. Now with the clean channel volume on zero, there is no guitar sound coming through the amp. WooHoo!

Unfortunately the hiss is still there. It is definitely in the effects loop section. If I plug my guitar in the Return the hiss is completely gone. That's with the guitar pot on zero. It is a fixed level, so it's not too bad. In fact I could probably eliminate it by putting a Master Volume pot in the effects loop. That would decrease the level of the hiss if I turned it down. Then I would just need to increase the amps clean Vol a little more. I may go ahead and try that, or just decide it is OK.

I spent an hour playing the amp all buttoned up with the reverb and everything. I even spent about 10 minutes on the gain channel, which sounds pretty awesome as well. That's something my Twin Reverb can't do?

I think I will try to finish the schematic from the pictures I took. I don't want to flip the board back and forth any more. I may just look for a way to bypass it entirely. Maybe try the effects loop master volume pot first, that would be the easiest thing to do.

It's just nice to know that all those years in college and 40 years working in electronics I can finally put it to good use! You know, something meaningful!
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