Multiple amps suddenly stopped working in stereo mode but still work in bridged

joey toribio

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Jun 13, 2022
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I've got 4 Accusonic/Show amps (2 x PSA-3900, 2 x PSA-31500), all four of them were working in all modes (stereo, bridged, parallel/mono). The following day I was testing them out and all of a sudden all four amps only work in bridged mode but not stereo mode or parallel/mono mode. Has anyone ever seen or heard of this before? I'm testing each one on their own so it shouldn't be a power supply/voltage drop issue. The only thing the four of them had in common is that they were all plugged into the same power bar running through a DBX Driverack PA2. So I tried them individually on a different power bar and a different outlet with a direct signal to each amp, bypassing the PA2 and still have the same issue. What would cause 4 individual amps to all lose stereo mode at the same time? I have verified the following during my trouble shooting:

  • speaker is working: used multiple working speakers with no luck
  • Speakon cable is working: wired correctly to pins 1+/1-
  • signal is working: balanced signal being sent via XLR
  • power supply is working: breaker has been reset
  • breakers on all amps are all reset
  • clip limit is on but i've tried it off as well with no luck
  • 30hz filter is off but i've tried it on as well with no luck
  • bridged mode: signal showing on both channels when channel A pot is used, as designed. sound is being sent to bridged output channel.
  • stereo mode: Signal is showing on both channels A and B. sound is not being sent to output channels A or B.
  • parallel(mono) mode: Signal is showing on channel A as designed. sound is not being sent to output channels A or B.
Here's a link to an older version of the amp that had binding posts. my version has 3 speakon terminals only 1 stereo pair and a 3rd for bridged mode. https://www.manualslib.com/download/1536675/Show-Psa-Series.html

pics of front and back:
20220613_170819.jpg
20220613_170806.jpg
There's probably an obvious reason but I don't know what I'm missing.

Thanks in advance for your help.
 

joey toribio

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See bullet #2. I verified +-1 wired correctly. Also used multiple cables. That's why I'm confused, it shouldn't work in bridged with a +-1 cable.

Update: it randomly started working in stereo again. All 4 amps. Nothing changed. It has to be a voltage issue or something related to the outlet as that is the only thing that could be fluctuating since I did not changing anything on the amps and i'm still using the same cable. Will try a power conditioner to see if it stabilizes things.
 

Tim McCulloch

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Jan 11, 2011
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See bullet #2. I verified +-1 wired correctly. Also used multiple cables. That's why I'm confused, it shouldn't work in bridged with a +-1 cable.

Update: it randomly started working in stereo again. All 4 amps. Nothing changed. It has to be a voltage issue or something related to the outlet as that is the only thing that could be fluctuating since I did not changing anything on the amps and i'm still using the same cable. Will try a power conditioner to see if it stabilizes things.
Hi Joey, welcome to Soundforums!

I really, really doubt it's AC power related. Why? Because EVERY MODE SELECTION is made by a mechanical switch. What comes from the power company doesn't enter into it. It's possible that dirty (oxidized) switch contacts are involved but for 4 amps to behave 'wrong' at the same time, and then a miracle occurs later, I'm not buying dirty switches.

You need some 'custom' wired speaker cables; is there a possibility that your cables got mixed up? Also was there a physical tear-down, move, and re-set with system? If so, it could explain the sudden miracle of proper operation. EDIT PS: I see you swapped cables around and did not change the apparent output. Hmmmm.

While AC power gets blamed for a lot things, it's almost never the problem so long as it's at the rated frequency and within voltage specs (under load) for the connected devices.
 

joey toribio

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Hi Joey, welcome to Soundforums!

I really, really doubt it's AC power related. Why? Because EVERY MODE SELECTION is made by a mechanical switch. What comes from the power company doesn't enter into it. It's possible that dirty (oxidized) switch contacts are involved but for 4 amps to behave 'wrong' at the same time, and then a miracle occurs later, I'm not buying dirty switches.
Agreed, it's not dirty switches. 4 individual switches can't all fail and resolve simultaneously on 4 amps that were tested both simultaneously and then independently.
You need some 'custom' wired speaker cables; is there a possibility that your cables got mixed up? Also was there a physical tear-down, move, and re-set with system? If so, it could explain the sudden miracle of proper operation. EDIT PS: I see you swapped cables around and did not change the apparent output. Hmmmm.
For stereo mode, custom cables were not needed. 1+- cables were used and working previously and then these same 10' cables suddenly no longer worked. The only thing that changed between when they worked and when they didn't work was the date and time. Custom cables wired 1+2+ would be needed for bridged.

I swapped cables as a trouble shooting procedure only after the malfunction. I tried the new cables in all 3 modes and only bridged worked. I tried the old previously working cables in all 3 modes and only bridged worked. When the problem resolved itself randomly, both sets of cables (new and old) worked in the 2 modes they were supposed to work in (stereo and mono).

The resolution did not come from a system change. It came randomly after some time had passed but no changes were made. What else would change over time that isn't hardware related? AC power is the only thing I can name.
While AC power gets blamed for a lot things, it's almost never the problem so long as it's at the rated frequency and within voltage specs (under load) for the connected devices.

I had gotten some brand new 25' cables and hooked it all up just to find nothing worked. I went back to the shorter 10' cables from the day before and they no longer worked either. Identical setup different day. That's when I started trouble shooting and isolating the amps the cables and the speakers to limit points of failure.
 

Art Welter

Senior
Jan 11, 2011
861
30
28
Florida
I've got 4 Accusonic/Show amps (2 x PSA-3900, 2 x PSA-31500), all four of them were working in all modes (stereo, bridged, parallel/mono). The following day I was testing them out and all of a sudden all four amps only work in bridged mode but not stereo mode or parallel/mono mode. Has anyone ever seen or heard of this before?
  • stereo mode: Signal is showing on both channels A and B. sound is not being sent to output channels A or B.
There's probably an obvious reason but I don't know what I'm missing.
Joey,

Probably a change in input drive cords or source is the problem.
As you wrote in DIY audio :
"I understand the amp is not a mixer but it's not difficult to sum a stereo signal to mono. I've done it very easily while building a 3.5mm stereo jack to XLR adapter."
Your input wiring (common to all four amps) is likely the problem.

You can "easily" connect (short circuit) the tip and ring of a stereo 3.5mm (or 1/4") TRS together, then wire the shorted pair to the XLR pin 2 (+), and the sleeve to XLR pin 3 (-) to sum to mono.
Not a good choice, the tip and ring should be isolated with resistors.

To function properly, a 3.5mm stereo jack to XLR adapter would require two XLR connectors, one for left, one for right.

The standard connection of a 3-pin XLR to a 1/4" TRS (AKA stereo jack plug) is:
pin 1(shield) to sleeve, pin 2(+) to tip, pin 3(-) to ring.

If a mixer stereo headphone out was used to drive the amp input using that configuration, the L/R of a mono signal (exactly 180 degrees out of phase) would cancel if driving the amp XLR switched to "stereo", while the inverting "bridge mono" connection would not.

https://qscprod.force.com/selfhelpportal/s/article/How-to-Connecting-stereo-outputs-to-mono-input

Art
 

joey toribio

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Jun 13, 2022
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Joey,

Probably a change in input drive cords or source is the problem.
As you wrote in DIY audio :
"I understand the amp is not a mixer but it's not difficult to sum a stereo signal to mono. I've done it very easily while building a 3.5mm stereo jack to XLR adapter."
Your input wiring (common to all four amps) is likely the problem.

You can "easily" connect (short circuit) the tip and ring of a stereo 3.5mm (or 1/4") TRS together, then wire the shorted pair to the XLR pin 2 (+), and the sleeve to XLR pin 3 (-) to sum to mono.
Not a good choice, the tip and ring should be isolated with resistors.
That cable was done correctly with resistors however this was not the source cable used in the setup. I merely mentioned that summing a stereo input in an amp wouldn't be difficult.
To function properly, a 3.5mm stereo jack to XLR adapter would require two XLR connectors, one for left, one for right.

The standard connection of a 3-pin XLR to a 1/4" TRS (AKA stereo jack plug) is:
pin 1(shield) to sleeve, pin 2(+) to tip, pin 3(-) to ring.

If a mixer stereo headphone out was used to drive the amp input using that configuration, the L/R of a mono signal (exactly 180 degrees out of phase) would cancel if driving the amp XLR switched to "stereo", while the inverting "bridge mono" connection would not.

https://qscprod.force.com/selfhelpportal/s/article/How-to-Connecting-stereo-outputs-to-mono-input

Art
Signal cable used during testing was a standard balanced XLR from a single balanced XLR source channel tested on each amp input channel. No summed cables or 3.5mm cables were used for testing, only balanced XLR.
 

Paul Lewendon

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Oct 8, 2019
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There is a very slim possibility it's a dirty/oxidized switch, under the right circumstances I've had the switches on a few units fail repairing things. Especially if the switches in question haven't been used and then you started.
Kind of like with old amplifier balance pots, it's sitting in the exact same position for years, and then as soon as you move it off that spot, the crackling starts and you have troubles finding that "normal" spot again.
Try moving all the switches back and forth a bunch of times, if it's oxidized that can temporarily remedy it, the best is to use a switch cleaning spray that also lubricates.
Also a slim chance, did you happen to send any phantom voltage into the inputs ?
I don't know if it would matter to the amp, but if it was sensitive to the voltage sent to it, it could potentially damage multiple units at a time but just on the setting you were on on the amps. If one circuit didn't totally fail it could get intermittent.
 

joey toribio

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Thanks for the suggestion. No phantom supply was sent. And if it did it would have originally gone to the dbx driverack pa2, not the amps, which was when I first noticed all 4 stopped working. I then tested them individually with a direct line in and bypassed the driverack. It's possible that phantom power could be sent to the amp at that time but the problem had already manifested, so most likely not phantom power.

The switch problem is unlikely that it would affect 4 individual amps simultaneously. The cause would have to be external of the amps.

The best suggestion I've heard so far is some kind of shared fault protection from the common power outlet that all the amps shared if exposed to the same power conditions. The fault protection would resolve if the external condition was resolved. Same concept as an external power surge would cause all 4 amps to trip circuit breakers simultaneously.
 

Art Welter

Senior
Jan 11, 2011
861
30
28
Florida
That cable was done correctly with resistors however this was not the source cable used in the setup. I merely mentioned that summing a stereo input in an amp wouldn't be difficult.

Signal cable used during testing was a standard balanced XLR from a single balanced XLR source channel tested on each amp input channel. No summed cables or 3.5mm cables were used for testing, only balanced XLR.
Joey,

What is the signal, and what is it's source ?
Was the identical signal source used when the amps worked normally as when they did not?
What "source channel" was used to drive each amp input channel?

Art
 

joey toribio

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Jun 13, 2022
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The source was the right channel output of a balanced XLR signal from a mackie 4 channel mixing board travelling through an XLR cable into a dbx driverack pa2 as a mono input into the left channel input. The driverack pa2 was configured for a mono input which then output 3 pairs of stereo outputs to the 3 amps (tweeter, mid, sub).

It was the same source when it was working and the same source when it stopped working.

After it stopped working I tried trouble shooting by bypassing the drive rack and sending the same source directly into either of the amp channels, one amp at a time, in stereo mode testing only one output channel at a time using a known working speakon cable. Neither channel responded. That is when I checked all three modes and discovered bridged mode was sending a signal through the bridged output but using a 1+- cable.

When the malfunction randomly resolved the input stayed unchanged. When it reoccurred the input stayed unchanged and then when it resolved the second and final time, the input remained unchanged. I think we can conclude it isn't a source issue.
 

Paul Lewendon

Member
Oct 8, 2019
49
9
8
52
British Columbia Canada
The source was the right channel output of a balanced XLR signal from a mackie 4 channel mixing board travelling through an XLR cable into a dbx driverack pa2 as a mono input into the left channel input. The driverack pa2 was configured for a mono input which then output 3 pairs of stereo outputs to the 3 amps (tweeter, mid, sub).

It was the same source when it was working and the same source when it stopped working.

After it stopped working I tried trouble shooting by bypassing the drive rack and sending the same source directly into either of the amp channels, one amp at a time, in stereo mode testing only one output channel at a time using a known working speakon cable. Neither channel responded. That is when I checked all three modes and discovered bridged mode was sending a signal through the bridged output but using a 1+- cable.

When the malfunction randomly resolved the input stayed unchanged. When it reoccurred the input stayed unchanged and then when it resolved the second and final time, the input remained unchanged. I think we can conclude it isn't a source issue.

Thanks for the suggestion. No phantom supply was sent. And if it did it would have originally gone to the dbx driverack pa2, not the amps, which was when I first noticed all 4 stopped working. I then tested them individually with a direct line in and bypassed the driverack. It's possible that phantom power could be sent to the amp at that time but the problem had already manifested, so most likely not phantom power.

The switch problem is unlikely that it would affect 4 individual amps simultaneously. The cause would have to be external of the amps.

The best suggestion I've heard so far is some kind of shared fault protection from the common power outlet that all the amps shared if exposed to the same power conditions. The fault protection would resolve if the external condition was resolved. Same concept as an external power surge would cause all 4 amps to trip circuit breakers simultaneously.
Yes the switches are a slim possibility, but still possible, under the right "perfect storm", like age, previous usage of the switches, etc. If a switch isn't used for a long time and then moved, it can cause issues. For some reason oxidization doesn't seem to happen directly at the position it has been sitting at, which is why things like balance potentiometers, and input switches on old home amps can be totally fine as long as they aren't used/moved from its "normal resting spot".
I've had old units that I've been fixing that were sitting in storage that every single moving connection(pots, switches) were cutting out or not working at all.
Unless your bridged mode uses a separate power circuit than the other settings, I can't imagine that being the problem, generally if I am remembering right, and different power circuit for different modes would be further down in the circuit, so unlikely for an external power issue to travel that far down to cause an issue in an amp.
 

joey toribio

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On the switches issue they were moved extensively when the system was working as I was testing different configurations when I first bought the system. I had to make sure all modes worked before paying which I did confirm. The issue arose a couple weeks after I had settled on stereo mode so the switches didn't move after that. When the issue surfaced the mode switch wasn't moved until after I started troublenshooting and checked bridged mode as part of the process of elimination. It was the only mode still working on all 4 amps at the exact same time.

One suggestion is that bridged mode doesn't remove signal from the 1- pin. It says to use 1+2+ for bridged but doesn't say what happens to 1- when it goes bridged mode. Perhaps there is still signal there even in bridged mode.
 

Tim McCulloch

Graduate Student
Jan 11, 2011
3,042
51
48
Wichita KS USA
On the switches issue they were moved extensively when the system was working as I was testing different configurations when I first bought the system. I had to make sure all modes worked before paying which I did confirm. The issue arose a couple weeks after I had settled on stereo mode so the switches didn't move after that. When the issue surfaced the mode switch wasn't moved until after I started troublenshooting and checked bridged mode as part of the process of elimination. It was the only mode still working on all 4 amps at the exact same time.

One suggestion is that bridged mode doesn't remove signal from the 1- pin. It says to use 1+2+ for bridged but doesn't say what happens to 1- when it goes bridged mode. Perhaps there is still signal there even in bridged mode.

Hi Joey -

The text is bold is correct for most power amps. As the channel negative output is not physically disconnected, there is typically a signal present.
 

joey toribio

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Hi Joey -

The text is bold is correct for most power amps. As the channel negative output is not physically disconnected, there is typically a signal present.
Ok so that would explain why bridged still works but I never found out why stereo stopped working. It has been fine since the initial 2 occurances that resolved itself.