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 drive rack PA2. So I tried them individually on a different power bar and a different outlet with a direct signal to each amp, bypassing the DRPA2 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:
pics of front and back attached.
Thanks in advance for any insight or trouble shooting suggestions. I feel like there is a very simple and obvious answer to this but I'm not sure what I'm missing.
- speaker is working: used mutiple working speakers with no luck
- speakon cable is working: wired correctly to pins 1+/1-
- signal is working: balanced signal being sent
- 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.
pics of front and back attached.
Thanks in advance for any insight or trouble shooting suggestions. I feel like there is a very simple and obvious answer to this but I'm not sure what I'm missing.
SOmething outside the amps. Something about your signal source or connection scheme perhaps.
That was my first thought which is why i isolated it back to basics. A single channel signal out from my mixer directly into a single amp using a single cable into a single speaker. no other amps are connected yet they all behave the same way when i test them individually.
yes, correct. the amp sends signal to +1 +2 bridged. Stereo is supposed to use +-1. I have verified that the cables are wired +-1 so they should work in stereo but they don't. they only work in bridged while wired +-1. I tried to rewire the cable in +1+2 but then it didn't work in bridged when it should have. how would that be?
The confusing part is that the whole system worked the day before with the same cables, same amps, same speakers, and same signal. The very next day all four amps had the same issue and nothing changed.
correction, I wired the cable +1+2 and it did work in bridged. i wrote earlier that it didn't but i had accidentally wired the cable -1 -2 which was why it didn't work.
ok so update. it spontaneously started working again. working in stereo on all 4 amps. again i've changed nothing. it has to be a voltage issue in my building. i'll try a power conditioner to see if that stabilizes things. i'll let you know if it happens again.
I might just have to. Lol. I'm stumped. It happened twice and resolved itself twice with no changes made.🤷🏽♂️
Check continuity on the speakons / cables. Look at the backside of the amp females. Connectors could be bad or perhaps a dirty Stereo / Bridge switch
Thanks for the tips. I checked continuity on the cables several times and used multiple known working cables to eliminate that variable. Connectors could be dirty but I think its statistically impossible for all 4 amps to malfunction simultaneously and resolve itself simultaneously if the female terminals were bad. The stereo bridge switch could be faulty but again wouldn't explain the simultaneous malfunction and resolution of all 4 amps. Has to be voltage related since it's the only thing the 4 amps share when they malfunction together.
When you test these amps, are you feeding it from a mixer in stereo? Try feeding only ONE side at a time. DOes the side making no sound now work? Likewise, take a signal and feed it to both left and right inputs with a Y-cord. I am not yet convinced it isn't a signal source problem. As you say, there is no mechanism for all four amps having the exact same trouble at the exact same time.
Initially when i first discovered the problem it was running stereo inputs but in order to trouble shoot, I tested each channel of each amp individually with a single cable and single balanced line signal. Channel A nor B worked in stereo mode but oddly the bridged output worked with a 1+1- pinned cable and should not have. This would indicate some kind of pin short at the bridged output since both 1- and 2- should have been dead. But after it resolved it self randomly, channel output A and B both worked again. like you said there is no mechanism across 4 individual amps being tested separately to produce identical malfunctions at the same time and then reverse itself. It has to be a wall voltage issue or something common, maybe a ground loop issue/lifted ground issue or something.
If you use some other amp in bridge mode, you might find both channels are working, they are just out of phase with each other. SO if one side is dead, the other side is still going, so a #1 cord would still work on that channel. A wise man said, "When you see hoofprints in the dirt, think horses, not zebras. (Or maybe when you hear hoofbeats...doesn't matter) I think it would take a serious wall voltage problem to do this selectively. And ground loops more likely to make it hummy.
Looking at your rear panel, I don't see the other two pins as being dead, I just see them as unused. The bridge connector tells you to use the two particular terminals.
I don't know your amp, but SOME when in bridge mode will disable one of the inputs. ANy chance here?
Looking at your rear panel, I don't see the other two pins as being dead, I just see them as unused. The bridge connector tells you to use the two particular terminals.
I don't know your amp, but SOME when in bridge mode will disable one of the inputs. ANy chance here?
These are all great suggestions. Thank you. Not sure it would make sense to disable one of the inputs in bridged mode as you would lose any music content that pans to that channel. I only use mono mode when the line signal has been summed at the source to keep musical content intact. The issue wasn't that the stereo channels weren't working when bridged mode was selected it was that it wasn't working when stereo mode was selected. I see your point that the bridged connector may still be live on 1- it's just not supposed to be used. So that would explain why bridged still works in stereo mode but doesn't explain why all 4 amps simultaneously malfunction in the exact same way at the exact same time. Is there some thing other than power and ground that would be universal to the amps when I'm testing them independently while eliminating the variabiles of cable continuity, signal path, and speaker? EMI/RFI? I can't think of anything else that could cause it. Voltage was measured at 120V-122V during the malfunction and during normal operating behavior so it doesn't seem to be that either.
In bridge mode, an amplifier used both channels together, so only one input can be used. SO we have to pick one input, it feeds a channel, and then a reverse phase version of the signal is fed to the other channel. That leaves nothing for the other input to do. If you have one side of a stereo signal connected to that input, it just goes no further. The amp is not a mixer, so if you feed the amp stereo but have it in bridge mode, ony one side of the audio gets amplified. You cannot play stereo through a bridged amp.
MY zebra reference was intended to suggest not looking for some weird esoteric odd phenomenon, it is going to be something very basic in the connections or similar. For instance, make VERY sure your speaker cables are as intended. I have had customers bring me amps with a "dead channel" only to find they were using a cable intended for 2. I'd put tape on each cord one verified as to its wiring so you can be sure which is which.
When I suggest using a mono input signal, I mean one single cord, not stereo wiring with mono content. If your amp won't make both channels in stereo, plug that cord into one input, then move that cord to the other input. That tests both channels with the same source.
And since bridge is the two channels driving the same load, if one channel stops, the remaining one will still show up in the speaker. That can be confusing until you check with a scope or something to see that both channels are driving the bridge out or just one.
MY zebra reference was intended to suggest not looking for some weird esoteric odd phenomenon, it is going to be something very basic in the connections or similar. For instance, make VERY sure your speaker cables are as intended. I have had customers bring me amps with a "dead channel" only to find they were using a cable intended for 2. I'd put tape on each cord one verified as to its wiring so you can be sure which is which.
When I suggest using a mono input signal, I mean one single cord, not stereo wiring with mono content. If your amp won't make both channels in stereo, plug that cord into one input, then move that cord to the other input. That tests both channels with the same source.
And since bridge is the two channels driving the same load, if one channel stops, the remaining one will still show up in the speaker. That can be confusing until you check with a scope or something to see that both channels are driving the bridge out or just one.
This might help, this modern amp has internally wired connectors especially for bridge. In the old days, to bridge an amp, you made a special cord and used the banana jacks. You connected channel A plus to the speaker terminal and channel B plus to the other speaker terminal. The ground terminals of both channels remain unused.
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. Almost all bridgeable car amps use stereo outputs and just bridge L+ and R-. However I believe you are correct about it choosing just channel A for bridged input since this is the only pot that is used in bridged mode.
I verified multiple times that the cable was wired 1+1-. I verified it with previously working cables, brand new cables, and stripped cables that were wired directly into the speaker terminals on the speaker end. I made sure to state that in my original post to eliminate that variable. It is not esoteric to find a common denominator. The common denominator could not be the cable because I tried multiple cables across the different amps at different times and got the same malfunction across all 4 amps at the same time. The most basic answer will be one that is common to all 4 amps at the same time and that could only be the power in my walls.
see reply #4 in this thread.
That is exactly what the pin diagram shows is happening with bridged mode. 1+ and 2+ are being used for bridged mode and according to the block diagram that appears to be the case with this amp.
When I work on an amp that is driving me nuts, I sometimes turn the amp 180 degrees. Like work on it from th rear instead of from the front. This gives me a new viewpoint. SOunds silly, but when something basic looks to be wrong and I cannot find it, many times it is because I looked at something long enough that it looked "right" even though it wasn't.
When you said this earlier, it concerned me, since you are automatically losing one side of a stereo signal when in bridged mode. You can mono up the source and feed it to one input, but otherwise that second input does nothing. SO I am not sure how we would be losing anaything.