Skip to main content

Amps suddenly stopped working in stereo but still work in bridged.

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
  • 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 attached.

amp 2

amp 1

There's probably an obvious reason but I don't know what I'm missing.

Thanks in advance for your help.

Comments

paulears Tue, 06/14/2022 - 23:12

Speakons. These amps are wired in a non standard way, so a cable that works in the bridged socket will not work in either of the two others. If you get output from the bridged speakon output, it is wired 1+/2- which is very weird. A connector that works in that middle socket needs to be wired differently. A correctly wired 1-/1+ cables should not do anything plugged into the centre socket. Maybe you have one dodgy wrongly wired cable. If the LEDs flash in the usual manner, this would explain why you hear nothing from the two normal outlets but do hear the bridged output. 

peter truverez Wed, 06/15/2022 - 01:10

paulears wrote:

Speakons. These amps are wired in a non standard way, so a cable that works in the bridged socket will not work in either of the two others. If you get output from the bridged speakon output, it is wired 1+/2- which is very weird. A connector that works in that middle socket needs to be wired differently. A correctly wired 1-/1+ cables should not do anything plugged into the centre socket. Maybe you have one dodgy wrongly wired cable. If the LEDs flash in the usual manner, this would explain why you hear nothing from the two normal outlets but do hear the bridged output. 

You're absolutely right. A properly wired +1-1 speakon should not produce any sound in the bridged socket. That's precisely why I'm stumped. The cables are correctly wired +1-1. I tried multiple cables all of which were verified wired correctly for 1+1-. These were the cables that worked in stereo mode just the day before. Suddenly they were no longer working in stereo but were working in the bridged socket. And that was across all 4 amps.

update: the problem spontaneously disappeared, reappeared, and disappeared one more time to all 4 amps at the same time, with no changes to the cables, amps, or signal. I think it may be a voltage issue from the wall... that's the only thing the 4 amps have in common every time they have malfunctioned and resolved.

Boswell Wed, 06/15/2022 - 09:59

If you have been completely rigorous in your testing, the only thing I can think of that has not already been mentioned is that you have intermittent contacts on the mode switch (parallel/stereo/bridged). However, It's unlikely that this would be the same problem on all the amplifiers suddenly.

That said, I have a grave suspicion about the wiring of the parallel mode as depicted on the rear panel. It's a no-no in the audio industry to wire power amplifier outputs in parallel and simply send the same input signal to both power amps, as the panel picture suggests. This would normally cause output tripping and/or fuse blowing.

I note that all the versions I could find of the instruction manual on-line do not show a wired-together option for these amplifiers. In addition, the photos do not show a B output Speakon, and have an A/B connector and a bridged connector. The parallel mode simply sends the A input to both amplifier channels, i.e. the paralleling is done at the input, not the output. The amplifier block diagram in the manual has at least one error in not showing anything feeding the B channel input when the switch is set to parallel.

peter truverez Wed, 06/15/2022 - 10:59

Boswell wrote:

If you have been completely rigorous in your testing, the only thing I can think of that has not already been mentioned is that you have intermittent contacts on the mode switch (parallel/stereo/bridged). However, It's unlikely that this would be the same problem on all the amplifiers suddenly.

That said, I have a grave suspicion about the wiring of the parallel mode as depicted on the rear panel. It's a no-no in the audio industry to wire power amplifier outputs in parallel and simply send the same input signal to both power amps, as the panel picture suggests. This would normally cause output tripping and/or fuse blowing.

I note that all the versions I could find of the instruction manual on-line do not show a wired-together option for these amplifiers. In addition, the photos do not show a B output Speakon, and have an A/B connector and a bridged connector. The parallel mode simply sends the A input to both amplifier channels, i.e. the paralleling is done at the input, not the output. The amplifier block diagram in the manual has at least one error in not showing anything feeding the B channel input when the switch is set to parallel.

I don't think it's sending the same input to both power amps in bridged mode. I think it's sending the respective channel inputs into the respective power amps and then bridging the output as shown in the diagram which is how every bridgeable car audio amp I've ever used has worked (see pic).

bridged amp

Your observation is correct. The photos on the back do not represent the 3 speakon connector outputs. The connectors are labeled separately from the diagram as "channel B",  "bridged", and "channel A" in that order right above each connector. The picture on the back that you mentioned is a legend to show pin outputs for each of the 3 modes, "Channel A/B" aka stereo, "Bridged", "Parallel (mono)". It's not there to label the outputs.

The block diagram is correct because you're not supposed to adjust the channel B pot in mono/parallel mode. Input is Channel A only and adjustment is to Channel A only, so channel B input never sees a signal even though the output is stereo to A and B. The channel B amp still sees the signal from channel A input. Bridged mode is the opposite, two signals in, one bridged channel out. The mode switch shares the Channel A signal to the Channel B amp after the 30Hz filter in the diagram.

Boswell Fri, 06/17/2022 - 08:50

There's a lot that doesn't add up in this thread.

Firstly, I would say you must forget what is done in car audio systems, particularly those that are designed for a 12V supply. That type of amplifier invariably uses H-bridge configurations on each channel to get 4x the power of single outputs from a relatively low voltage rail.

In a mains-powered amplifier such as the PSA-3900, supply voltage is not a problem, and in the low/mid power range amplifiers, the specified output power can usually be met without having to use an H-bridge configuration.

This is the block diagram of the PSA-3900 from the PSA user manual:

You will see that three Speakon outputs are shown, just as you have on the rear of your actual amplifier. However, a couple of pages further on in the manual is this drawing of the rear panel, which shows a mix of only two Speakon connectors and binding posts:

It could be that the rear-panel drawing shows the higher-powered models in the range and the block diagram shows the lower/mid models, but no reference is made in the text to the differences.

Going back to the block diagram, you will see that the configuration slide switch (upper centre in the diagram) shows the B amplifier feed selectable between the B input (for stereo mode), no input when set to the middle position, and the A input taken through a -1 gain (inverter). This last one gives the bridged mode, where you have to use the middle Speakon, as that, according to this diagram, is the only one where both amplifier outputs are available. This differs from the photo of the rear panel of your amplifier, as does the function of the middle position of the configuration switch. It's possible there is a link missing in the diagram from the input to the -1 gain amplifier to the centre position of the slide switch to give a workable parallel mode.

The upshot of all this is that I don't think it's possible to give a definitive answer to your question, nor a reason why you got no output in stereo and parallel modes. It could be that you triggered a shutdown of certain sections of the protection circuitry (not shown in the block diagram). If you have the amplifiers working correctly again, it may be that you have to put this behaviour down to a strange protection fault, and that it may not recur.

In one of the range high-power amps that I designed under contract, I went to great lengths to put explanations and diagrams of the protection circuitry in the draft user manual, principally so the user could understand better what happened under various fault conditions. The manufacturer chopped all that out of the printed manual, saying it was proprietary information. Keep the users guessing.

 

peter truverez Fri, 06/17/2022 - 09:02

Boswell wrote:

"There's a lot that doesn't add up in this thread.

Firstly, I would say you must forget what is done in car audio systems, particularly those that are designed for a 12V supply. That type of amplifier invariably uses H-bridge configurations on each channel to get 4x the power of single outputs from a relatively low voltage rail.

In a mains-powered amplifier such as the PSA-3900, supply voltage is not a problem, and in the low/mid power range amplifiers, the specified output power can usually be met without having to use an H-bridge configuration.

This is the block diagram of the PSA-3900 from the PSA user manual:

You will see that three Speakon outputs are shown, just as you have on the rear of your actual amplifier. However, a couple of pages further on in the manual is this drawing of the rear panel, which shows a mix of only two Speakon connectors and binding posts:

It could be that the rear-panel drawing shows the higher-powered models in the range and the block diagram shows the lower/mid models, but no reference is made in the text to the differences.

Going back to the block diagram, you will see that the configuration slide switch (upper centre in the diagram) shows the B amplifier feed selectable between the B input (for stereo mode), no input when set to the middle position, and the A input taken through a -1 gain (inverter). This last one gives the bridged mode, where you have to use the middle Speakon, as that, according to this diagram, is the only one where both amplifier outputs are available. This differs from the photo of the rear panel of your amplifier, as does the function of the middle position of the configuration switch. It's possible there is a link missing in the diagram from the input to the -1 gain amplifier to the centre position of the slide switch to give a workable parallel mode.

The upshot of all this is that I don't think it's possible to give a definitive answer to your question, nor a reason why you got no output in stereo and parallel modes. It could be that you triggered a shutdown of certain sections of the protection circuitry (not shown in the block diagram). If you have the amplifiers working correctly again, it may be that you have to put this behaviour down to a strange protection fault, and that it may not recur.

In one of the range high-power amps that I designed under contract, I went to great lengths to put explanations and diagrams of the protection circuitry in the draft user manual, principally so the user could understand better what happened under various fault conditions. The manufacturer chopped all that out of the printed manual, saying it was proprietary information. Keep the users guessing."

 

You're correct, the manual is for an older version of that model but it was the only manual I could find for this amp. The block diagram may not be representative of my amps anymore.

It is possible that there is a fault protection that occurred in the amps due to a power source related condition. All 4 amps are the same make and line so they may all respond to the same power source condition with the exact same fault protection.

Thank you for your response. It was extremely informative.

 

Tags

x

Register