Can anyone diagnose cause of amplifier fault?

[Sossages]

Can anyone please help diagnosing cause of excess voltage in amplifier causing one of the output channels to blow



Denon PMA-2500NE Amplifier fault history in detail;-



Works fine from new for nearly 6 months. No glitches. Used daily.

Connections;-

SACD player both analogue RCA and Digital Co-ax

Turntable to Phono input

Mac via USB

AV receiver to HT Bypass (EXT.PRE)



Fault 1

For no reason, having been used without issue the previous day and for the previous few months, a minute or two after turning on and working fine it cuts out and blinks red. Volume only at 9 o’clock. Source digital co-ax 1 in from SACD player. All other supplies in either turned off or on standby. Amplifier circuit has failed.



Fault 2

Back from repair connect only digital co-ax 1 to SACD (and possibly analogue RCA to SACD??) and speakers and play SACD player, volume at 9 o’clock

1st time on works fine for one or two minutes and then cuts out and blinks red. Amplifier circuit has failed.



Fault 3

Brand new replacement amplifier using existing power lead.

Connect everything up including HT Bypass (EXT.PRE) but everything either off or on standby. Turn it on with volume at zero. Fine. Scroll through input selector and notice hum on HT Bypass which there had never been on the first amplifier before there had been any faults. Seconds later it cuts out and blinks red. Amplifier circuit has failed. The AV Receiver was in standby. Note the AV receiver was not connected to HT Bypass when fault 2 occurred. I did not scroll through to HT Bypass immediately before fault 1 or 2 so don’t know if there was a hum.



All faults occurred when the amp was cold and on for a only minute or two and the volume low. Speaker cable connected correctly - indeed all connections as they were when fully working.



History of fault testing;-



Speakers tested on various amplifiers - All good



Speaker cable tested with Pioneer A400 - All good

They are factory Airlock terminations with banana one end and spades the other. No visual damage



AV Pre Out input and Turntable input tested with Pioneer A400 - All good



Denon DCD-1600NE SACD player tested with Pioneer A400 analogue RCA - All good. Note not tested with digital co-ax as no digital input. Fault 2 occurred with digital co-ax from SACD player.



All testing done from the same electrical socket as was used when the fault occurred - All good. This socket was also used for other things when the amp was away for repair with no problems and other components on the same ring main (hifi, AV receiver, TV etc) never had any issue on the 3 times the amp fault occurred. Also checked by an electrician.



Someone kindly suggested;-

“I don't think there is a problem with the speakers, because even if there was an issue, with volume at 9 o'clock, over current protection would switch amp off without causing any damage to the output transistors. Outer bit of Coax's socket is internally connected to ground. Your SACD was connected to that socket every single time, right? Many points in the amplifier circuit have predefined, safe voltage values compared to the ground=0V. If that gound has a positive value (+1,2,3V..ect), voltage of those safe points will change. Unfortunately, rapid red flashing means high DC voltage on the speaker outputs aka shorted power transistors. Obviously this is just my theory, but I would take a closer look at SACD player.“



Thank’s to Coronavirus I have not had the chance to have the SACD player tested under warranty (Sontec engineers are fuloughed), however does the fact it is working ok with analogue RCA rule this out? What are other possibilities?



I am needing to get the amplifier repaired, however once done I don’t want a repeat of the fault and need to find out the cause

 

[TrevC]

If the A400 works in place of the Denon I would reject it and replace it with a different model.

 

[Sossages]

Thank's for the reply. It may unfortunately come to that which would be a great pity as - when it works - is an excellent amp (as it should be for £2k) and has a top phono stage, DAC and also HT bypass which few others offer. It is also paired with the visually matching SACD player so substitution would be preferably avoided.
I am out of my knowledge zone when it comes to fault cause and wonder it it may be the SACD digital co-ax out of the SACD player or the digital co-ax cable itself as these are the only things I am unable to test with the A400 as it has no digital in (I also don't have a separate DAC) and it was this being used when the faults occurred. If these are ok then my basic logic would point back to the amps having an inherent fault, despite it being repeated with a 2nd one of them. I am ignorant though so there may well be something I am overlooking or unaware of that would cause high DC output voltage so if anyone else knows more please enlighten as this has been going on since March

 

[Al ears]

Why digital coax from the SACD player?
If that device is decoding the discs surely analogue out is all you need?
There's no point in having both, however not saying this is cause of issues, although this connection does seem to be the common denominator

 

[Sossages]

I was listening to SACD via analogue out thanks to Sony copyright with SACD and CD via digital out to the marginally improved DAC in the amp

 

[Blacksabbath25]

Sounds to me somewhere in your chain of what you have connected with is tripping the amplifier into protection mode .
Anyway possible just running the amplifier And then add CD player but No speakers wired up and see if the amplifier runs for an hour like this then add speakers and see if it runs .

 

[Sossages]

It's not going into protection mode i.e. where it can be turned back on again. When repaired the 1st time it blew it needed new output transistors. I assume (perhaps wrongly) this is what has happened since.
Once repaired it will very much be a case of attaching one thing at a time - but the reason for this thread is to try and find the cause without repeat damage...

 

[Blacksabbath25]

It's not going into protection mode i.e. where it can be turned back on again. When repaired the 1st time it blew it needed new output transistors. I assume (perhaps wrongly) this is what has happened since.
Once repaired it will very much be a case of attaching one thing at a time - but the reason for this thread is to try and find the cause without repeat damage...

Well my first thought was protection mode as I own a Yamaha which can do the same thing .
How long will it run for if you just had the amplifier on ?
Do you think you might be getting surges of electricity from you mains ?
Failing this then the amplifier is faulty again which is strange

 

[Sossages]

How long will it run for with just the amp on?
Don't know as I never tried that - I wasn't expecting the possibility of repeat issues. I can't try it at the mo as not working - needing repair. Will try this once repaired.
No surges from mains as nothing else on this ring was affected at the time, before or since. Also has been tested by electrician. Also happened the very 1st time it was turned on for both the 2nd and 3rd fault both weeks apart and weeks apart from 1st fault when nothing else on that socket or ring had any issues. Also has surge protection at the fuse board. I am not going to get a surge to cause this that doesn't affect anything else a minute after turning on for the 1st time twice weeks apart (faults 2 & 3) on a surge protected ring. I have my trusty old A400 connected to the same socket with no issues. No issues ever with the AV receiver, Bluray, Sub, TV, SACD player or turntable all on the same ring.
edit: the SACD player is also connected to the same double socket with never any issues

 

[Dom]

Get your electrics checked, could be a dodgy wall socket?

 

[Sossages]

For fear of repeating myself again they have been. All good

 

[Dom]

Okay, I guess I was just trying to help.

 

[Al ears]

I was listening to SACD via analogue out thanks to Sony copyright with SACD and CD via digital out to the marginally improved DAC in the amp

Unfortunately I don't think that is recommended as does seem to be the source of your issues.
The analog out is only going to be the CD layer of a disc.

 

[Sossages]

"The analog out is only going to be the CD layer of a disc."

Wrong
Analogue out is the only way to get the SACD layer out of a DCD-1600NE

Analogue out is currently working fine with the A400.
I am currently unable to get digital out until the PMA-2500 is mended so I have a DAC to feed. This is one possibility of the source of the problem that I haven't been able to test and as you say has been a consistent with all faults

 

[Deleted member 108165]

Why are you using both digital and analogue out from the CDP to the amp??

 

[ClarkNovember]

Why are you using both digital and analogue out from the CDP to the amp??

He’s already answered that: analogue out for SACDs (using CDP DAC), coaxial for standard CDs (using amp DAC). Whatever, it seems unlikely this would be causing the amp to fail.

 

[Deleted member 108165]

It appears to be the same DAC chip in both amp and CDP so this is a pointless exercise.

This is pointing to a connection fault of some kind unless the OP has been seriously unlucky with two amps. There are some very knowledgeable guys on here but think this will need a hands on approach with someone who is more electronics savvy than any on here.

 

[TrevC]

If everything works on the Pioneer amp but doesn't on the Denon then it can only be a problem with the amp. Perhaps the speakers have an impedance dip that the Denon doesn't like.

 

[Sossages]

Thank's for your inputs everyone
Re the impedance the speakers are Monitor Audio Gold 300 series 4 and the system had been working for 6 months no probs
It is correct to say the chip is the same, however I understand the 2500 has upgraded parts compared to the 1600. Does this make a difference in sq? The only way to tell is by listening to tell if a pointless exercise or not.... which is exactly what I was doing. Curiosity is, after all, a sign of intelligence. Conclusion being a smidgeon of improvement with concentrated listening, assuming of course no subconscious confirmation bias.

 

[Blacksabbath25]

Thank's for your inputs everyone
Re the impedance the speakers are Monitor Audio Gold 300 series 4 and the system had been working for 6 months no probs
It is correct to say the chip is the same, however I understand the 2500 has upgraded parts compared to the 1600. Does this make a difference in sq? The only way to tell is by listening to tell if a pointless exercise or not.... which is exactly what I was doing. Curiosity is, after all, a sign of intelligence. Conclusion being a smidgeon of improvement with concentrated listening, assuming of course no subconscious confirmation bias.

I would be handy if we knew the amplifier would run for an hour without anything hooked up to it to rule out the amplifier

 

[Sossages]

It would indeed. I am currently chasing the retailer to get it repaired so that this can be done. I have even suggested to them that they do this after repair and before return to myself.
Does anyone know anything about vu meters and testing the digital output on the SACD player? I know nothing about this but would try and get hold of a meter if I could be advised

 

[Blacksabbath25]

It would indeed. I am currently chasing the retailer to get it repaired so that this can be done. I have even suggested to them that they do this after repair and before return to myself.
Does anyone know anything about vu meters and testing the digital output on the SACD player? I know nothing about this but would try and get hold of a meter if I could be advised

Best getting it checked by an engineer who has that right gear and know how but your amplifier shouldn't go wrong like that unless it wasn't fixed right the first time round