Amplifier fault. Ideas on cause please


Novice Member
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.


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

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.

On a previous post Dobrykamil 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?

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
I've got the same Amp and the only time it happened to me, it was a fault on the speaker cables. Think it's the Amp's short circuit protection circuit at work. It was well over a year ago and I'm racking my brain to remember exactly what it was? Perhaps disconnect the speakers and switch the Amp on. See if it happens then? If it doesn't connect one speaker and power up. Repeat with just the other channel's speaker connected.
If I remember exactly what caused it, will post again.
Good luck!
My speakers are biwired and I'd had them out. Reconnect them and crossed over the + & - on one channel's speaker tweeter. So crossed polarity on a speaker, was my problem.


Novice Member
Thank's Ron. Did your amp need repair i.e. parts replaced or did it self protect and could be turned back on?

Thing is with mine speaker cable hadn't been touched between the times before when it was working perfectly and when out the blue it blew the output channel. Further more the cables were then disconnected to send for repair then reconnected for the same problem to occur. The cables have since been used on a different amp (A400) with no issues and the speakers have also. Unfortunately I can't test the amp as it currently doesn't work. Covid-19 is slowing everything up re repair
My Amp was OK and never had any further problems. The short circuit protection, should ensure no damage happens to the Amp. Unusual to have a problem with Denon kit. Maybe it's a problem with the short circuit protection function itself? Worth trying it with the speakers disconnected and gradually turn the volume control up.


Novice Member
Will do that once it's repaired.
Don't think your issue was the same as mine as speaker cable (or anything else) hadn't been disconnected between it working one day and it suddenly not working the next and they were connected correctly every time the problem occurred - which actually caused damage
Thank's for the suggestion though

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