Help me find the faulty component

[hyperknot]

I got back a response from Audiophonics, they identified a bad capacitor and changed the 6 identical ones to make sure it's good. They were super fast, amp arrived yesterday, was already fixed and tested for 4 hours by today.

 

[hyperknot]

So I've done some DC measurements before installing the fixed amp.

DAC out
RCA 1.6/0.9 mV
XLR 3.2/2.2 mV

Amp out / speaker binding posts:
55/36 mV

Are these values OK?

 

[Marvin55]

So I've done some DC measurements before installing the fixed amp.

DAC out
RCA 1.6/0.9 mV
XLR 3.2/2.2 mV

Amp out / speaker binding posts:
55/36 mV

Are these values OK?

Acceptable but not excellent (in my opinion FWIW). The error seems to be in the DAC, and the amp is reproducing it.

 

[Doodski]

So I've done some DC measurements before installing the fixed amp.

DAC out
RCA 1.6/0.9 mV
XLR 3.2/2.2 mV

Amp out / speaker binding posts:
55/36 mV

Are these values OK?

What amp is it? The offset at amps is usually a little lower but some amps do run a 50mV offset by spec. Let's see if we can find a service manual.
EDIT: I read the entire thread. You should ask the service depot and the manufacturer what is the spec for offset.

 

[Count Arthur]

What amp is it? The offset at amps is usually a little lower but some amps do run a 50mV offset by spec. Let's see if we can find a service manual.

This one: https://www.audiophonics.fr/en/powe...tereo-ncore-nc252mp-2x250w-4-ohm-p-14185.html

It was in the first post.

 

[markstein]

DC may also be encoded in the music if not properly produced. RME references an example with a DC of -8dBFS! (read their ADI 2/4 manual, chapter "shocking").
It makes definitely sense to have a DC filter in your DAC, and you should activate it.

 

[Doodski]

This one: https://www.audiophonics.fr/en/powe...tereo-ncore-nc252mp-2x250w-4-ohm-p-14185.html

It was in the first post.

Yes, after I made that comment I read the entire thread. I don't think we are going to find a service manual for that class D amp. I edited my first comment. Thanks for the heads-up.

 

[Count Arthur]

https://www.hypex.nl/media/7f/85/fa/1708610358/NC252MP_04xx_R13.pdf

 

[Doodski]

https://www.hypex.nl/media/7f/85/fa/1708610358/NC252MP_04xx_R13.pdf

View attachment 386789

That is interesting although it does not specify the offset voltage at the speaker terminals when there is no input to the amp. This information specified max 50 Volts at the open collector output. Not sure what the open collector output is exactly too.
I checked out the link you provided and read the <pdf> and did not find a speaker output offset voltage. I am not aware of any adjustment used for calibrating the offset too.

 

[somebodyelse]

This information specified max 50 Volts at the open collector output. Not sure what the open collector output is exactly too.

To whatever the implementer wants to use for indicating a DC error. Perhaps an LED and current limiting resistor between J6.3 (standby power) and J6.7 (DC Error open collector output), or to a microcontroller pin with a pull-up resistor.

 

[Ropeburn]

So, I tested this with the same amplifier, connected to DAC via XLR (all pins connected). Simple 4000-count multimeter set to mV range, supposed precision +-1% plus 10 digits. Directly measured at speaker output with banana wire.

Starting with amplifier unplugged from mains: ~2mV. Likely to be residual charge, drops to 0 and very slowly discharges when using speakers as load. DAC plugged in but soft powered off.

Plugging in mains and switching on amplifier (but soft power off): ~46mV. Not great but uncritical. Near identical to OP's measurements

Soft power on (idle state): 11.4mV.

DAC powered on: 0.5mV. Smaller than measurement error. Edit: drops to 0.0 after a minute.

So there's only something going on in the soft power off condition. At full idle state, the chain's DC output is near perfect.

 

[Doodski]

@hyperknot is this amp still under warranty?

 

[Cbdb2]

Oh... looking around online for a manual for the amp, I noticed this somewhat concerning notice on the link you posted, @hyperknot:

View attachment 374439

3) you might want to check your signal (input) source for a DC offset(?).

Since when is 12V DC into a speaker not a problem? Less than 50mV is the typical maximum. A quick and dirty DC test if you dont have a volt meter: Turn your Amp on (make sure its working) than with no signal disconnect the the speaker. Do you hear a pop or see the woofer moving? If you do there's probably to much DC out of your amp. Reconnecting the speaker will do the same thing only the woofer will move the other way.

 

[Cbdb2]

I measured the DAC, playing "silence" on normal levels. DC offset is 0.2 mV.

Your Amp could still put out a DC offset with no DC input if its faulty.

 

[mhardy6647]

Since when is 12V DC into a speaker not a problem? Less than 50mV is the typical maximum. A quick and dirty DC test if you dont have a volt meter: Turn your Amp on (make sure its working) than with no signal disconnect the the speaker. Do you hear a pop or see the woofer moving? If you do there's probably to much DC out of your amp. Reconnecting the speaker will do the same thing only the woofer will move the other way.

Indeed, thus my comment that the snippet I posted was somewhat concerning.

 

[hyperknot]

@hyperknot is this amp still under warranty?

It's over warranty now, unless they gave some kind of extension because of the fix. But everything works fine at the moment.