Expected voltage on binding posts of NC252MP

[Veganfloyd]

Hi,

I just got a hold of a used 3-channel amplifier based on NC252MP. As advised earlier on this forum, I tested it with a multimeter before hooking it up to speakers. I am getting 0.29 volts on channels 1 and 2 and 0.4 on channel 3. Upons switching off, the voltage goes up to 1.3-1.4 V for a second and gradually declines over the course of 15-20 seconds. This is much different than 24 mV (please note the different scale) on my NC252MP stereo amp from the same company (Nord). It also does not have a similar hike in the voltage when switching it off.

The above are the highest measurements that I have come across and the first time that I had to switch from the mV setting to V to get a reading. Also, the amp has a high-pitched/buzzing kind of sound when turned off that can be heard from a short distance. This is also unlike my stereo NC252MP as well as NC502MP amps from Nord.

Is there any cause for concern or am I safe to test it with speakers?

 

[staticV3]

can you upload a picture showing the multimeter and where each of the two probes is touching?

just to make sure that we're all on the same page.

 

[DVDdoug]

It seems kinda' high but does it come down with speakers connected?

 

[mcdn]

NC252MP has DC coupled inputs. So it could be DC coming from the inputs. Try testing with the inputs shorted.

 

[Veganfloyd]

can you upload a picture showing the multimeter and where each of the two probes is touching?

just to make sure that we're all on the same page.

Thanks for your response. Please find the picture attached.

 

[Veganfloyd]

NC252MP has DC coupled inputs. So it could be DC coming from the inputs. Try testing with the inputs shorted.

Thank you. I tried my best googling around 'testing with the inputs shorted', but I could not figure out what you meant. Can you please be more specific as to what I should do with my multimeter in hand?

 

[Postlan]

It's not about multimeter. Inputs of the amp have to be shorted before measurement.

 

[Veganfloyd]

This gets me as far as knowing that it is my amp that I don't know what to do with.

Can you please tell me what specifically I need to do? I am not familiar with the term. Further googling with amp terms gets me as close as having a guess that it involves connecting inputs with one another. If this is right, how can I do it with a three channel amp. Also, there are both RCA and XLR inputs. Do I just pick either and need some sort of three way cable for it?

 

[mcdn]

[Edited to remove quote]

Shorting the inputs means that for the channel you are testing, you connect the Signal+ to Signal-, thus ensuring there is no voltage difference between them. For an RCA input that means Signal+ and Ground. You can buy them or make them. To make one just take a cheap RCA lead, cut it a couple of inches away from the plug, strip a bit of insulation off, and twist the two bits of copper together.

If this is happening with XLR inputs selected and nothing plugged in then there is almost certainly something wrong with the amp. Nord connects the sockets directly to the NC252MP input buffer, so there can be no offset provided by the Signal+ and Signal- pins just hanging out in free air.

Assuming you're using RCA inputs it's a bit more complicated, as they use the chassis ground as the signal ground, and the chassis ground can also be connected to power ground, so offsets can come from a few places. There are four test conditions to try:

1. Nothing connected to inputs (presume this is what you have done already?)
2. Shorting plugs connected to inputs
3. Source connected to inputs but powered off
4. Source connected to inputs and powered on but silent

So measure all four ways and report back!

 

[Veganfloyd]

[Edited to remove quote]

Shorting the inputs means that for the channel you are testing, you connect the Signal+ to Signal-, thus ensuring there is no voltage difference between them. For an RCA input that means Signal+ and Ground. You can buy them or make them. To make one just take a cheap RCA lead, cut it a couple of inches away from the plug, strip a bit of insulation off, and twist the two bits of copper together.

If this is happening with XLR inputs selected and nothing plugged in then there is almost certainly something wrong with the amp. Nord connects the sockets directly to the NC252MP input buffer, so there can be no offset provided by the Signal+ and Signal- pins just hanging out in free air.

Assuming you're using RCA inputs it's a bit more complicated, as they use the chassis ground as the signal ground, and the chassis ground can also be connected to power ground, so offsets can come from a few places. There are four test conditions to try:

1. Nothing connected to inputs (presume this is what you have done already?)
2. Shorting plugs connected to inputs
3. Source connected to inputs but powered off
4. Source connected to inputs and powered on but silent

So measure all four ways and report back!

Thank you very much! I got the caps and have the results. RCA and XLR results refer to the switch position at the back of the amp and, if applicable, use of either XLR-RCA cable (Monoprice) or RCA-RCA. The source inputs were the RCAs on my AVR for back surround speakers as I had them sitting empty.

When doing test #4 with XLR, I heard a thump when switching the amp off. It was not loud but I found it a bit worrying, Sounded a bit like speaker thump, but that does not seem plausible unless a spike in the output that I described in my initial post upon switching off could have traveled through the cable via the AVR to the speakers. So I would guess something on the AVR side made that noise.

The results are for channels 1 / 2 / 3.

1. Nothing connected:
RCA: 0.280 / 0.281 / 0.391 V
XLR: 0.012 / 0.011 / 0.031 V

2. Shorting caps:
RCA: 0.007 / 0.012 / 0.024
XLR: no shorting caps available

3. Amp on, connected to RCA inputs, source off (unplugged)
RCA: 0.073 / 0.094 / 0.185 V
XLR: 0.143 / 0.146 / 0.200 V

4. Amp on, connected to RCA inputs, source on:
RCA: 0.068 /0.092 / 0.183 V
XLR: 0.140 / 0.143 / 0.196 V

I would very much appreciate an interpretation, ballpark values that should be expected in these measurements and whether or not advice whether the amp seems safe to use or if I should return it.

 

[mcdn]

The numbers with the shorting caps in place and the XLR numbers with nothing connected look fine. They prove that the amp module is not creating the DC offset. For comparison I checked some of my own amps and they were between 0.003 and 0.009.

The Nothing Connected numbers for RCA do look wrong. Your speakers aren't going to catch on fire as V^2/R = 0.28*0.28/4 = 0.02 watts, but it does suggest a grounding problem.

BEFORE doing the next bit, make sure the speakers are connected, then unplug the amp from the mains and wait 2 minutes.

The next check would be the resistance between the RCA ground (so the shell, not the centre pin) and the earth pin on the power socket - that's the middle pin. This should be 0 ohms or close to it.

 

[Veganfloyd]

The numbers with the shorting caps in place and the XLR numbers with nothing connected look fine. They prove that the amp module is not creating the DC offset. For comparison I checked some of my own amps and they were between 0.003 and 0.009.

The Nothing Connected numbers for RCA do look wrong. Your speakers aren't going to catch on fire as V^2/R = 0.28*0.28/4 = 0.02 watts, but it does suggest a grounding problem.

BEFORE doing the next bit, make sure the speakers are connected, then unplug the amp from the mains and wait 2 minutes.

The next check would be the resistance between the RCA ground (so the shell, not the centre pin) and the earth pin on the power socket - that's the middle pin. This should be 0 ohms or close to it.

Thank you very much, again.

With the multimeter set to the 20k ohm position, I was able to get a reading of 2.60, and I believe this is in kiloohms. I gather that this is far off the expected value. Can you please explain what this means?

Thank you for doing the math for the watts. Does the spike to 1.3-1.4 V upon switching off described in my initial post not reach dangerous levels though?

Some other observations:
1. The high-pitched/buzzing kind of sound that I described initially can be heard from 2-3m, not just a short distance that I described initially. It is subtle when not putting your ear close to the amp and possibly perceptible or more pronounced to younger ears only. It seems to get my ears tired in a similar manner as listening to an overly bright recording - this is with just the amp on in a quiet room.
2. The buzzing gets quieter when switching the power off (including disconnecting from the mains) but it stays on for 5-10 minutes.
3. The thump that I described in post #10 when switching off the amp connected via XLR-RCA to AVR: I am now pretty sure it was coming from the speakers connected to the AVR (no speakers were connected to the amp). With benefit of hindsight, I cannot see anything other than speakers making that sound.
4. Something seems a bit loose inside. As I was positioning the amp for the various measurements, I heard a speck of metal or plastic rattling around. It does not seem completely loose running around freely though.

 

[mcdn]

@Veganfloyd something is definitely not right, and the amp needs returning. XLR ground and RCA ground should have 0 ohm resistance to mains ground. The 1.4V spike won't damage anything but it's also not expected. There's also no way an NC252MP module should make mechanical noise unless there is a wiring fault. So contact Nord and I am sure they will fix/replace.

 

[Veganfloyd]

Thank you. Just to clarify, this was a used amplifier - not bought directly from Nord. Just to emphasise that there is no suggestion of any fault from Nord per se.