Finding source of different sound output between channels

I’ve got a several dB difference in volume between channels. I’ve narrowed the likely culprit down to two possibilities, with the spdif run between Oppo and dac being the most likely (the other is one of the dac’s outputs). Swapping in another spdif cable got the same result. Since I don’t have another source to sub in for the Oppo, is there way with a multimeter to check what’s arriving for each channel on the destination end of the spdif cable? Thanks,

A multimeter is not capable measuring measuring audio levels of a digital signal.

I would suggest playing a test CD with tone on it. Set your multimeter to AC and compare the levels of the left and right analog audio signals coming out of your dac.

Thanks. That won’t tell me whether dac or Oppo, but might eliminate the preamp.

So how does one test a player’s digital output for a proper signal?

Follow up: Playing 120Hz tone, dac outputs each show 1.0V (200V scale). The only things after that are a preamp and active speakers. I know it’s not the speakers. I tried the multimeter in the preamp’s XLR and RCA outputs with AC but didn’t get anything.

I would suggest reducing the scale of your multimeter 20V or less to get more resolution.

You are able to measure the dac’s output so it makes no sense to not be able to measure the preamp‘s output.
Try again?

On the XLR connectors the maximum voltage will be measured between pins two and three.
if you measure between pins one and two or between pins one and three the voltage should be 1/2 of the pins 2 to 3 reading.
Sometimes the signal on pin two or three is missing. That results in 6db less signal voltage.

How did you determine the speakers are not the problem?

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Thanks. I’ll check the preamp again. 200V is the lowest AC band on my Extech MN35 multimeter. The meter shows tenths of a volt, which I assume would substitute adequately for a lower voltage band. If amps would work, there is a 10 amp setting. The difference in output travels to the other speaker when channels are swapped.

Swapping cables left to right/right to left down the signal path will eventually show you what the culprit is. If the signal truly is several dB less in one channel, it shouldn’t be hard to detect with just listening tests.
Just my experience, I’d be really surprised if the Oppo’s COAX output is at fault. I’m not sure I’ve ever heard of an instance where a channel’s output was lower in a digital signal.

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Since 200 V is the lowest scale you have, it is probably adequate for this test.
The amps function is of no value in this case.

Are you using XLR cables to feed your powered speakers?

Yes, XLR to ATC SCM19A.

It took awhile, but I was able to get 0.1V from each preamp XLR output, although I couldn’t get a reading from the other end of the cables. I then swapped XLR’s and the cables are ok, which would suggest one speaker. But that speaker’s amp module was inspected by the distributor in Dec 2019 and found fine, so I’m not ready to go there yet. There is another possibility, Oppo aside.

The Lampi dac came back from a warranty repair in Jan-Feb 2020 with the top analogue (RCA) output nut completely tightened. I tightened it some on the inside, but couldn’t get it as tight as the others because a solder joint is in the way. With locking connectors, that nut was apt to keep coming loose, and the other day it turned enough to cause some bare wires to touch each other or the center pin, killing the sound on that channel. I pulled them apart, and am going in shortly this morning to get the nut tightened by a tech (requires de-solder, re-solder).

In looking at it, I noticed something else. See the photo below, after I pulled the wires apart (the RCA outputs are on the left). I noticed that the top output (the troubled one) has a single white wire (yellow cover) soldered to the center pin, with blue and white wires soldered to the tab (ground?). But looking below at the other channel (below), while blue and white wires go to the tab there too (view blocked), two wires – blue and red – also go to the center pin. My question is, why the difference between the center pins and does it matter? I’ve got that question into Poland and the U.S. tech that worked on it, but haven’t heard back. Any knowledgeable techs that can speak to this?

Btw, this reminds me that the dac can be directly hooked to the speakers – I’ve got adapters – which will simplify troubleshooting some by cutting out the preamp.

Just to mention why I’ve thought it was the Oppo or Oppo to dac run: The first day I was setting up the subs, when I reseated that cable, especially on the Oppo end, output evened up between the channels. But then the difference would soon come back (that connection has to be perfect or it has problems). Then I tried another coax cable but still different outputs. Btw, the channel difference using test tones and DecibelX on an iPad Mini is ~4dB at 120 Hz and ~6 dB at 60 Hz (the speaker specs say it’s down 6 dB at 54 Hz).

Turns out that the top RCA center pin connection is tenuous and maybe the pin is broken, along with the sloppy use of bare wires that invites grounds. So just replacing the port would not really speak to the problem. Plus, the tech noticed that a resistor solder joint on the dac board, with a wire going to the outputs, has thermal heat discoloration, unlike those around it. Have to deal with Lampi.

Did the technician measure the analog voltage output of both channels?

No, we didn’t get that far. I don’t know if he would have had a source to drive it. He’s experienced with audio and small dacs but it’s primarily a computer shop. I would have needed to pick up a port from Radio Shack (if not have them do the work).

He would have replaced the port and all those bare wires – sloppy work by a Lampi NA tech and a potential and perhaps actual source of unwanted grounding to the case. But then he noticed that the Ground resistor’s solder joint on the dac board showed thermal heat damage. There’s a wire from that joint that goes to the outputs. At that point, he said this should go to someone who knows the dac’s circuitry, rather than him spend some hours pulling it apart and figuring it out.

But you measured 1.0 V out of both channels of the dac earlier.
If that was a valid measurement, then the dac’s outputs are both good.

If it were me, I would remove the XLR connectors from the self powered speakers.
I would play a one kHz tone.

I would then significantly increase the volume on the preamp in order to get a higher voltage at the XLR connectors.

I would then measure between pins two and three of both XLR connectors that normally plug into the powered speakers.

If the voltages are significantly different, say 6 V and 3 V I would work backwards and find out what’s causing it.

If they are essentially the same then I would have to rethink the whole situation.

I’m between a rock and a hard place. The dac’s top RCA port needs changing. Some of the wiring to it and the other needs replacing, and there’s thermal damage to the solder of the ground wire coming from the board. It was sloppy work by a NA tech. I could have the local Radio Shack just change the port, if they were willing to do that alone, but I don’t know if they would. But that would leave other things undone and unchecked. The tech I saw at another shop said no, he’d want to dig into the whole circuit to make sure everything was ok. LampiNA was willing to reimburse me, but not for three to four hours work at $80/hr. So it seems the best all-around thing to do is to have Lampi look at it, and then see what turns up – and doesn’t – about the volume issue. NA is in contact with Poland about sending it back.

Ok, while the output port still needs sorting for the long haul on the Lampi dac, I’ve gotten some hold on the channel volume issue. Simple story, pink noise shows no difference, midrange and lower test tones do. Remember, all this arose during process of trying to set up subs. Here’s a table of results and a link to two photos showing what my listening room looks like (doesn’t include full height bass trap in back right corner). Testing was done from listening position on end of sofa closest to camera, with iPad held in front of eyes, facing speaker in question. Listening position is ~103" (2.6 meters) from each speaker.!AqsCKHVTj0ydpUeJ7YLsmrOmraq2?e=B8pLuV

You should measure a meter away from each speaker to get rid of room nodes.

Not clear why, since I don’t listen a meter away. And this is about what I hear from my listening position, isn’t it? It seemed enough to me that pink noise registered the same between speakers at the listening position to know that components, cables and speakers are not the problem. My room is far from optimal and that’s not going to change. It’s not worth fighting, except to add some first reflection point scatter pieces.

If you’re satisfied your room is causing the dB imbalance, great. Otherwise measuring near field helps you rule out your speakers as the source of the imbalance.

I checked that by swapping leads.