What's gone wrong? massive DC offset in left channel of f CD player

[AlexM]

Something really strange here - any ideas what has gone wrong?

My Cambridge Audio 851C, which I use with my Squeezebox Touch has been acting oddly.

I have noticed a channel imbalance, with the right channel being significantly louder than the left. The imbalance followed the channel when I swapped the interconnects left to right, so this hopefully eliminates problems with cabling, amps or speakers. I swapped the 851c for the CA DacMagic 100 and my own Musicstreamer II+, and both were fine.

I returned the CD player for service via Richer Sounds before Christmas, and it was returned two weeks later with no fault found. When I reinstalled the player in my system, the channel imbalance was still obvious. I also noticed that the stepped attenuator on my CJ ET3 preamp caused a loud clicking through the speakers when increasing or decreasing the volume or pressing the mute button. This made me wonder if there was a DC offset problem with the output stage, so I decided to do some measurements.

I used the signal generator function in the SB Touch to produce a 1Khz sine at 0Db (maximum) I did some measurements at the unbalanced outputs, and found a DC offset of 0.7mv in the right channel, and -5.4V (!!) in the left. Output level was 2.18v on the right channel, and 1.45v on the left.

And then things get quite strange - when I took the player back to the shop, it was plugged into their demo system (Cambridge 8-series amp), and there was no channel imbalance apparent. I had taken my multimeter with me, and the DC offset was still there. I compared my unit's measurements to their demo unit, and this showed no DC offset at all (<1mv on both channels). Mine still measured -5v ish on the left and 1mv on the right. I couldn't retest the the output level as I didn't have a means to create a test signal that is the same between channels.

Questions are: what has gone wrong that would cause massive differences in DC offset between left and right, along with a big channel imbalance in my system?. Why would this not to be apparent in another, despite the DC offset?.

Any thoughts?

Alex

[StanleyB]

There can be a few reasons for the problem, so I'll describe the ones that I remember from my days as a repair engineer.

1. One of the opamps in the analogue output section after the DAC chip could be faulty, or the positive supply could be missing.
2. You got a broken ground connection on the RCA/phono socket on the channel that is faulty.
3. The DC blocking capacitor in the signal path between the faulty channel and the RCA centre pin connection is faulty.
4. I don't know the CDP in question, but I would have whipped it open and compare DC voltages between the left and right channel opamps with no music playing. If the DAC chip is a balanced output one, then it would have two opamps per channel. One of these could have gone faulty, thereby only giving you an output on just one. This would be about half of the signal on the working channel, which would sound like an imbalance.

[AlexM]

Stan,

What a great reply.. Many thanks for that.

It does use dual differential DACs, and uses a servo control for the DC offset at the output rather than a simple DC blocking cap. Your explanation makes sense - faulty op amp or missing supply. If one of the op amps ona channel had gone down, would a single ended output see the whole waveform at half amplitude or the positive or negative phase?. Would the balanced outputs still work? I didn't test them. .

The weird thing is that it sounded good in the shop - no obvious channel imbalance, but there was still a big DC offset. In my system it seemed to upset the preamp - partial loss of signal and click/thump when adjusting the relay switched volume control.

Thanks again for your input.

Regards,
Alex

[StanleyB]

Let's put it this way; if I was a repair engineer with very little time to diagnose a fault like that, my first job would be to compare the volatges around the opamps in the left and right channel. You mentioned a DC offset servo, and that is something that I would have a look at as well. I had a servo fault on an item that took me days of head scratching to figure out. The symptom was a positive offset voltage, and the fault was with a capacitor in the negative rail. In your case you have one good channel working, which helps a lot.
If you lived within an hour's drive from me I would have asked you to drop in and put the kettle on whilst I had a quick look at it.

[agk]

I had a 751C with the same issue and after buggering around gave up. The build on Cambridge kit is very hit and miss in general it would seem.

[AlexM]

Andrew,

That's interesting... do tell more. Did you try to get it fixed?. Cambridge audio says they haven't heard of it being a common problem, but given my 840c died in a similar way, there may be something in the design of the output stage that lends itself to problems. The build quality is actually very good, but all electronic devices can go wrong I suppose. No reason to think that there is a fundamental quality or manufacturing issue.

How was the situation resolved in your case?. it sounds as though it wasn't from what you're saying. What exactly were you experiencing?

Stan, thanks - it has gone back to richer sounds for them to take another look, this time with the job sheet updated to include my measurements. We'll see, but there is no way that level of DC offset and difference in the output measurement can be returned as 'no fault found' again. Strong words may need to be had if so. It all sounds a bit... complicated.. dual differential DACs, DC offset servos etc. One supposes that that complication serves some audible purpose..

I'll update when I have heard back again..

Many thanks,
Alex

[AlexM]

Ok, so it is back from Richer Sounds after a second attempt to fix it. All is well again - replacement DAC board FOC as an out of warranty repair. To be precise I paid £35 for an out of warranty repair but they waived the charge for the parts (£300-ish!). Once again, very good customer service from them... top marks. I got a call from the engineer who was working on it, who asked me exactly what the problem. He suggested replacing the DAC board to be sure to avoid any possibility of an intermittent fault. I'm very happy to have it fixed, but slightly disappointed to not know exactly what failed.

Anyway, So far so good: measured 0.7mv DC offset in one channel, and 1.2mv in the other, output is 2.2v a 0db with a 1k sine test signal. The weird thing is that when I tried to demonstrate the fault in the shop with their CA system, it sounded fine... no channel imbalance at all. Is it possible that their amp has a blocking cap on the input, and mine doesn't?. I would have thought that all preamps do, but maybe not.

Anyway, a happy ending :)

Regards,
Alex

[Barry]

Glad to hear you are happily enjoying music again.