SB3 volume output lower on left than right channel

[Tagore]

Have an SB3 that is unfortunately out of warranty. The volume output is different (lower on left than the right channel) both through the rca outputs and also the headphone socket on the SB3.
Have tried updating the firmware and returning the unit to factory settings but neither helped.
I suspect a hardware problem but have not been able to find any reference to similar issues on the forum with the SB3.
Anyone have an idea where the problem might lie? Might it be a simple job to trace to the faulty circuit part and component that is causing this?

[gizek]

Did you perform any tweaks on SB ? There is no balance control on SB, isn't there ? What happens when you go with volume all the way up or almost down - any changes in balance.

[Tagore]

I haven't done any tweaks and the L&R channel output imbalance remains the same whatever the volume output level.
For those that know the SB3, is there a particular part of the circuit / component that is likely responsible for the output difference?
Would send for servicing here but understand that where I live, Singapore, Logitech don't support SB3 servicing.

[gizek]

Is left, lower output channel in any way distorted ?

[Phil Leigh]

If there is a hardware problem it is likely to be in the handful of components that comprise the final analogue stage right next to the analogue output sockets.

[Tagore]

thanks for the responses. if I get bold enough might open it up or let an electrician friend of mine take a look at it.
the left channel out put is not at all distorted just a lower in volume than the right. I would use balance but my pre-amp doesn't have it and the software plugin that I found for the SB3 to alter volume balance between left and right loads but doesn't have any effect.
Shame they don't service it here and also that the SB3 is now hard to buy new as well. not keen on the duet really. oh well, will try and find one in the UK rather than here in Singapore.

[seanadams]

The fact that it occurs on both the RCA and headphone outputs really narrows down the problem because it rules out a dozen components in the RCA drivers.

I haven't seen this problem before, but I would say it is most likely the PCM1748 DAC or the 10uF cap connected to pin 7 of that chip.

Those are readily available parts but the DAC is not easy to replace.

Here's a partial SB2 schematic that I posted years ago. SB3 is the same but the component numbering is different, and there is no alternate footprint for the AKM part.

http://web.archive.org/web/20060616043837/http://www.seanadams.com/dac.pdf

[Tagore]

Appreciate the link and suggestions. Have replaced ICs and capacitors in other equipment before so am confident of doing the repairs. Any pointers for how to confirm if either of these are indeed the faulty components?

[seanadams]

Appreciate the link and suggestions. Have replaced ICs and capacitors in other equipment before so am confident of doing the repairs. Any pointers for how to confirm if either of these are indeed the faulty components?

Play a sine wave test tone and measure RMS volts to ground, before and after the caps. If the left is different from the right before the caps, then it's the DAC, if it's different after the caps then its the caps. I might be oversimplifying, but that's where I would start.

[peterbell]

Thanks for the information here, Sean.

This has given me the required knowledge to fix my SB3 which was showing a 20dB loss on the left channel. The fault did, indeed, turn out to be the decoupling cap between dac and op amp.

As a quick fix, I removed the decoupling cap, eliminating the op amp from the circuit, and took the dac output directly to the second decoupling cap, immediately prior to the lp filter at the output sockets.

[cutandaweb]

Me too

In my Squeezebox touch december/2010 the volume output is different also.

[eCo]

This exact problem has now happened to my SB3 as well. I'll give this fix a try. Thanks for the info.

[Stu-e]

I just had a quick look at the SB2 schematic Sean posted earlier and I am very concerned about the decoupling caps C7 and C39. The schematic symbols for the caps imply they are polarized and studying the circuit they are fitted with incorrect polarity. This will probably cause low frequency popping or rumble noises and eventually premature failure of the caps.

The inverting opamp buffer circuit has an input reference of 5V connected to the cap negative pin. The output of the DAC is somewhere between 0V and 5V connected to the cap positive pin.

I'm going to open my SB3 and check this out for myself. I think I've got a faulty power connector anyway which I can fix while I'm there.

Stu-e

[Stu-e]

Yup
C16 and C20 are reverse biased on my SB3.
I'll start a new thread

Stu-e

[eCo]

I'm happy to report my SB3 is now fixed. Though only the left channel was affected, I replaced both c16 (R-Channel) and C20 (L-Channel) with new 10uF/16v caps (Vishay 097 RLP 7) as I concur with Stu-e's assessment about polarity.

Accordingly, I soldered the new caps in with the negative lead towards the pcm1748 - opposite to the schematic and the original caps orientation. The procedure was quite simple, but some soldering skill and an iron with a micro-tip are helpful.

This thread will undoubtedly come in handy for electronics' savvy consumers of SB3's who've yet to experience what appears to be a likely failure mode.