• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

CXA-61 High(ish) DC offset on one channel

jackpate

jackpate

New Member
Hey all. I purchased a Cambridge Audio CXA-61 a year ago to use in a bedroom. It has been a good performer for streaming. For background, I frequently tinker on vintage hifi stuff, Pioneer SX, '60s Sansui, Sony STR, etc.

Having recently brought some old gear back to life, I had my multimeter out and was curious about the CXA-61 DC offset. Left channel=4mV. Great. Right channel 18mV. Odd. Took multiple measurements with no inputs, letting it idle, A/B, using different meters even. Same results.

After patiently waiting for Cambridge to give me a clear answer (they do not share service manuals because of "intellectual property"), I was informed that +/-20mV is "within spec." Fair enough, but this seems like poor quality control for the channels to be so disparate. For such an item, I do not find it acceptable to be "on the edge" of the tolerance window.

Am I wrong in thinking this?
 
Register to hide this ad
No!, is there a way to adjust it? How about the idle current for each channel, what is that? Or is DC offset the same? Normally that can be adjusted. My old NAD 317 was 20 mv +/- 2 I think, so 10%.
 
rothwellaudio said:
Yes, you're wrong. 18mV is practically nothing. That fact that it isn't as good as the frankly stellar 4mV of the other channel is no great shame.
Click to expand...
Thanks for this point of view. I am used to getting things rather precise in my tinkering, so this just seemed surprising. Again, thank you.
 
Sandman1221 said:
No!, is there a way to adjust it? How about the idle current for each channel, what is that? Or is DC offset the same? Normally that can be adjusted. My old NAD 317 was 20 mv +/- 2 I think, so 10%.
Click to expand...

I have inquired about an adjustment. As they have stated the unit is "within spec," I do not have any recourse for warranty, unfortunately. I am not a fan of going in willy-nilly without diagrams. I'll update if I hear back from them. This is disappointing, obviously. I was thinking of moving the CXA-61 to the main room for a while, but now I want to figure out what is causing this disparity in DC offset. It's an expensive item for right/left to be this far apart, in my opinion.
 
The trim pots that some amps have for getting the DC offset to zero are something of a fudge (in my opinion). They really just introduce one error to compensate for another. A more sophisticated way to get the DC offset to zero is by using a DC servo, but that introduces more complexity and isn't something that can be retrofitted anyway.
In the most common type of power amp circuit the DC offset would be zero if the two transistors which make up the long tail pair were perfectly matched and the two resistors through which current flows into the bases of those transistors were perfectly matched too. Also, the DC blocking capacitor used in the feedback loop would have to have zero leakage. In the real world the lack of perfect matching results in a DC offset at the speaker terminals. Really, 18mV is not bad at all.
Would you prefer both channels to have 18mV offset so that both channels are the same?
 
From Micro back in 2019, about his recapped amp not sounding as good as a second he just recapped.

Micro said, "Mystery solved. I assumed that my idle current was 15mV, the way I adjusted it 4 years ago. But for some reason it dropped to 7mV so I had power amp way out of specs. First sign were stone- cold heat sinks after 15min. of warming up before bias adjustment. Once I put it back to (low) normal of 18mV I do not even need other unit to compare it, sound difference is obvious. Lesson learned, will recheck bias at least once a year."

So check the heat sinks for each channel after a 15 min. warm up to see if they are the same temp.
 
Last edited:
Don't know the price point of the amp, but as previously mentioned, the multiple factors that "create"DC Offset, come into play. Do you expect a mass production line to perfectly match, at voltage, the resistors and transistors in question? Hardly a reasonable ask for a producer. Pop the lid and see if there's multiple trimmers on the circuit board. If there's only one pair of trimmers, I'd bet they're for bias, not DC Offset.
 
jheu02 said:
Do you expect a mass production line to perfectly match, at voltage, the resistors and transistors in question? Hardly a reasonable ask for a producer.
Click to expand...
As a side note, Quad's famous "current dumping" circuit was somewhat baffling to many engineers when it was first introduced and people are still arguing about how it really works. Quad's claim was that it produced higher fidelity than had been possible before, but from a production point-of-view the big advantage was that there were no trim pots to fiddle about with. There was no need to have each amp powered up for a warm-up period before adjusting bias current, then waiting for stabilization before resetting bias. Loads of production time saved.
None of that is relevant to the OP's amp though.
 
You must log in or register to reply here.
Back
Top Bottom