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σ22 low output voltage?

sigma11, sigma22

σ22 low output voltage?

Postby wobblebonk » February 21st, 2020, 1:52 am

So I built a 4 channel β22 about a decade ago and I had intended to get a nicer case and eventually use 1 σ22 per pair of boards but I never got around to it. Anyhow, I noticed a channel imbalance after not having used it in a couple years. It got stored somewhere while I put in some hardwood flooring. I opened up the amp and couldn't see an obvious problem but when I look at the psu I get +- 12.something V with no load instead of +-30 (r10 is 6.81k), I checked the mutlimeter with some other PSUs and batteries and the DC v reading seems accurate there. I'm sure it was +- 30v when I made it and I think it's just not getting enough power now. I am not sure what might cause this, but maybe now is as good a time as any to put the amp and psus in a pair of rack mount cases and use a separate σ22 per pair of b22s? Other than the low V and channel imbalance everything "works" with no other obvious problems though I am not using it now until I fix the psu...
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Re: σ22 low output voltage?

Postby amb » February 21st, 2020, 5:18 am

±12V is too low for the β22. The amp will not be performing properly even if it seems to work.

Disconnect all β22s from the σ22 and re-measure the σ22’s output voltage to see if the low voltage condition persists.

The next steps will depend on the result of this test, so check this first.
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Re: σ22 low output voltage?

Postby wobblebonk » February 21st, 2020, 10:53 am

I am at work right now but for the +-12v with no load, I meant no β22 is hooked up, not no headphone on the β22, it was actually much lower than that with the β22 hooked up.

I've left it disconnected since I saw the V was so low. I won't be able to check anything else while I'm at work but I already know the answer to that... and that the transformer is still putting out >30v for each side to the input of the σ22.
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Re: σ22 low output voltage?

Postby amb » February 21st, 2020, 12:47 pm

OK. Check the two 0.47Ω resistors on your σ22 to see if they’re still good. Your DMM may not read accurately at that low a value but they shouldn’t measure way off.

Then, with power on, measure the voltage across D5 to make sure it’s still 12V (assuming you have a BZX55C12 there).

If any of these are bad then replace them. If not, then let’s dig deeper.
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Re: σ22 low output voltage?

Postby wobblebonk » February 21st, 2020, 8:30 pm

So one side's .47Ω resistors were like millions of ohms... not sure what happened there but replacing them restored my V to +- 29.8.
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Re: σ22 low output voltage?

Postby amb » February 21st, 2020, 10:01 pm

Great, problem solved.
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Re: σ22 low output voltage?

Postby wobblebonk » February 22nd, 2020, 8:45 am

Thanks for the help.
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