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hameay 8th January 2012 03:03 PM

speaker dc offset question
Happy new year everyone.

I've built Mark's ZCA MOSFET amp (my first amp build). Had it running open and working fine on an MDF board for a couple of months but finally got round to building a case for it. Everything's in the case now but I'm getting a 12V DC offset (same as MOSFET bias voltage) on the speaker terminals. I'm very dubious about hooking the speakers up to it as, from the way I understand it, a DC offset on the speaker terminals is bad, right?

p.s. ZCA schematics here: DIY Class-A 2SK1058 MOSFET Amplifier Project

DF96 8th January 2012 03:21 PM

Yes, bad. You have a wiring short or a faulty output capacitor.

hameay 8th January 2012 03:35 PM

Cheers - thought so. Glad I contained my enthusiasm and didn't hook it straight up to the speakers!

I've tested the output caps with my DMM and they seem to be working OK (shorted caps, then DMM set to resistance across caps, resistance starts near zero and increases steadily).

Could it be the way I've wired the 2SK1058 MOSFETs? Wired as in schematic, but the source has two routes to ground, one via a connection from source pin to ground, and the other via the MOSFET flange to the heatsink and chassis (since I haven't insulated the interface between MOSFET and heatsink).

system7 8th January 2012 03:58 PM

I'd confirm the output capacitor is connected with the right polarity, which is +ve to the +12V FET output if I follow the circuit, and connect a 1K 1/2W resistor across the speaker terminals and remeasure the offset. It might fall away to nothing if it's just a tiny leakage current affecting a high impedance probe.

Electrolytics will allow a considerable current to flow if wired with wrong polarity though, and not last very long. :)

Always possible the circuit is oscillating with no load too.

DF96 8th January 2012 03:58 PM

I'm not familiar with that device, but grounding the source twice is unlikely to do any harm unless it provokes instability.

Did you connect the output electrolytic capacitor the right way round?

Are you just measuring leakage current? A big electrolytic will pass a little current, plenty enough to drive a DMM. Try putting a low value resistor (say, 100-1K or whatever you have in stock) in place of the speaker and measure the voltage again. If now low, nothing to worry about.

system7 has same thoughts!

hameay 8th January 2012 04:11 PM

I've quadruple (!) checked the polarity of the output caps and they're wired correctly.

As suggested, I wired a 1K 1/2W resistor across the speaker terminals and measured the offset voltage (i.e. connected my DMM across the resistor). The DC voltage quickly (a few seconds) drops to a few mV.

Perhaps I'm misunderstanding offset voltage. Originally, I put my DMM across the speaker terminals (no resistor) and got 12V DC...

So, since with the resistor the offset voltage is only a few mV, is it safe to connect up the speakers?

hameay 8th January 2012 04:55 PM

Thanks for your help DF96 and system7 - appreciated. I hooked it up to the speakers and it's working fine :)

DF96 8th January 2012 08:07 PM

Electrolytic caps always pass a little current. Not enough to harm a loudspeaker, but enough to show up on a high impedance DMM.

AndrewT 8th January 2012 08:53 PM

It's better to ask, rather than us seeing Members' tales of woe.

hameay 8th January 2012 08:55 PM

That's a useful thing to know as in all the stuff I've seen it usually says something like "check there's no DC voltage across the speaker outputs", without the caveat of "with a low value resistor across the outputs". Thanks once again.

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