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Old 11th April 2004, 08:31 PM   #1
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Default Strange offset behavior in Aleph 5

In one channel of an Aleph 5 I'm debugging, the DC offset on the output initially swings negative (when it's switched on) by as much as 20 volts, then over the next 10 seconds or so, it recovers to around zero and eventually settles on a range of about 40-48mV. Then, after the amp is switched off, the offset creeps up towards the positive rail. Unfortunately this makes it impossible to connect a speaker and listen to music. It's using Kristijan's PCBs and a power supply around +/-41V with 0.55-0.65A bias per FET.

What could cause this? I could understand a consistent offset if the input differential pair wasn't matched, but I'm out to pasture on this one. Thanks.
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Old 12th April 2004, 06:57 AM   #2
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Hi,

is the other channel ok?
Do you have a load connected (resistor)?

William
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Old 12th April 2004, 04:20 PM   #3
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The other channel works fine (but hot!).

I didn't think of connecting load resistors because I don't have suitable ones, but I just tried connecting three 22 ohm 1w resistors in parallel to the output. When I do this the initial offset spike is a couple volts positive (normal - just like my other amps), then it slowly (10-15 seconds) settles to around +40mV. And then on power-off the offset goes back up, but by less than 1 volt. I am still not sure whether I should connect a speaker and see if it plays music. If the current source is working correctly then I could assume it won't damage anything, but...

Thanks
Michael
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Old 12th April 2004, 04:36 PM   #4
MikeW is offline MikeW  United States
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Default Q3

Check the zener and resistors around Q3. It could be a bad zener or cracked solder joint.
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Old 12th April 2004, 04:50 PM   #5
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I just tried connecting a speaker, and the amp seems to play music normally (and good sounding) with offset quickly stabilizing at 40mV. I would still like to get rid of it, but it's not too big of a problem (after all, Pass specifies <100mV offset). The current source for the input stage is working fine.

Also, I noticed occasional clicking sounds from inside, presumably from the thermal expansion and compression of the wires and heatsink. It's strange, and it makes me worry that a solder joint could go bad. Anyone experienced that before? Does it go away after a "break-in" ?
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Old 12th April 2004, 05:47 PM   #6
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If you shut off an Aleph without a load, you charge the
feedback cap, and when you turn in back on, this charge
gets actively removed by the amplifier. You can reduce this
by placing some resistance across the cap to slowly discharge
it, or by seeing to it that there is some DC resistance value
to the load. A 1 volt transient on turn on/off is very normal.

The ticking noise from thermal expansion is normal, and is
usually the heat sinks expanding and contracting slightly.
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Old 12th April 2004, 06:01 PM   #7
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If it really bothers you, you can use a relay to short the load until the amp has settled. In my headphone amp the load is shorted until 10 seconds after power-on, and at power-off the load is shorted before the power supply fails. All you need for such a circuit is a suitable relay (with enough current-handling ability to survive the transient), a 555 timer, an AND gate, and some passives.
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