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- May 14, 2014
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- No such things as bad days, just bad moments
I found a second NAD receiver in the trunk of an off to be recycled car in as many months. Newer and smaller than the first and seems to work fine except for bulbs to backlight the display (done) and the volume control cleaned. No phono section (bummer) but it uses a toroidal (and as the sticker says) Custom Manufactured For NAD "Holmgren" transormer.
Only tried it with Minumus 7's so far and sounds fine but before hooking the speakers up, I checked for DC offset. I actually did that right off the bat, connecting the DMM even before pushing the power button on for the first time.
The reading started high >200mv then dropped steadily until after 30 seconds or so. Then it rose and fell fairly fast, going into the negative but not higher that 35 ~ 40 millivolts that I noticed. Both channels act the same.
This afternoon, I did some Google searching and read that this might be normal. Capacitor coupled amps were cited as producing unstable DC offset but I thought those were things of the past.
The few amps I've checked for DC offset all had a steady reading. Any speculation if varying DC offset voltage can be expected? And how can one tell if an amplifier is capacitor coupled or not?
One thing I have not yet done is check the bias voltage.
EDIT: This morning I checked and adjusted the bias. SM said "to between 25 and 30mv". After 1/2 hour warmup they were both just a few mv below, but I nudged them up to 30.
2ND EDIT: After more internet scouring for info about the integrated on which this receiver is based, it does appear to be capacitor coupled. It was referred to as an unusual design that uses a 4,700mf in the output stage. So since all else seems well, I'm going to consider the fluctuating DC offset as normal.
Only tried it with Minumus 7's so far and sounds fine but before hooking the speakers up, I checked for DC offset. I actually did that right off the bat, connecting the DMM even before pushing the power button on for the first time.
The reading started high >200mv then dropped steadily until after 30 seconds or so. Then it rose and fell fairly fast, going into the negative but not higher that 35 ~ 40 millivolts that I noticed. Both channels act the same.
This afternoon, I did some Google searching and read that this might be normal. Capacitor coupled amps were cited as producing unstable DC offset but I thought those were things of the past.
The few amps I've checked for DC offset all had a steady reading. Any speculation if varying DC offset voltage can be expected? And how can one tell if an amplifier is capacitor coupled or not?
One thing I have not yet done is check the bias voltage.
EDIT: This morning I checked and adjusted the bias. SM said "to between 25 and 30mv". After 1/2 hour warmup they were both just a few mv below, but I nudged them up to 30.
2ND EDIT: After more internet scouring for info about the integrated on which this receiver is based, it does appear to be capacitor coupled. It was referred to as an unusual design that uses a 4,700mf in the output stage. So since all else seems well, I'm going to consider the fluctuating DC offset as normal.
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