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Old 8th September 2005, 09:26 PM   #721
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In my AlephX, motorboating occurs when bias pots are turned all the way down. I suggest playing with V1, V2 and see what happens.

/Hugo
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Old 12th September 2005, 10:56 PM   #722
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Another question from a friend who is building an Aleph-X. This amp can be run at lower power (or put on standby) by lowering the idle current. Now, if you have a power transformer with multiple primary taps, can you do the same thing by switching taps? That would drop the rail voltage, but I would suspect would not affect the idle current (or would it?). Nonetheless, it does represent a lowering of dissipation.

Is this feasible or is there something stupid that I'm not seeing?
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Old 13th September 2005, 01:06 PM   #723
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Hi,

lowering the voltage without changing anything else will raise the bias (as experienced when powering up the X through a variac).

I would just turn it off if not needed. Sound quality is very good from cold and putting it on standby wonīt change the warmup behaviour very much (abs. dc offset).

William
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Old 13th September 2005, 10:09 PM   #724
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The issue is not so much standby mode, but using less than maximum power depending on the speakers. So the idea is switching both voltage and speakers, so that it's not necessary to run the same dissipation for (say) speakers that work OK with 50W as one would for speakers that require 150 W. But the amp would be capable of returning to 150W easily when higher power requirement speakers are in place.
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Old 13th September 2005, 10:25 PM   #725
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ok,

I still think this would be not that simple (but possible). You would have to setup two modes (high power/low power) and have settings for:

1. bias (two pots)
2. ac-current-gain (two pots)
3. diff pair bias (one pot) to set abs. dc offset

This could be done by setting one mode, measuring the resistor values, setting up the other mode, measuring the resistor values and then switching 5 resistors per channel plus the supply voltage

The changes to the supply voltage canīt be too big otherwise other resistors will have to be changed as well.

I would keep it a bit simpler and only change the 5 resistors without changing the supply voltage. If changing speakers only happens two times a year I would fit these resistors in little plug-in modules and change those when needed.

William
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Old 13th September 2005, 10:26 PM   #726
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Danke schoen!
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Old 14th September 2005, 08:02 PM   #727
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I think that's one of those places where you start using op
amps to actively control the absolute DC offset. As you change
the bias, the absolute DC needs adjusting. The "typical DC
servo" you see in AudioXpress and elsewhere could be adapted
for this job.
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Old 14th September 2005, 08:05 PM   #728
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Hmmmm, servo.... servo.... where have I seen this before?
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Old 14th September 2005, 09:20 PM   #729
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MST3K! Tom Servo.
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Old 14th September 2005, 09:23 PM   #730
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No, Crow.
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