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Old 11th January 2011, 09:01 PM   #1
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Default Help powering up a quasi amp for the first time

Hi guys,
I rebuilt an old amp which was originally full complementary in quasi complementary.I used 2 pairs of c5200.
I already tested the amp with +/-4v(funny,but the sound was clear before clipping).I connected it to a +/-30vdc power supply with a 60W bulb in series with the primary winding.I set the bias pot to max R,but when i measured the voltage drop across the emittes R's(0.22R),i got 35mv.Isn't this figure a little too high for class AB?How do i reduce the idling current as am intending to use +/-50 volts?
Regards.
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Old 11th January 2011, 09:52 PM   #2
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Can you take a screenshot of the VAS and Vbe mulitplier so I can see how the pot works ?

If the VAS uses a constant current source then the power supply shouldnt matter much as the current should stay the same through the Vbe multiplier.

On my quasi design I could get the bias voltage down to zero. Could be your design has got extra resistors in to limit the bias range.
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Old 12th January 2011, 04:50 AM   #3
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Default Not the case

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Originally Posted by nigelwright7557 View Post

If the VAS uses a constant current source then the power supply shouldnt matter much as the current should stay the same through the Vbe multiplier.
This doesn't seem to be the case,with +/-50v,supply am measuring 43mv.The bulb is still in place.I just measured the rail voltages and they are just at 16.9v.
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Old 12th January 2011, 05:39 AM   #4
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Just wanted to let you know:i quadrabled the value of a resistor in series with the pot and the value is now down to 2 mv with rails at +/-40v.I was wondering if this could distabilize the bias circuit?
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