Biasing question - totem-pole OTL

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Thermionic Idler
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#1 Biasing question - totem-pole OTL

Post by Thermionic Idler »

Denizens of AT...

I've reached the stage on my Beast build where I have to connect up the biasing pot. And I'm trying to figure out which way around to hook it up.

In principle it's straightforward - connect the wiper and one end together to ground, and the other end to the marked point on the PCB (which is as much guidance as the documentation gives). However, when biasing the amp, I want to ensure that a clockwise rotation leads to an increase in the DC voltage across the speaker terminals (so - to +). So I know that if the voltmeter reads too negative, I need to turn clockwise to correct it.

To get things the right way around so that it behaves in this way, I need to sanity check my understanding of the output stage.

I can't publish the actual circuit as it's copyrighted, but for the purposes of my question, the output stage can be considered to be the same as this published Glass Audio circuit:

Image

The A- Bias voltage (on the positive bank) is varied to get as close to 0v DC at the speaker. It's usually set to about -45V.

Now, if my understanding is correct, if A- Bias is brought more positive, towards zero, say to -40V, this will lead to a more positive DC voltage between speaker hot and ground. Similar to the way a cathode follower works.

I think I'm right but I just want to check this. Many thanks in advance.
Deck: Garrard 301 - Audio Origami PU7 - Van Den Hul Black Beauty
Phono: Whest PS30RDT
Linestage: AMB α24/LCDuino
Power amps: 'Newton' 300B PP based on a design by Lynn Olson
Speakers: FF225WK 'Big Met' Metronomes c/o Cressy Snr
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Nick
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#2 Re: Biasing question - totem-pole OTL

Post by Nick »

Yes the a bank are cathode followers so the voltage on the cathode will follow the bias voltage.
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Thermionic Idler
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#3 Re: Biasing question - totem-pole OTL

Post by Thermionic Idler »

Thanks Nick!
Deck: Garrard 301 - Audio Origami PU7 - Van Den Hul Black Beauty
Phono: Whest PS30RDT
Linestage: AMB α24/LCDuino
Power amps: 'Newton' 300B PP based on a design by Lynn Olson
Speakers: FF225WK 'Big Met' Metronomes c/o Cressy Snr
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Mike H
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#4 Re: Biasing question - totem-pole OTL

Post by Mike H »

Image

8)
 
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