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Old 24th March 2003, 06:41 PM   #11
rljones is offline rljones  United States
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I've built several class B amps over the past couple of years. Depending upon the components/topology used, I've noticed that some of them seem to literally have a bias null point. That is, when adjusting bias using a distortion analyzer, there appears to be a THD minimum (greater THD at both lower bias and greater bias). Other amps appeared not to have this minimum. Instead, they show greater THD at low bias, which decreases at some value, and then only further decreases at greater frequencies and greater powers, with a bias approaching a class AB designation. I therefore think each amp design needs to be examined for its own unique distortion characteristics with both measurements and listening tests.

I too, as NP explained, tend to go slightly above the just acceptable bias point if the heatsink-design goal will tolerate the greater setting.

As an example, one recent amp I made has an acceptable THD with a bias of about 50 mA. Going from 50 mA to 90 mA lowers THD from 0.0075 to 0.006%. Further increases, say up to 200 mA, slightly lowers the THD to about 0.0045% (all values for 1W into 8ohms). From this figure, THD doesn't seem to change even when increasing the bias to the +1A range. I cannot hear any difference when going above the 80 mA bias point. Since I wish to run the amp as efficiently as possible, I've settled on the 100 mA range for this particular design.

Regards, Robert
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Old 24th March 2003, 08:07 PM   #12
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paulb,
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Whether you get a distortion minimum at a particular bias current will depend upon the amp design and the devices used. It is a complicated system. It is quite possible that a low bias will measure better than a higher bias.

You need to listen to the design.

There are many distortion mechanisms at work, simultaneously, in a PP output stage. Some will improve with bias and some will get worse. It depends which you are measuring. You ears give you the net effect of these mechanisms and so the bias point that sounds best may well be different from the bias point that a particular measure predicts as best. Hence Nelson's observations.
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Old 24th March 2003, 08:38 PM   #13
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Default The sweet spot!!!

Quote:
I've noticed that some of them seem to literally have a bias null point. That is, when adjusting bias using a distortion analyzer, there appears to be a THD minimum (greater THD at both lower bias and greater bias).
Yes!!..but this null point is strongly dependent of the impedance of the load!!!
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Old 24th March 2003, 09:34 PM   #14
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Default Re: The sweet spot!!!

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Originally posted by Tube_Dude


Yes!!..but this null point is strongly dependent of the impedance of the load!!!

I have seen no evidence that the 'sweet THD spot' is load dependent.....may be the case......however, it is clear that overbias is better than underbias, which is where your amp. is likely to drift, if you bias for the said 'sweet spot'.....better to go for class AB.....

Self's schemes for maintaining thermal stability over the long term are not very convincing......eventually bias will drift.....i would rather drift into Class AB than class C....
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Old 24th March 2003, 09:59 PM   #15
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I've never designed conventional audio power amplifiers (just one very unconventional one, see Electronics World February 1996), but my guess is that for a complementary emitter follower configuration, the optimum bias is the one which causes a voltage drop of a specific number of kT/q's across the emitter resistors, probably something close to ln(2)*kT/q across each emitter resistor.

In this equation, k is Boltzmann's constant (1.38065E-23 J/K), T is the absolute temperature and q is the electron charge (1.6022E-19 C). All of this assuming that the transistors are not in high injection in the quiescent point and that the influence of the emitter and base bulk resistances is negligible.

For MOSFET output stages and output stages with local feedback around two transistors in each half (CFP), I'd guess the optimum bias is not very reproducible, being dependent on the precise transistor parameters.

Marcel van de Gevel
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Old 24th March 2003, 10:23 PM   #16
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Default paulb

check check out my A-B amp from wireless world in the solid state thread.
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Old 24th March 2003, 10:32 PM   #17
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Default Re: paulb

Quote:
Originally posted by AMPMAN
check check out my A-B amp from wireless world in the solid state thread.

Where??

...cannot seem to find it..
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Old 25th March 2003, 01:36 AM   #18
paulb is offline paulb  Canada
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Quote:
Originally posted by traderbam
paulb,
I have a brother who lives in Calgary!
Oh yeah, him. He says to say hi.

I have a brother who lives in Calgary too.
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Old 25th March 2003, 09:27 AM   #19
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Quote:
Originally posted by MarcelvdG
I've never designed conventional audio power amplifiers (just one very unconventional one, see Electronics World February 1996), but my guess is that for a complementary emitter follower configuration, the optimum bias is the one which causes a voltage drop of a specific number of kT/q's across the emitter resistors, probably something close to ln(2)*kT/q across each emitter resistor.

In this equation, k is Boltzmann's constant (1.38065E-23 J/K), T is the absolute temperature and q is the electron charge (1.6022E-19 C). All of this assuming that the transistors are not in high injection in the quiescent point and that the influence of the emitter and base bulk resistances is negligible.

For MOSFET output stages and output stages with local feedback around two transistors in each half (CFP), I'd guess the optimum bias is not very reproducible, being dependent on the precise transistor parameters.

Marcel van de Gevel

...AAHh the great Mr Van de Gevel....I've read your work...i am sure glad to find you on this forum....

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Old 25th March 2003, 11:20 AM   #20
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look in solid state for 07-06-2002 or perform search for ampman
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