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Old 9th October 2003, 11:38 PM   #11
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Designing a smart class-a amplifier with all the benefits and few of the disadvantages is certainly desirable and there have been many commercial designs claiming to have achieved this, but how close do any of these designs come? Why aren't we all listening to 'new class a' type amplifiers?

I think in many cases the performance of this type of amplifier is dominated by the extra control circuitry rather than the class-a aspect, and without care you could end up with class-a/b in disguise.

The idea is certainly worth pursuing but analysing such a design could prove difficult and it is unlikely to ever out-perform a true full-time fixed bias/PSU class-a design since the extra complexity will not do anything to improve sound quality.

Having said that it is tempting to want to have a go.

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Old 9th October 2003, 11:43 PM   #12
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>The original Aleph 1 had an automatic idling mode to reduce
dissipation when not in use (after 10 or 20 minutes) but
we immediately took it out because everybody complained.

How 'bout your original Threshold "swinging class A" ? Wasn't that the idea with it ? ............ mike
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Old 10th October 2003, 07:34 PM   #13
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I always thought that the main benefit of highly biased class A was low level resolution. In Pass amps using mosfets, the higher you bias, the better the sound quality. So, it seems like you would be shooting yourself in the foot by reducing bias for low level signals. I could see running a good amount of bias, and increasing it for more output power, but I think you would hurt the sound by reducing it too far.
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Old 10th October 2003, 09:07 PM   #14
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Default No so Dutch idea

In my opinion (having succesfully built my second JLH-amp last summer) Kees' idea might be worth exploring a bit more. However my first note would be that the design should not go for maximum reduction of power consumption, but for some siginificant reduction.

Besides Bias-current reduction, Rail-voltage reduction should be considered (this is however not a reduction when using one fixed mains transformer)

One idea might be just to relate bias to the position of the volume control. (crude, but not totally insensible)
With volume between 7 o'clock and 9 bias is halved. Over 9 bias is full.
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Old 11th October 2003, 08:44 AM   #15
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Default Re: No so Dutch idea

Quote:
One idea might be just to relate bias to the position of the volume control. (crude, but not totally insensible)
With volume between 7 o'clock and 9 bias is halved. Over 9 bias is full.
[/B]
But the output level of Source equipment varies and is not standardised yet. Hence, the position of the volume control is not necessarily an indication of amplifier drive.
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Old 11th October 2003, 02:57 PM   #16
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Default Re: Re: No so Dutch idea

Quote:
Originally posted by Samuel Jayaraj


But the output level of Source equipment varies and is not standardised yet. Hence, the position of the volume control is not necessarily an indication of amplifier drive.
Correct Samuel,

However there are some standards (which are not adhered to very well) eg
a normal line output should have something like 0,7V @ 0dB (a CD player makes 2V@0dB).

And of course you have CD's which don't come anywhere near -12 dB and there are ones which do contain 0dB signal.

So my idea should work, has limitations, but it has some simplicity too.

Jos
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Old 11th October 2003, 03:53 PM   #17
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Quote:
Originally posted by nobody special
[I]t seems like you would be shooting yourself in the foot by reducing bias for low level signals.
I agree, and this is why I prefer and use a simple mute/standby switch to limit power consumption between listening sessions. The front end and all power supplies stay "live" so warmup/break-in time is minimal. Very basic, and works like a champ.
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Old 12th October 2003, 01:34 AM   #18
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Default sliding bias control chip

The Linear Technology chip LT1166 is a sliding bias control chip for mosfets, it maintains the product of the upper and lower currents as a constant. Also provides dual slope SOA feature.
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Old 12th October 2003, 04:39 AM   #19
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I think I can lay claim to the original patent on this subject,
and you'll notice that I tossed the idea after 2 years. If
you want to adjust the bias, I recommend a knob and a
meter on the front panel.
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Old 12th October 2003, 08:22 AM   #20
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I think the core issue of the idea of Kees (hi Kees) is that music is highly dynamic. Adjusting the bias to the average signal level means that at high level peaks it is underbiased, unless you have a very fast acting bias control. I would be very sceptical on the quality aspects of a bias control that adjusts itself along the instanteneous signal level, however.

But I have never tried it, maybe somebody finds the holy grail of bias control.

Jan Didden
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