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Metaxas Solitaire III Bias
Metaxas Solitaire III Bias
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Old 30th November 2018, 12:50 PM   #21
RM is offline RM  Norway
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I`m inclined to say just play it as it is.

You see the bias problems are also due to the fact that Metaxas uses two seriescoupled 1N4001 diodes as control element. These are glued into epoxy, which is not normally a problem, but it is in this instance. Because you have so small heatsink area you can`t really get a stable temperature and as a result the bias will fluctuate.

The spread in bias values are due to differing data on the transistors.

A better way would be to utilize a Vbe transistor, as this can be properly bonded to the heatsink.

R
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Old 1st December 2018, 10:59 AM   #22
Depaj is offline Depaj  France
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The bias is actually quite stable, I'm just not that happy with the wildly spread bias values (the original transistors actually seem to be more poorly matched than the on semi I replaced them with).

Both amps have some oscillation on bottom and top half (more so on the bottom part) of the sine wave when I get to about 3/4 of the max rated power. It's a little worse on the one I repaired but only very very slightly so I'm guessing this is normal for this design (I must add this is with the top cover removed).
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Old 2nd December 2018, 08:25 AM   #23
AKSA is offline AKSA  Australia
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I believe this is Bode Nyquist instability, Depaj. The only way to fix this is to increase the lag compensation cap - usually ceramic NPO or even silver mica - around 25%.

The amp has insufficient phase margin; that is, at the high pole we loop gain is at unity the phase shift if OVER the 180 degree from audio frequencies. This means negative fb presented to the negative feedback node the fb is now POSITIVE and the amp is in an oscillator.

If you can identify that cap, it is easy to fix, BUT, the sound will be different, 'slower', and possibly a 'dead' midrange.

Setting this cap precisely is one of the difficult aspects of good design. I know Metaxas; he is a clever guy, but he cannot be told. He has been told about this issue but refuses to acknowledge it. Such is life, people are people......

Nice looking amp, too.

Hugh
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Old 2nd December 2018, 09:15 AM   #24
Depaj is offline Depaj  France
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Hello Aksa,

Yes Rm suggested this in a previous post. The cap (styroflex) was originally 10pF, I already ramped it up to 56pF. As it is known to be somewhat unstable I did this little mod before actually testing the amp so I don't know if the oscillation was as bad before and if it sounded any different than it did before (which is pretty nice on my pair of ditton 66).

Did I go too high, not high enough ??
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Old 2nd December 2018, 09:26 AM   #25
AKSA is offline AKSA  Australia
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Yes, I would not go over 22pF. 56pF could make it very slow and dull.

Hugh
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Old 2nd December 2018, 09:28 AM   #26
Depaj is offline Depaj  France
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Wouldn't that make it oscillate even more to reduce it again from where I stand now ?
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Old 2nd December 2018, 10:30 AM   #27
RM is offline RM  Norway
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I think so too.

I once read about an American amplifier that it was designed on the ragged edge of stability. It would appear that it was not alone on that edge.

You might want to try 56-68pF between BC on the drivers. It does wonders on ML 23.

R
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Old 2nd December 2018, 10:54 AM   #28
AKSA is offline AKSA  Australia
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Try it. 56p is too much and will overcompensate and ruin the sound.
The drivers are in emitter follower mode; no gain. Oscillation arise in the voltage gain and to be honest we are all guessing without a good schemat.

HD
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Old 2nd December 2018, 11:21 AM   #29
Depaj is offline Depaj  France
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I will try to reduce it to 22pF but I have to order them. I can't get the polystyrene caps here, what would you suggest, silver mica or ceramic NPO ?
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Old 3rd December 2018, 10:10 AM   #30
AKSA is offline AKSA  Australia
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Silver mica is the best......

Hugh
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