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Old 14th December 2012, 12:02 AM   #61
fas42 is offline fas42  Australia
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I've just taken to looking more closely at the Stereophile amplifier measurement pages, by JA, and the distortion graphs can be very illuminating in terms of predicting how the amps will perform subjectively.

There are 4 things I'm now looking at:

* THD at precisely 1W into 8 ohms: this varies from -70 to nearly -100dB for pretty decent to excellent, SS, units
* How precisely linear the distortion vs. power curve is: less premium units have a decent upward bulge midway along, crossover distortion issues. The hyped Halcro looks rather wonky once real power levels are hit; in comparison the Soulution unit is beautifully straight right to clipping
* The flatness of the THD vs. frequency curve: the "engineered" units are ruler flat; others gracefully start to climb in distortion heading towards 20kHz
* And finally, how THD varies with load, especially the 2 ohm setting

Combine those 4 factors, and it's quite straightforward to connect the subjective reactions of people exposed to the units with the objective measurements. So, for example, the Halcro unit was not liked by many people and to me it's very clear from those graphs why this was so ...

Frank
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Old 14th December 2012, 02:40 AM   #62
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Flat THD out to 20kHz at anything near full power or is this 1 watt?

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Old 14th December 2012, 03:02 AM   #63
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Decent power levels, 20 to 80 watts. As an example of an "engineered" THD performance there's Bryston: Bryston 28B-SST monoblock power amplifier Measurements | Stereophile.com. And for superbly linear to clipping, Soulution: Soulution 710 power amplifier Measurements | Stereophile.com. Finally, an all-out assault on optimum performance, MBL: MBL Reference 9011 monoblock amplifier Measurements | Stereophile.com.

Krell is a bit of an also-ran in this crowd: Krell Full Power Balanced 350mc monoblock amplifier Measurements part 2 | Stereophile.com ...

Frank
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Old 14th December 2012, 06:55 AM   #64
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On more than one occasion it has been noted that snide comments and personal "put downs" of other members have made there way into some of the postings.

If this doesn't stop, penalties will be handed out.

Keep to the topic !
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Old 14th December 2012, 07:43 AM   #65
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Edmond Stuart View Post
Elvee: just 26mV (in case of BJT's).

Cheers,
E.
Indeed for optimum linearity, but people paralleling many devices may have a different agenda, like thermal stability, accuracy of sharing, tolerance to mismatch, etc
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Old 14th December 2012, 08:30 AM   #66
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BTW, Mismatch (in particular 'controlled mismatch') is not as bad as one might think. It might even lower the distortion. Think of multi-tanh or similar circuits.

Cheers,
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Old 14th December 2012, 11:01 AM   #67
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by Edmond Stuart View Post
BTW, Mismatch (in particular 'controlled mismatch') is not as bad as one might think. It might even lower the distortion. Think of multi-tanh or similar circuits.

Cheers,
E.
Very smart: changing a problem into a feature, I love it.

I personally couldn't rely on chance alone, Murphy would make that impossible, but a nice hand-picked spread could certainly help smooth out and straighten the transfer curve.
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Old 14th December 2012, 11:18 AM   #68
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Further review of this thread has led to more posts being pulled and warnings handed out.
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Old 14th December 2012, 03:10 PM   #69
Elvee is online now Elvee  Belgium
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Here is class AC for comparison: it offers a no-compromise wide class A region, but without the penalty of an increased THD when that range is exceeded.

It strikes me that it can be likened to Edmond's idea of statistical spreading, but discretized down to two elements.

The same principle could be applied to a larger number of devices pairs of suitable Is, and fitted with corresponding series resistors.


This example uses schottky diodes to minimize the class A quiescent (even with this trick, it is still 0.7A, quite a "hot" class).
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Old 14th December 2012, 04:34 PM   #70
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An alternate diode connection, allowing a lower quiescent current (less than 300mA):
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