"Trevor Wilson" wrote in message
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"Robert Morein" wrote in message
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"Trevor Wilson" wrote in message
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"Robert Morein" wrote in message
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"Trevor Wilson" wrote in message
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"Robert Morein" wrote in message
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**It's OK. Most do, when I present them with that information.
However, it is the ONLY explanation which makes sense of the
"softness" associated with Haflers, Perreaux and other early,
standard MOSFET amps. Even with the level WAY below clipping, even a
crappy BJT amp (like a late model Phase Linear) the lack of dynamics
in the MOSFET amps is immediately noticable. You, yourself, have
acknowledged in your statement that they are "soft" sounding.
Even the DH-200 was specced
at something like 0.2% distortion at full power. If the amplifier
compressed a full power sine wave, the result would not be a sine
wave.
**Except that I am not talking about sine waves. I am talking about
fast rise time, assymetrical transients. Just likke the stuff we get
in music. The Hafler does a fine job of reproducing sine waves, just
like any other MOSFET amp. It's music that it stuggles with.
I appreciate your effort to explain the characteristic sound, but I
would need to see a gapless explanation.
**OK. I'll try to explain later. Briefly, however, consider the
effects, at chip level, when a MOSFET is subject to the heating via a
fast rising transient (which causes lots of current to flow). The
MOSFET very briefly tends to reduce this current, via the negative
tempco of Gm. The NFB loop will tend to counteract this effect to some
degree, but, IMO, not all that successfully. Very high bias designs
(ala Pass, et al) get around this problem by operating the chip at
constant high temps. Thus, the effect is not noticable. Low bias
MOSFET amps do suffer with this compression effect. Sound reasonable?
Not proven, but definitely reasonable, ie., within consideration. But
if we listen to Francois, low-biased MOSFETs may be in the negative
temperature coefficient region.
**Absolutely, which is why for MOSFETs to sound reasonable, they MUST be
biased on hard. Very hard. At least ten times harder than a BJT. And
then, all you get is a linearity which is approximately similar to a
BJT.
Trevor, one of us has a sign reversed. According to the stuff Francois
brought forth, a MOSFET has a negative temperature coefficient if the
gate-to-drain voltage is below a certain level, ie., which would tend to
imply that if the bias is below a certain level, a negative temperature
coefficient exists. Above some level, be it bias or signal, a positive
coefficient exists. See
http://www.irf.com/technical-info/guide/device.html, where the
coefficient refers to the voltage drop. This means to me that for bias
below a certain level, the thermal effect would tend to do the reverse of
compression; the error would tend to magnify transients. For bias above a
certain level, the temperature coefficient goes positive, which means
that the thermal effect, if it is significant, does indeed compress the
signal.
**I assumed we were discussing the Hitachi MOSFETs. They suffer the
negative tempco of Gm more severely than the IRF devices.
I didn't know there was a difference, but I'll look.
So compression may occur due to thermal effects, but it seems to me from
the above that one cannot draw the conclusion that high bias = good, low
bias = bad. In general, bias works by the equivalent of Taylor expansion
around a nonlinear function; the expansion is locally linear around the
point established by the bias current. The size of the linear region
around the expansion point is a neighborhood that is small in comparison
to the local rate of change of the function. High bias makes the
expansion locally linear because the scale factor of the expansion is
determined by the bias current.
Physically, the reasoning is like this: High bias makes the junction
hot, and the hot junction loses heat proportional to the temperature
differential divided by the thermal resistance. The hotter the bias
current makes the junction, the less significant the variable heating
caused by the signal.
**I would agree with that.
The above is in support of your opinion, but with somewhat different
reasoning. At the same time, I maintain that there are enough successful
MOSFET designs out there to indicate that even if the linearity is
nonlinear in nature, feedback can make it work.
**Really? Name one, really good, low bias MOSFET amp. Not a second rate
amp, but a really good one. One which has received universally excellent
praise. The only MOSFET amps I know which have garnered really good
reception are high bias models.
I've never heard a low bias bipolar amp that sounded excellent to me.
Therefore, there are none that received universally excellent praise
But
seriously, there is too much individual preference to make the question
valid. At best, an amplifier can have a following. Take a look at this page:
http://audioreview.com/mfr/hafler/am...6_1583CRX.ASPX Notice how
many users find qualities in these amplifiers, even the traditional "DH-"
designs, that apparently outweigh your sensitivity to their particular
artifacts. Pay particular attention to the DH-500. Considering how
obnoxiously noisy the fan of a DH-500 is, the sound must have quite an
allure to those devoted to it. That's how I feel about my XL-600(s), anyway.