"Trevor Wilson" wrote in message
...

"Robert Morein" wrote in message
...

"Trevor Wilson" wrote in message
...

"Robert Morein" wrote in message
...

"Trevor Wilson" wrote in message
...

"Robert Morein" wrote in message
...

"Trevor Wilson" wrote in message
...

"Robert Morein" wrote in message
...

"Trevor Wilson" wrote in message
...

"Robert Morein" wrote in message
...

"Trevor Wilson" wrote in
message
...

"Robert Morein" wrote in message
...

wrote in message
ups.com...
I just bought a clean Macintosh MA 6100 integrated amp at a
bricks and
motar auction. I haven't done anything with the 6100 other
than
plugging it in (all the lights work and it didn't blow a
breaker) and
it will be a few days before I can put it in my system and
make an
evaluation. I currently have a Hafler 220 amp and Hafler 110
preamp. I
know that the Mac was built to the highest standards of the
mid 70s,
but even my Hafler, a modest piece of equipment, has poly
caps, metal
resistors, better pots etc. If I like the way the Mac sounds,
would it
be worthwhile or even desirable to upgrade individual
components? My
system is two channel, music only and I listen to FM, CDs and
records.

Don't assume that the Mac can be made to sound better. The
Hafler has newer semiconductors. Rapid progress was made in
those years.

**Not so much. The Hafler uses MSOFETs. Rather poorly
implemented.

I disagree. The Hafler can sound pretty good, matched with bright
speakers.

**Nope. The Hafler has the typical compressed sound quality of all
MOSFET amps of it's time. The XL280 was better.


The Mac,
however, uses BJTs, but is crippled in other ways. Toss of the
coin time.

I'd agree with that.

They're both pretty ordinary amps. A late model Phase Linear
would be a much better (and cheaper) choice.

The Phase Linears are nicknamed "Flame Linear". Never have so
many amplifiers of one brand blown up or caught on fire so
regularly. They cannot be recommended for any use.

**I did say: LATE MODEL Phase Linears. Early model PLs had
stability problems, if retro-fitted with fast(er) output devices.
They went up in smoke. Late model PLs did not suffer this problem.
In any case, there are better choices, but I was trying to make a
point about how bad both amps mentioned actually are.

What have you got against the DH-220 ? It sounds soft, perhaps not
to your taste, but "bad" ?

**It compresses the sound, like all MOSFET amps of it's time.
Compression is distortion. Distortion is bad.

I don't know of any quantifiable distortion, except for that which
occurs as the output voltage approaches the rails. That's pretty high
up.

**You said it yourself: "It sounds soft, perhaps not to your taste,
but "bad" ?" The "softness" you notice, is compression. Halfers
possess an exemplary frequency response, so that is not an issue. It
is compression distortion which is the problem. It can be heard on
Perreaux amps, Haflers and a whole host of others of the time.

It compresses as the voltage reaches the rails.

**Perhaps, but that is not what makes it "soft" throughout the rest of
the range.

With respect to compression, I have to dispute.

**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. Your conjecture might be right, but
there's so much of that in this backwater field. In order to explain why a
classic MOSFET design can reproduce high amplitude sine waves but not
transients, if in fact that's what happening, some more work has to be done.
A good explanation doesn't mix the various possibilities together; it
separates them out. Nelson Pass says, "
The Mosfet designs on the market are also Class AB designs. The transfer
curve of Mosfets reveals serious nonlinearities at low bias

currents, resulting in crossover nonlinearity in push-pull designs. This
design flaw makes for a sonic signature that many have referred to as

"Mosfet mist", where a loss of detail is apparent."

His explanation is different from yours; it is unlikely that both
explanations would contribute in approximately equal proportions. And he
isn't necessarily right. Here's my own personal anecdote.

The Hafler XL-280 is an improved version of the DH-200/220, but it looks
very similar. It has six output devices/channel; the bias current is the
same 100ma/device, it has JFET inputs, but it looks pretty much the same --
except that there is a small air plate varicap in the output. This is/was to
be used with a comparator bridge lent by the dealer to null the difference
between the amp input and output. I've played with it, although recently,
I've left it untouched. The amp is flat far above the audible range, with a
slewing rate of 75 v/us, but it is said to have a peak at 170 kHz, adjusted
by the cap. This amplifier does not have the liquid mist of a DH-200; at
least one review, perhaps Audio, stated that it had the greatest amount of
low level detail of any amp they had heard. Nevertheless, the amplifier does
sound a little flat, or undynamic.

But when the amps are bridged, the sound changes completely. I use a pair
with the NEAR 50m's. In bridged mode, they sound like a very, very good
amplifier. What happens when an amplifier is bridged? Does the damping
factor halve? The XL-280 has a larger cousin, the XL-600, with a similar
circuit, the same air varicap, and 8 devices/channel, run at 100ma/device.
It has a damping factor of 80, approximately half that of the XL-280. The
XL-600 has enjoyed considerable popularity in U.S. studios, as a very good
all-around amp. It sounds indistingushable from the bridged XL-280's. I have
both in my rack, but I use the 280's in the winter, because they have no
fan.

I wen through the above to show that the MOSFET mist, which still afflicts
certain modern designs, such as the ADCOM pieces, can apparently be
diminished or eliminated to the point where the result is considered highly
desirable, at least by some ears, while still maintaining the traditional
zero-voltage gain output stage.


The distorted
sine wave would be decomposable into a Fourier series with higher powers.
The extreme of this form of compression results in something
approximating a square wave. Please explain how an amplifier can compress
while having excellent harmonic distortion.

**Er, because it can.

Did I see your hands wave? Please say that again with your hands in your
pockets


Because of gate capacitance and on resistance, MOSFETS do compress,

**Their compression, I believe is more to do with the negative Tempco of
Gm.

but in
order to meet rated distortion at rated power, they do this at power
levels above the rated power. This is not an egregious flaw; the amp
lacks headroom in a manner similar to Class A amplifiers. But the classic
MOSFET design also provides a benefit similar to pure Class A bipolar
amps; extremely low distortion at low power.

**Only because it uses massive amounts of Global NFB. Due to the high
levels of distortion at low currents, MOSFETs need to be biased on real
hard, or be used with huge amounts of NFB (usually Global). Except for
very high bias designs, there are no (to the best of my knowledge) Zero
Global NFB MOSFET amps.


The result is an ideal
clipping characteristic.

**Sort of.

I'll admit it has a characteristic sound, but imho,
you come down hard on what some of us consider a very useful
characteristic. I can drive one of the classic Hafler amps with a
source that has a very high peak-to-crest ratio, such as certain piano
music. At high volumes, any other amp exhibits some audible clipping.

**Nope. Haflers are no different to many other amps, WRT clipping. Clip
them hard and they sound harsh and nasty. Possibly less so than many
amps, but nasty, nevertheless. BTW: There are many other schemes which
exist to keep an amp (BJT) from exhibint unpleasant clipping
characteristics. Most involve sacrificing a few Volts of headroom. NAD
is one such promoter of "soft clipping" type systems. There are others.
They cost a little more, but can provide demonstable benefits. The best
systems keep the anti-clipping system outside the feedback loop.

The Hafler, never.

I have a NAD PE receiver, and my perception is that it does not clip as
cleanly as a Hafler.

**Which model?

7225PE

ALL of the PE NADs are utter horrors. They clip badly. Soft
clipping notwithstanding. In any case, I used the NAD as an exmaple. The
NAD scheme is primitive and not all that good, but it does work. NADs clip
more gracefully than most of their competition.

When driving Acoustat panels with a DH-200 or XL-280, a
deliberate twist of the volume knob full clockwise (which the Acoustats
can tolerate, they're rated at 117db), produces no audible clipping. The
amplifier has to clip, but does not manifest a typical clipping
signature. In mentioning Hafler, I specifically mean only those based on
the original Hafler design: the DH and XL models. When Rockford bought
Hafler, and then Acoustat, they eventually replaced the original Hafler
design completely with the Transnova circuitry. As much as I like them,
these amplifiers do clip audibly, because they do drive the MOSFETs
straight to the rails.

**They use a higher Voltage rail for the drivers?

The rail voltage in the Transnova is 72V. However, because the outputs have
voltage gain, the drivers are operated from a regulated 25V rail.