"Arny Krueger" wrote in message
...
"Bob Morein" wrote in message


"Chad Williams" wrote in message
om...

In researching solid state integrated amps/receivers I've come across
several proponents of old receivers, circa '75-early 80's, who say
that these solid state systems are every bit as good as anything
being made now.

Certainly by the mid-late 1970s and early 1980s really good-sounding SS
power amplifiers had started to really proliferate.

To your ears, perhaps. I put it early eighties.
While the Phase Linear amplifiers had good sound, they couldn't manage
thermal runaway.


While I don't disbelive this statement, I'd like to
understand why this is the case. Was the build quality simply better
back then? Are the transformers higher quality???

[snip]
However, in terms of quality of the amplifier, it is definitely
false, for several reasons:

1. Designs in the 70's suffered from transient intermodulation
distortion. Around 1979, this was discovered and eliminated.

TIM is just excess nonlinear distortion at high frequencies.

No, it's not. The definition includes the word transient. It is
distinguished from steady-state TIM in that it is measured when the
amplifier is not in steady state. Read he
http://www.zero-distortion.com/techn...cations_04.htm

In the early
days of SS amps there was a tendency to give power ratings for power amps
that were too close to the actual capabilities of the equipment. Most SS
amps have output decoupling networks that improve amplifier stability, but
also cause losses that decrease the maximum undistorted power output of
the
amp at 20 KHz by 0.5 to 1 dB.

This is obfuscation. The cause of TIM is excess loop gain at high
frequencies, and there is a specific remedy.

[snip]
The TIM myth was
effectively deconstructed at that time.
I disagree.



2. Bipolar transistors suffer from "thermal runaway", which occurs
when a small area of the junction heats up locally and becomes more
active than the rest of the transistor. Once it starts, the
transistor is quickly destroyed.

Again this is false. Thermal runaway is a large-scale effect that involves
the entire transistor junction. It has been known and managed about as
long
as there have been bipolar transistors, or more than 50 years.

Permanent damage of transistors due to small-scale localized heating is
instead known as "Secondary Breakdown".



IMPORTANT***IMPORTANT***IMPORTANT:

Arny, in a BJT, THERMAL RUNAWAY AND SECONDARY BREAKDOWN ARE IDENTICAL.
Secondary breakdown is merely the physical phenomena which causes loss of
current control.
In this case, you are confusing the same phenomena viewed differently.
"Secondary breakdown" is the term used by a device physicist.
"Thermal runaway" is the term used from the systems & control point of view.


The only solution available in the 1970's was brick-wall current
limiting.

Again this is false. In the 1960s and to this day, SOA protection circuits
were widely used, but these circuits monitored both the voltage being
dropped across the output devices and the current flowing through them. If
one finds audio transistor design manuals from the 1960s as well as modern
manuals, SOA protection circuits that are controlled by both voltage and
current are described.

In my experience, I have not found an amplifier made in the 60's or 70's
which did anything other than brick-wall limiting.
I'd be interested in some examples.


However, amplifiers which use
this kind of protection cannot handle the dynamic range of a CD at
greater than low volume.

This is also false. SOA limiting parameters are determined by the SOA
limits
of the amplifier's output devices.

Based upon my experience with Marantz and Heathkit, I disagree. But as I've
said, I'd be interested in some counterexamples. Crown, perhaps?


SOA circuits tend to be activated by real-world speaker loads, but are
less
likely to be activated in resistive load testing. Therefore, some power
amps
with relatively high power ratings such as the original Crown DC-300 were
sold that arguably lacked sufficient SOA for handling loudspeakers that
had
the deadly combination of low impedance, high reactance, and low
efficiency
at frequencies where music tends to have a lot of energy.

SOA limiting was often observed while playing LPs, not just CDs as stated
above.

For example the "Some Amplifiers Sound Different" article that I
co-authored and appeared in High Fidelity News and Record review is
basically a story about an expensive (Audio Research) solid state power
amp
that had just been highly reviewed by TAS, but in fact had SOA issues with
certain (Acoustat) speakers. These listening tests were as I recall, based
on playing a LP of the Eagles' "Hotel California" at a pretty high level.
Back the volume off a dB to a more modest but still loud level, and the
problem went away.

3. The noise figure of bipolar transistors dropped about 10 dB around
1980. Prior to that, equipment had an S/N ratio of around 70 dB.
After 1980, S/N ratios of 90 dB and greater became the norm.

This is a meaningless statement because SNR is only meaningful when
referenced to an operating level. If one measures the SNR of amps and
preamps made in the 1960s, 1970s or even 1990s the line level circuits
tend
to have SNRs that are better than tubed equipment and are in the 90+ dB
range.

Apparently, you can't hear hiss.

If one measures their phono inputs the SNRs are more like 70 dB and
up which is logical because the operating voltage levels are lower. There
have not been any difficulties with well-designed transistor amplifiers
being noisy excessively noisy since no later than the late 1950s. So not
only is it a meaningless statement, it's just plain wrong.

4. In 1981, David Hafler was the first to build an audio amplifier
with Hitachi's new "power MOSFET."

Again the date is all wrong. Trivial searching shows that the DH-200 was
introduced in 1979.
I'll give you the date.

However the DH-200 was not the first Hitachi-MOSFET
power amp, just the first popular-priced kit.

This was a major watershed in amplifier design.

While the DH-200 and many successive MOSFET amplifiers are fine
amplifiers,
in fact they don't as a rule sound better than competitive, well-designed
bipolar designs. MOSFETs have long been popular, but have never dominated
the marketplace for high quality power amplifiers. They have their
advantages and their disadvantages...

Concurrently, new methods of protecting bipolar
transistors were implemented.

Previously debunked, and false.
No, true from personal experience.
I have had several amplifiers of that era that clip hard as a result of the
current limiting.


What has happened is that the SOA of SOTA
power transistors and ICs has undergone steady improvement.

True with respect to discrete devices. With respect to ICs, there has been
substantial work in more sophisticated safe area protection, which I
reference below.

Therefore, it's
possible to build an effective high powered amplifier with fewer and
physically smaller output devices. For example I have an upgraded Dyna 400
that equals or exceeds the reactive-load handing capabilities of the stock
Dyna 416 with half as many, but far more modern output devices.

That's a terrible amplifier. Your fundamental problem is a lack of hearing
acuity.

Arny, you're SO full of ****. Here's a reference to a National data sheet:
National Semiconductor's bipolar-output parts (Table 1) incorporate a
dynamic SOA-protection mechanism, called SPiKe, which stands for self-peak
instantaneous Kelvin temperature-protection circuitry. National claims the
circuitry makes the ICs nearly impervious to damage from instantaneous
temperature peaks and overvoltage and overcurrent conditions.
You can read the above at the EDN website:
http://www.e-insite.net/ednmag/archi...1795/17df1.htm
and the datasheet at http://www.national.com/an/AN/AN-898.pdf#page=9
Also of interest is http://www.national.com/an/AN/AN-261.pdf#page=2, which
dates an attack on this problem to 1981.


The Philips Power Division has a relevant document:
http://www.semiconductors.philips.co...es/APPCHP7.pdf
"When a power transistor is subjected to a pulsed load,
higher peak power dissipation is permitted. The materials
in a power transistor have a definite thermal capacity, and
thus the critical junction temperature will not be reached
instantaneously, even when excessive power is being
dissipated in the device. The power dissipation limit may
be extended for intermittent operation. The size of the
extension will depend on the duration of the operation
period (that is, pulse duration) and the frequency with which
operation occurs (that is, duty factor)."

and

"Conclusion
A method has been presented to allow the calculation of
average and peak junction temperatures for a variety of
pulse types. Several worked examples have shown
calculations for various common waveforms. The method
for non rectangular pulses can be applied to any wave
shape, allowing temperature calculations for waveforms
such as exponential and sinusoidal power pulses. For
pulses such as these, care must be taken to ensure that
the calculation gives the peak junction temperature, as it
may not occur at the end of the pulse. In this instance
several calculations must be performed with different"

IBM has a document dated 1977 that refines safe area calculation:
http://domino.watson.ibm.com/tchjr/j...c?OpenDocument

Both Hafler's and Strickland's MOSFET
designs had interesting qualities that raised the bar for bipolar
designers.

Credit needs to be given to Hitachi's engineers who laid out many of the
circuit designs and parameters for building MODFET power amps and provided
them along with the devices. "Name" high end audio engineers generally
don't
innovate much of anything in the way of power amps, they just tune and
repackage circuits that are already widely used and/or suggested by device
manufacturers.

This is bull****. And what the hell is a MODFET ?