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Arny Krueger
 
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Default old solid state circa 70-80's`

"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.

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???


No, the power transformers were arguably worse. There have been some changes
in power transformer design that has led to smaller, lighter power
transformers with greater power handling capacity, or equal-sized
transformers with superior capacity. However, the power transformer is just
one more thing that has to be designed and specified, and a wide range of
options are available, and have been available and exercised for decades.

I notice on ebay that a lot of these "vintage" models aren't even
that cheap. Some sell for well over $100. For another $100-200 you
could have something new, with new technology and a remote. So why
would you buy old?


Sentimentality.

It's not true.
If you're into vintage equipment, and you want that sound, or that
memory, then, of course, it might be worth it to you.


Right, the sentimentality issue.

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. 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 decrease in power output is audibly
insignificant, but can cause a substantial increase in distortion to be
measured if the amp's power rating is too close to the actual maximum
capabilities of the amp. Thus the appearance of what seemed to be TIM showed
up in many power amplifier tests in the late 60s and 70s.

The date information presented above is grossly inaccurate. A check of the
AES database for technical papers shows that Otala's first TIM article was
published in 1972, and that by 1979 Otala was publishing some of the last
papers in which he vainly attempted to defend the audible significance of
his earlier claims. Yes, Otala failed to adequately defend his theories and
only ignorant people place any credence in them.

Note that by 1982 Clark had published his first ABX listening test paper. On
the 1982 AES convention exhibits floor Clark demonstrated ABX listening
tests of a TIM simulator that was originally designed and promoted by a
well-known TIM advocate, I seem to recall it was Walter Jung. These
listening tests, and all successive reliable listening tests ever done since
show conclusively that for TIM to be an audible problem, it has to be far
more severe of a problem than had ever been observed in real-world solid
state amplifiers of even mediocre quality levels. The TIM myth was
effectively deconstructed at that time.

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".

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.

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. Early Germanium devices didn't have much
SOA capacity and failed often. The early mass-market silicon output devices
(The 2N3055 family) had adequate SOA for building power amps in the 20-30
watt power range, but were marginal for building larger power amps unless
used in multiples which is expensive.

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

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.

The result was an informal competition which lead to rapid
advances in amplifier design. This continued up until about 1991.
Since 1991, high end amplification has shown no significant advances,
although variations occur from time to time.


Actually, many ca. 1990 and later power amps include no circuit refinements
that were not well-known in 1980 and possibly 1970 or even 1965. The biggest
tangible changes have involved the power capacity/size/cost of the output
devices, and how the circuits are packaged (ICs versus discrete designs).
The power levels at which IC power amps are effective has continued to
slowly improve and is now arguably in the range of one or more 100's of
watts. This is up from 5 or 10 watts in the early days of audio IC power
amps.

Home theater has had a negative impact on amplifier design.
Nevertheless, there is one company, Pioneer, which makes MOSFET
receivers of heavy construction that are notable for reproduction of
music.


There are some basics in power amps. Their power supplies tend to be large,
expensive and heavy. Power output stages tend to need large heat sinks.
There are ways to minimize these costs, but those means are themselves
costly. Power amps are now essentially commodity items. Such competition and
technical innovation as exists mostly relates to power, size, and weight
considerations.

The key years were 1981-1982. The CD propelled an advance in the
state of the art.