Mr. Arny may enjoy this:
- From: suckerton2@xxxxxx
- Date: Thu, 5 Jun 2008 17:43:19 -0700 (PDT)
DYNACO
THE ULTRA-LINEAR
AUDIO TUBE AMPLIFIER
Copyright © 2000 - 2004 Stanley Fay White
FORWARD
It is with reluctance that I take up a pen to critique this sturdy
veteran of the early years of High Fidelity. This amplifier in kit
form outsold everything else in the marketplace for a number of years.
This amp still has many fans. I myself am a reluctant fan of the
original. Though possessing many engineering defects, the Mark II and
III were the best dollar valued amplifiers available at the time,
That was then, this is now. We are in a "re-visiting mode" of tube
amplifiers (in the new century) .by the audiophile industry. There is
no attempt here to put down the vintage models for what they were in
their time. This is rather a critique to show why the ancient circuits
are inferior to modern day technology and are no longer competitive.
I have great respect for the Model T Ford (an example of old
technology). Most people don't know it, but the Model T (and some
versions of the Model A) had automatic transmissions! There were other
features that made the Model T the most popular car of its era. The
Model T was produced relatively unchanged over a period in excess of
16 years. In that period, most of the bugs were worked out. In its
final year of production, the Model T was available in Detroit for
$270.00. I wouldn't buy a Model T to drive today--technology has
passed it by. The same can be said of the Dynaco, the kit that had a
run of many years. So let us examine why the Dynaco was so popular in
its day and what if anything of its design remains relevant today.
OVERVIEW
The Dynaco amplifiers (and pre-amp) were born in the ferment of an
audio revolution. The phonographs of pre-war (World War II) America
were outdated by 1945 and woefully inadequate. The public was asking
for and wanted more. Led by a devout group of amateur radio buffs, and
a group of audio buffs, (they grew in to what became known as
"audiophiles") these groups powered the drive for the upgrade of home
sound systems. In the early days, there were no audio stores or even
audio departments as such. All audio equipment was purchased from
radio parts distributors, such as Concord Radio, Radio Shack, Newark
Electric and Allied Radio. These distributors in turn resold to local
radio repair shops and ham radio gear outlets, whilst also selling
equipment off the floor themselves. This was the early milieu. It was
very disorganized.
Engineering? Most of the audio equipment designers of the era were
retread radio engineers. The criteria for these radio retreads were:
if it produced intelligible sound, it was acceptable. This was
temporarily acceptable by the public for the speakers the radio
amplifier people designed for were no better. Into this disorganized
mess, Williamson offered am upgrade set of standards for an audio
amplifier that are still used by many. Williamson, then a young fellow
(working for a receiving tube company) put forth the idea that in
order for an audio amplifier to produce quality sound, it had to have
a response beyond the used audio range. (This was required to control
the sum and difference frequencies produced in the audio spectrum.)
Williamson proposed using a transformer such as the 36 section
Partridge that had a frequency response from 10 Hz. to 100 kHz.
Williamson was in the tube business. He designed an amplifier that
used a lot of tubes and didn't produce much power (10 watts max).
Though most of the Williamson parts were difficult to obtain from
local feather merchants, many audio enthusiasts somehow managed to
build this Williamson circuit from the Williamson schematic. (An
analysis of the Williamson is contained in other articles) . The
Williamson sparked a vigorous debate that was aired in the then
current audio magazines (such as Audio Engineering). The relative
advantages between pentodes and triode front ends were hotly debated,
as were other numerous other Williamson parameters. The relative
merits of triode output Vs pentode output were disputed vehemently.
The use of power supply oil filled capacitors was considered quaint.
They were woefully deficient in filtering capability. Around this
time, McIntosh introduced his class B amplifier, adding another
dimension to the discussion--to B or class A was the question. There
were numerous marginal designs that appeared around this time that can
be reviewed from old audio magazines of the time. This was the stage
upon which the DYNACO and POWRTRON amplifiers entered around 1952 AD
AUDIO - THE NEXT GENERATION
Williamson pretty much settled the question of triode versus pentode
for front end use. Pentodes were a product of the radio design era.
The criteria for a good radio design of the 1930"s was high gain. The
goal was "reception". Radio enthusiasts would stay up all night
searching for static filled voices of radio stations as far distant as
possible. The "gain" in the radio circuit determined how far away a
station could be and still be picked up. Radio speakers had so much
distortion that the distortion of the electronics was unimportant. In
the early years, alnico had not been invented. The flux available in
the speaker voice coil gap was pitiful. When loudspeakers improved
with the arrival of alnico and the source material (LP records, FM
etc. appeared), suddenly the quality of the audio amplifier became a
factor in the design of a Hi Fi system. The BROOK (all triode)
amplifier enjoyed a good sales record for a while, but it only put out
10 watts max and didn't satisfy audio buffs for very long.
There is an aside to all of this. In the 1930's recording engineers
decided that response above about 800 Hz. was meaningless. The reason
for this was that record needle noise combined with high distortion in
speakers above 800 Hz. combined to make high frequency response a
liability. When Bill Putnam (Universal Recording Studio) produced "Peg
o' my heart" with the Harmonicats and an echo chamber in 1947, the
flaw in the thinking re high frequency response became evident. Audio
has not been the same since.
The availability of the McIntosh amplifier (50 watts) pushed the
industry to come out with higher-powered amplifiers. The Hafler and
Keroes article in Audio Magazine, pointed the way to higher power and
lower distortion. The "Ultra-Linear" circuit was not well understood
by the general public, but they liked the sound of constant power
output, it improved the transient response of an audio system. The
Peerless output transformer was an immediate success. A brief
digression here might be useful.
In tube audio amplifiers (running class A to AB) there are three
general modes of operation. Triode tubes like the 2A3, 807 (triode
connected) or similar are VOLTAGE amplifiers. The impressed voltage
signal on the output tubes is voltage amplified by the output tubes.
The voltage is more or less linear, but the current varies according
to load. This mode of operation is also inefficient; ten-watt triode
amplifiers are the norm.
Beam power tubes, (6L6, 5881, EL34, KT88) etc. are CURRENT amplifiers.
The impressed voltage signal on the output tubes is current amplified
by the "output " tubes. The tubes tend to keep the load current
constant, while the voltage across the load varies as the resistance
of the load varies. Hafler and Keroes designed an output transformer
by which beam power tubes produced CONSTANT POWER out, when fed by a
voltage signal. This was accomplished by a judicious tap on the plate
transformer, which fed into the screens of the output tubes. The
resultant load line produced constant power out (within limits). It is
a bit whimsical that a VOLTAGE feedback loop was impressed around this
constant power mode of operation, reducing the effect of the ultra
linear configuration. The Dyna amplifiers so confused the operation of
the output tubes, that the ultra linear concept was not fully
realized. The Dyna amplifiers never delivered constant power to the
speaker load.
BACKGROUND OF DYNA DESIGN
Stan White brought out his POWRTRON amplifier (a constant power
design) in 1952 using the Hafler-Keroes Peerless transformer in the
output. The POWRTRON amplifier is described in other articles. Hafler-
Keroes knew of the POWRTRON design when it came out. A short time
later, Hafler (under the name of Dynaco) produced the Mark II
amplifier (mainly in kit form). An examination of this design shows
that it is a watered down version of the POWRTRON. The distortion was
compromised as well.
Though pentode tube front ends had been shown to be inferior, Hafler
designed them in his amplifier anyway as a compromise in getting
enough gain to run the amplifier with two stages of front-end
amplification. He also reduced the power supply filtration by using
only one choke (and this of vast inferior design). The power
regulation suffered, causing distortion in transient power response.
The phase splitter was used simultaneously as an output tube driver.
The difference in impedance between the two halves is significant.
This presented a design problem because of the miller effect in the
output tubes. Due to the difference in impedance between the drivers,
the high frequency response was seriously affected above 10 kHz. due
to the amplitude imbalance between the halves of the output stage.
This sort of problem is amplified when negative voltage feedback is
put around the entire amplifier. The difference in drive amplitude
between the two sides is further exacerbated with the insertion of
feedback around the whole amplifier.
The DC drift was minimal because there was no DC amplification in the
circuit. As there were only one set of coupling capacitors in the
circuit, it was DC stable (copied from the POWRTRON). The power
transformer was quite adequate, as was the output transformer (a D.
Hafler design). The amplifier was quite compact, and as the number of
parts used was frugal, the maintenance problems on the amplifier were
minimal. The life performance of this amplifier was well beyond the
norm for that period.
The Dyna had two interesting features. It had provision to balance the
tube current in the output stage, and the use of grounded cathodes on
the output stage was state of the art.
Summarizing, this was a good mine run amplifier at the time at minimum
cost. The efficacy of the design was proven by the widespread
acceptance of the amplifier among audiophiles.
That was Then
This is now. Transformer design has advanced considerable since 1952.
The 60 kHz. top end response of the Dyna output transformer is
outdated. Numerous output transformers are now available that have a
top end response of 300 kHz. This increased bandwidth is important
because of sum and difference frequencies generated during
amplification. These unwanted frequencies need to be controlled for
good performance. The low end of the transformer response has also
been improved to 10 Hz. creating a better low-end response in the
amplifier.
No quality front end uses pentodes any more. They generate too much
distortion. The modern phase splitting system mimics transistor
design. The phase splitter system is balanced impedancewise and
amplitudewise. Balanced drive to the output tubes (over a wide
frequency range) is important. Power supply regulation is also more
important. Transient response has become recognized as being very
important. Tubes (with a properly regulated supply) have a superior
transient response to transistors. This is one reason the younger set
is choosing tubes over transistors today. The original front-end
response was comparable to the response of the output transformer.
Inasmuch as the transformer performance has increased, the front-end
response has also had to improve to keep pace with the quality of the
output transformer. The Dyna front end is woefully inadequate. Some
are even resorting to the Williamson front end as a compromise, but
the Williamson front end no longer meets the modern criteria.
Heat is still a problem with tubes. This must be designed for.
Excessive heat causes fires. The Dyna used a steel chassis. This is
still superior to aluminum for shielding.
An amplifier built to Dyna specs. costs as much as those built to
modern specs. The original Dyna advantage of cost is no longer a
factor. A selenium stack for the bias voltage is no longer good
design. Selenium ages and the voltage across the stack will change
with age, causing an unwanted change in the bias voltage on the output
tubes. The remaining design factor of the Dyna, the ultra linear
output transformer is still viable. This design, when incorporated
with a suitable feedback system drives the speakers with constant
power and increased transient response.
A final point is of interest. EVEN the original Dyna, with all its
faults sounds better than most transistor amplifiers. The kids know
this. This is why tubes are coming back in style for good audio
reproduction for the home. To those who are contemplating buying a
tube amplifier system, be careful to properly ventilate the tubes.
They get hot!
http://www.stan-white.com/a_doc_dyna_1.htm
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