"Audio Empire" wrote in message
On Mon, 28 Mar 2011 06:13:29 -0700, Arny Krueger wrote
(in article ):
"Audio Empire" wrote in
message
You mean Like the Dynaco ST-120 running hard into class
'B' with it's VISIBLE crossover notch?
I have an all-orgional Dyna ST 120 and have tried all
sort of schemes to measure or hear any such thing.
AFAIK, this is an audiophile myth. It may have happened
in equipment that needed maintenance, but it was not a
standard feature of properly-operating equipment.
We've been down this road before.
The facts you present actually support my experiences.
Dyna eventually fixed the problem and it one point, they even offered a
kit of
parts for doing so. I even posted a list of those parts
for you.
No need since that information has been on the internet for a long time.
If you have a later ST-120, they don't exhibit
the problem,
You ought to be clear about this before you make those global statements.
if you have one made before they finally fixed it, they did.
I've tried to get one of those in my possesion, and have failed. IME they
are unobtanium.
Or the early McIntosh SS deigns that used coupling
transformers
A comprehensive archive of McIntosh schematics and
service manuals can be found he
http://www.tubebooks.org/mcintosh_data.htm I find no
McIntosh SS amps with coupling transformers. Perhaps you
can find some?
They were sold under the name "Mac" to differentiate
between the tube (McIntosh) and the transistor (Mac)
gear. They abandoned the "Mac" name in the early
Seventies,
Irrelevant. There's plenty of schematics of McIntosh SS amps at the cited
location, and none of them have coupling transformers.
Driver transformers were widely used in the early days
of the evolution of SS power amps, with surprisingly
good results. Manufacturers that used them included
Acoustic Research, Heath, Altec Lansing, etc., etc.
These parts were called on to handle relatively small
amounts of power and therefore were easily overdesigned
and overbuilt. They overcame the expense and relatively
rarely of complementary driver and output devices. They
were eliminated as a cost-saving move when appropriate
(complementary - similar transistors that were available
as both NPN and PNP parts) became widely available at
low cost.
Dynaco (ST-120, ST-80) and H-K (Citation 12) used
complementary drivers to drive their output transistors.
That became the standard way to do things, once complmentary drivers became
cheap and readily avaialble. The only signficant change since then has been
the use of true complementary output devices instead of compound pairs where
only the low-powered devices were complementary.
It worked fine, but when one 2N3055 "went" it usually
took the two driver transistors with it (and often the
other 2N3055, as well).
If one fixes a lot of blown output stages one finds that this pattern
continues to this day. If you blow an output device, it often takes the
direct-coupled driver with it. Some output devices seem to have built-in
fuses, which fail in open circuits instead of shorts. This tends to make
failures less catastrophic.
The Crown SS power amps had conservatively rated SOA
protection circuits that contributed to their
longetivity. As long as you stayed clear of highly
reactive speaker loads, they sounded fine.
Matter of opinion. I never thought that the Crown D150 or
the D300 sounded "fine".
It seems like one has to do careful DBTs with the relevant equipment to
dispel many of these audiophile myths.
Something you couldn't say of the early Dynacos or the
Harman-Kardon Citation 12, or any other 40-60
Watt/channel amps using 2N3055 output devices...).
The Citation 12 did not use 2N3055 output devices. Its
output devices were RCA 40636's which were similar, but
then so were very many other silicon power transistors
of the day.
The kit I built certainly used 2N3055s,
Last-minute parts substitutions at the factory are possibe 40636s are
pin-compatible with 2N3055s. In fact these devices were highly variable
when produced and selected by testing on the production line. The specified
tolerances were broad enough that type number substitutions, or remarking of
devices was often an option.
The Dyna 120 was originally shipped with 2N3055 output
devices but they were quickly upgraded by Dyna to 2N3772
types which were an uprated device. My Dyna 120 appears
to have been factory built and shows no signs of parts
replacements or other maintenance. It came with 2N3772
output devices.
And was a later model that did NOT have the heavy class
'B' biasing, and thus no crossover notch.
Except that there was no offical later model, just the simple fact that the
parts in the boxes changed from time to time.
Every one that I ever looked at had the crossover notch, It's easy to
see on the oscilloscope with a sine wave test tone. By
the time Dynaco "fixed" the ST-120, most of the
audiophiles that I knew (including myself) had moved-on.
The very early Dyna's that were tested by Audio magazine and High Fidelity
magazine in 1966-67 lacked these alleged obvious faults.
IIRC, it was Bob Orban who showed me how to fix mine. He
came up with a re-biasing scheme which was similar to
Dyna's later fix (he had a ST-120 as well).
The offical Dyna mod is called "The TIP mod" probably because it involved
upgrading some transistors with parts whose numbers started out "TIP" (for
TI Plastic).
The problem
with the 2N3055s that Dyna used was that you couldn't
just replace them with off-the-shelf replacement parts.
Dyna selected the 2N3055s for V-sub-BE (I believe) and
you had to buy your replacements from the factory (they
weren't even marked as 2N3055s). I got tired of the
goddamn thing blowing first one channel and then the
other, so I moved on to a used Citation two, (which was
pretty bulletproof) and I liked the sound better than
that of the ST-120, as well.
I stongly suspect that many of these alleged audible differences would
disappear were proper DBT listening producedures actually be used. There's
an interesting exchange on Audio Asyluym where someone started scouting up
the details of the TIP mod to address audible distortion, but the person
asking the questions was able to resolve the problem by tightening the
speaker cables.