Hi:

Has it occurred to you that these design teams you cite had many conflicting
constraints they had to balance and OA sonic performance was just one of
them and maybe not the most important one. In the case of the AA151, low
cost was clearly a consideration. Surely, having been the owner of one of
these amps for 40 years you're intimately familiar with the strange line
level input design on this amp. For those who aren't, the line level inputs
are padded down by 24 dB at the input sockets and then fed thru the input
selector switch to what amounts to the second stage of the magnetic phono
preamp. I know of no sonic advantage to this scheme but it does result in
simpler (cheaper) input switching. The price is additional noise on the
line level inputs. Then there's the single cathode bias resistor for the
four output tubes. I know of no sonic benefit to this biasing scheme but it
does save the cost of an additional power resistor and cathode bypass cap.
It also just about demands a matched quad of output tubes to prevent one
tube from going ballistic. As you know, the conventional approach would be
to put each pair of output tubes on their own bias resistor. As you also
know, some of the top classic designs such as by Mullard even went as far as
individual cathode bias resistors for each output tube.

By inference, you are a professional engineer and as such, being an amateur,
I'd really appreciate hearing your opinions regarding the sonic advantages
of the line level input scheme and the four into one cathode bias
arrangement for the output tubes.

To me the bottom line here is that today's hobbiest doesn't have to deal
with the cost issues that constrained the original designers. In a
cost-no-object environment I'm certain that the original AA151 design team
would have done things a lot differently in many areas including the
addition of a filter choke or two, a more optimum output stage bias scheme
and definitely a different line level input circuit. Just because their
hands were tied doesn't dictate that ours should be now. If you want to own
and listen to a piece of equipment that would not be out of place in a
museum, that's fine. Others want to try for improved performance and on the
AA151 that's relatively easy to do. And since it's relatively uncollectable
and cheap to obtain, why not?

--
Steve


firedome wrote in message ...
I'd leave this amp alone...it sounds extremely good just as it is, IF
properly restored...I'm constantly amazed at the amateur engineers who
think they know better than the original design teams responsible for
creating many of the best of the vintage amps...I've had one of these
amps for 40 yrs. and it still sounds great...leave it be!
Roger in NY