[[email protected]]

Stan White designed an amplifier with power rather than voltage
feedback, called "Powrtron", sometime in the 1950s. It appears in the
"Audio Anthology" books chronicling audio development and reprinted by
OCSL/The Audio Amateur. Mr White seems to be still going at it, to
judge from a website.

But is power feedback compatible with crossover networks in modern
speakers? Power feedback seems similar to the variable damping feature
which was offered in the late 1950s as an option on several
amplifiers.

That feature seems to have died out. Apparently variable damping made
a mess out of the earliest speakers designed using Thiele-Small
parameters, which assume a perfect voltage source.

Has anyone any differing or additional commentary? It seems like an
interesting detour interms of technical history.

[Chris Hornbeck]

On Tue, 10 Jun 2008 15:41:18 -0700 (PDT), wrote:

Stan White designed an amplifier with power rather than voltage
feedback, called "Powrtron", sometime in the 1950s. It appears in the
"Audio Anthology" books chronicling audio development and reprinted by
OCSL/The Audio Amateur. Mr White seems to be still going at it, to
judge from a website.

Is Stan White still alive? The website is pretty strange, but
could be from someone invoking his name. Stranger things have
happened. He was a geezer when I was coming up, so he'd be
about a million now.


But is power feedback compatible with crossover networks in modern
speakers? Power feedback seems similar to the variable damping feature
which was offered in the late 1950s as an option on several
amplifiers.

That feature seems to have died out. Apparently variable damping made
a mess out of the earliest speakers designed using Thiele-Small
parameters, which assume a perfect voltage source.

Has anyone any differing or additional commentary? It seems like an
interesting detour interms of technical history.

Modern audio design is for the lowest possible source impedances.

Anything else suffers from "specificities", the lack of being
general and applicable to all circumstances. In the 1950's
everything was Wild West and pretty much hand-built to the
job, so some cuttin'-'n-tryin' wasn't so fatal a flaw.

Today, it's uniformly a liability (and was back then too, but
sometimes other issues dominated) but is interesting for its
historical perspective. The technologically dark days before
T/S and Linkwitz/D'Appolito thinking seem as far away as
Galileo. How soon we forget. Arf!


Much thanks, as always,

Chris Hornbeck
"True, but only for large values of zero."
-Mike Rivers

[Serge Auckland[_2_]]

"Chris Hornbeck" wrote in message
...
On Tue, 10 Jun 2008 15:41:18 -0700 (PDT), wrote:

Stan White designed an amplifier with power rather than voltage
feedback, called "Powrtron", sometime in the 1950s. It appears in the
"Audio Anthology" books chronicling audio development and reprinted by
OCSL/The Audio Amateur. Mr White seems to be still going at it, to
judge from a website.

Is Stan White still alive? The website is pretty strange, but
could be from someone invoking his name. Stranger things have
happened. He was a geezer when I was coming up, so he'd be
about a million now.


But is power feedback compatible with crossover networks in modern
speakers? Power feedback seems similar to the variable damping feature
which was offered in the late 1950s as an option on several
amplifiers.

That feature seems to have died out. Apparently variable damping made
a mess out of the earliest speakers designed using Thiele-Small
parameters, which assume a perfect voltage source.

Has anyone any differing or additional commentary? It seems like an
interesting detour interms of technical history.

Modern audio design is for the lowest possible source impedances.

Anything else suffers from "specificities", the lack of being
general and applicable to all circumstances. In the 1950's
everything was Wild West and pretty much hand-built to the
job, so some cuttin'-'n-tryin' wasn't so fatal a flaw.

Today, it's uniformly a liability (and was back then too, but
sometimes other issues dominated) but is interesting for its
historical perspective. The technologically dark days before
T/S and Linkwitz/D'Appolito thinking seem as far away as
Galileo. How soon we forget. Arf!


Much thanks, as always,

Chris Hornbeck
"True, but only for large values of zero."
-Mike Rivers

I don't know about Power Feedback, but some amplifiers in the '50s/ early
60s had adjustable positive feedback as well as by-then conventional
negative feedback. Pye's professional monitoring amplifiers had this in
particular.

The idea is to create a negative output impedance (the voltage rises with
increasing load, rather than drops as is usual)
to offset the highish output impedance of valve amplifiers. I incorporated
this into a valve amplifier I designed in the '70s, and it worked a treat. I
could adjust the output impedance from a few ohms positive to negative. If
the sense point was brought out to a terminal, and a sense wire was added to
the 'speaker cables, you could ensure a zero output impedance actually
across your loudspeakers after a length of 'speaker cable.

I had no problems with the positive/negative feedback system, and I wonder
why it wasn't more common, possibly because it needed to be adjusted for
each amplifier and so add to manufacturing cost.

S.

S.
--
http://audiopages.googlepages.com

[Chris Hornbeck]

On Wed, 11 Jun 2008 10:31:27 +0100, "Serge Auckland"
wrote:

I don't know about Power Feedback, but some amplifiers in the '50s/ early
60s had adjustable positive feedback as well as by-then conventional
negative feedback. Pye's professional monitoring amplifiers had this in
particular.

The idea is to create a negative output impedance (the voltage rises with
increasing load, rather than drops as is usual)
to offset the highish output impedance of valve amplifiers. I incorporated
this into a valve amplifier I designed in the '70s, and it worked a treat. I
could adjust the output impedance from a few ohms positive to negative. If
the sense point was brought out to a terminal, and a sense wire was added to
the 'speaker cables, you could ensure a zero output impedance actually
across your loudspeakers after a length of 'speaker cable.

I had no problems with the positive/negative feedback system, and I wonder
why it wasn't more common, possibly because it needed to be adjusted for
each amplifier and so add to manufacturing cost.

Maybe a better description of the "Powertron" and such was that
output current was sensed in addition to output voltage. Some
magic elixir was then dialed in.

It seems strange to us, a generation after elegant algebraic
models of loudspeaker response were made, but in those Dark
Ages, before modern analogies to LC filters were available,
an awful lot of *literal* cut-'n-tryin' was the order of the
day.

My favorite example from that era is the Klipsch model "Cornwall",
surely dated now, but still loved in some circles. Paul designed
the ported box entirely by cutting wood, assembling a box, and
measuring in the best way available at the time (and this was before
they had their bigger anechoic room) and iterating.

Decades later, in the 1970's, when better instrumentation was
possible, the cut-'n-try final version could be measured to be
a durned-near-enough-to a QB3 alignment.



These archaic things seem foolish, like wondering how gravity
works, until we remember that *we* don't know how gravity
works either. Life's always a work in progress. Science is
the same thing, except without the verneer of belief.

Much thanks, as always,

Chris Hornbeck
"True, but only for large values of zero."
-Mike Rivers

[[email protected]]

On Jun 12, 10:26 pm, Chris Hornbeck
wrote:
On Wed, 11 Jun 2008 10:31:27 +0100, "Serge Auckland"



wrote:
I don't know about Power Feedback, but some amplifiers in the '50s/ early
60s had adjustable positive feedback as well as by-then conventional
negative feedback. Pye's professional monitoring amplifiers had this in
particular.

The idea is to create a negative output impedance (the voltage rises with
increasing load, rather than drops as is usual)
to offset the highish output impedance of valve amplifiers. I incorporated
this into a valve amplifier I designed in the '70s, and it worked a treat. I
could adjust the output impedance from a few ohms positive to negative. If
the sense point was brought out to a terminal, and a sense wire was added to
the 'speaker cables, you could ensure a zero output impedance actually
across your loudspeakers after a length of 'speaker cable.

I had no problems with the positive/negative feedback system, and I wonder
why it wasn't more common, possibly because it needed to be adjusted for
each amplifier and so add to manufacturing cost.

Maybe a better description of the "Powertron" and such was that
output current was sensed in addition to output voltage. Some
magic elixir was then dialed in.

Schematics are readily available.

Many classic speakers were iteratively designed and perform as
intended with amplifiers of lower damping factor. In fact many were
designed to be driven from local transformers of a 70 or 100 volt
constant voltage line.

[Eeyore]

wrote:

Stan White designed an amplifier with power rather than voltage
feedback,

Sounds unlikely and can't see the point anyway.

Graham