"Don Pearce" wrote in message
...
On Thu, 4 Oct 2007 18:51:36 +0300, "Iain Churches"
wrote:


"Dave Plowman (News)" wrote in message
...
In article ,
Don Pearce wrote:
In tube power amps, the difference between wide
band and "A" weighted was often found to be 12dB,
and this was, IIRC something of a rule of thumb.

Iain

Such a difference would suggest a great deal of 1/f noise.

Fairly typical of AF valve circuits. Hum.

A common misconception.
With a properly designed PSU C-L-C-R-C and with
attention paid to layout and ground buss, one can build
valve power amps with no hum or thermal noise even EAS
(ear against speaker) And it plays music to please the
most discerning ears.

My 50W PPP tube amp has a noise floor of 80µV
that's -106dB

http://www.kolumbus.fi/iain.churches...em/C50_002.jpg


Not a misconception, Dave is right - yours is hardly a typical valve
amp. My impression of the huge majority of the valve amps I have come
across is that you just know when they are switched on; there is
always that "liveness" about the speakers, usually a mixture of hum
and noise.

Of the thirty or so members of the "gramophone society" to which
I belong, more than half have valve amps. I cannot think of a
single member who has a system where one can even hear a
hint of hum/hiss at the listening position. Most systems are
silent with ear against speaker.

With the exception of the Cheepies, the quality of valve amps
offered by the many bespoke builders is very high indeed.
I would say the performance of my own is typical.

Iain