On 2007-10-04, Iain Churches wrote:

"Dave Plowman (News)" wrote in message
...
In article i,
Iain Churches wrote:
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

That's not typical.

I have a Lowther LL26 (EL 34's PP) 26W at 0.1% THD.
It is half the power of my PPP amp, but still the noise
floor is only 120µV a very presentable -98dB.

Very presentable indeed. I guess I look on amplifier noise floors
as follows:

- specified WRT full power they indicate the absolute maximum dynamic
range available from a system. I'm afraid I'm keen on good dynamic
range.

- specified WRT the nominal 1 W into 8 ohms (2.83 V RMS) you can add the
speaker sensitivity and approximately check if the hiss will be
audible (at 1 m or at the litening position by correcting at 6 dB for
each doubling).

I have heard systems (SS systems) in the past, even at dealers, that
had quite audible noise from the 'speakers and wondered why. It seems
perfectly possible to do an engineering check to see if a system will
exhibit a number of avoidable limitations like this.

I fixed a system some years ago which had a Hafler DH-100 pre-amp with
20 dB of gain from the AUX input, connected to a Quad 405 with its high
gain - somewhat untypical of the US power amps with which the Hafler
might have been designed to work, and some high-ish sensitivity 'speakers.

The combination was noisy. I guess no-one designed it. It just got
assembled. I had to reduce the gain of the preamp (checking it for
stability) to 10 dB, when it just became silent at the listening position.

--
John Phillips