"John Phillips" wrote in message
...

In article i,
Iain Churches wrote:

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.

That is the reason I think it more sensible to quote noise
floor levels in µV rather than in dB, as a power amp is seldom if ever
running at full power.

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.

I had a similar experience with a mu-follower valve/tube preamp
(gain 25dB) running into a 50W PP tube amp (input sensitivity 600mV)
and full range speakers which had a sensitivity approaching that of
Lowthers (SPL 100) The source was a Quad 66 CD player.

Taking the preamp out altogether, and fitting a DACT stepped
attenuator to the power amp, resulted in a very good sounding
system, with no standing hiss or hum.

Iain