On Thu, 11 Nov 2004 21:36:58 +0000, Trevor Wilson wrote:

snip

THD is meanongless in the real world. It is a great method of producing
comparisons between amps, but there are so many other factors to be
taken into consideration that, below something like 5%, it is completely
inaudible.

**Bull****. Way back when I was a trainee tech, some of us experimented
with some pretty crappy speakers and discovered than 1% THD was audible.
Other experimenters have suggested that around 0.1% is a reasonable
threshold for average listeners. Critical listeners may be able to detect
far less.


You can produce two signals with identical THD%. One will have very
audible distortion and the other won't. It depends on the relative
strengths of the harmonics. This is reproducible under test conditions by
adding harmonics to a pure tone. The signal with the higher level of low,
even harmonics will sound purer. In the light of this, how can you state
that comparison of THD is meaningful *in listening tests*? For amp
designing and comparison measurements, yes, but not for listening. Your
listener will quickly pick up on, say, third harmonic if the second
harmonic is at a low level but as the second harmonic level increases it
masks the third harmonic problem. The THD goes up but the sound appears
"purer" to the listener. Obviously, I am talking about test tones here,
not real music, but that tends to introduce other masking effects anyway,
as the relative values of the harmonics are detected differently at
different frequencies. By all means design for low THD as this affects
the entire system, but a high THD does not necessarily mean that the amp
sounds bad or that the effect of the high THD is audible.

snip
My first trf used battery valves &
ran from suitable 90v and 1.5v batteries!

**I'll betcha you didn't use an infinite impedance detector. Yours used
a standard diode one, right?


Hell - I don't remember much about it now! It must have been over 40 years
ago! It was a regenerative detector (with a "reaction" control) just to
keep things simple for a first set. All I can remember for certain is that
it had a 1T4 valve in it. I seem to remember adding another one or two
valves later, but one of them was a DL91 to drive a speaker. The other was
probably an RF stage to isolate the oscillator from the aerial! Oh yeah -
it had a Denco coil on a paxolin former. Why did I remember that? I can
even picture the coil now!

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info