----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.co.uk>
To: <amps@contesting.com>
Sent: Wednesday, March 05, 2003 2:48 AM
Subject: Re: [Amps] Why hasn't solid state replaced tubes?
> Zyg Skrobanski wrote:
> >Hi Ian,
> >
> >>That's the key question, because you have been talking all the time
> >>about 3rd order IMD, which is close to and underneath the speech signal
> >>itself.
> >
> >Does a transmitter with poor 3rd order IMD therefore have any noticeable
> >distortion in the voice quality due to the speech intermod products
within
> >the audio bandwidth?
> >
> Often it doesn't, because the only IMD you'd hear inside the normal
> receiver passband would be coming from the low-frequency components of
> the speech signal, and would be covered by the higher-frequency
> components.
>
> That's why a "hi-fi" report on your speech quality is absolutely useless
> as an indicator of IMD.
>
> You have to tune away from the signal in order to see what's really
> happening. There is a step-by-step guide on my web site (URL below)
> under "No More Splatter!"
>
>
> >>A 3rd-order IMD figure on its own is no guide to what the amplifier
> >>sounds like further out.
> >
> >If the system/amplifier is IMD'ing in the audio frequencies, then
> >intuitively, the problem causing that IMD is probably also causing
splatter
> >products outside the audio bandwidth too.
> >
>
> Probably, but the audio frequencies are the worst possible place to look
> for the IMD.
Actually they are meaningless Ian because of all the harmonic distortion,
amplitude changes, and IM that occurs before and after the narrow filters.
The rest of the system dominates the distortion we hear on frequency, while
troublesome IM products are only noticeable off frequency.
The biggest single mistake people make when checking for bandwidth
complaints like clicks and IM distortion is to listen on frequency, or to
use a bandscope.
73 Tom
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