[1000mp] audio filters for the Mark V

[Ian White, G3SEK]

J. Mc Laughlin wrote:
>    I do not know what "the hiss" is

Just to clarify this point: the "hiss" that everyone talks about is 
noise at audio frequencies *above* the roll-off of the IF filters.

The noise originates in the IF stages after the filters... maybe in the 
455kHz analog notch filter, which is never completely disabled, only 
de-tuned.

It isn't a serious hindrance to communication, but it is quite 
noticeable in unmodified FT-1000s, and also the FT-990.

Some people don't care about it, while others find it very irritating. 
(I certainly do, because it's a constant reminder of another stupid 
design error that Yaesu refuse to recognise, and that kind of thing bugs 
me. YMMV, of course.)

The audio lowpass filters will kill the hiss completely, but you have to 
adjust the audio cutoff frequency whenever you switch IF filters. The 
Inrad IF amplifier mod masks the IF noise, but causes a permanent 
degradation of close-in IMD performance.


-- 
73 from Ian G3SEK