[AMRadio] re: Apache audio not good

[Shirli Sieb]

As stock, the mic  input impedance of the Apache is around 500K ohms - way
too low for any crystal mic.
The existing coupling caps are all around 510 pf - again too small for
proper voice response.

When Heath originally designed the Apache, the design  target was to use the
rig with the Electrovice 638 series __DYNAMIC__ mics. I've used my own stock
Apache with hi-z dynamic mics and it starts to sound acceptable. (Using a
Shure 55, it actually sounds good.)  Using the stock rig with a D-104  will
always result in "peaky" audio, - with unpreamped D-104's, because the input
Z is too low, and with preampted D-104's because the mic has it's own peak
in the "articulation range" portion of the voice spectrum.

The standard fix is to change the input grid resitor from 470K OHMS to a
minimum of 5 megohms.  This will allow any crystal mic to be correctly
loaded, and thus produce good bass response.  Next, change ot all the
coupling caps to at least .05 or higher to allow this new bass response to
pass.

Finally, due to the clipper and it's filter, the best waveform you can
expect is a rounded triangular wave, due to the clipper and it's
phase-shifting filter.
Bypassing the clipper (just pop out the 6AL5), will at least allow some
minor control over phase and assymetry if there is any.

Most of the audio mods for a DX-100 are also suitable for the Apache.

Good luck,

73,

Ed - VA3ES


> -----Original Message-----
> From: Ron Samchuk
> Sent: Tuesday, April 09, 2002 12:35 PM
>
>
> Hi again...
>     Thanks for all the advice and suggestions received so far