[AMRadio] Audio response and Long distant QSO's (was: Suggestions Please)

[Donald Chester]

>From: "Jeff Edmonson" <w5omr at w5omr.shacknet.nu>

I'm hoping that Don can come in here, and remind me how it is/was
>he was peaking the audio at around 3kc, and then rolling the whole thing
>off at 3.5kc (or slightly higher).  And, what's the bottom end?
>

I do it by using a rising pre-emphasis between curve in the mic preamp, 
starting at 800 cps and peaking 8-10 dB at 3 kc/s and above.    Also, I use 
a D-104 mic (for highs) and a dynamic mic (for lows) mixed together in 
phase.  The low-pass cutoff is accomplished using passive L-C filters.  With 
the flip of a switch I can choose  between a gradual hf cutoff filter at 5-6 
kc/s, a very sharp cutoff filtor at 3400 cps, or no lowpass filter at all.  
The bottom end goes down to 30 cps or so, but the scope shows visible 
distortion on a sinewave below about 40 cps.

With the 3400 cps filter in the circuit, the response curve is flat from 40 
cps to 800 cps, then rises steadily up to about 2 kc/s and  begins to 
flatten off beyond 3 kc/s, but hits a brick wall at 3400 cps.  I  bought the 
filter at a military surplus store in Washington DC back in 1973 ( I wish I 
had purchased half a dozen of them). There is virtually no attenuation at 
3300 cps; at 3400 it is over 25 dB down, and at 3500 cps you can't detect 
the signal on the oscilloscope.
The only other filter I have ever seen that has that sharp a cutoff is one 
designed by W2WLR that he sent me to evaluate.

One thing that is often overlooked is that the audio system capability needs 
to extend one octave above and below the intended frequency response of the 
program material, to minimise phase distortion effects.  This is per 
information in the United Transformer Corp. catalogue.  For example, if you 
intend to transmit high quality voice audio from 100 to 3500 cps, your audio 
chain from the mic through the modulator stage needs to be as flat and 
distortion free as possible from 50 to 7000 cps.  Good "communications" 
quality audio 300-3000 cps would require the transmitter to be capable of 
150-6000 cps (that is almost exactly the audio specification of the BC-610). 
  Any limiting of frequency response should be done using shaping circuitry 
at low levels, not by limiting response at high levels by using transformers 
with poor frequency response, or putting capacitors across interstage or 
modulation transformers, or similar gimmickry often seen in ham rigs.

My system seems to work well with my own voice.  However, when N3DRB came by 
for a visit, he operated my station, and everyone told him his voice didn't 
sound nearly as good as it does on his rig at home.  Each operator should 
make sure the rig is taylored individually to his or her voice.

Don K4KYV

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