On 4/29/2010 3:03 PM, Stuart Rohre wrote:
> Russell,
>
> Thanks for the links to comments about the Corsair audio.
>
> I used a Corsair II that a friend had, and it was super on his favorite
> mode, CW. It sounded fine to me on SSB as well.
>
> That said, there are better audio chips that could replace the Audio IC
> in the model. If that is too much conversion for you, simply change the
> audio coupling capacitors to reduce the high frequency response to the hiss.
Coupling capacitors only affed the low frequeny roll off. It takes shunt
capacitors to roll off the highs.
> Particularly subject to failure is the series R and C network on the
> audio IC output pin to ground, that is supposed to limit high frequency
> response in older Audio ICs.
That and keep the output amp loaded at frequencies where the speaker
goes highly inductive (like 20 KHz) so it won't oscillate. If cap or
resistor are open a scope will likely show some ultrasonic parasitic
oscillations that won't help the audio quality.
>
> The first reference writer clearly did not understand that for a
> communications / ham receiver use, you do NOT want "High Fidelity", but
> you want communications audio punch. This is achieved by NOT passing
> alll audio frequencies
> but limiting the audio to the voice band 300 to 3 KHz. This was all
> proved in Acoustics back in the 1930's. Typically, you want smaller
> series coupling capacitors to alter the frequency range being passed to
> the speaker.
That raises the low frequency cutoff, but has no effect on the HF side.
If taken to an extreme it emphasises the HF of the audio spectrum.
> And check that the aging speaker has not warped and thus is causing
> distortion if it has become off center, or the cone detaches from the
> edge mount.
>
> The second reference has some useful suggestions to tailor transmit
> audio as well as receive audio in Corsairs.
>
> The other contributor to hiss is the age of electrolytic capacitors in
> the Corsair or even Corsair II's if they have not been replaced in less
> than 5 years. You may find lack of filtering by original electrolytics.
> Any electrolytic such as used in ham radios has a limited lifetime. It
> probably will degrade slowly enough that the failure will not be evident
> until there is a large amount of hum or audio feedback, but it is worth
> checking if the filtering is adequate.
>
> The Corsair II's are one of the most desirable older Ten Tec radios.
> That has been stated on this and other reflectors for many years.
>
> Audio is very subjective to hams. You should try to listen to a radio
> you are considering, with both speaker and good quality communications
> headphones, to evaluate the audio fairly. Ensure you are not turning up
> RF gain too high and thus adding noise during a test. Sometimes on
> older radios, the correct RF gain is lower than where the S meter
> operates as you would expect. I tend to give S meter reports by my ear,
> as some rigs have a Scotch reading on the S meter. There were no clear
> standards for S meters back in the day of the Corsair. It seemed to do
> as well as other brands but it was not digital precision, since it was
> an analog circuit.
> I have seen hams who could not hear audio I could clearly hear on their
> radio. At Field Days, folks often miss calls I hear and work, because
> each of us has differing skills in "brain filtering" noise imposed upon
> a signal.
> Newer hams need to listen to HF bands for many hours and under differing
> conditions to condition their ears to optimum "Communications Quality",
> if they want to succeed with weak signal work.
>
> -Stuart Rohre
> K5KVH
>
>
73, Jerry, K0CQ
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