On 8/21/2018 6:25 AM, Bob McGraw K4TAX wrote:
If you are finding the headphone audio cleaner than the speaker audio,
the audio amp which drives the headphones and speaker is the same
device. This would seem to indicate the audio amplification is working
correctly.
There are several common causes of audible hiss in headphones. One is
the use of headphones with very high voltage sensitivity. Others are the
RF and AF gain adjustments you noted.
Audio can be distorted at the lower amplitude limits of a gain stage or
an A/D or D/A converter, so the same RF and AF gain adjustments apply.
In general, most radios are not designed to run at either limit of their
AF and RF gain controls -- they generally work/sound best mid-range
settings. In the old days, before our receivers had decent AGC, it was
common (and good) practice to run AF gain moderately high and ride the
RF gain control. This is still good practice, but "more is NOT better"
-- that is, AF gain should NEVER be all the way up for this setting, and
is likely to make noise in the audio chain (hiss) audible.
If you have an external speaker plugged into the headphone jack, the
headphone jack is isolated via resistors and the AF amp will not drive
an external speaker.
The resistors are there to protect the amplifier output from being
shorted when a headphone jack is inserted and/or removed.
Try an amplified speaker such as computer desk
top speakers.
The internal speaker itself may be defective thus the source of the
distortion you are hearing.
An external speaker can be distorted if it is overdriven (by turning up
the AF gain in the radio too high). It will also be noisy (hiss) if it's
gain is turned up too high, and/or if AF and RF are mis-adjusted in the
radio.
Something like 45 years ago, I was a "troubleshooter for hire" for audio
and video systems, and a common problem was improper settings of audio
gains and levels through a complex signal chain. The most common problem
was power amps turned all the way up, and other parts of the system
operating far below proper levels, making their noise audible. The
solution was simple -- feed a max rated level signal at system inputs
and adjust gains in the system so that every stage is operating just
below clip.
73, Jim K9YC
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