I just got this email back from Jack at TT.
Greg,
The high pitch tone is about 40dB below no signal, that is with the
volume control at zero, and about 70dB below S9 signals. This
measurement was with an audio spectrum analyzer. The phone output
is comprised of two LM 360 amps. The volume is controlled before the
amps. The amps are running full out. The tone is probably picked up
from some of the digital signals.
You mentioned that some do not seem to have the problem. I think it
may be due to the type and impedance of the phones in use. The
output of the audio amp is constant voltage. If you have low impedance
(8 or 4 ohm) phones, the acoustic output is much greater than higher
impedance (32ohm) phones. This could make a difference of 10 to 15
dB. I would suggest, for an immediate solution, to make an adapter with
68 to 100 ohms in series with each phone. If you prefer I will have one
made and sent to you. Let me know. I also think the click on keying
may be due to the same problem.
Jack, K4JUJack Burchfield
Ten-Tec
1185 DOLLY PARTON PKY
Sevierville, TN 37862
phone 865-453-7172
I am going to do a quick adapter with trimmers this evening and find the
value where the whine disappears (hopefully it does). I will post the R
value of what I find. Do not want to pad it too much as this will also
affect the audio available from the monitor circuit which is pretty low even
with 8 ohm phones.
73
Greg
AB7R
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