After completing my TT 1201 Desk Mic kit,
I experienced abismally low audio output ----
not enough to drive my Corsair II,
and not enough to drive my Scout,
even with their mic gain pots at max.
Turns out that the electret mic element is under-biased .
Bias resistor R11 is 22k --- about ten times to large, starving the mic
element of necessary bias-current.
It therefore cannot develop the typical 10~30 mv that you can expect from
this type of transducer.
Dropping R11 by a factor of 10x , from 22k down to 2200 ohms, brought the
output up enough to easily drive the CII and Scout to full power !
The second problem is low frequency roll-off.
Capacitor C2 ( .022 uf ) and the combined resistance of R2 and R14 ( approx
8800 ohms ) set the low end 3db corner of the
TL061 op-amp circuit at about ~ 800 hz.
Which is a bit high.
This corner frequency should be well below 300 hz.
Changing C2 by a factor of 10x from .022 uf to .22 uf
will move the 3 db low end roll-off below 100 hz.
I suspect both of these wrong component values arose from simple typos.
Looks likeTT slipped a decimal point. This probably happened when the 705
microphone was offered as a kit and the internal TT documentation was
re-written for the masses.....makes one wonder if TT has ever built one from
their own kit instructions ?
Since I made these changes, the 1201 (aka 705 ) seems to be performing
marvelously.
Anyone else built this kit ?
Anyone comments / comparisons on the 705 factory model ?
73,
- Phil KF6ZZ
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