[TenTec] Design flaws in the TT 1201 Desk Mic.....

[KF6ZZ@aol.com]

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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