I built the TT 1201 kit a couple of years ago. It works fine with my Corsair
I, as built with 22K resistor. I did not make the change in the Corsair
circuit, so I'm using it with a 9V battery.
Winston K4CWQ
----- Original Message -----
From: <KF6ZZ@aol.com>
To: <tentec@contesting.com>
Sent: Monday, September 04, 2000 02:20 AM
Subject: [TenTec] Design flaws in the TT 1201 Desk Mic.....
>
> 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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>
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