[TenTec] Orion mic preamp question.

[Robert Carroll]

John-
I have an MD2 and am interested in your results.  Let me know how the preamp
works out.

Bob W2WG

-----Original Message-----
From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]
On Behalf Of John Perkins
Sent: Wednesday, January 12, 2005 1:52 PM
To: tentec at contesting.com
Subject: [TenTec] Orion mic preamp question.

I wish to use my Yaesu MD-1 dynamic mic with my Orion, however 
the output at 5mv is too low. The Orion Mic gain needs to be at
max 100 to talk about 1 foot from the mic and I like to have some
gain in hand. I prefer not to use the SP in normal use.  
So I am building the mic preamp from the ARRL handbook 2005, 
page 14.44  (low-noise TL071 in place of the TL081).

Now we know that the Orion Mic socket is as follows:
1 -  mic input
2 -  mic return
3 -  PTT
4 -  +10v DC (no load)
Shell - RF ground    AND    PTT return (& 10v return)

(Handbook Figure 2-4 is incorrect.)

I will power the preamp from pin 4 which I calculate will give 
about 7v after the drop from the 2,2k resistor in the Orion. 
The TL071 opamp typically uses 1.4 mA.

We now have only 2 audio pins out of the preamp - Mic and Ground 
so I am obliged to short pin 2 and the Shell (RF ground).  OR   
forget pin 2 - that is the question!
I don't think we can forget the Shell plus braid if we wish to be 
RF immune! I shall test both, but would be good to know if anyone 
else is using a mic preamp. Of course, one could produce a 
Mic in/out pair by adding an isolating transformer to the  preamp 
out and ground - surely not necessary.

I have stripped out the guts of the MD-1 base except for the nice 
PTT switches which go to pin 3. At present I am using it with a 
shielded audio transformer which works well except as previously 
stated, the output is on the low side.  My old Yaesu FT-102 has 
much more mic gain than the Orion so there was plenty in hand.

I have taken extra RF shielding precautions such as bypassing the 
MD-1 mic connector with copper braid, removing the paint from 
the mic head rim so the metal housing shows zero ohms to ground.  
Ferrite beads on all inputs and outputs plus RF bypass caps on
the opamp. I am now awaiting delivery of the chip before testing.

Any ideas welcomed.
John   G3PPB

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