[TenTec] It the 715 speech processor for my Omni C?

[Dr. Gerald N. Johnson]

On Sun, 2009-11-15 at 17:26 -0500, Walt K8CV wrote:
> Like a true ham the first, and probably wrong, idea is a transformer! Low to
> high impedance line transformer?
> 
> Walt K8CV
> 
> -----Original Message-----
> From: tentec-bounces at contesting.com [mailto:tentec-bounces at contesting.com]
> On Behalf Of Ivin Flint
> Sent: Sunday, November 15, 2009 1:16 PM
> To: tentec at contesting.com
> Subject: [TenTec] It the 715 speech processor for my Omni C?
> 
> I have been looking at the TenTec 715 speech processor on the Ten Tec web
> site and now saw a review on it in the latest QST.  From what I read it
> would be very effective on a radio that does not already have speech
> processing in the transceiver.  
> 
> I wonder if it would be effective in my Omni C?  I use a Electrovoice 606
> high impedance mic.  The article says it takes any microphone with a
> impedance of 50kohm or less so I guess there is one problem.  The hook up
> cables from the rig to the processor sound like none are made for the stereo
> plug mic jack in the Omni C.  I think the manual of the Omni C calls for a
> high impedance mic so maybe this isn't really for the vintage that I use.  
> 
> I though bringing it up here might help more than just myself.  Any ideas?
> 
> Ivin W9ILF

The 715 manual doesn't talk about older rigs. The audio input impedance is 
sure 47K set by a resistor to ground at the first audio amp input. The EV 606 
prefers 250K to 5M according to its spec sheet. There surely is lots of
gain at that point, so hooking a 200K or 220 K resistor in series with
the hot microphone input should take care of the impedance match without
too much signal loss. A high impedance microphone has lots of volts
compared to a low impedance microphone so the 5:1 voltage drop shouldn't
be a problem.

Looking at the 715 schematic some more, the output comes from R30 (I
think, the on line manual is FUZZY) a 1K pot, probably the radio drive
set pot. If it won't give enough voltage to drive the high impedance
radio, that 1K pot is in series with 22k that could be reduced or
shorted (though then the 1K pot might ought to be 20K) to raise the
voltage as much as 20 times. There's no problem driving the radio's high
input impedance from a low impedance source so long as that source has
enough voltage. Reducing or shorting R31 looks like it would give plenty
of voltage, better if R30 was changed to a higher value though. 

It would be nice for the older radio if TT made a high impedance input
option (by means of the simple series resistor or an option to make that
47K about 270K) and a high impedance output (by raising the value of R30
and maybe lowering the value of R31) option. That output option could
use a 10K pot all the time and for low impedance (and so low level) with
provisions for adding a 1K resistor in parallel with R30. That's how I'd
attack it if I wanted to.

73, Jerry, K0CQ