[TenTec] Pegasus mike input is Hi-Z, unbalanced!

[Robert & Linda McGraw K4TAX]

Oh me oh my.  First most audio professionals do not consider 10K ohm as high 
impedance.  Inputs such as used on the older radios, Heath's, Collins and 
the like have their input Z in the order of several megs.  As to signal 
levels there are three basic groups:  mike level, line level, and high level 
or speaker level.  None are compatible with the other which says that one 
should not be connected to the other one.  I'll not dive into the topic of 
source impedance vs. load impedance and the like.

Now, as to running a broadcast audio chain into the Pegasus, first a 
broadcast audio chain will have a source impedance of 10 to 25 ohms which is 
ultimately designed to operate into a load Z of 500 ohms or higher.  This is 
done such that when multiple loads are desired to be connected across a line 
Z output such as the output of a broadcast chain, then the loads should be 
of bridging impedance typically in the order of 10K ohms or so.  This allows 
up to 10 loads to be safely connected across one source with out loading or 
affecting the source output level.  In all cases we are dealing with line 
level outputs connected to line level inputs.

Next issue, a broadcast chain output is line level usually +4 dBM to +8 dBM 
nominal level reference 1 mW into 600 ohms.  The Pegasus input mike level is 
designed to operate typically in the -60 dBM to -40 dBM range.  The point 
being, one can not run line level signals into a mike input without great 
risk of overdriving the first audio stage.  Usually the gain control is 
after the first stage and the adjustment of such will not serve to solve the 
problem as related to first stage overdrive.

Solution: build an unbalanced T network with about 45 dB attenuation for 
600 ohms input to 600 ohms output and terminate the output in 600 ohms an 
then feed that direct to the mike input of the Pegasus.  That is equivalent 
of using a bridging input across a terminated output.  Perfectly correct 
technically.  No matching transformer is needed and only 4 resistors 
required.  For the T network the input and output series R values are 
calculated at 593 ohms and the shunt to ground is 6.7 ohms.  Use 1/4 watt 
values and the closest standard 5% value resistors will work just fine.

Alternatively: one can build a 3 resistor PI network designed to operate 
between 600 ohms and 10K ohms and provide some 45 dB attenuation.  In this 
case use a 628 ohm input R to ground, a 217787 series R, and a 10480 shunt R 
to ground on the output side.  Feed this direct to the input of the Pegasus. 
Values are calculated and the nearest standard value 1/4 w carbon R will 
work just fine.  Again, no transformer is needed.

As to transformers, a matching transformer is just that, one designed to 
transfer maximum energy from one impedance to another.  In the case of 
feeding a line level signal to a mike input this is exactly what is NOT 
needed.  Some method of attenuation is required because of the difference in 
levels between the two devices of 40 to 60 dB.  If one believes that a 
transformer is needed for ground loop isolation then a 600 ohm to 600 ohm 
transformer should be used between the output of the broadcast chain and the 
input of the resistive attenuator.  This places the transformer at the 
higher signal level and thus minimized the chance of hum introduction at the 
transformer.

73
Bob, K4TAX




> I've been attempting to run my broadcast audio chain into my Pegasus
> with limited success.  I'd been informed, variously, that the mike input
> to the Ten Tec rig was low impedance - balanced (which is what I need),
> high impedance unbalanced, low impedance unbalanced and high impedance
> balanced :-).  The manual I have is devoid of information and I have no
> schematic.
>
> So, I called Gary, N4CJX, at Ten Tec -- the "Peg tech".  He says the
> mike input is high impedance (about 10K) unbalanced -- which is the
> usual amateur practice (strangely).  He's also sending me a schematic
> (yippee!).
>
> Anyway, armed with that knowledge, I'll employ a matching transformer
> and pad and all should be well -- unless I run into more RF incursions.
>
> I just thought I'd pass along the information in case anyone else is
> curious.