[Elecraft] D 104

[Don Wilhelm]

  Jim and all,

Looking at the schematic for the K3 mic input, it is an op amp  with a 
gain of 10.  It looks to be a classic differential amplifier with a 10k 
resistor in both the + and the - inputs.  That would produce an input 
impedance of about 20k ohms.  Matching is not necessary (nor desirable).

So yes, it is not a "600 ohm input impedance" - it is the nominal 
impedance for a typical driving device which will produce sufficient 
voltage swing for maximum input - that is a LOT different than "matching 
impedance".

In audio applications, this is quite common - the driving impedance is 
not matched to the input impedance of the receiver.  Instead, the 
"impedance" of the driving device will dictate the voltage and current 
required for operation.  The op amp only cares about the voltage 
reaching the input of the IC.  The driving voltage will meet the input 
impedance of the op amp and current will flow.  As long as the driving 
voltage is within the range of voltages expected by the transceiver 
designer, then the impedance of the driving device does not matter either.

In other words, the K3 mic "input impedance" is a derived value, and has 
nothing to do with reality - it all has to do with the voltage and 
current characteristics of the driving device (in this case, the 
microphone).

In other (2nd other) words, do not calculate based on the nominal "input 
impedance" of the receiving op amp, it can be very different than stated 
- the voltage swing is what really matters.

Jim is correct - it is quite common to have high input impedance devices 
in the audio world - maximum power transfer is NOT the goal, but an 
appropriate voltage swing is required to drive the device, and that is 
often dictated by the capability of the driving device, and not by the 
input device.  Impedance matching is not required.  That has been the 
case since op amps came into common use in the audio field - no 
impedance matching, just provide enough voltage swing to operate the op 
amp as the designer has required.  Low impedance driving devices may not 
provide enough voltage swing - so the "600 ohm" specification can be 
construed as stating "a typical 600 ohm driver  of the type expected 
(typical microphone output) will provide sufficient voltage swing for 
proper operation" -- it says nothing about the actual input impedance of 
the receiver.

This is quite different than transmitter PA stage output circuits which 
require matching to (typically) a 50 ohm impedance for maximum power 
transfer.

73,
Don W3FPR

On 6/4/2011 4:03 PM, Jim Brown wrote:
> On 6/4/2011 12:16 PM, Grant Youngman wrote:
>> That would be a COMPLETE AND TOTAL waste of time. The K3 mic input Z is 600 ohms nominal per spec.
> Nominal is the operative word here. It's designed to have enough gain
> for the low impedance mics commonly used with ham gear. If you study the
> schematic, you see that the mic input stage of a K3 is NOT 600 ohms,
> it's an opamp, which is a fairly  high Z load. The input Z is at least
> 10K, probably more like 15K -- there's a 10K resistor in series with the
> input!
>
>> The D-104 element needs 500k minimum and better no less than 5M. Last I heard the input stage of a K3 was not a 12AU7/12AX7.
> This whole issue of impedance matching in audio is widely misunderstood.
> In a passive mic (that is, one that is unamplified), it mainly affects
> gain and sensitivity. The high-Z D104 is a very hot mic, and the K3 has
> a LOT of available gain, so you can afford to lose a lot with the
> mismatch.  The second way that impedance CAN affect the sound of a
> passive mic is frequency response (due to stray L and C in the capsule
> or output network), but the K3 has VERY good and very flexible TXEQ,
> which is very likely to be able to correct for any response deviations.
>