[Kenwood] Matching transformer for mike

Fri Jul 10 15:41:29 EDT 2020

If the mic audio sounds good on the headphone output of the compressor
then you need to focus on getting the output level into the radio mic
input right. As Richard said an attenuator should do the trick. No need
for a transformer. 
There are many versions of resistor network attenuators you could use.
Some google searches on T pad, L pad, pi pad should turn up some
information on resistor values needed for the amount of attenuation
necessary. How much do you need? Depends on the output level specs of
the device and in the input level specs of the transmitter. Attenuator
resistor value data will be be dependent on impedance, I suggest using
600 ohm impedance values. Impedance matching is not super important here
though. Radio's mic input impedance is probably in the kOhms. If you
like use a 600 Ohm shunt on the output of the attenuator. 
Or you could just use a potentiometer as a voltage divider. Maybe a 10
kOhm pot. Once you determine a setting that works by experimentation
replace the pot with fixed value resistors.
You may also want to try using only a 50 Ohm termination (dummy load)
for the transmitter to see whether RF getting into the device is a
problem. Using the termination instead of your antenna system should
reduce any RF feedback problem. If you determine that RF feedback is a
problem then you need to figure out how to stop it.
DE N6KB
On 2020年07月10日 07:45, Richard Knoppow wrote:
> That makes sense because the line output is at a much higher level. There should be specs fro this thing. Do you have an instruction book for it? If not check BAMA.
> You should not need a transformer for it on either input or output sides.
> There is much confusion about microphone impedance partly because the conventional spec is for the microphone as a source impedance and partly because its not very consistent. The American system is to state the source impedance of the mic. The input to the amplifier being much higher. The British also give mic impedance as a source but assume the amplifier provides a termination. The BBC published a technical note justifying this method but it seems to be that it lowers signal to noise by 6db and is not a good method. For microphones with internal amplifiers, which includes all condenser and electret mics, the impedance is generally of little consequence. Level is important. High impedance mics generally put out more voltage. The use of a transformer in old equipment was primarily to increase the voltage at the amplifier input. I am not sure what the practice is now, much of the terminology was established in the vacuum tube era.
>> On 7/10/2020 3:54 AM, Steve H wrote: Thanks Rich, yes, its got a mike and line level option and inside the compressor on the main board there is a switch that changes the output
> from line or mike. I was told to put that into the mike position, which I did, and it made a huge difference and eliminated alot of humm
> and artifacts from the signal.
>> This is the mike I am now using:
> https://www.newark.com/stellar-labs/cm-2250/studio-condenser-cardioid-microphone/dp/53Y1921 Since its designed around needing phantom power, and the 528E can supply it, figured it was a good match. When I tried it with just my headphones monitoring the output of the compressor, it really did sound great.
>> But no matter what I did last night with it set like that, could not get a whole lot of modulation into the Kenwood where it was an upgrade over the MC50 Kenwood mike. If I put the mike gain on the Kenwood past 3, I get lots of distorting and oscillations without even speaking into the mike. I can cut the mike gain control or the main level out from the 528E down to eliminate it, but at the cost of the percent modulation decreasing way down so its nearly unreadable.
>> So far the MC50 mike sounds way better than what I can get from this setup and it seems like more mike gain would get me more % modulation, however
> as I get it better and hit 3, it just distorts. If I put the Kenwoods built in compressor on, I can get past 3 and modulation will improve as I go up, but even with the mike gain at max, the modulation % is just not very robust at all.
>> Steve Hearns
> KA2PTE
>> ----- Original Message ----- From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
> To: <kenwood at mailman.qth.net>
> Sent: Thursday, July 09, 2020 11:18 PM
> Subject: Re: [Kenwood] Matching transformer for mike
>> Level are important. If the output level of the processor is high you need a resistive attenuator which will also serve to match the impedance.
> In general, American practice is to state microphone input impedance as the expected source impedance, usually there is no need to match impedances because the source is a voltage source not a power source. For instance, in historical broadcast equipment and sound equipment designed to work from low impedance microphones the the typical input impedance was on the order of 5K ohms or more while the source impedance was on the order of 150 ohms. For equipment that runs at line level the impedance is usually a terminating impedance, source impedance can be the same or sometimes not.
> See if the processor is meant to drive a microphone input or a line level input. If a line level input you need an attenuator to get the level down to mic level, maybe as much as thirty db. The exact impedance is not critical.
>> On 7/9/2020 7:53 PM, Steve H wrote: I am trying to hook up a Symetrix 528E voice processor to my Kenwood Hybrid 830S and was told that the mike input is about 150 ohms
> to the rigg and the processors output is 600 ohms.
>> Other hams I think use a matching transformer, I guess 4 to 1? Anyone out there have any experience selecting a matching xfmr for the Hybrids?
>> Steve Hearns
> KA2PTE
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> WB6KBL
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