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Common gate impedance matching

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I14R10

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I have a ham radio that uses microphone that has 50k ohms impedance. But I want to connect a microphone that has 600 ohms impedance instead of that 50k ohm one. I read that you can make an impedance matching circuit with JFET, basically this schematic.

Rd=50k, Rs=600 ohms Can it be done this way?
 

Audio circuits do not match impedances.
Usually the input impedance of a preamp is much higher than the impedance of a microphone so that the level is not cut in half. A power amplifier has an output impedance of 0.04 ohms or less to drive an 8 ohm speaker. The very low impedance of the amplifier damps resonances in the speaker. Radio antennas are matched to the cable and RF amplifier.
In my 37 years audio career I worked with many microphones and preamps for them and I have never seen a Jfet common-gate preamp circuit.
 
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    I14R10

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Audioguru - This is the user manual of my radio **broken link removed**. On page 7 it says that "the microphone input is designed for high impedance (50k ohms) microphones only. And on page 11 it is shown that microphone needs matching transformer. I don't doubt that you know what are you talking about, but I am wondering why would Kenwood explicitly state this in user manual.
And common gate circuit wouldn't be used as a preamplifier, just as a impedance matching circuit. I can use matching transformer, but I wanted to know if I can use JFET.
 

Kenwood does not say that the mic matching transformer steps up the voltage of a low impedance mic a lot since the transmitter is designed with low mic gain for the high signal level from a 50k ohm mic.
I have never seen a 50k ohms mic and I have never seen an input designed for one. Maybe a 50k ohms mic is a standard 600 ohms mic with a stepup transformer inside.
Your Jfet circuit also steps up the low signal level from a low impedance mic a lot, similar to a transformer.
 
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Maybe a 50k ohms mic is a standard 600 ohms mic with a step-up transformer inside.
Click to expand...
Exactly. Microphones with built-in step-up transformers have been used with tube amplifiers. Direct connection of a low impedance microphone would involve too much noise and hum. Believe it's more than 40 years that I've seen one of these "50kohm" microphones.

As already explained, the transformer purpose isn't impedance matching. It can be well replaced by a gain of 10 or 20 low noise voltage amplifier. There's no benefit of using a low input impedance amplifier.
 
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So, what should I do? Like I said, I have a mic with 600 ohm impedance and I want to connect it to the radio. I would like to avoid using amplifier since I don't want additional power supply. Would a simple transformer do the trick?
 

I see some audio transformers with 1:10 - 1:15 windings ratio available from catalog distributors, at different quality and price levels. Consider that a cheap transformer without magnetic screen is prone to pick up mains transformer hum.

They are made e.g. by www.TriadMagnetics.com
 

None of the Triad audio transformers are anywhere near 600 ohms to 50k ohms. Most have "exceptional performance" but produce no bass.
 

As far as I understand, the application is speech only (radio transmitter). In so far a transformer with small magnetizing inductance might work.

Personally I would start without any transformer, checking if the microphone sensitivity is already sufficient.
 

I would do FvM's experiment with no transformer.
If that doesn't work then you would need a transformer or you might consider a simple preamp such as this.
It has adjustable gain and will likely give better performance than a single transistor amp.
You can power it from a 12Vdc wall-wort.
 

I'll try it without the transformer. The mic that I'm talking about is this one **broken link removed**. It should have -72dB sensitivity. More than it's required in radio manual. It's not very expensive, but it sounds good.
 

The radio expects the mic to have a sensitivity from -50dB to -60dB but the 600 ohm mic has a sensitivity of -72dB which is -12dB to -22dB (average of -17dB) less which is quite a lot of difference.
You might need to "eat" the mic (touching your lips) and shout.
 

Most modern radio transceivers have some kind of fairly effective automatic level control, which is why I suggest to just to try it.
 

Warpspeed said:
Most modern radio transceivers have some kind of fairly effective automatic level control.
Click to expand...
Yes, mainly to prevent horrible over-modulation from an audio level that is too loud. But the audio level from the wrong impedance mic will be much too low.
Try it anyway.
 

The manual describes VOX operation. This activates transmission when you talk into the mic, loudly enough to turn on VOX. Perhaps a higher impedance mic (50k) produces sufficient amplitude to do this. On the other hand when testing low impedance dynamic mic's, I find they only put out a few tens of mV, and then I must speak loudly.
 

I misinterpreted what microphone sensitivity means. I thought that -72dB is better sensitivity than -50dB. My radio has Mic Gain potentiometer, but I don't know how much it can amplify the input from the microphone. I really might look into buying that amplifier that somebody suggested here.
 
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