# Amplifier Circuit help, impedance matching

• DIYEngine235

#### DIYEngine235

TL;DR Summary
getting a low impedance mic (7 ohm) to work with a high impedance radio (2.2 k ohm).
I've been working on a personal project to connect my old H10 series david-clark headset to my UV-5R radio. Up to this point, I've been using the headset purely as a set of headphones to listen in, but I want to be able to transmit with it as well. The problem I'm running into is that the impedance of the microphone is 7 ohms (measured) while the impedance of the radio is 2.2kohms (speced). I've done a little research and know I need to match the impedance of the mic to get it to work with the radio and I did find a basic circuit that I could wire into my setup, but it's not working.

Below is the circuit that I've put together. Source voltage is 3.2V (measured) from the radio.
I'm using a J310 as the amplifier.
R1 is 150 ohms
R2 is 1 M ohm

When I hit transmit, I do get a connection, so I know it's getting a signal. I just think my input signal from the mic is still too small. Do I have the right resistance values?

It's also entirely possible that I'm not remembering my electrical circuit theory that I learned in college correctly. It was a while ago.

In my opinion a FET is a poor choice to drive with a low impedance source. How much have you researched various schemes to do this?

DIYEngine235 said:
R1 is 150 ohms
R2 is 1 M ohm
Why have you contradicted the resistor values or identifiers on the diagram?
Do you have a mic that works with the radio?
Did you try connecting the 7 ohm mic directly to the 2k2 mic input, and was it very quiet?

It looks to me like the operating point at ##V_{gs} = 0V, V_{ds} = 10V## is 24 - 60mA. In 150Ω this would make 3.6 - 9.0V, if it could. I think your device is saturated. That might be an issue...

Read all of the datasheet for this device.
[disclaimer: I didn't, because I'm lazy and it's your problem, not mine]

On a more practical note, buy an IC. Don't build your own amplifier unless you want to learn about transistors by making one that will never, ever, ever, be as good as what TI or Analog Devices can do for less than $5. Low Z microphones compared to high Z are typically low signal voltage. It's quite easy to see the signal voltage on a scope with a high Z but signal level on a low Z such as a 500 ohm mic will be considerably lower. I'd have no reason to believe something with a 7 ohm Zout would be any different. May not be visible at all. My suggestion would be to look for various preamp circuits online. First thought would be to use a bipolar transistor in common base configuration. - If you were to use an actual transformer you're transforming the impedance by 314:1. A voltage gain of 17.7 is the equivalent. DaveE DIYEngine235 said: the impedance of the microphone is 7 ohms (measured) How did you measure this? Averagesupernova said: First thought would be to use a bipolar transistor in common base configuration. That is correct. Common base BJT, or common gate FET amplifier circuits, have low input impedance, higher output impedance. The low impedance mic is connected to the emitter or source of the transistor, the output is taken from the collector or drain. This is a confused problem, since power must be provided to an amplifier, which must be biased to drive the load. Is there a circuit provided for the Mic input on the radio? This is the topology of the circuit I would expect when adapting from an electret to a low impedance microphone. DaveE said: Yes and no. The headset I'm using is the military version that I was issued when I was working aircraft maintenance in the US Air Force. The main difference is the mic, which at the time, was a low impedance mic that would aid in filtering jet engine noise during use. Basically, a lot of the ground com systems on the aircraft use old analogue filters and the low impedance mics work well with those. The closest is actually the H10-76, but the mic mine use is an M-87, not a DC-87. https://www.davidclarkcompany.com/files/literature/230-DC MiltaryBro 3-14.pdf I guess I could explain my little project better. My end goal is to have a circuit I can wire directly to the adapter I already have for the radio. Yes, they have jacks that supposedly work, but I'm not about to fork over$100 to try something out.
Replacing the mic is also out, since I'd like to keep the one I have because I know it works well in a high noise environment.
And besides, where's the fun in buying something when I could make it?

Averagesupernova and DaveE
Baluncore said:
Why have you contradicted the resistor values or identifiers on the diagram?
Do you have a mic that works with the radio?
Did you try connecting the 7 ohm mic directly to the 2k2 mic input, and was it very quiet?
I was typing while half asleep. Parent life.
Yes, the mic input to the radio works.
The M-87 mic doesn't put out enough voltage to even register as a signal. I have no reason to believe the mic is broken. The M-87 is a military specification mic that is known for durability. Short of smashing it with a hammer, there's not much you can do to hurt it. I have used this headset and mic in the pouring rain, subfreezing snow and burning dessert sands, all while four TF-33 jet engines were running and getting excellent sound.

Baluncore said:
That is correct.
Common base BJT, or common gate FET amplifier circuits, have low input impedance, higher output impedance.
The low impedance mic is connected to the emitter or source of the transistor, the output is taken from the collector or drain.

This is a confused problem, since power must be provided to an amplifier, which must be biased to drive the load. Is there a circuit provided for the Mic input on the radio?

This is the topology of the circuit I would expect when adapting from an electret to a low impedance microphone.

View attachment 340111
I'm trying to avoid using a second power source since I want the radio and headset to be a portable system. In other words, I can clip the radio to my belt and throw my headset on and walk around while talking. If I have to wire in a power source, I will, but I REALLY want to avoid it.

DaveE
DIYEngine235 said:
I'm trying to avoid using a second power source since I want the radio and headset to be a portable system.
I suspect the radio provides power to an electret microphone, with the required components now probably part of the radio.
My schematic in post #8, shows how to interface a low-impedance moving coil microphone, or a speaker used as a microphone, to a late model radio by using only four components, in an external circuit that may fit in the plug, and that does not require a power supply.

Averagesupernova
Baluncore said:
I suspect the radio provides power to an electret microphone, with the required components now probably part of the radio.
My schematic in post #8, shows how to interface a low-impedance moving coil microphone, or a speaker used as a microphone, to a late model radio by using only four components, in an external circuit that may fit in the plug, and that does not require a power supply.
Oh, I see now. As I mentioned, I'm working on limited sleep today.

I'll give that setup a try and let you know how it goes. I'll have to order some capacitors though. That's one reason I was trying to accomplish this with just resistors. Though my, admittedly flawed, understanding is that the capacitors function more as filters than anything else in this type of circuit? Is the capacitor necessary?

DIYEngine235 said:
Is the capacitor necessary?
Yes, it is essential. The value can be anything from 1uF to 470uF. Below 1uF, the base frequencies will be attenuated. Pinch a tantalum capacitor, or an electrolytic capacitor from a dead circuit board. I expect the voltage gain will be about 35dB which may be a bit low.

You might consider a simple audio coupling transformer, with the correct ratio, as suggested here.
Averagesupernova said:
If you were to use an actual transformer you're transforming the impedance by 314:1. A voltage gain of 17.7 is the equivalent.

Averagesupernova said:
If you were to use an actual transformer you're transforming the impedance by 314:1. A voltage gain of 17.7 is the equivalent.
Baluncore said:
You might consider a simple audio coupling transformer, with the correct ratio, as suggested here.
Yep. Might not work, but it's easy to try and then build. You may need coupling caps with it so you don't short out the input at DC.

I can't say the transformer was an actual suggestion. More of a way to get a ballpark idea of voltage gain required. @Baluncore has a good circuit posted in my opinion. I looked through a couple of old handbooks I have laying around and was disappointed to find very little information. The only thing I found was op amp based with a 500 ohm terminating resistor that could be switched in to ground for low Z microphones.

I would still try a uPower op-amp. Powered from the amp input bias. Here's one I sketched up quickly, it definitely needs review, somethings likely to be wrong. But, the point is that you can run real amplifiers off not much power.
https://www.analog.com/media/en/technical-documentation/data-sheets/max9910-max9913.pdf

PS: Maybe an issue with cross-over distortion? IDK. You could bias it into class A operation with a pull up resistor, but that costs power that you might not have.

Last edited:
Here is the schematic for the UV-5R radio.
MICin is pin 11 on U2. J1 is the MIC and PTT interface.

It has R70, a 2k2 pull-up to 3.3V when enabled by Q16. You may have to set a bit to enable that supply voltage to the microphone.

Looks like it would work with my post #8 circuit. Change my R2 from 100k to 47k for the 3.3V supply.

DaveE and Averagesupernova
Baluncore said:
Here is the schematic for the UV-5R radio.
MICin is pin 11 on U2. J1 is the MIC and PTT interface.

It has R70, a 2k2 pull-up to 3.3V when enabled by Q16. You may have to set a bit to enable that supply voltage to the microphone.

Looks like it would work with my post #8 circuit. Change my R2 from 100k to 47k for the 3.3V supply.
Excellent!!! I miss the days when manufacturers published schematics.

PS: BTW, I think they screwed up that audio jack schematic symbol. It shows (to me) that the microphone(s) are disconnected with a floating amp input when the plug is inserted. But we can just pretend they actually built it the right way . Anyway, it looks like you've got a 3.3V, 100Ω power source available for a "real" amplifier.
https://www.cuidevices.com/blog/understanding-audio-jack-switches-and-schematics

Last edited:
DIYEngine235 said:
TL;DR Summary: getting a low impedance mic (7 ohm) to work with a high impedance radio (2.2 k ohm).

I've been working on a personal project to connect my old H10 series david-clark headset to my UV-5R radio. Up to this point, I've been using the headset purely as a set of headphones to listen in, but I want to be able to transmit with it as well. The problem I'm running into is that the impedance of the microphone is 7 ohms (measured) while the impedance of the radio is 2.2kohms (speced). I've done a little research and know I need to match the impedance of the mic to get it to work with the radio and I did find a basic circuit that I could wire into my setup, but it's not working.

Below is the circuit that I've put together. Source voltage is 3.2V (measured) from the radio.
I'm using a J310 as the amplifier.
R1 is 150 ohms
R2 is 1 M ohm

View attachment 340100

When I hit transmit, I do get a connection, so I know it's getting a signal. I just think my input signal from the mic is still too small. Do I have the right resistance values?

It's also entirely possible that I'm not remembering my electrical circuit theory that I learned in college correctly. It was a while ago.
I think the terminal marked + connects to the radio mic socket.
DIYEngine235 said:
TL;DR Summary: getting a low impedance mic (7 ohm) to work with a high impedance radio (2.2 k ohm).

I've been working on a personal project to connect my old H10 series david-clark headset to my UV-5R radio. Up to this point, I've been using the headset purely as a set of headphones to listen in, but I want to be able to transmit with it as well. The problem I'm running into is that the impedance of the microphone is 7 ohms (measured) while the impedance of the radio is 2.2kohms (speced). I've done a little research and know I need to match the impedance of the mic to get it to work with the radio and I did find a basic circuit that I could wire into my setup, but it's not working.

Below is the circuit that I've put together. Source voltage is 3.2V (measured) from the radio.
I'm using a J310 as the amplifier.
R1 is 150 ohms
R2 is 1 M ohm

View attachment 340100

When I hit transmit, I do get a connection, so I know it's getting a signal. I just think my input signal from the mic is still too small. Do I have the right resistance values?

It's also entirely possible that I'm not remembering my electrical circuit theory that I learned in college correctly. It was a while ago.
I think the circuit is similar to that used for electret microphones, where power is supplied from the mic socket on the radio. In such a case the resistor in the radio which supplies the "bias" current forms the load resistor for your amplifier. I think your dynamic microphone should basically work in your circuit but the 150 Ohm resistor is an emitter bias resistor and should be bypassed with a large capacitor. I have found a link to an old circuit which uses a bipolar transistor but looks functionally correct to me. https://www.epanorama.net/circuits/dynamic_to_electretinput.html. I also suggest, as a simple check, that if you connect a multimeter on Ohms range to your microphone you should hear a click.

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