Impedance matching mic to preamp

[Qmavam]

I'm building Elliott Sound Products mic preamp, Low Noise Microphone Preamp
The write up says, "With the component values shown, impedance matching is correct for a 600 Ohm mic" I'm using the PU5024, https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf. This mic has an output impedance of 2.2k. I'm striving for Low noise, I don't know if impedance matching is important to noise. I don't want the cost of a transformer. Do I just connect this and be done, or is there some way to get a better match?
Is there a best practice for this?
Thanks, Mikek

 

[shabtek]

i am assuming the datasheet circuit for electret capsule
why is the output impedance 2.2k? does it have to do with loadR? probably
In ESPre what if you omit the 1200Ω R1? than the mic is not isolated (in AC terms) from the output of the pre if turned down; could R1 be moved to the other side of the pot? could a cap be moved or eliminated?

the esp pre is not optimized as abbey may have said

 

[abbey road d enfer]

The write up says, "With the component values shown, impedance matching is correct for a 600 Ohm mic" I'm using the PU5024, https://www.mouser.com/datasheet/2/334/AOM-5024L-HD-R-1219369.pdf. This mic has an output impedance of 2.2k

Then the write-up is very incorrect.
The actual output impedance is that of a FET's drain, which is quite high (several dozens kiloohms).
The feeder resistor determines the output impedance of the combination. They recommend powering with 3V through a 2.2k resistor, but it could be 6V through 8.2 kiloohms or 24 V through 44 kiloohms, or any combination that sets the current at 500uA. Maybe 500V through 1 Megohm...

"impedance matching is correct for a 600 Ohm mic" is an oxymoron.
The correct matching for 600 ohms is 600 ohms. Period. The preamp has an input impedance of about 1300 ohms. This doesn't match. Impedance matching is a concept that optimizes power transfer and phase response in long lines ( several miles) and video apps.
Now dynamic microphones are supposed to be bridged, which means they should be loaded with at least 10 times their nominal impedance. That's why many preamps have an actual input impedance of about 2 kohms, for accomodating 200 ohm mics. For a 600 ohm microphone, it would need to be at least 6 kohms. It's far from being the case.
Now there is another very important aspect of impedance, which is the OSI - Optimum Source Impedance - which conditions the noise performance of the mic/preamp combination.
I'm too lazy to make the calculation, but I guesstimate the OSI of this preamp to be about 1.5 kiloohm.
And anyway, even presented with a mic with an impedance equal to the OSI, the noise factor would be at least 2, which means 6dB noisier than what could be achieved wit a more adequate circuit.

. I'm striving for Low noise,

Then I wouls be tempted to say "don't use this circuit."
However, considering that the mic itself has a rather high intrinsic noise, I don't think it would make a significant difference in practice.

I don't know if impedance matching is important to noise.

Matching to the OSI is important. However, as in most cases, the overall performance is conditioned mainly by the transducer and the input stage (which is the FET inside the capsule).

Is there a best practice for this?

You need to learn about noise in electronic circuits.

 

[Qmavam]

Then the write-up is very incorrect.
The actual output impedance is that of a FET's drain, which is quite high (several dozens kiloohms).
The feeder resistor determines the output impedance of the combination. They recommend powering with 3V through a 2.2k resistor, but it could be 6V through 8.2 kiloohms or 24 V through 44 kiloohms, or any combination that sets the current at 500uA. Maybe 500V through 1 Megohm...

At the moment I have a 30V supply with a 56k resistor.

"impedance matching is correct for a 600 Ohm mic" is an oxymoron.
The correct matching for 600 ohms is 600 ohms. Period. The preamp has an input impedance of about 1300 ohms. This doesn't match. Impedance matching is a concept that optimizes power transfer and phase response in long lines ( several miles) and video apps.
Now dynamic microphones are supposed to be bridged, which means they should be loaded with at least 10 times their nominal impedance. That's why many preamps have an actual input impedance of about 2 kohms, for accomodating 200 ohm mics. For a 600 ohm microphone, it would need to be at least 6 kohms. It's far from being the case.
Now there is another very important aspect of impedance, which is the OSI - Optimum Source Impedance - which conditions the noise performance of the mic/preamp combination.
I'm too lazy to make the calculation, but I guesstimate the OSI of this preamp to be about 1.5 kiloohm.
And anyway, even presented with a mic with an impedance equal to the OSI, the noise factor would be at least 2, which means 6dB noisier than what could be achieved wit a more adequate circuit.

I have noted that increasing the feedback resistor of that preamp increases the input resistance. Should I set the feedback so I get 22,kΩ input resistance and then drive the mic with 3V and a 2.2KΩ resistor. Then just add another stage of amplification to get the gain needed?

Then I would be tempted to say "don't use this circuit."
However, considering that the mic itself has a rather high intrinsic noise, I don't think it would make a significant difference in practice.

I thought 80db was a pretty high S/N ratio! What is a lower noise mic?
I'm trying to do that, but truthfully I don't have the math skills to do well at it.

Matching to the OSI is important. However, as in most cases, the overall performance is conditioned mainly by the transducer and the input stage (which is the FET inside the capsule).

You need to learn about noise in electronic circuits.

I'm trying to learn more about noise, but truthfully I don't have the math skills to do well at it. But I'll lumber along anyway.
Thanks, Abbey

 

[abbey road d enfer]

At the moment I have a 30V supply with a 56k resistor.

The result is that the oepen (no load) sensitivity is increade by about the same ratio as 56/2.2 or about 28dB. But then the output impedance is about 50 kiloohms.
The preamp has an impedance of about 1300 ohms, so it attenuates the signal by a factor 50/1.3 or about 32 dB.
The comination of attenuation and gain results in an almost identical overall sensitivity.
That is normal, since the FET in the capsule acts as a current generator, and the circuit as a virtual-earth current amplifier

I have noted that increasing the feedback resistor of that preamp increases the input resistance.

Yes. It also increases the gain.

Should I set the feedback so I get 22,kΩ input resistance and then drive the mic with 3V and a 2.2KΩ resistor.

That would result in no gain.

Then just add another stage of amplification to get the gain needed?

I already told you that there's better things to do with two transistors.

I thought 80db was a pretty high S/N ratio! What is a lower noise mic?

It computes at about -104dBu, which is 25dB noisier than a 200 ohms resistor, and 20 dB noisier than a merely half-decent preamp.

I'm trying to do that, but truthfully I don't have the math skills to do well at it.


I'm trying to learn more about noise, but truthfully I don't have the math skills to do well at it.

Understanding phase cancellation requires only basic knowledge of trigonometry.
Noise is more complex, but you can do with learning a few basic receipes.
You just need to be confident the giants on which shoulders we stand knew their stuff.
How do you think I got away all these years?

 

[Cranehazard]

That is an awesome sounding preamp by ESP love it on Tom's and guitar cabs the feedback loop makes it hard to clip the input, so it works good even though it has a high output impedance. It works great as is with all kinds of mics. If you use the version with the op amp it will have low output impedance also. Only 40db of gain so it's only good on loud stuff. It wants to oscillate at low gains. You can cure this by limiting the bandwidth.

 

[abbey road d enfer]

That is an awesome sounding preamp by ESP love it on Tom's and guitar cabs the feedback loop makes it hard to clip the input, so it works good even though it has a high output impedance.

As it is the output impedance is about 20 ohms, which is fine by me.

It works great as is with all kinds of mics. If you use the version with the op amp it will have low output impedance also. Only 40db of gain so it's only good on loud stuff. It wants to oscillate at low gains. You can cure this by limiting the bandwidth.

Well, you may love how it sounds, with which I won't argue with you, but its performance doesn't stand out by any metrics, particularly in terms of noise (-121dBu EIN) and THD (0.7%).
Keep in mind the OP wants low noise performance.

 

[Qmavam]

As it is the output impedance is about 20 ohms, which is fine by me.

I'm not sure if this discussion is of the one I built or #66. I measure the one I built to have an output impedance of 450 ohms. Although the waveform gets distorted and needs an increase in R4 to 20k to get rid of the distortion. This also reduced the gain of 40 to 20. I have a audio autoformer with 13 taps that I use, it seems the 2.5k" input to 150Ω output feeding my headphones gives maximum audio,

Well, you may love how it sounds, with which I won't argue with you, but its performance doesn't stand out by any metrics, particularly in terms of noise (-121dBu EIN) and THD (0.7%).
Keep in mind the OP wants low noise performance.

FWIW, I can hear the noise difference between a 100Ω resistor and a 12K resistor across the input at open loop gain. (about 3500). I don't hear any difference
between a 100Ω resistor and a short on the input. Maybe when I get the better transistors.
Mikek

 

[abbey road d enfer]

I'm not sure if this discussion is of the one I built or #66.

What is #66?

I measure the one I built to have an output impedance of 450 ohms. Although the waveform gets distorted and needs an increase in R4 to 20k to get rid of the distortion. This also reduced the gain of 40 to 20.

I can't comment on that since I don't know what schematic it refers to.

I have a audio autoformer with 13 taps that I use, it seems the 2.5k" input to 150Ω output feeding my headphones gives maximum audio,

You use a mic preamp to drive headphones?

FWIW, I can hear the noise difference between a 100Ω resistor and a 12K resistor across the input at open loop gain. (about 3500).

Of course you can.

I don't hear any difference
between a 100Ω resistor and a short on the input.

Since there us a 1.2k resistor at the input, tehre is nt much difference between 1200 ohms (input shorted) and 1300 ohms (with 100r resistor)

Maybe when I get the better transistors.

I seriously doubt it. The input transistors is biased at about 150 uA, which results in an OSI of about 5 kohms including teh 1.2k input resistor, meaning that below 3.8 kohms, there is not much reduction in noise potentially only 3 dB..

 

[Qmavam]

What is #66?

I built Elliot Sound products #11, I thought #66 was mentioned, maybe not. ESP Projects Pages - DIY Audio and Electronics

I can't comment on that since I don't know what schematic it refers to.

You use a mic preamp to drive headphones?

So far, I can add a driver, but I need a limiter of some type so I don't over drive the input on big unwanted signals.

Of course you can.

Well, I wasn't sure with this amp as bad as you keep pointing out that I could hear resistor noise over the amp noise, so it was a surprise to me.

Since there us a 1.2k resistor at the input, there is not much difference between 1200 ohms (input shorted) and 1300 ohms (with 100r resistor)

Good point.

I seriously doubt it. The input transistors is biased at about 150 uA, which results in an OSI of about 5 kohms including the 1.2k input resistor, meaning that below 3.8 kohms, there is not much reduction in noise potentially only 3 dB..

What happens if I reduce the 1.2K resistor, how much resistance is required? Why is that there if it creates such problems.

 

[abbey road d enfer]

I built Elliot Sound products #11, I thought #66 was mentioned, maybe not. ESP Projects Pages - DIY Audio and Electronics

#66 is a totally different animal, since it uses transistor pairs in Szlikaï topology, which does not require the infamous input resistor.

So far, I can add a driver, but I need a limiter of some type so I don't over drive the input on big unwanted signals.

I don't get it; what does it have to do with noise?

What happens if I reduce the 1.2K resistor, how much resistance is required?

This resistor defines the input impedance of the preamp and is essential in controlling THD.
Making it smaller will not only load the microphone in a way that it sounds bad, it will also increase gain and distortion significantly.

Why is that there if it creates such problems.

The circuit relies on Voltage-to-Current negative feedback for controlling distortion and gain, but has a big drawback. In order to get enough negative feedback, the impedance it sees has to be relatively high, which is achieved in that case by adding the 1.2k resistor, and resistors add noise.
It's simply the wrong topology for low-noise design.
This is well known.
There are many sources that compare opamp-based inverting and non-inverting topologies and it is known that the inverting topology has a marked noise disadvantage.
The two-transistor ESP preamp is an inverting circuit.

 

[Qmavam]

#66 is a totally different animal, since it uses transistor pairs in Szlikaï topology, which does not require the infamous input resistor.

Yes Abbey, I just wanted to cleaify what amp we were discussing.

I don't get it; what does it have to do with noise?

I want a limiter, maybe clipper, because I'm hurting myself with high gain and headphones. Absolutely nothing to do with noise.

This resistor defines the input impedance of the preamp and is essential in controlling THD.
Making it smaller will not only load the microphone in a way that it sounds bad, it will also increase gain and distortion significantly.

Thanks

The circuit relies on Voltage-to-Current negative feedback for controlling distortion and gain, but has a big drawback. In order to get enough negative feedback, the impedance it sees has to be relatively high, which is achieved in that case by adding the 1.2k resistor, and resistors add noise.
It's simply the wrong topology for low-noise design.
This is well known.
There are many sources that compare opamp-based inverting and non-inverting topologies and it is known that the inverting topology has a marked noise disadvantage.
The two-transistor ESP preamp is an inverting circuit.

I was hoping for a simple transistor circuit. I do have some TL071 Opamps, But, I have an order going to digikey, do you have a favorite low noise circuit with an IC I could put on the order? Also if its lower gain then I'll need an amp to drive headphones. Also I would like have a lower noise mic to compare to what I have now, any recommendations? Questions are open for anyone to answer.
Thanks, Mikek

 

[Garagescience]

Don’t want to hijack this thread, BUT..

Way above my head electronics theory-wise, but am curious what comes out of this discussion.
I have some 5024s as well that I’d like a low noise preamp (and PIP) for. Though I’ll skip the headphone amp, and take a stereo line out into my portable recorder’s line-in (and headphone monitor from that).
Please, carry on!