Simple mic pre with phantom power - results?

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
Hi all,
I am a noobie here and hope to get some answers to make my life easier. I have a couple of old Panasonic 61A mic capsules and wish to make suitable simple mic preamps with phantom power. I have seen the following circuit all over the web. I have all the parts on hand, but have found no specs for freq resp, noise and distortion. I will put some caps across the battery to help reduce noise. Has anyone measured this circuit for freq resp, noise and distortion? What can I expect? I plan to use the circuit to measure loudspeakers during DIY design and construction.


1708360529748.png
I can convert the circuit to use 2N3906 transistor and have the ability to use the Linkwitz cut capsule technique with a negative supply to the capsule. How would that affect freq resp, noise and distortion?

Some web examples of this circuit include an analog 10K pot across the output. How would that affect utility, freq resp, noise and distortion?

I thank you all for any info or help you can give.

ab

 

Irving

Joined Jan 30, 2016
3,887
Last edited:

Audioguru again

Joined Oct 21, 2019
6,693
Your very high resistance for R1 is starving the mic for current, try 10k or less if the 9V is low.
The Linkwitz mod is for an electret mic in very loud sounds (inside a drum or inside a piano).Then the mic produces less sensitivity and less overload distortion.
 

Attachments

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
Welcome to AAC!

How much noise & distortion can you tolerate? A single transistor pre-amp isn't going to be particularly great.

The generally suggested solution here is one of the many single chip modules with MAX9814 or MAX4466 chips.

https://www.amazon.co.uk/HiLetgo-Electret-Microphone-Amplifier-Adjustable/dp/B07DRGF8C2
https://www.amazon.co.uk/HALJIA-High-peprformance-Microphone-Compatible-CMA-4544PF-W/dp/B07QM7HRP1
Hello Irving. And thanks for the input. I did some searching here in the US and found the following.
Amazon.com: DIYmalls GY-MAX4466 Electret Microphone Amplifier Sensor Sound Module Adjustable Gain Breakout Board for Arduino (Pack of 2) : Electronics

It could be a really cheap way to play the game of measurement mic. I could remove the mic from the board, place it on the end of a suitable tube. Provide electrical connection at the end of the tube and start measuring. But this type of circuit is commonly designed with a freq response tailored expressly to pass the human voice to earphones and such. I was unable to find freq response, noise or distortion measurements for any of the products that offered this type of service. Other similar products used amplifier chips rather than preamp chips and thus would produce rather poor results for measurements. Some of them use agc or avc and would be an anathema for measurement. Any thoughts about what could be used?

Thanks ab
 

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
Your very high resistance for R1 is starving the mic for current, try 10k or less if the 9V is low.
The Linkwitz mod is for an electret mic in very loud sounds (inside a drum or inside a piano).Then the mic produces less sensitivity and less overload distortion.
Hello Guru. And thanks for the input.
Your pic says you use 10K and filtered 9V. Could you describe your filter? I will use the 10K.
I am using the Behringer UCA222 and the M-Audio Transit as USB outboard sound cards. Both are super excellent. But neither has a mic in. They are both line level inputs. Will I have any problems using the above modified circuit to properly drive the line inputs?
FWIW, a long time ago I talked Joe D'Appolito into calibrating one of my 61A's. He charged me $25! I know those cal files will be of little use to you but it might be fun to compare them with what you have. In any event I will be happy to send them to you for posterity.
Thanks for any help you might give. ab
 

MisterBill2

Joined Jan 23, 2018
18,522
The circuit in post #1 is not compatible with phantom power in any way, shape, or form. In addition it does not show a balanced output, and certainly it will not survive 48 volts of phantom power.
You will not see a correct circuit on the cartoon channel. (You Tube) probably not ever anything even close. Check the website "Schematics for free"/audio/microphones, I think. They have actual circuits that actually work.
 

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
The circuit in post #1 is not compatible with phantom power in any way, shape, or form. In addition it does not show a balanced output, and certainly it will not survive 48 volts of phantom power.
You will not see a correct circuit on the cartoon channel. (You Tube) probably not ever anything even close. Check the website "Schematics for free"/audio/microphones, I think. They have actual circuits that actually work.
Hi Bill,
Perhaps it was not clear to you, but the phantom power we have been talking about is not the pro audio 48V type that ultimately feeds a balanced input to pro audio gear. We are talking about phantom power for electret capsules that may need only 2.5 to 9V bias. Indeed, pro audio mics fitted with electret capsules must take the pro audio 48V phantom power and reduce it to 9V or less to prevent frying the electret capsule. People who DIY design, build and measure loudspeakers routinely use unbalanced line inputs fed by 9V phantom power electret capsules. This URL might be helpful to you.
System Test (linkwitzlab.com)
All I am trying to do is make a simpler version than the Linkwitz one.
 

MisterBill2

Joined Jan 23, 2018
18,522
Hi Bill,
Perhaps it was not clear to you, but the phantom power we have been talking about is not the pro audio 48V type that ultimately feeds a balanced input to pro audio gear. We are talking about phantom power for electret capsules that may need only 2.5 to 9V bias. Indeed, pro audio mics fitted with electret capsules must take the pro audio 48V phantom power and reduce it to 9V or less to prevent frying the electret capsule. People who DIY design, build and measure loudspeakers routinely use unbalanced line inputs fed by 9V phantom power electret capsules. This URL might be helpful to you.
System Test (linkwitzlab.com)
All I am trying to do is make a simpler version than the Linkwitz one.
OK, Now it is clear what the goal is. The first thing is to understand that it IS NOT "PHANTOM POWER" !!! The microphone with the electret capsule is using remote power, which is very reasonable, but it is not even similar to phantom power. That is because the power appears across the signal circuit, whereas in phantom power the voltage is common to both signal wires, not across the signal pair, but between that pair and the shield conductor.
So the two schemes are very much different, although they serve similar functions. Microphone amplifier circuits using remote power are simpler, most of the time, and almost always use a voltage much less than 48 volts. It happens that they are also a lot easier to find from reliable sources.
One caution is that the published Linkwitzlab circuit will certainly be damaged if it becomes connected to an active Phantom Power input. It is not a "remote power" microphone amplifier, it uses two 9 volt batteries.
The circuit shown in post #1 can be used as a remote powered microphone amplifier, but it will needs a cable with two wires plus the shield, and it will need a three-circuit connector plug, and there will be an issue if a non-amplified microphone is connected to that input. But the circuit looks OK , although I have not verified the resistor values.
And the two circuits shown in post#10 ARE true "phantom Power"microphone amplifiers, a fair bit more complicated.
 
Last edited:

Irving

Joined Jan 30, 2016
3,887
I was unable to find freq response, noise or distortion measurements for any of the products that offered this type of service
To a certain extent those parameters depend on how you use the chip. The raw chip performance is in the data-sheets though it does take some interpretation, for example the MAX4466 will give 20dB of gain and almost constant phase shift from 10Hz to 80kHz, though this will depend on the output load impedance (higher = better, but increases potential for increased input noise at following stage) and other components, see this chart:
1708427054063.png
 

Audioguru again

Joined Oct 21, 2019
6,693
Thank you again for your reply. Your filter will be part of my coming build.
Last, do you think this passive circuit will be able to suitably drive line level inputs?
Thank you
ab
A microphone has a very low signal level for nearby speech, therefore it needs a preamp that has a voltage gain from about 50 times to 200 times. your transistor had a voltage gain of only 10 times or less. Use an audio opamp.
 

Attachments

Ya’akov

Joined Jan 27, 2019
9,165
Welcome to AAC.

No matter what you use you will have to calibrate it if you want to use it for measurement.

That being the case, unless the frequency response is very odd, it doesn’t really matter. So long as it doesn’t roll off before the lower and upper limits you hope to measure, the calibration will allow you to compensate for whatever the curve looks like.

On the other hand, when you can buy a calibrated microphone for $35.00 that almost certainly exceeds anything you would make with your spare parts, it starts to look like wheel spinning to pursue doing that.
 

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
Welcome to AAC.

No matter what you use you will have to calibrate it if you want to use it for measurement.

That being the case, unless the frequency response is very odd, it doesn’t really matter. So long as it doesn’t roll off before the lower and upper limits you hope to measure, the calibration will allow you to compensate for whatever the curve looks like.

On the other hand, when you can buy a calibrated microphone for $35.00 that almost certainly exceeds anything you would make with your spare parts, it starts to look like wheel spinning to pursue doing that.
Hi Ya'akov. And thanks for your reply.
I already have an ECM 8000 and a Eurorack UB802 mixer to use it with. The 8000 is not calibrated and its need for a mixer to provide 28V makes the measurement process unnecessarily complex. I do have a calibrated wm61a and am looking for the simplest way to use it accurately. The 9V powering circuit provided by Audioguru seems ideal but I have no feedback on the question of whether it can properly feed and unbalanced RCA type input.
Thanks. ab
 
Last edited:

Ya’akov

Joined Jan 27, 2019
9,165
Hi Ya'akov. And thanks for your reply.
I already have an ECM 8000 and a Eurorack UB802 mixer to use it with. The 8000 is not calibrated and its need for a mixer to provide 28V makes the measurement process unnecessarily complex. I do have a calibrated wm61a and am looking for the simplest way to use it accurately. The 9V powering circuit provided by Audioguru seems ideal but I have no feedback on the question of whether it can properly feed and unbalanced RCA type input.
Thanks. ab
OK, so I am wondering, then why you don’t just build a phantom power supply for it (as low as 15V according to the specs) since that only requires an appropriate DC supply which you can get for a few dollars if you don’t already have one, two connectors, and a handful of passives.

But, before this goes too far afield—the project is yours to define and this is a diversion, so carry on and good luck with whatever route you take.
 

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
To a certain extent those parameters depend on how you use the chip. The raw chip performance is in the data-sheets though it does take some interpretation, for example the MAX4466 will give 20dB of gain and almost constant phase shift from 10Hz to 80kHz, though this will depend on the output load impedance (higher = better, but increases potential for increased input noise at following stage) and other components, see this chart:
View attachment 315723
Thanks for the input.
 

Thread Starter

oneoldude42

Joined Feb 19, 2024
23
OK, so I am wondering, then why you don’t just build a phantom power supply for it (as low as 15V according to the specs) since that only requires an appropriate DC supply which you can get for a few dollars if you don’t already have one, two connectors, and a handful of passives.

But, before this goes too far afield—the project is to define and this is a diversion, so carry on and good luck with whatever route you take.
Ya'Akov, thanks for the reply.
And yes, this has become a sort of diversion. I only have one calibrated mic capsule and it is not in the ECM 8000. So i am trying to do the best I can. Thanks - ab
 

MisterBill2

Joined Jan 23, 2018
18,522
A single low-noise IC amplifier using a remote power supply would not be complex and could easily fit inside many styles of microphones. But I would not consider any of the stuff shown in the amazon add, even if it were free.
An amplifier that would operate with a single external supply will not be complex. A single op-amp pre-amplifier can provide the gain you require.
 
Last edited:
Top