Balanced Microphone Amplifier

Thread Starter

Arjune

Joined Jan 6, 2018
196
Will my attached circuit function well as a balanced condenser microphone amplifier and with also a dynamic mic plugged in with an XLR plug? I'm thinking the phantom power voltage would be 12 volts. I want to use an LF353 opamp with +/-12v for power 1664697411485.png
 

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Audioguru again

Joined Oct 21, 2019
5,160
Condenser microphones have an extremely high impedance and need their 48V phantom power and preamp also to be extremely high input impedance. Therefore the preamp, balancing and phantom power supply are usually built inside them.

Your preamp is not balanced and does not have an extremely high input impedance.
A dynamic mic never has a DC voltage on it and its preamp has a mid-low input impedance.

Maybe your mic is an electret type that has 48VDC built into the electret material and has a Jfet with an extremely high input impedance of 1G ohms as an impedance converter?
 

MisterBill2

Joined Jan 23, 2018
13,115
Usually phantom power IS 48 volts, sometimes it is not. And it is always the same on both signal wires. And it is actualy NEGATIVE with respect to the shield pin#1.
Unfortunately "condenser Microphone" does not tell us much. An electret condensor mic is an entirely different thing, anyway.
So really we need a much better explanation of what you have.
There are quite a few good circuits published but not on yootoob.
 

Ian0

Joined Aug 7, 2020
6,286
I am presuming that you are referring to a condensor studio microphone such as a Shure SM86, not a condensor microphone insert from a telephone handset.
48V phantom power IS standard for studio microphones. It is supplied on both pins 2 and 3, generally with 6.8k supply resistance.
A condensor microphone has a preamplifier built-in (otherwise there would be no need for the phantom power) and can easily drive the 10k input impedance of a mixing desk.
Because phantom power is supplied on both pins 2 and 3, and dynamic microphones have no connection to pin 1, then they will perform correctly if the phantom power remains connected.
Your circuit needs the phantom power on both pins 2 and 3 along with the coupling capacitor, and the 27k resistor is superfluous.
It will work, but the noise will be disappointing. LF351 is really designed for high impedance inputs and has high voltage noise of 18nV/√Hz. This will be amplified by 470 times by your circuit and you will get quite a noisy output.
(The LF351 has excellent current noise of 10fA/√Hz, but that is of no advantage at all in this curcuit.)

Now, are you using the LF351 because it is what you have on hand? If so, you can turn it into an excellent microphone preamp with the addition of a couple of transistors? Or are you looking for a suitable IC to use? If so, then look at SSM2019, THAT1510 or INA217
 

DickCappels

Joined Aug 21, 2008
9,297
Looking at your ripple clipper (2N3904 circuit) a few observations:

A low value resistor in series with the collector can limit the current through the transistor at turn-on without affecting the way the circuit works.

The 2N3904 has a FT of 300 MHz and will very likely oscillate if used as shown. A common way of dealing with this is to put a resistor (100Ω) between the base of the transistor and the rest of the base circuit. The resistor lowers the Q of the base circuit and reduces the chance or oscillation. A lower FT and quieter transistor would be a better choice.

A 0.01 uf multilayer ceramic capacitor from the collector of the 2N3904 to ground, or better a lower value capacitor from the collector to the base, will keep the circuit connected to the collector from working as a tank circuit thus preventing the the circuit from turning the stage into a Colpitts oscillator.


There may be a slight advantage of putting the 4.7k resistor very close to the emitter of the 2N3904.

Elsewhere in the circuit: You will need power supply bypass capacitors on the power supply pins of the op amp.
 

Ian0

Joined Aug 7, 2020
6,286
This is an interesting article, mainly to promote the ThatCorp devics
https://www.thatcorp.com/datashts/AES129_Designing_Mic_Preamps.pdf
but if you scroll down to page 28 and the section "What's in the triangle?" there's a circuit which would improve yours by a huge amount by reducing the noise, but only adds two transistors (although is says NE5532, it will work nicely with an LF353)
If you can spare two more transistors the circuit on page 35 will improve it again, by reducing the distortion.
There's another version of the page 35 circuit here.
https://sound-au.com/project66.htm
 

Ian0

Joined Aug 7, 2020
6,286
Shure Dynamic Microphone pin 1 is case ground (so please don't cut off the ground pin on the AC plug on the PA amp!) ;)
Pin 1 is always ground on a XLR. The socket contact is not as deep as pins 2 and 3 so that it mates first.
If this is a standalone preamp, then the ground should connect to the ground of the next stage. A 100Ω resistor to mains earth may also be useful, in case the ground of the next stage is not earthed.
 

eetech00

Joined Jun 8, 2013
3,418
Pin 1 is always ground on a XLR. The socket contact is not as deep as pins 2 and 3 so that it mates first.
If this is a standalone preamp, then the ground should connect to the ground of the next stage. A 100Ω resistor to mains earth may also be useful, in case the ground of the next stage is not earthed.
Just saying that on a shure dynamic mic it is chassis ground, PA system mic input grounds the case of the microphone for safety reasons. So you don't get shocked on the lips by non-grounded PA system (like when someone cuts off the third ground prong on the AC power cord).

1664726114201.png
 
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drjohsmith

Joined Dec 13, 2021
502
Will my attached circuit function well as a balanced condenser microphone amplifier and with also a dynamic mic plugged in with an XLR plug? I'm thinking the phantom power voltage would be 12 volts. I want to use an LF353 opamp with +/-12v for power View attachment 277487

https://www.ti.com/lit/pdf/sboa320

https://www.electronics-lab.com/project/phantom-powered-micro-phone-pre-amplifier/

https://www.analog.com/media/en/technical-documentation/obsolete-data-sheets/139484016ssm2017.pdf
 

MisterBill2

Joined Jan 23, 2018
13,115
How does a singer standing on a carpeted wood platform a foot above a wooden floor, not touching any instrument, get a shock on the lips from a ground connected microphone?? I have suggested that it is static from the synthetic carpet, since the guy with the acoustic guitar gets shocks as well. It requires a return path to get a mains voltage shock. And when I seriously grounded the mic stands the shocks got worse. Of course, the mic holders are all plastic, rather good insulators.
AND a shock from mains power keeps on shocking until you break contact. A static electricity shock is over once the charge has dissipated. That is a simple way to tell them apart.
 
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eetech00

Joined Jun 8, 2013
3,418
How does a singer standing on a carpeted wood platform a foot above a wooden floor, not touching any instrument, get a shock on the lips from a ground connected microphone??
How do you know what the vocalist will be standing on?
Could be on a wet floor...

I have suggested that it is static from the synthetic carpet, since the guy with the acoustic guitar gets shocks as well. It requires a return path to get a mains voltage shock. And when I seriously grounded the mic stands the shocks got worse. Of course, the mic holders are all plastic, rather good insulators.
AND a shock from mains power keeps on shocking until you break contact. A static electricity shock is over once the charge has dissipated. That is a simple way to tell them apart.
they won’t get a shock if the equipment is properly grounded.
 

Thread Starter

Arjune

Joined Jan 6, 2018
196
I have LF353 available. I think only pin 2 of the XLR Jack should have phantom power and should have a positive voltage. If there's power to pin 3 there won't be a differential voltage to power the electret element. Pin 3 is a circuit return and is negative with respect to pin 2. Pin 1 would be the shield connection for the cable of the microphone. These condenser microphones are very expensive and I don't have one to test it with my circuit so I cannot guarantee that I'm correct. I think there is an attempt to change the theory with false information so that the physical configuration of reality would change miraculously to alter mental understanding. I posted this video on YouTube
 

Ian0

Joined Aug 7, 2020
6,286
I have LF353 available. I think only pin 2 of the XLR Jack should have phantom power and should have a positive voltage. If there's power to pin 3 there won't be a differential voltage to power the electret element. Pin 3 is a circuit return and is negative with respect to pin 2. Pin 1 would be the shield connection for the cable of the microphone. These condenser microphones are very expensive and I don't have one to test it with my circuit so I cannot guarantee that I'm correct. I think there is an attempt to change the theory with false information so that the physical configuration of reality would change miraculously to alter mental understanding. I posted this video on YouTube
Wrong.
Both Pins 2 and 3 have identical phantom power. The condenser element is powered between the phantom power and ground. It is deliberately done as a common mode voltage so that the differential amplifier cancels out any signals that come from the power supply.
 

ronsimpson

Joined Oct 7, 2019
2,385
Read the "THAT Corp" paper in #6. They know more than all of us. I have copied their ideas many times. They like to teach.
Also, post #10, middle link. TI makes good parts and have good papers. Their papers are not as educational.
I have schematics of high-end audio mixers. That is a good place to look. If a company makes a living making mic-preamps for decades, chances are they know something.

A youtube video is not a good source.
 

Thread Starter

Arjune

Joined Jan 6, 2018
196
Wrong.
Both Pins 2 and 3 have identical phantom power. The condenser element is powered between the phantom power and ground. It is deliberately done as a common mode voltage so that the differential amplifier cancels out any signals that come from the power supply.
I'm sorry I stand by my belief.
 
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