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Old 9th Apr 2024, 1:37 pm   #1
ScottBouch
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Default Dynamic microphone to phantom powered electret input

Hi all,

I have been looking into using a Beyer Dynamic DT109 headset fitted with a dynamic microphone with my computer for MS Teams meetings. I'm using a Sound Blaster X-Fi Go! Pro USB sound card that is designed for an electret microphone and provides a phantom power output.

So, I'm looking to make a simple/small amplifier circuit to provide the interface, and found a possible solution online.

I want to discuss this simple circuit, using a single transistor to modulate the input to the sound card:

https://www.epanorama.net/circuits/d...tretinput.html

I can see that the transistor base will have a small AC voltage applied to it from the dynamic microphone. However this will need a positive bias applying so that the whole AC signal is used, and not clipped below 0V, so the decoupling capacitor and resistor are added.

As the transistor's action is to load the phantom power supply in response to audio signals, the supply to the bias resistor will be modulated, therefore the positive bias provided by the resistor to the transistor base will also be modulated.

This appears to me as having the action of negative feedback, as when the transistor is conducting, the collector voltage will fall, and as a consequence so will the base voltage as the bias voltage falls, reducing the transistor conductivity. Can anyone confirm if I'm on the right track with this?

The author states that it does not produce a very "Hi-Fi" sounding result. Does anyone have any alternative suggestions as to why this simple circuit would sound a bit distorted? Is it just a matter of playing around with component values to avoid clipping if the base was to fall down to 0V? Or really are we into the territory of op-amps, separate power supply, etc..? I'm ideally looking to utilise the phantom power and keep it as simple/elegant as possible.

Microphone specification is in the attached datasheet for the DT 109 headset.

Cheers, Scott
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File Type: pdf DAT_DT109_EN_A4.pdf (415.6 KB, 64 views)
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Old 9th Apr 2024, 7:00 pm   #2
frsimen
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Default Re: Dynamic microphone to phantom powered electret input

You are right, there is negative feedback applied via the base bias resistor and this is a very basic example of a virtual earth amplifier.

The problem with the circuit is that it has a very low input impedance, probably too low for the microphone to be happy. The specification for the microphone calls for a load impedance of 1k ohm. Connect a 1k resistor between the microphone and the input capacitor and you have sorted that problem out.

The frequency response might still be a little odd, but increasing C1 to give a lower roll off frequency at the bass end will help, bearing in mind the 1k ohm input resistor. At higher frequencies, the microphone inductance may come into play, causing the high frequencies to be reduced. What the gain will be depends to an extend on the value of the resistors feeding the phantom power, as they become the collector load for the amplifier.

Adding a resistor between the transistor collector and the phantom supply, say 1-2.2k, may reduce the distortion if that is a problem.

Paula

Last edited by frsimen; 9th Apr 2024 at 7:03 pm. Reason: Collector resistor added.
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Old 9th Apr 2024, 7:45 pm   #3
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Default Re: Dynamic microphone to phantom powered electret input

To connect my Shure SM10A-Koss Pro/4AA headset to my USB audio interface, I researched various boosters like the Cloudlifter, Alctron MA-1, FEThead etc. I made one on matrix board inside an aluminium extrusion with XLRs on either end, powered by the audio interface's phantom power. Here's the thread on it with some circuits.

Embarrassingly, I discover I never updated that thread with the result, which has been under test since then on my desk, working well for meetings and online chat... I'll add it to the list.
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Old 10th Apr 2024, 8:30 am   #4
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Default Re: Dynamic microphone to phantom powered electret input

Thanks Paula, that advice is very helpful.

And thanks Uncle Bulgaria... The three diagrams in thee other post appear to be for balanced mic circuits, for my purposes I think i will only need 50% of one of them, as the computer sound card input in unbalanced, similar to the circuit in post 1.

I see FETs are used in these circuits as opposed to BJTs, this may be a good idea for reducing the load on the microphone element, as Paula suggested adding resistance to the BJT circuit for this reason.

All of this information has given me good food for thought, thank you both very much.

Cheers, Scott
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Old 10th Apr 2024, 8:43 am   #5
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Default Re: Dynamic microphone to phantom powered electret input

Rather than trying to add an amplifier, may I respectfully suggest that a simple coupling capacitor will probably do the job? The output level of a dynamic microphone and an electret microphone are not very different, so all you need to do is stop the DC power supply from the audio card's microphone input getting to the dynamic microphone's coil. A capacitor of a few microfarads between the microphone output and the sound card input will do that absolutely fine, and the results should be as hi-fi as they can be.

Chris
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Old 10th Apr 2024, 9:46 am   #6
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Default Re: Dynamic microphone to phantom powered electret input

Of course, your soundcard has a 3.5mm jack? The USB interface I have has a weird dual socket that can do XLR and 1/4". Since the mic came with an XLR I made the balanced one all the way through.

FETs appear to be common in this application - there are certainly lots of balanced possibilities around in commercial circles for in-line phantom-powered microphone boosters.
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Old 10th Apr 2024, 1:45 pm   #7
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Default Re: Dynamic microphone to phantom powered electret input

Quote:
Originally Posted by cmjones01 View Post
Rather than trying to add an amplifier, may I respectfully suggest that a simple coupling capacitor will probably do the job?
Hi Chris,

I have just tried this with a 2.2uF capacitor, and while it does work, I was able to quietly record my voice in Audacity with the gain turned up, but under Windows/MS Teams, I can't crank the gain up enough to make myself heard to others.

It was well worth a try, and thank you for the recommendation, it could have saved a lot of time, but I think I will need to use an external FET circuit in this instance.

The phantom power is +5V. It actually appears to be a stereo input, the 3.5mm jack tip and ring both present +5V by internal pullups and have resistance between them, suggesting independent inputs. I shall join the L/R channels as I am only using one microphone, both L and R can see the same.

Cheers, Scott.
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Old 10th Apr 2024, 1:47 pm   #8
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Default Re: Dynamic microphone to phantom powered electret input

Quote:
Originally Posted by Uncle Bulgaria View Post
Of course, your soundcard has a 3.5mm jack? The USB interface I have has a weird dual socket that can do XLR and 1/4". Since the mic came with an XLR I made the balanced one all the way through.

FETs appear to be common in this application - there are certainly lots of balanced possibilities around in commercial circles for in-line phantom-powered microphone boosters.
Yes, should have said, it's a 3.5mm socket. Appears to be a stereo input too (see post above).
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Old 10th Apr 2024, 8:12 pm   #9
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Default Re: Dynamic microphone to phantom powered electret input

For our work Teams meetings we were given uncomfortable headsets with a microphone built in. Being resistant to wear such contraptions I made my own using the USB part but breaking out the headphone and microphone parts separately. I used a set of vintage JVC headphones and a separate 1960's Philips microphone. People on the other end said I sounded distant and often crackling too. I was thinking of adding a circuit as described here but realised in time that due to the crackling from the 3 pin DIN to 3.5mm socket adapter and huge gain (plus impedance mismatch) I was getting much less 'time wasting' calls as the caller would really have to work to hear and may get a dose of heavy duty crackling at any time...so it has stayed as is

So if you are being bothered too much by Teams calls, something to bear in mind.
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Old 11th Apr 2024, 7:44 am   #10
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Default Re: Dynamic microphone to phantom powered electret input

Hi Ian, now thts a fun approach! But really I do rely on Teams to talk to customers, so could so with getting something decent working.

I have sketched up two potential circuit designs, I now just need to order a suitable FET type, and find an hour ot two to experiment with them.

I have perfectly fine headset for talking, bit they are not so Hi-Fi when listening to music. I wanted to use my Beyerdynamic DT109 as they are the best (sound) quality headphones I own, they sound lovely. So its just a matter of interfacing to the boom microphone.

My other main set is a modern Bose noise cancelling set of headphones with mic in the cable, and really for the money they cost, the sound quality isnt as good as the Beyerdynamics from 30/40 years previous!

Cheers, Scott
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Old 12th Apr 2024, 10:46 am   #11
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Default Re: Dynamic microphone to phantom powered electret input

Hi all,

I have never really played with JFETs (or FETs) before, and just wanted to ask a basic question please to help my design.

I have been looking at the datasheet of the 2SK170 N-channel JFET as it was used in one of the example circuits, it appears to require a negative voltage to be applied to the gate WRT source voltage.

I am envisaging the JFET drain to be connected to the +5 V phantom powered sound card input, and the source at 0 V, via a resistor.

https://www.mouser.com/datasheet/2/408/6909-57550.pdf

Could someone confirm if I'm reading this correctly? It seems that the gate needs to be taken negative with respect to the source in order to make it conduct? If so, this would explain the resistor between the source and 0 V to lift the Source voltage up.

The microphone nominal voltage is ~2.5 mV (assumed RMS) (ref DT109 datasheet), so I can place the JFET source at say + 10 mV by selecting a suitable resistor to place between the source and 0 V. But this will only be true when the JFET is conducting, when "open circuit" this source voltage will fall toward 0 V (with no current passing through the source resistor), and if the source voltage falls below the gate voltage it will be impossible to "turn on" again.... so, should I also add a resistor from the source up to the drain (+ 5 V when non conductong) to form a potential divider to prevent this situation?

The example circuits don't seem to include a resistor as I am suggesting (effectively across drain and source), so I am wondering if I am worrying about something unnecessarily?

I suppose if the JFET has a small amount of internal leakage, some current may pass through when "turned off" which may be enough to lift the source up away from 0 V?

I should add though that the microphone element will be producing an AC signal, so if one side is connected to 0 V, the microphone signal will swing 1.25 mV above and below 0 V, the 'below 0V' part helping this situation...

Cheers, Scott.
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Last edited by ScottBouch; 12th Apr 2024 at 11:05 am.
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Old 13th Apr 2024, 7:25 am   #12
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Default Re: Dynamic microphone to phantom powered electret input

Hello Scott,

You are going to struggle to get that FET to work here, I think. The characteristics of FETs are similar to a low voltage triode valve. With the source and gate both at zero volts, the drain to source current will be at its maximum. Making the gate more negative than the source will reduce the current. That’s the opposite to what you have stated.

The big problem is that there is a very wide range of Drain to Source current for the device that you have chosen and an equally wide range of negative gate voltage needed to keep the current in the region that you want, in the region of 0.5 to 5 or more volts of gate bias. Some devices just will not work in a circuit with only 5 volts to play with, where you have no control of the drain load resistor.

When you said that you were using phantom power, I hadn’t realised that the supply voltage was 5V. The original BC547 circuit can be made to work well enough. All you need to do is to monitor the collector voltage of the transistor. Ideally you are looking for 2.5-3V. If the voltage is too low, try adding a resistor from the base to 0V, starting with 100k. Lower values will increase the collector voltage, higher values will reduce it. If the voltage is too high, reduce the value of the 470k resistor. With so many unknowns it’s not possible to give a sure fire design in this case.

Paula
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Old 13th Apr 2024, 9:18 am   #13
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Default Re: Dynamic microphone to phantom powered electret input

As just said by frsimen, the biassing characteristics of FETs vary wildly within even the same type number. This makes them rather difficult to use, especially when you're trying for simple circuits. Also pinch off voltages can go well over your available 5v supply.

Another factor is that FETs not only offer very high impedances at their input (not really of any value with low impedance things like moving coil microphones) but they also only offer their best noise performance when operated from high source impedances.

You can get some specialist FETs with low Vp and current-mirror bias arrangements which fix the bias difficulties, but don't affect the suboptimal noise circumstances. They do have one big disadvantage. The manufacturers have decided that the time is up for these parts and they are obsoleting them as fast as they can.

In that simple circuit, the bias resistor is used rather crudely and it also forces an amount of feedback on you. This limits your available gain, and from what you're saying, you're fighting to get enough gain.

The answer is to have a two (bipolar) transistor circuit. With a two-transistor feedback pair, you can get stable bias, all powered from phantom source, a suitable input impedance to make dynamic microphones happy and have plenty of gain.

Designing one was one of my last tasks at a recent employer, so they own the design rights and it's not mine to share, I'm afraid. Glider pilots ore proud of their nice, quiet cockpits. They like to run their radios into a cabin speaker and have a dynamic microphone on a gooseneck beside their head. They don't like headsets. The firm makes VHF COM radios, and they're all designed around electret microphones. Hence the need for a tiny adaptor box for gliders. With phantom power it lives in a cable with oddball sized jack plug and socket, no power cables, not even any need for an on/off switch.

I got this working with a wide variety of moving coil microphones... aviation ones and even a few recording studio types. I made the gain switchable with a DIP switch so we could tell purchasers the switch settins for various popular microphones.

So, it can be done and the sound can be good. You have first to let go of the idea of doing it with one capacitor, one transistor and one resistor.

David
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Old 14th Apr 2024, 8:25 am   #14
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Default Re: Dynamic microphone to phantom powered electret input

Phantom power 24v to 48v is usually switchable on AI's/SC's.

Andy.
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Old 14th Apr 2024, 10:56 am   #15
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Default Re: Dynamic microphone to phantom powered electret input

If it's bias voltage for an electret it won't be 48V or 24V and will be in single digits, probably as little as 5V if the USB power is not got a boost circuit added.
One thing to remember is that the input on the creative audio interface is probably stereo and the output from the microphone will be balanced to a simple XLR-3.5mm adaptor will not work properly and for best performance you'll need a balun, or make a "pseudo-balanced" lead, or there are leads that can be that only take one half of the balanced signal but you'll lose some level doing that.
Personally I would look for a cheapo professional audio interface that has an XLR input, this is cheap enough from a reputable manufacturer, https://www.thomann.de/gb/esi_u22_xt.htm
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Old 14th Apr 2024, 5:32 pm   #16
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Default Re: Dynamic microphone to phantom powered electret input

Hi all, I think I may have been misleading with using the word 'phantom' power which is commonly associated with stage/studio microphones. There is no XLR/stage equipment in use here.

My situation is phantom powered, but at 5V by a 3.5mm socket in a computer sound card. There is a L (tip) and R (ring) input, so two independant mic inputs, but I shall just connect these in prallel as I only need mono.

My microphone is a communications boom mocrophone on a headset, which happens to be a 200 ohm dynamic mic.

Cheers, Scott
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Old 14th Apr 2024, 5:37 pm   #17
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Default Re: Dynamic microphone to phantom powered electret input

Thanks to all for the advice on FETs, and how using them at 5V may be a bit hopeless in this circumstance.

I shall explore BJT options instead, as Radio Wrangler said, a two stage amplifier is probably best, thanks.

Many thanks to all so far!

Cheers, Scott
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Old 14th Apr 2024, 8:09 pm   #18
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Default Re: Dynamic microphone to phantom powered electret input

A typical 5v phantom is likely to have enough current available for a 2-stage amp. I got it done with 6v open circuit available.

David
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Old 16th Apr 2024, 12:10 am   #19
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Default Re: Dynamic microphone to phantom powered electret input

Quote:
Originally Posted by ScottBouch View Post
Hi all, I think I may have been misleading with using the word 'phantom' power which is commonly associated with stage/studio microphones. There is no XLR/stage equipment in use here.
The system that you are talking about is normally called Plug-In Power.

https://help.rode.com/hc/en-us/artic...%20microphones.

or

https://www.soundonsound.com/glossary/plug-power-0
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Old 16th Apr 2024, 1:51 pm   #20
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Default Re: Dynamic microphone to phantom powered electret input

James is of course correct, and those links are very helpful.

Unfortunately it is often, wrongly, referred to as Phantom power. Very annoying to those of us who have worked in pro-audio !
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