phantom powered mic preamp schematics

[Matt Syson]

Hi
What type of capsule (dynamic / electret/ condenser)?
Look on web for things like Neumann . AKG or whoever's mic amps for ideas.
Matt S

[thomasdf]

Thabks Matt
It's a cheap electret. I'm going to check the micbuilder group for schematics.
If anybody has a good idea concerning what schematics I could use, feel free to tell me!
Thanks

[Matt Syson]

Hi
You could use a small transformer to balance the audio and strip the DC from it or even just a moderately cunning arrangement of resistors and capacitors with two or 2 small silicon diodes to act as a voltage regulator (if the electret wants only a couple of volts).
The output from an electret is already 'buffered' and at a mederate level so you don't actually NEED an amplifier in there.
Matt S

[thomasdf]

I think I made a mistake, this doesn't looks like electret. I guess it's dynamic, it was taken off a old cassette recorder.

Here's a few pics to help!





It has " 4 Z 5" written on it...

[nosebleedaudio]

Looks like an condenser to me, the kind you find in a boat load of cassette recorders ect..Have used them many times in T/B mics in consoles ect, cost around $2.00 at Radio Shack..

[Matt Syson]

It's an electret most likely.
Apply about 1.5 Volts positive to the yellow wire, ground on the 'screen' of the wire and the output is blue.
To run it from phantom power:
Wire a cap, say 10uF 100 Volt with negative to the blue wire and the positive to pin 2 of an XLR.
Using a 10K resistor from pin 3 of the XLR take it to the 'hot' end of the 1K resistor.
From the junction of these 2 resistors fit 2 or 3 1N4148 (or any small silicon diodes) in series, or even JUST a red LED, which will light up.
Fit another 10uF 100 volt cap from ground (pin 1) negative to oin 3 positive. This will 'unbalance' the line in audio terms.
So that is 2 resistors, 2 capacitors and 2 diodes (or 1 red LED).
Matt S

[thomasdf]

Originally Posted by Matt Syson

It's an electret most likely.
Apply about 1.5 Volts positive to the yellow wire, ground on the 'screen' of the wire and the output is blue.

Sounds OK to me.

To run it from phantom power:
Wire a cap, say 10uF 100 Volt with negative to the blue wire and the positive to pin 2 of an XLR.
This is OK
Using a 10K resistor from pin 3 of the XLR take it to the 'hot' end of the 1K resistor.
What is that 1K resistor you're talking about?
From the junction of these 2 resistors fit 2 or 3 1N4148 (or any small silicon diodes) in series, or even JUST a red LED, which will light up.
OK
Fit another 10uF 100 volt cap from ground (pin 1) negative to oin 3 positive. This will 'unbalance' the line in audio terms.
So that is 2 resistors, 2 capacitors and 2 diodes (or 1 red LED).
This is OK too

I'm trying to draw a schematics out of your explanation that is pretty clear, except I don't get the 1K resistor point. I'm stuck for now, but this already helps a lot!

Many thanks

[Matt Syson]

Hi
The 1K is a mistake from an earlier thought where some electrets have only 2 connections, being ground and power. The 'power' connection is supplied through the typically 1K resistor and you take the audio from the junction of power and the capsule.
If you draw up what you think I mean I will happily elaborate.
Matt S

[thomasdf]

Ok, here's what I draw for now on. I still don't really get the 1K stuff. Is it soldered between the yellow cable and the hot blue one? ... Seems weird to me

[Matt Syson]

Hi
Take the mic screen to pin 1 then add the 10K and diodes (or an LED). You can omit the 1K, it was a mistake when I wrote it.
Matt S

[thomasdf]

Ok, I'm still lost here. Do I need to use a 1,5V battery??

edit: when I tap on it, I get sound only between the "hot" blue cable and the yellow one... I definitly am completely lost :D

[Matt Syson]

Hi
You could use a battery but that is not the point of phantom power!
No, the 48 volts from pin 3 should go through a 10K resistor onto the yellow wire of the mic. I would suggest a red LED connected from the yellow wire/10K resistor junction (anode of Diode) and the cathode to pin 1.
This will then light the LED and will set up a voltage of about 1.8 Volts on the yellow wire, powering the capsule.
So, to recap from your diagram:
Remove the 1K and fit a 10K in it's place.
Take the 'disconnected' end to pin 3 of the XLR.
Fit a LED with anode (+) to the yellow wire.
Connect the cathode (-) to pin 1 of XLR.
As mentioned before, take the screen of the capsule to pin 1 of the XLR.
Now it should at least work. The LED should light and you should get 'sound'.
Matt S

[thomasdf]

Ok, thank your for your patience. I draw this schematics. I guess it's the right one, I'm pretty sure the "ground" connection from the pin1 to the mic body is OK, but I'll have to try with the electret body

[Matt Syson]

Hi
Yes pretty much BUT as drawn you have the LED wired the wrong way up.
To test this you would disconnect the yellow wire and plug it in and the LED should light up. Now you can reconnect the yellow wire.
Matt S

[Matt Syson]

Hi
You realise this is an 'experimental' setup, it SHOULD work but please don't cry too much if you damage the electret unit, they are quite cheap so not a huge loss.
Matt S

[thomasdf]

Ok. Don't worry, I don't care if the electret is out of order, as you said it's pretty cheap, and I got this one for free... I'm just tinkering here, and I'd like to see how it sounds, especially on hi hat.

[Matt Syson]

Hi
I was rushing out earlier. To protect the capsule I would put 2 more diodes in 'inverse parallel' from the blue (output) wire to screen. This would prevent the switch on or 'plug in' current spike from the 10uF output capacitor from damaging the electronics inside the electret capsule.
Inverse parallel meaning parallel, but one with anode to screen and the other cathode to screen.
Matt S

[thomasdf]

Ok, so that would give us this?





What diodes do I need to use?

[Matt Syson]

Hi
Yes, this should at least work.
Diodes can be 1n4148 or any small signal silicon diodes. They are only there to stop the 48 Volt 'spike' when you power it up which may damage the transistor inside the mic capsule.
Matt S

[thomasdf]

Ok, great! Thank you very much.
I'll keep you updated and will try to upload pictures of it and sound samples!

[thomasdf]

It works gRReat! I've been testing it at home with my hifi amp and it sounds really good.
I will do a proper recording in studio this week, I just need to build the body of the mic now.

Many thanks!

[busselik]

Originally Posted by Matt Syson

Hi
I was rushing out earlier. To protect the capsule I would put 2 more diodes in 'inverse parallel' from the blue (output) wire to screen. This would prevent the switch on or 'plug in' current spike from the 10uF output capacitor from damaging the electronics inside the electret capsule.
Inverse parallel meaning parallel, but one with anode to screen and the other cathode to screen.
Matt S

Hi. i know this is an old thread but i was wondering if you could help me with an electret with only 2 connections as you mentioned before. What changes should i do to the schematic? thank you in advance