Convert SE mic to balanced ???

ksor

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I have some SE (=not balanced) "electret" mics - they are old and were cheap back then but I'm very satisfied with them for recording acoustic guitars - just my kind of sound !

There is not much signal when I use them with my balanced input modul - compared to other mics with BALANCED output.

I wonder if it's recommenable to convert them to balanced output to get higher output level from them.

They are housed in a tube ... diameter 18-20mm, so the "conversion" has to be some NON-tube electronics  :-\

Any comments and ... maybe some schematics (with PhantomPower) ?
 

ksor

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Winetree said:
Direct Box
Plug the unbalanced mic out into a direct box.
Take the balanced out of direct box and plug  into your mic pre.

Yeah, but i would prefer to include the "DI-box-electronics" in the mic  :-\ !

I don't think there is room for a trafo in the mic-housing  :-\

A direct box is primarily used to convert a high impedance instrument signals to a low impedance balanced mic-level signal - right ?

Where do I find a "DI-box-schematic" for a mic ?
 

Violinist

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ksor said:
Winetree said:
Direct Box
Plug the unbalanced mic out into a direct box.
Take the balanced out of direct box and plug  into your mic pre.

Yeah, but i would prefer to include the "DI-box-electronics" in the mic  :-\ !

I don't think there is room for a trafo in the mic-housing  :-\

Where do I find a "DI-box-schematic" for a mic ?

Active o Passive one?
 

Winetree

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A D.I. input can be any unbalanced signal.
An active direct box is'nt going to phantom power your mic.
It only powers the active electronics in the direct box.
I assume the mic has a 2 connector 1/4" plug. How was the mic powered unbalanced? With a battery probably.
Use the battery to hear what the mics sound like.
 

ksor

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Winetree said:
A D.I. input can be any unbalanced signal.
An active direct box is'nt going to phantom power your mic.
It only powers the active electronics in the direct box.
I assume the mic has a 2 connector 1/4" plug. How was the mic powered unbalanced? With a battery probably.
Use the battery to hear what the mics sound like.

Yeah, the mic has a 1,5V battery now - I don't know what kind of electronics it has today.

I thought the "membran-unit" could be used in some "new" electronics with balanced output and ALL included in the exsisting housing and then PhantomPowered.
 

RuudNL

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I did this once with an 'el-cheapo' (but good sounding)  TTI 7301 electret microphone.
It had an attached cable, that I removed.
The inner diameter of the tube was just the size of a 3-pin DIN connector (with pins and a threaded ring) as used on the  'N' norm microphones at the time. I simply glued it in place with 2-component epoxy cement.
There was just room to include a small transformer. I also added two resistors, a capacitor and a zener diode.
This all fitted in the compartiment where the 1.5 V battery used to be.
Result: a microphone with a balanced output and phantom powering. No need to excange batteries anymore!

(Or was it a TTI 7103?  :-\ )
 

ksor

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RuudNL said:
I did this once with an 'el-cheapo' (but good sounding)  TTI 7301 electret microphone.
It had an attached cable, that I removed.
The inner diameter of the tube was just the size of a 3-pin DIN connector (with pins and a threaded ring) as used on the  'N' norm microphones at the time. I simply glued it in place with 2-component epoxy cement.
There was just room to include a small transformer. I also added two resistors, a capacitor and a zener diode.
This all fitted in the compartiment where the 1.5 V battery used to be.
Result: a microphone with a balanced output and phantom powering. No need to excange batteries anymore!

(Or was it a TTI 7103?  :-\ )
I like such a posting ! ;D ;D ;D

... but unfortunately you haven't got the schematic any more  :'( :'(

I like my electrets too because I think they sounds good for recording acustic guitars.
 

RuudNL

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It must have been something like this:

electret.png


I used a 2.1 V/500 mW zener. A bit more than 1.5 V. from the battery, but it doesn't 'hurt'....  ;D
Capacitor is 47 uF or 100 uF / 6 V.
You want something like 4 mA going through the zener diode.
Phantom power is 48 V over 2 internal resistors of 6800 ohm.
Voltage drop should be 48 - 2.1 V = 45.9 V.
If we want a current of 4 mA, the series resistor should be 11475 ohm.
We have already 3400 ohm (2 resistors of 6800 ohm parallel for DC) so we need an extra 8075 ohm
In practice: use 2 resistors of 15 K.ohm.  ( 2 x 15 K.ohm // = 7.5 K.ohm)
Now the current will be 4.2 mA. That's OK!

I like my electrets too because I think they sounds good for recording acustic guitars.
I have used a Behringer B-5 microphone for acoustic guitar. A cheap but excellent microphone.
A real condenser microphone with very low noise. And phantom powering of course!

By the way: I would be a bit hesitant to use a DI box for a microphone...
DI-boxes are meant for instrument level signals. I am afraid that the signal to noise ratio will suffer if you use it with a microphone!
 

zephyrmic

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RuudNL said:
I did this once with an 'el-cheapo' (but good sounding)  TTI 7301 electret microphone.
It had an attached cable, that I removed.
The inner diameter of the tube was just the size of a 3-pin DIN connector (with pins and a threaded ring) as used on the  'N' norm microphones at the time. I simply glued it in place with 2-component epoxy cement.
There was just room to include a small transformer. I also added two resistors, a capacitor and a zener diode.
This all fitted in the compartiment where the 1.5 V battery used to be.
Result: a microphone with a balanced output and phantom powering. No need to excange batteries anymore!

(Or was it a TTI 7103?  :-\ )

I had some electret mics that used 1.5v. batteries, and the capsules were OK and the bodies were too good to throw out, so I used a small piece of veroboard and made a Schoeps style circuit, like the Alice mic circuit, etc., to fit in the space once occupied by the battery. I used another small scrap of veroboard directly behind the capsule to mount the FET and associated resistors, with the FET gate soldered directly to the capsule terminal of the TSB-165A capsule. That prevented any high impedance leads being too long and degrading frequency response. Other mics of mine had an internal FET, so I just used the last bit of the Schoeps circuit with the two PNP transistors in the emitter follower circuit and their associated components. They worked out really well, and it was not too difficult to fit the components in the battery space. You could even use a captive mic lead with a suitable plug on the end, for example an XLR 3 pin male plug, and that is a solution to modifying the mic body too much. I could post a pic or two to illustrare, or, alternatively, you can find them in the Yahoo micbuilders forum, Photos section under the name Zephyrmic's DIY/6mm Cardioid Phantom Power Mod 1&2, and the pics of the TSB-165A mics in the bodies of a cheaper electret mic. I am very happy with how these sound. Hope that helps.

Kindest regards,

zephyrmic
 

RuudNL

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In case you use the Schoeps circuit, it is very important to match the PNP transistors for Hfe and Vbe.
Otherwise you will get a differential voltage between the two signal wires.
This is not really a problem when you use electronically balanced microphone inputs, but transformer balanced inputs won't like the DC current in their primary winding...
Also select them for noise.
Generally the 2N5401 is used, but I found that this type is sometimes noisy. (Depending on the batch you get.)
I had good results with the BC560B.
To test them I simply build the last stage (transistors only), connect the circuit to a good microphone amplifier and measure the noise with my 'good-old' Nakamichi T100. Also listen for irregular noise. (So called: popcorn noise or hail noise.)
 

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