help with Zener diodes and audio

[tempus]

Hey all;

Not exactly stompbox, but close: I'm using an electret mic on my acoustic guitar for live shows. The particular mic I'm using (like many if not most electrets) is spec'd for a max of 10v. As I'm sure you're aware, most professional systems with phantom power provide 48v. I've built a custom power supply/preamp that delivers the rated voltage, but I'd like to have more compatibility with other systems, so I tried dropping a 5.6v zener from pin 2 to ground. It worked, but dropped the output considerably. Is this because the Zener is conducting a portion of my signal to ground? Also, if this were the case, would I not also hear some distortion (the sound was not noticeably didtorted)?

Thanks

[robthequiet]

I believe that Rod Elliot has an article about direct boxes that speaks to the issue of phantom power and different supply levels, here.

[anotherjim]

need to know more about your particular electret please. 2 or 3 way connector?

[tempus]

Thanks for the replies.

I checked out Rod's schematic, but it's powering a DI box, and from what I can tell, the audio is isolated from the zener by the 6.8K resistors. Is this correct? Also, wouldn't this increase the output Z of the box unacceptably?

Jim - it's a 2 connector mic, but I'm using a 3 connector cable (XLR) in a balanced impedance setup. Here's the schem:

[anotherjim]

Thanks, is the electret capsule between XLR 1&2?

Seen this?
http://www.epanorama.net/circuits/microphone_powering.html
If you've searched the web on this, you probably already found those in the link above. Those schemes don't allow the electret capsule to be directly grounded. If it's only 2 wire into the mic, you can't use them - you need a 3rd connection for screen ground around the capsule.

Something neater, that does allow the capsule ground...


Using more common components, this...

[PRR]

> 5.6v zener from pin 2 to ground. It worked, but dropped the output

6dB. Half the signal is shorted. The active Zener is ~~30 Ohms dynamic impedance, which is near-zero in terms of 2K FET loads.

Why do you need 5V power? As you say, "all" electret capsules can stand 10V from a few K of resistance.

If you must do 5V, change your regulator. LM7805.

If "5.6V" is a Government Spec, use LM7805 with a plain diode in series with the Gnd pin.

[tempus]

Thanks for the replies.

Jim - I'm trying to set this up inside the XLR jack that connects the mic to the mixer, and don't have the means to etch an SMD board, so the electronics in the 2 schematics (although helpful) won't fit.

PRR - The voltage doesn't have to be 5.6, that was just the value of the Zener I had lying around. Also, I can't just use a regulator, for 2 reasons; it won't fit inside the jack, and it would be inline with the audio, which, although I've never tried it, I don't think would work (would it?). I had thought about putting resistor in series with the regulator (inside the jack), but figured that would raise the ouput Z of the mic unacceptably. Is this the case?

Thanks again

[tempus]

Hang on - or would putting a resistor in series with the Zener actually lower the ouput Z of the mic, as it would be in parallel with the 2.2k resistor leaving the regulator?

I should also clarify - the schem I attached in the earlier post is from the custom preamp/power supply I mentioned in my original post. The 7809, the 2.2K resistor, and the 3 pinned symbol are all in that box, with the 3 pinned symbol being the XLR female that the mic plugs into.

[PRR]

[anotherjim]

Would you try...

XLR cold (3) to zener cathode. Zener anode to XLR ground.
Electret power feed resistor from Zener cathode to Electret + wire. IIRC, Panasonic recommend 2volts across their electret capsules, so you might select the resistor to give that with your chosen zener voltage (max of 10v).
Cap large enough (10uF seems popular) to AC couple Electret + wire to XLR hot (2). Cap polarity ~ the XLR hot will be more positive than the electret +. The cap should be 63V rated.
Electret ground to XLR ground (1).

This is not a balanced circuit.

[anotherjim]

If anybody is interested, here's what's on my breadboard...


Switch on noise when phantom power first enabled is rather raucous for about 5 seconds as the blocking cap C1 charges.
The second 9v1 zener, D2, is there because when phantom power is applied, C1 initially behaves as a short circuit, placing the full hot wire voltage on the electret & over-riding the regulated voltage from zener D1. D2 prevents this endangering the electret. Once C1 is fully charged (P48 on the XLR side, about 3v5 on the electret), D2 has no effect.

You might get away with a 50v rated C1, but one of my mic pre's has 53volt phantom, so 50v is a tad too close for comfort IMHO.

[tempus]

Thanks for all your help on this guys.

It looks like I'm either going to have to put this thing in a box or bite the bullet and just go with an electret that has a separate terminal for V+.

Anotherjim - have you actually tried this? How does it sound (is it noisy, etc)? Also, how does it sound when something less than the standard 48v is applied? I know of gear where phantom power may be more like 15-20v.

Thanks again

[anotherjim]

Tried on breadboard. It sounds like a... a microphone! Only listened to room sounds, but it's clear & sensitive. I'm using a Panasonic full range omni capsule (WM63A?) - don't have a commercial electret mic with 2 wire connection. Although it's not balanced signal, the pre-amp still rejects the AC hum picked up on the open breadboard.

About its low volts phantom performance, if I add a capacitor across D1 to store some voltage, it carries on working a few seconds after phantom is switched off until under a volt, so it shouldn't mind a low voltage feed. The zener only limits the volts to its rating, it doesn't prevent the voltage going lower than that.