Noisey T12 to Phantom Convertor

[corgan4321]

Hello.

I just built one of these: http://www.uneeda-audio.com/phantom/p48t122.jpg

It works nicely however I'm getting some white noise using an old Sennheiser MKH mic. I tested the mic with an external PSU hooked up just with the 180R resistors and the blocking resistors and that's much quieter. Is this possibly a noisey zener diode situation? Only differences in my circuit and the Uneeda one is I used bipolars for the DC blocking and a 82uF 25V for the parallel cap with the zener.

Thank you!

 

[corgan4321]

I've been reading about how zeners above 3 or so volts are very noisey. Any thoughts on replacing the 1N5242b with four 1N5225B's in series?

 

[ccaudle]

Hello.

I just built one of these: http://www.uneeda-audio.com/phantom/p48t122.jpg

It works nicely however I'm getting some white noise using an old Sennheiser MKH mic. I tested the mic with an external PSU hooked up just with the 180R resistors and the blocking resistors and that's much quieter. Is this possibly a noisey zener diode situation? Only differences in my circuit and the Uneeda one is I used bipolars for the DC blocking and a 82uF 25V for the parallel cap with the zener.

Thank you!

Have you verified that the zener was marked correctly and you actually have 12V at the junction of the 2.2K and 180 resistors? Even if the zener was noisy, the combination of the 1K parallel resistance of the feed resistors with the 82 uF cap should make a single pole filter with a 2Hz -3dB point that will filter the noise.
Now that I had a second thought on that, I'm not sure if that calculation works for the cut-off frequency of a cap on a zener, the zener may lower the effective impedance feeding the capacitor. I may need to research that a bit.
Can you tack on a couple extra capacitors to see what effect that has? That would help determine if the noise is being generated in the zener.

 

[Ivan K.]

Hello.

I just built one of these: http://www.uneeda-audio.com/phantom/p48t122.jpg

It works nicely however I'm getting some white noise using an old Sennheiser MKH mic. I tested the mic with an external PSU hooked up just with the 180R resistors and the blocking resistors and that's much quieter. Is this possibly a noisey zener diode situation? Only differences in my circuit and the Uneeda one is I used bipolars for the DC blocking and a 82uF 25V for the parallel cap with the zener.

Thank you!

 

[Ivan K.]

Hi, My company used to make T-powered microphone power supplies for Sennheiser and Schoeps mics using 2 * 9V batteries and an LM723 regulator. The Sennheiser noise spec--if I recall from a very long time ago, was 1 uV p-p and we had to select the 723s to meet or beat this number.

This is the long way 'round to saying that I'm not sure any Zener will do the trick. Possibly, one of the 3 terminal CV devices or one of the latest regulators designed for RF circuits. Of course, now you no longer have such a simple design!

 

[corgan4321]

Have you verified that the zener was marked correctly and you actually have 12V at the junction of the 2.2K and 180 resistors? Even if the zener was noisy, the combination of the 1K parallel resistance of the feed resistors with the 82 uF cap should make a single pole filter with a 2Hz -3dB point that will filter the noise.
Now that I had a second thought on that, I'm not sure if that calculation works for the cut-off frequency of a cap on a zener, the zener may lower the effective impedance feeding the capacitor. I may need to research that a bit.
Can you tack on a couple extra capacitors to see what effect that has? That would help determine if the noise is being generated in the zener.

Yes, the zener is functioning properly and giving me my 12V to power the microphone, it's just giving me noise with it! It's more of a shhhhhhhh kind of white noise or random noise, not a low frequency thing. I'll tack on some more parallel capacitance to see if it does anything but I'm skeptical...

 

[corgan4321]

Hi, My company used to make T-powered microphone power supplies for Sennheiser and Schoeps mics using 2 * 9V batteries and an LM723 regulator. The Sennheiser noise spec--if I recall from a very long time ago, was 1 uV p-p and we had to select the 723s to meet or beat this number.

This is the long way 'round to saying that I'm not sure any Zener will do the trick. Possibly, one of the 3 terminal CV devices or one of the latest regulators designed for RF circuits. Of course, now you no longer have such a simple design!

Thanks very much for your info! That's what I feared about the zeners in this application. I'd love to get something quiet that can run off of phantom power, these old MKH series mics seem very high quality and would be a nice convenience vs. building a small 12v linear supply.

Would something like this work do you think? https://www.mouser.com/ProductDetail/Microchip/LR8N3-G?qs=fAemwt7pb8mfOvPhoAlXTA==

 

[Ivan K.]

Thanks very much for your info! That's what I feared about the zeners in this application. I'd love to get something quiet that can run off of phantom power, these old MKH series mics seem very high quality and would be a nice convenience vs. building a small 12v linear supply.

Would something like this work do you think? https://www.mouser.com/ProductDetail/Microchip/LR8N3-G?qs=fAemwt7pb8mfOvPhoAlXTA==

Also, remember that the maximum power you can draw from a phantom supply is quite limited--perhaps 1/4W depending on the source & resistor size. My MKH405 need 10V at 5 mA, or 50 mW, so you should be able to do something with a couple of 9 V Batteries and a quiet regulator that will have a long life and be isolated from everything. See https://www.richardhess.com/manuals/sennheiser_MKH_104-105-404-405-804-805.pdf. The LR8 should work, but if they they don't give you noise specs--there's a reason! Look for low noise regulators and less than 1 uV of noise.

The MKH line are great mics, especially for exteriors and rugged conditions. The circuit was actually invented by Stephan Kudelski of Nagra fame and given to Mr. Sennheiser. They use an FM circuit that will never arc as can all the older 150V U-47s, C-12s etc. in humid conditions, i.e outdoors when recording in the rain or jungle. Even 48C mics are not immune.

 

[Bo Deadly]

it's just giving me noise with it! It's more of a shhhhhhhh kind of white noise or random noise, not a low frequency thing.

How are you quantifying the noise level? With enough gain you're going to hear hiss. Put the mic 1 meter away from a speaker playing a tone or music at a moderate level, adjust the mic pre gain to get +4dBu or whatever an appropriate level is for your recording rig and start recording. After 10 seconds, pause the recording, turn off the speaker (and really power it off so that it's silent) and record another 10 seconds of silence. Listen to the with and without signal and decide if the without part is really too noisy.

 

[tinn]

The noise of the zener is generated with the internal dynamic resistance of the zener.
It is about 7 Ohms. Your 100u(82u) capacitor has to filter a noise source with
that impedance and fails. I would try a 1000u capacitor and check the result.
As well you may use an additional resistor between cap and Zener (47Ohms).
Raising the dynamic impedance of the zener with 47 Ohms might work as well.
(Zener and 47 ohms serial)
This circuit does not work with mics that draw up to 10mA (some Neumanns).
with 6mA it should be fine.

Best Tito

 

[ccaudle]

Yes, the zener is functioning properly and giving me my 12V to power the microphone, it's just giving me noise with it! It's more of a shhhhhhhh kind of white noise or random noise, not a low frequency thing. I'll tack on some more parallel capacitance to see if it does anything but I'm skeptical...

I think you are right to be skeptical that just adding a little capacitance will help, the zener effective impedance is pretty low.
This post on DIYAudio has measurements for some zener diodes and LEDs. The 12V zener he used was only about 0.3uV noise, which doesn't seem very high. Do you have specs on the noise output of the linear power supply you used?
Noise measurements of zener diodes

There can be quite a bit of variation between different diode constructions, so perhaps the one you are using is particularly noisy compared to the ones tested in that DIYAudio post.

This post has some good general information on measuring power supply noise, including some measurements of 3-terminal regulators that probably aren't directly relevant in the current situation:
Noise in voltage references

Would something like this work do you think?

The spec for that Microchip regulator doesn't mention noise at all, so saying one way or the other would just be guessing on no basis.
Although in general, if a parameter is not in the datasheet, it isn't measured, it isn't guaranteed to be the same from batch to batch of devices, and the designers probably didn't care about it at all when working on the part. So no mention of noise in the datasheet is not a good sign if you are looking specifically for a low noise device.

 

[corgan4321]

The noise of the zener is generated with the internal dynamic resistance of the zener.
It is about 7 Ohms. Your 100u(82u) capacitor has to filter a noise source with
that impedance and fails. I would try a 1000u capacitor and check the result.
As well you may use an additional resistor between cap and Zener (47Ohms).
Raising the dynamic impedance of the zener with 47 Ohms might work as well.
(Zener and 47 ohms serial)
This circuit does not work with mics that draw up to 10mA (some Neumanns).
with 6mA it should be fine.

Best Tito

Thanks Tito,

I tried a 2200uf and it definitely brought the noise down a lot. I also tried a 47ohm resistor in in series with the cap, is that what you meant? It seemed to make it worse. The 47R made no difference when in series with the zener.

 

[corgan4321]

Also, remember that the maximum power you can draw from a phantom supply is quite limited--perhaps 1/4W depending on the source & resistor size. My MKH405 need 10V at 5 mA, or 50 mW, so you should be able to do something with a couple of 9 V Batteries and a quiet regulator that will have a long life and be isolated from everything. See https://www.richardhess.com/manuals/sennheiser_MKH_104-105-404-405-804-805.pdf. The LR8 should work, but if they they don't give you noise specs--there's a reason! Look for low noise regulators and less than 1 uV of noise.

The MKH line are great mics, especially for exteriors and rugged conditions. The circuit was actually invented by Stephan Kudelski of Nagra fame and given to Mr. Sennheiser. They use an FM circuit that will never arc as can all the older 150V U-47s, C-12s etc. in humid conditions, i.e outdoors when recording in the rain or jungle. Even 48C mics are not immune.

I happen to have a couple of old 723 regulators around so I'll give them a try with a couple of 9v batteries and see how they compare to the zener. Should this basic circuit work from the datasheet?