Watts Up, Ohmie ?

[MicMaven]

Quick Technical Question:

What will happen (i.e., go wrong) if I power an electret condenser microphone designed for 48v with just 10 volts?

Background:

BM-700/800 condenser microphones are designed to operate with either 48v from audio gear, or just 5-10v from a computer sound card. What can/will happen if I operate a condenser microphone expecting 48v on an unbalanced audio circuit providing just 10 volts? While I COULD just try it and see what gives, I do not like surprises, especially surprises that might cost serious money to fix, just to satisfy my curiosity. If nothing bad looms on the horizon, it opens the door for multiple applications. Is the answer be different for an electret condenser than a true condenser? (I assume the answer to this last bit is yes, but I don't profess to know for sure.)

Any traction? James - K8JHR

 

[Khron]

Depends entirely on the circuitry between the capsule and the output.

 

[ccaudle]

The bias connection from a computer soundcard is completely different. I don't see how a balanced output made for phantom works with a PC soundcard.
Do they actually have an XLR to 1/8" plug adapter, or separate connection for soundcard?
You may have to pull the mic apart and trace out what circuit you have.

 

[MicMaven]

Um ... OK... I get where you guys are coming from. Perhaps a real world example would help clarify the situation:

Imagine a radio transceiver with an 8-pin Foster jack with three pins assigned to microphone input. Pin 1 is Mic signal, Pin 2 provides 10v DC, and Pin 3 is Mic Ground.

A BM-700/800 is designed to work with either 48v on an audio device, or with just 5 volts bias voltage. Connect the radio's Pin 1 to Pin 2 of the microphone's XLR connector. Connect the radio's 10 volt Pin 2 to Pin 3 on the microphone's XLR connector. Connect Pin 3 on the rig to Pin 1 on the microphone's XLR connector - and Voilà, it works! But then, again, BM700/800 microphones are designed to work with less than 48 volts, although they sound substantially more robust with 48v.

So, what if I connect a CAD GXL-1800 to the rig using this same wiring scheme? I presume it is not intended to operate on less than 48 volts, but then I do not know what happens if I do. Will that damage the CAD GXL-1800? Will it seem OK for awhile, and be toast in the long run? What about a JLI Alice OPA board with an electret condenser capsule in the same scenario? Will that be OK. Damaging the radio would be disastrous and expensive. Damaging the microphone would only be expensive. I appreciate some things will work and seem OK for awhile, but not necessarily long term. (I am not even considering doing this with a true condenser microphone as I am reasonably confident it requires all 48 volts full time.)

I am not using an adapter, although I suppose the cable I cobbled together is, itself, an ersatz adapter.

While I appreciate the comment, I hope it does not boil down to MY tracing the circuit inside each microphone, as I am a rookie technician and that is still above my experience and pay grade at this point.

Footnote: I understand bias voltage on a sound card is different from phantom voltage on a mixer or preamp. Also, while BM-700/800 microphones typically include a cable with 3-pin XLR on one end, and 3.5mm TRS phone plug on the other end, I assembled my own cable with a 3-pin XLR-connector on one end, and an 8-pin-Foster plug on the other end. (We can safely ignore the other 5 pins on the Foster plug, because they concern radio functions aside from audio input.)

I hope this clarifies the question. Thanks for pitching in. James - K8JHR

 

[kingkorg]

If a mic will work at all it will probably exhibit much lower headrom, and much higher THD. I don't see how you could damage it.

 

[MicMaven]

If a mic will work at all it will probably exhibit much lower headrom, and much higher THD. I don't see how you could damage it.

OK, I understand that. Whew. I may well proceed and just try it. I will let you know how it goes.

Funny you should mention THD. Even high end radios of this type sport audio specifications as high as 10% THD! While that is a worst case spec, you might agree .1% THD is not very good. But then, considering transmit audio and RF bandwidth is a mere 3000 Hz, it is not exactly Hi Fi, and I probably sustain substantial THD in any case. :-(

Thanks, gang, for weighing in on this one. I prefer to ask questions, before shooting ... myself in the foot.

I will let you know how it goes. James - K8JHR

 

[k brown]

Aside from computer sound card, etc. - anytime you see a mic's specs state the operating voltage to be something like "9-52v", it means the incoming phantom is being zenered or regulated down to something just below 9v, so performance would not improve one bit by giving it anything more than 9v.

An exception is some electrets which have the option of operating on an internal battery; performance is often lower when working off the battery rather than phantom (which should be stated in the specifications).

 

[ccaudle]

performance would not improve one bit by giving it anything more than 9v.

True, but that is assuming the power supply is applied common mode, both pins connected to power. In the usual scheme for phantom powered mics there are two equal value resistors from pin 2 and 3 to a zener or regulator. If instead pub 3 is grounded the voltage will start off from a voltage divider, so your 10V supply now is no more than 5V.

 

[k brown]

. . . which is why I said "aside from computer sound card, etc."

 

[Murdock]

I'm also puzzled about lower polarization voltage. I have a measurement mic from Rohde & Schwarz here which can, according to the flyer, be polarized with either 150V or 15V... when using 15V it can withstand 166dB pressure with a k of 10% and when polarized with 150V it can withstand "only" 150dB but with a k of 2%.

 

[ccaudle]

I'm also puzzled about lower polarization voltage.

Lower polarization voltage results in lower capsule sensitivity, i.e. for a particular SPL there will be a lower voltage at the impedance conversion circuit which buffers the capsule.

when using 15V it can withstand 166dB pressure with a k of 10% and when polarized with 150V it can withstand "only" 150dB but with a k of 2%.

That seems to indicate that the capsule delivers the maximum voltage the impedance conversion circuit can handle at 150dB SPL when polarized with 150V. 15V is 20dB lower than 150V, so makes sense that the circuit could then handle nearly 20dB higher SPL at 15V. The higher distortion probably indicates that it hits the limit of capsule linearity around that point.

 

[kingkorg]

I'm also puzzled about lower polarization voltage. I have a measurement mic from Rohde & Schwarz here which can, according to the flyer, be polarized with either 150V or 15V... when using 15V it can withstand 166dB pressure with a k of 10% and when polarized with 150V it can withstand "only" 150dB but with a k of 2%.

Genuinely interested, how do you generate 166db or even 150db of clean signal?

 

[ccaudle]

Genuinely interested, how do you generate 166db or even 150db of clean signal?

What do you mean by "clean" signal? Do you mean for testing the microphone?
The two ways I know of are by electrostatic screens placed in front of the diaphragm, or for single or only a few frequencies, you have a motor driven piston with the microphone sealed to the tube containing the piston.
Those at least are the ways used to calibrate SPL sensitivity of measurement microphones. Understanding distortion performance of the electrostatic actuation method seems more straight forward to me than of the piston method. Or rather, I know reasonably well the accuracy limits of electrical measurements of the driving signal for the electrostatic method, but I don't know how well you can measure the mechanical performance of the piston method, because any deviation from sinusoidal motion would be distortion in the signal generator.

 

[molke]

One possibility would be a so-called ‘Impedanzsprungrohr’, which is tubes with different cross-sections.


Higher pressure at the end is just a side effect. More important is that rooms with different cross-sections apparently no longer “develop harmonics”. As a practical implementation, Holger Pastillé claims to have reached 157 dB SPL at 1 kHz and a THD of about 0.05 % up to 150 dB SPL with this kind of setup. Other frequencies are not that great (over 1 % for 321 Hz, but 128 Hz is still below 0.25 % at 150 dB).

 

[kingkorg]

What do you mean by "clean" signal? Do you mean for testing the microphone?
The two ways I know of are by electrostatic screens placed in front of the diaphragm, or for single or only a few frequencies, you have a motor driven piston with the microphone sealed to the tube containing the piston.
Those at least are the ways used to calibrate SPL sensitivity of measurement microphones. Understanding distortion performance of the electrostatic actuation method seems more straight forward to me than of the piston method. Or rather, I know reasonably well the accuracy limits of electrical measurements of the driving signal for the electrostatic method, but I don't know how well you can measure the mechanical performance of the piston method, because any deviation from sinusoidal motion would be distortion in the signal generator.

Yes i meant distortion free source at these levels.

 

[ccaudle]

I don't really have hands on experience with those calibrators, but the B&K Measurement Microphone Handbook has some nice diagrams.