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Author Topic: Electrical testing of tube microphones?  (Read 660 times)

mutterd

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Electrical testing of tube microphones?
« on: March 06, 2017, 02:19:08 am »

Hey guys,

Im wondering what the best procedure is to do frequency response, THD and self noise tests of tube microphones.

I searched the archives and could find many posts with tutorials on how to do tests WITHOUT proper test gear - but I actually a full test rig including an AP System One and a Sound Tech 1710a..

thing is - I haven't done much design/test work with microphones, so any advice or guidance would be much appreciated.

Ive read about swapping the capsule for a similar value cap for self noise - is that the best way? otherwise seems straight forward - a noise measurement across the output of the mic. Whats an acceptable ballpark figure in dBv?

But for frequency response - generator output across the grid resister? that seems problematic as the low source impedance of the generator output doesn't really mimic the capsule... Whats a better option?

and for THD? what sort of input level to the amplifier circuit?

and for the power supply? acceptable ripple/noise for the B+ and Heater?

thanks so much guys.





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Jim Williams

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Re: Electrical testing of tube microphones?
« Reply #1 on: March 06, 2017, 11:15:59 am »

Contact AP for guidelines, they have them.
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Kai

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Re: Electrical testing of tube microphones?
« Reply #2 on: March 07, 2017, 06:36:42 pm »

 For what reason do you want to do those measurements?
 The answer to this question is important for choosing the right test procedures.
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mutterd

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Re: Electrical testing of tube microphones?
« Reply #3 on: March 08, 2017, 06:22:48 am »

Thanks so much Jim - I contacted them today and they were able to give me a few tricks I didnt know.

that, and in digging thru the site i found lots of golden nuggets for later reading!

but to be clear - Im asking about concepts of termination - more specifically, how to terminate the low source impedance signal generator to the amp stage to get meaningful measurements.

Kai - The reason I'm asking is I have designed a new power supply for tube microphone - Id like to be able to make a full set of measurements (signal to noise/frequency response/phase shift/THD) for the mic (measurements on the power supply I'm comfortable with)

By hooking the generator across the capsule, coupled thru a 2uF film cap Im able to plot a frequency response graph that looks like what I would expect, and able to get THD numbers that make sense with the signal level, gain and output transformer.

While I am happy with the measurements I'm getting - I could make an argument that this method loads the circuit differently than that of the capsule - so I'm wondering if you guys have either:
a. a better/proper way.
b. an explanation as to why this would be acceptable.

also -

thanks guys.

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radardoug

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Re: Electrical testing of tube microphones?
« Reply #4 on: March 08, 2017, 03:49:41 pm »

If you connect through a value that is much greater than the capsule value as you are doing, you skew the results due to the low impedance of the generator. Better to use a cap about the same value as the capsule.
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mutterd

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Re: Electrical testing of tube microphones?
« Reply #5 on: March 08, 2017, 06:43:56 pm »

Thanks so much Doug, I appreciate that!

would you mind explaining your reasoning?

thats where I started with this, which may well be the correct way - but...

in my case the 2uf is not there to mimic the capsule in any way, but rather as a coupling capacitor, which blocks the DC polarization voltage from finding ground thru the (much) lower impedance generator output.

The capsule in the mic has a capacitance of 85pf - that in mind - absolutely the -3dB point of the filter that is created by the series cap (2uf in my case vs 85pf) and the shunt grid resister is much different (4Hz vs 94Hz)...

but after 180Hz or so - should they not respond the same - so is that where my measurements are skewed? Just frequencies <180Hz?

But isn't that then just a termination error? The whole point is to not have the means of measurement affect the measurements (impossible i know, but that is the goal)... To not load the circuit with the measurement apparatus...

So then were back at - is it even an acceptable means of measurement?

Saying that terminating the generator thru a similar value coupling cap somehow more closely resembles actual circuit behaviour assumes that the capsule is as frequency and phase linear as the generator and that the internal capacitance of the capsule acts as a capacitance in series between a perfect generator (the capsule) and an amplifier stage...

right? can we make those assumptions?

am I way off here?

thanks guys.




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Kai

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Re: Electrical testing of tube microphones?
« Reply #6 on: March 09, 2017, 06:14:38 am »

It's as simple as this:
You have to couple the generator to the amplifier (with removed capsule) by a high quality cap (preferable Styroflex or PP) of the exacting value of the capsule to get proper results, if you want to exclude the capsule from the measurement.
For signal to noise measurement of the whole microphone it's absolutely necessary to have a very quiet ambience, e.g. an isolation box within a quiet room, because the capsule contributes to the noise in several ways.
A properly calibrated sound level meter is necessary to create a reference sound level.
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radardoug

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Re: Electrical testing of tube microphones?
« Reply #7 on: March 09, 2017, 02:39:46 pm »

You can regard the capsule as a pure capacitance of say 50pF. The generator is different, having a finite output impedance. When you couple this to the DUT (Device Under Test, K.H.) with a large capacitance, then you are driving the amplifier from a low impedance completely unlike the capsule.

This means that amplifier noise will be much lower in the DUT. So this may have effects on noise and distortion measurements. As an advantage, the normally very high input impedance is swamped by the generator's low impedance, meaning noise figures will be very good. Remember the normal amplifier (the condenser mic's impedance converter, K.H.) has an input impedance in the range of 500 megohms to several gig ohms, depending on design.
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Jim Williams

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Re: Electrical testing of tube microphones?
« Reply #8 on: March 10, 2017, 11:19:32 am »

You forget that the capsule presents a lower source noise spec than an open circuit reading the very high bias resistor. It's wired in and creates a high frequency AC load to ground just as a test capacitor would do. This is why the noise measurements will be quite a bit lower with the capsule than without as long as there is no air movement.
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