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Best way to test if a microphone is damaged

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Best way to test if a microphone is damaged

Postby Amontillado » Sun Jun 24, 2018 1:30 pm

Hi all, I'm thinking of hiring out a couple of mics I have lying around, make a bit of extra dough.

I'm just planning ahead to when they're returned by a customer - obviously I can just plug the thing in to check it still works, but is there such a thing as damage that might only be evident at higher volumes?

It's one thing to do some quick 1-2-1-2-ing but I'm less keen on doing death metal screams or wacking snare drums in the living room of a Tuesday evening just to check the diaphragm is still good or whatever.

Also I heard multimeter testing can damage the coils, any truth in that? I've got an SM-57 and a Rode NT1. Cheers
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Re: Best way to test if a microphone is damaged

Postby Hugh Robjohns » Sun Jun 24, 2018 3:05 pm

Yes, testing a dynamic mic with a multimeter can potentially damage the voice coil or diaphragm suspension or both.

You don't need to do anything particularly complicated to check a mic, especially one as robust as the SM57. Just record some voice that you know well, and listen back. If its distorting or bass-light due to (very unlikely) diaphragm damage you'll hear it. Same with the NT1.
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Re: Best way to test if a microphone is damaged

Postby ef37a » Mon Jun 25, 2018 6:21 am

The Shure mics also contain transformers and these can be magnetized by DC from a meter.

The continuity range of most DMMs pushes enough current to light LEDs quite brightly. Handy for On Board Testing but bad news for certain transformers!

Look at any traff mnfctrs site and you will see they say "do not test with DC". Long been a no-no for tape heads of course.

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Re: Best way to test if a microphone is damaged

Postby Tim Gillett » Mon Jun 25, 2018 7:16 am

Are you sure about this Dave? My DMM cant put out that much current on Diode Check or the Resistance ranges. Also I occasionally test tape head coil continuity with my DMM. Tape heads might become minutely magnetised but that's about all IME.
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Re: Best way to test if a microphone is damaged

Postby ef37a » Mon Jun 25, 2018 8:26 am

Tim Gillett wrote:Are you sure about this Dave? My DMM cant put out that much current on Diode Check or the Resistance ranges. Also I occasionally test tape head coil continuity with my DMM. Tape heads might become minutely magnetised but that's about all IME.

Yus Tim! I used to 'buzz' the cableforms back to the front panel on B's S1 amps. V quick way to prove the wires, LEDs and PCB. 'Snot blindin!
That was with a Fluke 83 on Diode test. The meter also has a 'squeaker' mode and I will confess to using that to test mic circuits (the mic squeaks!) but then all my mics are pretty cheap **** . I would not do it to my Reslo RB!

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Re: Best way to test if a microphone is damaged

Postby Hugh Robjohns » Mon Jun 25, 2018 10:45 am

Tim Gillett wrote:Are you sure about this Dave? My DMM cant put out that much current on Diode Check or the Resistance ranges.

The voltage and current applied to the device under test will vary considerably between different designs the multimeter, as well as the selected resistance range setting.

Modern digital meters tend to operate with lower currents and are therefore generally 'safer' than old mechanical Avos... but passing any DC current through a sensitive coil or small transformer is patently inadvisable and could potentially result in damage or magnetisation. This kind of testing was emphatically warned against during my initial BBC engineering training, and it's just not something I would ever recommend today -- there are much better and safer ways of testing these kinds of components when necessary with AC signals and proper impedance testers.

More importantly for the OP's situation, any kind of metered testing is completely unnecessary.

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Re: Best way to test if a microphone is damaged

Postby Tim Gillett » Tue Jun 26, 2018 1:04 am

ef37a wrote:
The continuity range of most DMMs pushes enough current to light LEDs quite brightly. Handy for On Board Testing but bad news for certain transformers!

Quantifying this a little might help.

For full scale deflection the old AVO apparently required a current of 16.6mA.
I guess that would have lit an LED but maybe not fully.

I just tested one of my workshop DMM's with another. My old Hioki DMM (made in 1983) had a Diode Check short circuit current of approx. 0.3mA, or less than 2% of the AVO FSD current. Havent typical LED's required an average working current around 20mA , or over 60 times greater?

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Re: Best way to test if a microphone is damaged

Postby ef37a » Tue Jun 26, 2018 5:28 am

There is a great difference Tim between the 'paper' working current of an LED and that which will produce useful indication. This is something some equipment designers have not realized and just gone for the book value and the result is often coruscating!

Some years ago I needed LEDs to indicate the presence of phantom power AT THE XLR. Naturally I did not want to waste hard won mAs so I bought a 'lucky bag' of LEDs from Maplin and went through them . I found many that were easily visible in normal room lighting at 1mA and 1/2 doz that were quite usable at 200 MICRO amps!

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Re: Best way to test if a microphone is damaged

Postby Tim Gillett » Tue Jun 26, 2018 6:53 am

I just checked a few junk box LED's using the Hioki's Diode Test. At first I thought they were not illuminating at all. It was only when I viewed them dead in front (of the inbuilt lens) and deliberately switched them on/off/on/off that I noticed the weak light. If I hadnt been watching out for it I'm not sure I would have seen it.

Re magnetising tape heads, a common Studer Revox repro preamp input direct couples to the repro head. The makers explain that some DC constantly flows through the head, but not enough to magnetise the head. This is my point about magnetisation. It helps to know what is a safe and not a safe value of direct current in any given transformer.

Sure for the inexpert, a blanket rule never to apply any DC to a signal transformer might be a good rule, but we as technicians should have a more complete understanding and know how to test various transformer/head windings with DC safely.
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Re: Best way to test if a microphone is damaged

Postby ef37a » Tue Jun 26, 2018 8:50 am

Well bully for Studer Tim but I would prefer not to pass ANY DC through my tape heads, transformers or indeed, pickup cartridges!

One can also assume a 'pro' company would expect tape ops to do a regular de-mag? I doubt there are many readers here that own a de-fluxer?

A thought though has occurred! We regularly read of mic pre amps in SoS that have various "warmf" or distortion controls. Some are valve based, others use overdrive techniques (afaik) and some talk of "transformer saturation"* but I have never read of using a DC bias to induce even harmonic distortion in a traff? If someone develops such a thing and makes a couple of mil', remember the old valve jockey who thought it up? !!

*Which, like "rms watts" is nonsense. If a valve, transistor OR transformer is "saturated" it has ceased to work and is just hard on. Normal valve circuits won't actually let you saturate the device, it is very bad for them....BUT! I guess like "rms watts" we are stuck with the bloody daft term.

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