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 Post subject: Microphone Interface
PostPosted: 04 Jan 2013, 16:27 
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Joined: 04 Jun 2008, 20:59
Posts: 4326
Location: Arizona, USA
Hi Everyone, I recently got a calibrated microphone for doing audio measurements. Unfortunately, I had no way to connect it to anything. So after doing a bit of a search on what it really needed to work I built a small interface for it. The microphone requires a phantom power source between 15 and 48 volts and has a 200 ohm balanced output. I used a standard wall wart that gave out 32 VDC no load. It was rated for 24 VDC, but the drain is so small that it was able to deliver the full 32 volts. The schematic shows how an AC wall wart and rectifier could be used. The number of sections in the filter is probably excessive, but I like to be sure the DC is clean. Yes I know that a phantom powered microphone should be able to reject PS crud, but I want to be sure what I measure is not contaminated by PS junk. The matching transformer is from Edcor and is under $7. It has excellent specs (20-20KHZ) and is well suited for this application. The box is an ABS type and also inexpensive. Build is not complicated or critical as the impedances are low. It might be sensitive to nearby EMI sources so a metal box might actually be better. I can feed the output into either my laptop running TrueRTA software or into the Velleman PC based DSO on my main PC. Results agree with previous ones made on cruder gear.

Good Listening
Bruce
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 Post subject: Re: Microphone Interface
PostPosted: 04 Jan 2013, 17:52 
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Joined: 08 Aug 2009, 03:11
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Location: Chilliwack, BC
Question.... why not keep the PSU on the mic side and isolate it from the output?

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 Post subject: Re: Microphone Interface
PostPosted: 04 Jan 2013, 18:03 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
Did you consider using 3 x 9v batteries and pherhaps a 78L24 regulator?

Clean as "white"... :P


Cheers,

Miguel


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 Post subject: Re: Microphone Interface
PostPosted: 04 Jan 2013, 18:24 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, In the mic the negative PS connection is also the shield terminal on the cable and mic itself. My original thinking was that it ought to be continuous to the end amp. As shown the shield is continuous from the mic to the actual amp input. In theory it should be possible to achieve good S/N without that, but since it causes no issues as built I left it in place. Commercial gear would probably offer a "ground lift" feature that would allow disconnecting the two shields.

The reason I didn't use batteries (possible as the drain is low) was that I never intended it to be used away from main systems and power sources. Plus I generally dislike using batteries as they always seem to be dead when I want to use the device they were supposed to power. :bawling:
A good thought for individuals that need portable test gear.

Good listening
Bruce

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 Post subject: Re: Microphone Interface
PostPosted: 07 Jan 2013, 21:29 
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Joined: 28 May 2008, 21:53
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Location: Winnipeg, CANADA
Hi Bruce, nice work. I also just got the bug for a microphone setup but I got lazy and went the turnkey approach with the Dayton OmniMic. The mic looks similar to yours. I hope to give it a whirl soon.

Cheers

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 Post subject: Re: Microphone Interface
PostPosted: 08 Jan 2013, 10:05 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Thanks, Sort of going OT, but using the device has taught me a bit about speakers. With the mic, interface,laptop and running TrueRTA in 1/6 octave mode I had some rather surprising results. With non-bipolar speakers set up ought to be a snap, but with the ESLs I have not so. It is interesting to watch the relative sound levels shift with rather small changes in speaker position. Using the "relative" setting that levels the background noise to a flat level and then feeding in pink noise you can really see how funky your listening environment is. Trying to get both ESLs set at the same time is a lesson in futility. Too many things change each time you move, twist or tilt one. Setting them individually works though as long as they are approximately in the same orientation and distance from the listening area. In my case using the mic and stuff works. The speakers ended up close to where my ears told me they would go (about 2 inches off from where I had them). Slightly asymmetrical in placement. Still the final test is how everything sounds. Great in this case. Also related is the correlation of the results with a program I use to determine room nodes. Yes ....the 56 HZ and 180 HZ ones are there. Very easy to see with the setup.

Good listening
Bruce

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