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Impedance matching Shure Beta 58A, connecting to iPod Touch
andy o
post Sep 8 2011, 10:10
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Hey guys, I've been reading up about connecting a Beta 58A which is 150 Ohm (pdf spec sheet) and my iPod Touch 4, which apparently has the same circuitry as iPhone 4, which supposedly doesn't detect any mic below 800 Ohm.

So I got the Hosa MIT-156 impedance transformer:
This adaptor is designed to match 600R output impedance to 2.5K input impedance. It may be used to connect a microphone to a portable recording device, camcorder, or PC.

I don't know the math of impedance matching so sorry if this is obvious. From that information, can you tell what will be the output impedance of the Beta 58 + MIT-156?

So far the Touch can't detect the rig (works for a computer mic I have though, so my cable adapters that come after the MIT-156 are working fine). And when I connect the mic to the PC via the mic inputs of either an Asus U1 or the built-in IDT sound device (on Windows 7). I get low signal and very loud hiss. What am I missing?
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post Sep 8 2011, 20:08
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I'm thinking the problem is with the adapters... somehow... It looks like you have all the right stuff... Are you sure the male-to-male adapters are stereo (TRS)? Do you have a continuity tester or ohmmeter? I think the tip on the TTRS connector needs to "eventually" connect to the tip on the Hosa plug.

I read the iPhone has the ground/mic reversed from a typical TRRS connector, so I just bought that adapter.
I'm not sure what "reversed" means... I believe the tip is the mic signal, and the sleeve is the ground... The two ring connections need to line-up and work with a regular TRS headphone plug . The DC polarity might be reversed for an electret condenser "computer mic", but your dynamic mic doesn't use DC power so the DC polarity doesn't matter.

It seems to me that lower impedance should be easier to "detect" than a higher impedance, but I don't know what the iPod is doing.

... to the PC via the mic inputs... I get low signal and very loud hiss. What am I missing?
Most computer mic inputs are rather low quality. I don't know about the iPhone/iPod. A different transformer could probably give you more signal. But, it depends on what you're recording too. If you are recording a live rock band, you can probably get-away with no transformer...

For quality recordings, a better solution is a mixer or preamp (with a low impedance XLR input) into the computer's line-in, or a USB interface with an XLR mic input. (Shure makes a little "microphone to USB" adapter, but it costs more than your PG58. sad.gif ) Or if you need something more portable, a digital recorder might be the solution.

I don't know the math of impedance matching so sorry if this is obvious. From that information,
The voltage ratio is proportional to the impedance ratio. 600 -> 2500 ohms will give you ~ 4x the signal (~ +12 dB). A 600 -> 10k transformer would be in the ballpark of +20dB.

The voltage ratio calculations only hold if you don't "load down" the transformer output (secondary) with a low-impedance load. And, you don't get a power boost with a step-up transformer.... Voltage goes up and current goes down.
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Posts in this topic
- andy o   Impedance matching Shure Beta 58A   Sep 8 2011, 10:10
- - mixminus1   Hmm, that Hosa adapter should at least be getting ...   Sep 8 2011, 16:29
- - Speedskater   Remember, a transformer cannot do both impedance m...   Sep 8 2011, 18:20
- - andy o   Thanks for the info. The other adapters are a 1/8...   Sep 8 2011, 18:48
- - DVDdoug   I'm thinking the problem is with the adapters....   Sep 8 2011, 20:08
|- - andy o   QUOTE (DVDdoug @ Sep 8 2011, 12:08) I...   Sep 8 2011, 21:38
- - mixminus1   Wait, andy o: you're taking the 1/8" TRS...   Sep 8 2011, 20:29
- - andy o   Gimme some time to process all the info I've b...   Sep 8 2011, 20:45
- - mixminus1   Hmm, without knowing exactly how the eBay adapter ...   Sep 8 2011, 22:08
- - andy o   No, you're right, I think it's wired exact...   Sep 8 2011, 22:24
- - andy o   Didn't work. I think something's funny wit...   Sep 8 2011, 23:23
|- - andy o   OK, I'm reviving this, cause, dumbass me, I ju...   Jul 27 2012, 06:38
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 27 2012, 01:38) I...   Jul 27 2012, 15:38
- - andy o   I'm getting a lot of hiss noise because the mi...   Jul 27 2012, 16:35
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 27 2012, 11:35) I...   Jul 27 2012, 19:44
- - mixminus1   Correct, the 58A is a dynamic, albeit a high(er)-o...   Jul 27 2012, 18:00
|- - andy o   QUOTE (mixminus1 @ Jul 27 2012, 10:00) It...   Jul 27 2012, 23:43
- - andy o   thanks for the recommendation, Arnold. I found it ...   Jul 27 2012, 23:59
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 27 2012, 18:59) thank...   Jul 28 2012, 00:03
- - andy o   I think that would be even better, I mean I was ju...   Jul 28 2012, 00:11
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 27 2012, 19:11) I thi...   Jul 28 2012, 10:49
- - andy o   What about this one AT2020? It's even cheaper,...   Jul 28 2012, 00:21
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 27 2012, 19:21) What ...   Jul 28 2012, 03:39
- - andy o   Great, thanks for the input.   Jul 28 2012, 03:59
- - andy o   I have some questions about mic specs, I'll op...   Jul 29 2012, 21:10
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 29 2012, 16:10) I hav...   Jul 30 2012, 11:40
- - andy o   I ended up getting the AT2035, found it for ...   Jul 31 2012, 00:58
|- - Arnold B. Krueger   QUOTE (andy o @ Jul 30 2012, 19:58) I end...   Aug 2 2012, 18:53
- - andy o   Thanks for the tip(s). I would never have consider...   Aug 3 2012, 02:26

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