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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:
QUOTE
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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mixminus1
post Sep 8 2011, 20:29
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Wait, andy o: you're taking the 1/8" TRS output of the Hosa adapter straight into the TRRS coupler, and then plugging the TRRS cable into the touch's headphone jack?

Ah, that's the problem: inserting an 1/8" TRS plug into an 1/8" TRRS coupler will short the back ring (closest to sleeve) to ground all the way through the rest of the signal chain, and therefore, the touch will see the TRRS plug as if it were just a TRS, and therefore just headphones.

In order for the touch to recognize a given plug as coming from something with a mic, the back ring connection must *not* be connected to ground - it can be nothing, it can be a data bus (which I believe is how it's done with the various in-line control pods), but it must not be shorted to ground.

So, you'll need to "remap" the output from the Hosa adapter so that it lands on only the right channel ring of the TRRS, but the back ring is left unconnected - IOW, exactly what I did with my camcorder cable. smile.gif

You'll need an 1/8" TRS female-female coupler to connect the output of the Hosa to an 1/8"-to-dual-RCA cable, and then an 1/8" TRRS-to-3 RCA camcorder cable and an RCA barrel to couple one of the RCAs from the 1/8"-to-dual-RCA to the right channel of the camcorder cable.

Edit: Some time ago at work, I remember seeing an 1/8" TRS female-to-dual RCA female adapter - a molded adapter, not a cable - in one of our A/V adapter "tackle boxes", which would be perfect for what you're doing, but honestly, I've never seen another...but it's got to be out there somewhere!

This post has been edited by mixminus1: Sep 8 2011, 20:42


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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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