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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, 16:29
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Hmm, that Hosa adapter should at least be getting you close enough to get some kind of signal into the touch. Its 600 ohm input rating does mean that the signal will be lower than optimal with the Beta 58 (or pretty much any other balanced dynamic mic), but again, you should be getting something.

I've got an iPhone 4, and just for a quick test, I took the headphone out of my external USB Sound Blaster (which should be down around 5-10 ohms) through a generic camcorder A/V breakout cable (3 RCAs to an 1/8" tip-ring-ring-sleeve), and going in on the right channel RCA, I was able to get sound into the Voice Memo app. Judging by the amount of attenuation I had to dial in in foobar2000, it appears to be a fairly typical mic level input, so I'm not sure why you couldn't get anything at all - what are the additional cable adapters you mention?

As far as why you're getting a severe level mismatch going into your PC, those "mic inputs" are typically somewhere between a true mic level (-60 dB) and true line level (-10 dB) sensitivity - -30 dB is pretty common, so it appears that the Hosa isn't stepping the signal up quite enough.

FWIW, since there are no specs given for the Hosa, here is the manual for a very similar adapter from Shure (that costs almost three times as much as the Hosa). The Hosa *should* be constructed very similarly to the diagram shown in the Shure manual..."should" being the operative word here. Note how the Shure has the option to lower the input impedance in order to increase the output level...I wonder how closely Hosa copied their design? wink.gif


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Speedskater
post Sep 8 2011, 18:20
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Remember, a transformer cannot do both impedance matching and signal level matching at the same time. If you pick a impedance ratio the voltage is what the math dictates.


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andy o
post Sep 8 2011, 18:48
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Thanks for the info.

The other adapters are a 1/8" male to 1/8" male TRRS cable, and this, with 2 female-to-female stereo TRRS adapters at the ends. I know there should be a simpler option, but the cables from e.g. KV Connection are much more expensive, and it should work the same. With other mics like the computer mic mentioned it does work fine, with stereo headphone output as well. I read the iPhone has the ground/mic reversed from a typical TRRS connector, so I just bought that adapter.

Unfortunately the Hosa doesn't seem to have the option to lower the input impedance. Here is all the info in the back of the package, and pics of the insides:
http://photos.smugmug.com/photos/i-QsJxW2T...i-QsJxW2T-M.jpg
http://photos.smugmug.com/photos/i-XSpCpHB...i-XSpCpHB-M.jpg
http://photos.smugmug.com/photos/i-2Rfp75N...i-2Rfp75N-L.jpg

The Touch doesn't also seem to detect a headphone output from a JVC SU-DH1 headphone processor that I tried to use as a preamp.
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DVDdoug
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.

QUOTE
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.

QUOTE
... 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.

QUOTE
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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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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andy o
post Sep 8 2011, 20:45
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Gimme some time to process all the info I've been given, but I don't think I'm doing this: "you're taking the 1/8" TRS output of the Hosa adapter straight into the TRRS coupler".

The 1/8" TRS output from the Hosa goes into a standard TRS mic input from the adapter I got on ebay (via a stereo female-to-female TRS coupler). That adapter is meant to split the TRRS of a standard iPhone headset into separate TRS connectors, a stereo headphone plus mono mic signal, to be used in PCs. I'm just using it in reverse. I could not find adapters as cheap that could do the same thing natively, but I figure it's the same.

[EDIT: I see that I made a mistake in my post above, the female couplers are TRS, not TRRS]

This post has been edited by andy o: Sep 8 2011, 20:50
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andy o
post Sep 8 2011, 21:38
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QUOTE (DVDdoug @ Sep 8 2011, 12:08) *
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 don't have these instruments, but the headphones work fine in stereo with these couplers and the ebay adapter.

QUOTE
QUOTE
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.
This is what I meant.

QUOTE
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.
I've seen this 800 Ohm number quoted in several places, but nowhere "official". KV connection seems to be a good source though. They were in contact with this guy, from whom I'm taking some tips (never thought I'd take audio tips from someone who's also telling me why I should marry in a church and not at the beach smile.gif)

QUOTE
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.
This project started when I bought the rather inexpensive Four Track app (and then FiRe 2). The goal is (was?) to do some decent recordings in the cheap. I already had the Beta 58A from my band days, so I'm trying to get the cheapest working rig. I'd get something like a Zoom H2N, but I'm not that serious into recording to justify it.

QUOTE
QUOTE
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.

The guy also links to the CP8201 (pdf spec sheet). It says 250->50,000 Ohms. I'm wondering if this would be better than say KV's own iPhone adapter. Unfortunately they don't give specs, but from the email exchange with the "Life is a Prayer" guy it looks like they at least designed it for use with mics like the SM58.

Thanks for your comments, it's really helpful.

This post has been edited by andy o: Sep 8 2011, 21:42
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mixminus1
post Sep 8 2011, 22:08
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Hmm, without knowing exactly how the eBay adapter is wired, my guess would be that it has the ring of the mic TRS shorted to ground, and if that Hosa adapter's 1/8" plug is wired like the Shure's - with tip and ring wired in parallel, both carrying the signal - then it would be shorting the signal directly to ground through the 1/8" plug's ring.

Try opening up the Hosa's 1/8" TRS plug and seeing how it's wired, and if there is indeed a jumper between tip and ring, cut it - keeping only the tip connected - as recommended in the Shure manual's troubleshooting section (good name!).

Edit: Kinda hard to tell from your photos of the Hosa adapter, but it does look like only ground and a single signal wire go into the 1/8" plug, so there has to be a jumper in there somewhere...

This post has been edited by mixminus1: Sep 8 2011, 22:16


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andy o
post Sep 8 2011, 22:24
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No, you're right, I think it's wired exactly like the Shure adapter on the TRS end. Tip and ring are soldered together there. I'll try what you suggest, thanks.
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andy o
post Sep 8 2011, 23:23
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Didn't work. I think something's funny with that Hosa adapter. I tried just jumping the ring 2 and sleeve of the TRRS cable going into the iPod, and the signal + ground coming from the adapter/mic and the internal mic was disabled, but I got some noise coming from the mic and no signal into the iPod. It worked fine when I did it wih the computer mic. I'm even more confused now.
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andy o
post Jul 27 2012, 06:38
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OK, I'm reviving this, cause, dumbass me, I just figured out at least part of my problems was that the plug in the iPod Touch was filthy. Dust had accumulated, and I just figured it out now cause it got so filthy that I got out a piece of lint that was completely blocking the end connection point.

I'm revisiting this also cause I just bought this, which I just couldn't pass up at the price (specs here). I connected the Beta 58A into that with a regular XLR-XLR cable, and there is audio all right. The problem is that it's still too low, even at the max level (this device uses batteries). I see that I still need to impedance-match this. The problem now is that the two transformer adapters that I bought before are either too low or too high (600-2.5k and 250-50k). I've been googling without any success, is there a transformer that will get my 150 Ohm mic close to the 2.4k of the iXZ device?

I was wondering about other mic options for this thing, anyone has any thoughts about its phantom power capabilities? I'm using garageband now, and recording acoustic guitar and voice mostly. I saw the AT 2021 recommended in another thread. Any thoughts?

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Arnold B. Kruege...
post Jul 27 2012, 15:38
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QUOTE (andy o @ Jul 27 2012, 01:38) *
I'm revisiting this also cause I just bought this, which I just couldn't pass up at the price (specs here). I connected the Beta 58A into that with a regular XLR-XLR cable, and there is audio all right. The problem is that it's still too low, even at the max level (this device uses batteries). I see that I still need to impedance-match this. The problem now is that the two transformer adapters that I bought before are either too low or too high (600-2.5k and 250-50k). I've been googling without any success, is there a transformer that will get my 150 Ohm mic close to the 2.4k of the iXZ device?

I was wondering about other mic options for this thing, anyone has any thoughts about its phantom power capabilities? I'm using garageband now, and recording acoustic guitar and voice mostly. I saw the AT 2021 recommended in another thread. Any thoughts?


Doesn't compute. The Beta 58A is a (high output) condenser mic. From what I can discern, it should have plenty of gain in this application. What are you doing with the mic and what are your expectations? Is the signal so low that you can't bring it up far enough in your editing software or what?
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andy o
post Jul 27 2012, 16:35
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I'm getting a lot of hiss noise because the mic signal is low, the level meter can't even get up halfway with the gain all the way up, and my mouth/instrument just a couple inches from the mic. The Beta 58A is a dynamic mic btw, unless I'm not fully understanding what "condenser" means. Maybe you're confusing it with the Beta 87A?
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mixminus1
post Jul 27 2012, 18:00
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Correct, the 58A is a dynamic, albeit a high(er)-output one, with roughly 6 dB more output than a standard SM58 due to the use of a neodymium magnet.

In the time between when I first posted to this thread and now, we actually got a few of those Shure A96F adapters at work, and I just tried it to get an SM58 into my iPhone 4.

Worked great (although I had to go in on the left channel of the TRRS breakout...not sure how it worked last time): level's a bit low with the 58 (again, a 58A would be about 6 dB hotter), but clean, with a little noise, but not bad.

It sounds like there's some kind of EQ on the iPhone's mic input (haven't RMAA'd it yet wink.gif ), at least a low-cut filter, but it would definitely be useable for "scratchpad" applications.

This post has been edited by mixminus1: Jul 27 2012, 18:00


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Arnold B. Kruege...
post Jul 27 2012, 19:44
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QUOTE (andy o @ Jul 27 2012, 11:35) *
I'm getting a lot of hiss noise because the mic signal is low, the level meter can't even get up halfway with the gain all the way up, and my mouth/instrument just a couple inches from the mic. The Beta 58A is a dynamic mic btw, unless I'm not fully understanding what "condenser" means. Maybe you're confusing it with the Beta 87A?



You're right - I checked again and I had misread the spec. I', familiar with the SM57 and it does take a high gain mic preamp to work unless the source is very loud. The device described above is short of gain to work with a mic like this.

A condenser vocal mic would have 10-20 dB more output, and give quite a bit of releif. The lowest such cost that I know of is this one:

CAD 195 Mic, a low cost sleeper with good tone

another source:

Another source says free shipping
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andy o
post Jul 27 2012, 23:43
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QUOTE (mixminus1 @ Jul 27 2012, 10:00) *
It sounds like there's some kind of EQ on the iPhone's mic input (haven't RMAA'd it yet wink.gif ), at least a low-cut filter, but it would definitely be useable for "scratchpad" applications.

Yes, I read that there's a filter which cuts lower freq's cause the mic is usually used for voice. Either Fire 2 or FourTrack (don't remember which), have an option to compensate for that digitally. That was with iOS4.3, I don't know how iOS5 changed it, but I think Garageband doesn't do it (could be wrong though).
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andy o
post Jul 27 2012, 23:59
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thanks for the recommendation, Arnold. I found it at Amazon for the same price. For this application, is there anything else I should know re: mics for stage use vs. mics for recording use? Any thoughts about that mic vs. the AT2021? Seems to have better specs, it's $20 more.
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Arnold B. Kruege...
post Jul 28 2012, 00:03
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QUOTE (andy o @ Jul 27 2012, 18:59) *
thanks for the recommendation, Arnold. I found it at Amazon for the same price. For this application, is there anything else I should know re: mics for stage use vs. mics for recording use? Any thoughts about that mic vs. the AT2021? Seems to have better specs, it's $20 more.



The AT 2021 is not a mic that compares with the Shure you mentioned. It is more for working at a distance from the instrument or vocalist. The Shure is a classic close-working vocal mic, as is the CAD 195.
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andy o
post Jul 28 2012, 00:11
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I think that would be even better, I mean I was just using the Shure cause it's the only one I had, but recording at some distance would be better for me I guess.
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andy o
post Jul 28 2012, 00:21
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What about this one AT2020? It's even cheaper, can get it for $70 at Amazon. Sorry for this many questions that might seem basic, but I'm struggling to grasp all these differences between mic types. I play nylon-string guitar, fwiw.

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Arnold B. Kruege...
post Jul 28 2012, 03:39
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QUOTE (andy o @ Jul 27 2012, 19:21) *
What about this one AT2020? It's even cheaper, can get it for $70 at Amazon. Sorry for this many questions that might seem basic, but I'm struggling to grasp all these differences between mic types. I play nylon-string guitar, fwiw.


Even though the AT 2020 is side-address and has a larger diaphragm, my comments about the AT 2021 also apply to the AT 2020. I own 4 AT 2020 and use them for acoustic instruments and as choir mics.

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andy o
post Jul 28 2012, 03:59
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Great, thanks for the input.
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Arnold B. Kruege...
post Jul 28 2012, 10:49
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QUOTE (andy o @ Jul 27 2012, 19:11) *
I think that would be even better, I mean I was just using the Shure cause it's the only one I had, but recording at some distance would be better for me I guess.


The CAD 195 will work well at a distance, or up close.
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andy o
post Jul 29 2012, 21:10
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I have some questions about mic specs, I'll open a thread later on after doing some searching. Any recommendations for online resources about this?
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