Microphone Impedance Matching - Using an SM58 on iPhone 4 [UPDATED]

Please read this article for background/more info: iPhone/iPad external microphones

8/25/12 Update: The Tascam iXZ is another good alternative to the KVConnection cable, and is more flexible than the VeriCorder or KVConnection adapters, though it's a little more cumbersome. It's also only $30 on Amazon.

1/6/11 Update: The VeriCorder XLR Adapter Cable includes +6.1dB attenuation so you can have a slightly amplified XLR input on your iPhone 4, and is a slightly better solution than the KVConnection cables (in my experience).

Update 2: KVConnection's latest revision to their XLR adapters (here and here) add in impedance matching! Rejoice, for you shall no longer need any extra adapters/transformers - check out my video sample with the new adapter :)

Update 1: I have finally gotten ahold of the RadioShack transformer, and it works beautifully! Below is a sample recording with the SM58 and my iPhone 4 (no intermediary devices besides the KVConnection adapter and the line-matching transformer):

Here's an image of the setup for this connection (in its most basic form):

Shure SM58 with adapters for iPhone 4

From top to bottom:

Update 2: I just heard from KVConnection that they are working on an inline impedance-matching transformer, and I tested another solution—recording through my Mac—and posted the resulting video (straight from my iPhone!) on YouTube:

From KVConnection: We are working on a iphone adapters with built-in impedance

matching transformers for the low impedance microphones like your Shure SM58 which has an output impedance of 150 ohms. The iphone needs to see 800 ohms or more of impedance to recognize that an external mic is plugged in.

Since I have a pretty good single-person interview solution for audio and video on my iPhone 4 (I'm almost finished crafting a nice tripod adapter as well, so I can grab my mini-tripod and iPhone, and have a stable platform), I am now working on finding a solution to the problem of recording interviews where one person would interview another, with a handheld mic.

There are a few battery-powered handheld condenser mics that would work out of the box (like the Nady SPC-25), but I would like to use some of the nicer dynamic mics I have on-hand with my iPhone, like the Shure SM58.

It seems that I might be able to do this with an impedance transformer—if I can get the 'low impedance' (~150 Ω) microphone output level of the Shure SM58 transformed into the required 'high impedance' (800 Ω +, according to KVConnection) of the iPhone's input, I should be able to use the SM58, or any other so-called 'Lo-Z' microphone with ease. And they'd look a lot nicer, too.

I'm considering the following adapters (and might be able to pick up the RadioShack adapter tonight):

  1. RadioShack A3F XLR -> 1/4" Plug Adapter/Transformer ($19.69)
  2. Hosa Line Matching Transformer - XLR to 1/8"($16.99)

The advantage of the second transformer is that it should work great with most other consumer recording devices (cameras, DV cams, other portable recorders, etc.), without needing yet another adapter to get to the final plug. I'm not a huge fan of the 1/4" TRS connector—I figure, if you need a plug that big, stick with XLR (even if it's not balanced) - it locks in, and looks nicer :-)

More information about Impedance:

Further reading

iphone
audio
recording
microphone

Comments

It's a possibility that it might work... My main question would be whether the iPhone line-level input provided enough gain. Do you have the specs on that? Back in the day I had tried getting an SM57 to work with a sound card and ended up building a simple op-amp amplifier circuit to bring the up the signal to a line level. (I'd estimate I needed around 40dB of gain for what I was recording at the time, IIRC.)

Seeing that I still can't find a local RadioShack with the transformer I need, I might just hack together a few resistors across a connection to see if it works in principle.

Jeff,
RE: Update 2: KVConnection's latest revision to their XLR adapters (here and here) add in impedance matching!
Are these "new" XLR to TRRS adapters the ones KV had been transforming and are they the XLR impedance matching ones?
Are you trying one out in your video?
Thanks,
George

Thanks Jeff!

What's your opnion on a Electrovoice RE50ND/-B Dynamic XLR Microphone with this new KV adapter and running it through 25 ft of mic cable. No battery or ohantom powered needed

This Electrovoice Mic has an impedance of 150 ohms. Do you think this mic has enough output to go throgh 25 ft of cable & this adapter to get a good audio input signal into the iPhone 4?

Please look at Electrovoice DBs (@ 1000 HZ) http://www.electrovoice.com/sitefiles/downloads/RE50ND-B.pdf

Versus

Shure DBs (@1000 HZ) http://www.shure.com/idc/groups/public/documents/webcontent/us_pro_sm58…

Hi Jeff,
Thanks for the great write up! It's been very helpful.

I just purchased the iPhone adapters below that you suggested today:
http://www.kvconnection.com/product-p/km-iphone-xtrs.htm
http://www.kvconnection.com/product-p/km-iphone-2trs.htm

I plan to use the XLR adapter with a dynamic microphone by Peavey that is around 500 ohms. Would you be able to confirm that the XLR adapter I bought will be one of the new ones you mentioned with the impedance matching transformer? Or, will I need to purchase a low Z to hi Z transformer adapter?

The other 3.5mm adapter I'll be using with an Audio-Technica ATR3350 lavalier microphone-- that's condenser and is around 1000 ohms, so that shouldn't be a problem.

You shouldn't need a transformer if you just bought the XLR adapter - but to verify, go ahead and reply to the sales email from KVConnection and ask them if you are getting the newer or older version of the adapter. They will probably respond within the day!

I have a iphone4 and would like to hook my microphone up with it for interwievs etc. I have a Shure PG81 with a 3,5 jack converter cable. But I didnt get it to work. Any idea ?

Hey Jeff,

Having read your article as I was searching for a way to connect my new iPhone4 to a decent mic, I bought one of the kconnection adapters:
http://www.kvconnection.com/product-p/km-iphone-xtrs.htm

I was excited because I thought I found a solution: I have a Behringer XM8500, which according to specs is equivalent (though cheaper and not as good) to your SM58 in many ways -- including 150ohm impedance.

The adapter arrived today, and I was disappointed to find it doesn't work with my iPhone -- the internal mic stays on. The adapter works with my iPad fine, but that's not what I'm looking for!

Any thoughts? Did I order the wrong one? Am I using a dud microphone? Is my iPhone being picky?

Answering my own questions: I contacted kVconnections, after checking with different mics and different phones and still getting nothing. The folks at kVconnections have been *brilliant*, offering advice on troubleshooting and -- when it seemed certain that the cable was indeed faulty -- sending a replacement immediately.

I should point out: I am in Australia. That's long-distance service at its best. Kudos to the kVc team!