Author Topic: I need some impedance assistance...maybe?  (Read 1893 times)

therecordingart

I need some impedance assistance...maybe?
« on: August 02, 2009, 10:04:59 AM »
I've been wanting to use my iPod Touch as a four track recorder when guitar parts pop up in my head. I built the following breakout cable that gives access to the mic in and headphone outs:

http://www.crunchgear.com/2009/05/28/diy-an-ipod-touchiphone-microphone-and-headphone-breakout-cable/

The iPod doesn't recognize the guitar as a microphone (obviously), but does recognize a small electret element so I know the cable is wired properly. Peterson created a cable for the iPod/iPhone so you can plug in your guitar, but you don't have access to the headphones.

What did Peterson add so the iPod recognizes the guitar as a mic?

Any help would be awesome!

Paul Marossy

Re: I need some impedance assistance...maybe?
« Reply #1 on: August 03, 2009, 12:03:22 PM »
I'm would think some kind of audio transformer would need to be used in conjunction with a guitar, but that's only a guess.

Transmogrifox

Re: I need some impedance assistance...maybe?
« Reply #2 on: August 03, 2009, 04:30:43 PM »
You probably need a DC blocking capacitor.  Usually an electret mic needs a DC bias, which is supplied from the mic input.  If applied directly to a guitar output, the input amplifier is probably not biased into linear operating range.

You probably need a 1uF cap in series with the guitar, negative polarity pointing toward guitar.  If that doesn't work right off, then you may need to fiddle with a resistor value on the itouch side of the cap.  10k is a good first guess...but it's better to build a proper preamp.  See this page:

http://www.epanorama.net/circuits/dynamic_to_electretinput.html

This circuit is simple to build into a small case or cable end.  You can use about any NPN transistor with hFE of 300-500.

2N5089 would be a good substitute.

If you use something like a 2N3904 (or other lower gain transistor) then decrease the 470k resistor to 270k or so.
trans·mog·ri·fy
tr.v. trans·mog·ri·fied, trans·mog·ri·fy·ing, trans·mog·ri·fies To change into a different shape or form, especially one that is fantastic or bizarre.

Paul Marossy

Re: I need some impedance assistance...maybe?
« Reply #3 on: August 04, 2009, 11:13:44 AM »
You probably need a DC blocking capacitor.  Usually an electret mic needs a DC bias, which is supplied from the mic input.  If applied directly to a guitar output, the input amplifier is probably not biased into linear operating range.

Yeah, that's a good point. I think it would probably need some sort of preamp that allows for level control because the guitar will probably sound horribly distorted straight into the input.