iRig (with DIY impedance matching cable?)

[srv]

@ Scruffyhound -- "I was thinking we would power off the iPhone, but since that pin is also the input I'm finding that confusing, perhaps you could clarify.
Are we using an external battery or is the mic port also the power source?"


The power and audio input to the iphone are both on the "mic" connection.

Wire the "mic" connection to power the circuit/FET
The same connection from "mic" MUST go through a capacitor to the FET output.
The capacitor will block DC and pass through the guitar signal.

(I'll try showing this below in ASCII)

FET power
-------------------------------------------------------------> mic
FET audio output                                           |
----------------------] [------------------------------ (also to mic connection on 4pin plug)
                 (capacitor blocking DC but passing AC signal)

I'm guessing the cap would be a typical output coupling cap eg: 1uF electrolytic
and it might be better if it's bipolar given that it will have ~+2.7volts on the 'mic' pin side

The 'ground' connection will be common to both DC supply and the audio output signal.

Hope that helps.

Matt.W



--EDIT--
I was just looking back at the drawing from PRR,
it could be that the capacitor part of the circuit is internal to the iphone
in which case it won't be required in the iRig adapter.
From the drawing it looks like FET power and output are connected together and the iphone separates these internally.

[Skruffyhound]

Thanks for the input, I should have time to look at this again this evening.
Raphael's comment above seems to say the same thing that the circuit inside the box is what is breadboarded and the rest is the guitar and iPhone, ie the cap and 2k2 and resistor to ground is in the phone.
So I am just about right, some things definitely need tweaking to hold noise down and I'm not sure that the pops are due to our circuit, I'll have to check and see how well my old 3G should be able to run this app.
More later.

[Skruffyhound]

       Just a quick report
      Starting the app on the speaker setting triggers oscillation, don't do that, start it in headphone setting  
In the original constellation (apart from R1, see below) I measure on the FET G=0V,S=0.32 and D=0.78
      Today I was getting oscillation on the Amp at any higher gain and/or volume settings. I switched the 4k7 on drain for a 10k which sorted that out and may have stopped a rhythmic artifact that sounded a bit like a cap discharging. I'm a hack, tell me why that worked. Now I have G=0,S=0.34,D=0.99, Oh and I just checked again and the rhythmic thing is still there but it disappears after a minute or two.
      There is still some noise and some clicks here and there with the amp. I have this software on my computer and as I remember the amps were quite noisy, but the clicks shouldn't be there. I have 510k now for R1 (PRR's schem) thought that should stop it clipping.
      The delay is sounding good, and quiet and the Noise filter works to reduce the overall amp noise.
      The iPhone mains power supply makes a huge racket when connected, which is a pain in the balls because I think this app really draws power.
      That's it for now, any suggestions, I'm all ears.

Edit: I tried taking the 4k7 out Paul, but sound quality went downhill.

[mattboy2]

Any more progress on this? I have been waiting with baited breath

[askwho69]

PRR is my Idol!

A2

[Skruffyhound]

Well it's still on my breadboard. I'd be happy to take some measurements, but any changes to the circuit (apart from trying a few substitutions) that I might make would be pure guess work. Better if the more knowlegable heads could suggest something, I'm out of my territory on this one.
   Thanks for the interest though. Great if we could get the diy version working as well as the original, also, as mentioned above, so it can be used to connect to PC/Mac.

[rafaeldmachado]

Sorry on my absence the last couple of weeks.

I'll try to breadboard the circuit as well in the next week and do some tries of different components. (I think I'll have a hard time finding a similar JFET here in my city... Gut feeling). I'll keep you posted.

If succeded I'll try to design a circuit board that will be posted as soon as possible here.

[Skruffyhound]

If you really get stuck I'll send you some. Where are you in the world that you can't find something similar to a J201?

[rafaeldmachado]

Right now I'm in Belo Horizonte / Brazil. The radio shacks here are, say, limited in some way. They are more oriented toward replacement parts than electronic components. I really don't know if J201 is that common for replacement, that's why I'm not very confident.

[Skruffyhound]

Then it's the internet, or me I guess. Though I would think a Tayda order would get to you first since I'm in Denmark.

[rschultz]

Hi,

  Been following this thread.  Check out this guy's Youtube about the Polytune app:
  http://www.youtube.com/watch?v=0YpiB5Xc4tw

  At marker 3:10 or so he shows 2 caps and a resistor... and that's it.  Granted the Polytune app only is input and doesn't output, but still it goes through the iPhone mic...

  Anyway, I am building one of these stompboxes but need to figure out what circuitry to put in for the mic.  The schematics in this thread before seem complicated... have you come to any conclusions?

Thanks.
Ryan

[Skruffyhound]

My response to a PM from Ryan :

Hi Ryan,
            I still have this on a corner of a breadboard. It works as is but I feel it could be better. I don't have the iRig, so the best would be if the OP could build this simple circuit and compare to the original.
            My main interest is not really with the Amplitube app. since I have all that on my computers. I was more interested in ways to use the iPhone as mobile recorder and sampler.
            Since last post on that thread I may have solved my problems by buying the Alesis Protrack, which I saw on special offer. It allows me to connect condenser microphones and guitar input with a buffer and record.
            I watched the youtube clip. I would say you should just use the components he used and make a mock-up  and see if you can get it to work. Tone as such may not be critical in this application, just frequency. So those components may just be a pulldown resistor and some smoothing caps. Volume may also be less important.
            It does appear to be TC electronic who made the video, so it might be possible to get someone from the company to give you some more info if you email them.
            I'm still hoping that the thread will yield some more info.
            Good luck
                          Aston
P.s. I think I will post this in the thread for other folks looking into this stuff.   

[Matt420740]

I came across this the other day, not sure if its what your wanting or not - http://www.instructables.com/id/iPhone-iTouch-guitar-cable-buffer/

[Skruffyhound]

Hey, there's some more info! 
Just had a look at it, seems very competent. When I get a moment I'll breadboard and check it out.
Thanks a lot.

[rschultz]

Hi,

  Been following this thread.  Check out this guy's Youtube about the Polytune app:
  http://www.youtube.com/watch?v=0YpiB5Xc4tw

  At marker 3:10 or so he shows 2 caps and a resistor... and that's it.  Granted the Polytune app only is input and doesn't output, but still it goes through the iPhone mic...

  Anyway, I am building one of these stompboxes but need to figure out what circuitry to put in for the mic.  The schematics in this thread before seem complicated... have you come to any conclusions?

Thanks.
Ryan

Here is the circuit he uses.  I have done this and it seems to work both in and out... but I have to turn my guitar gain down below 1/2 or it distorts.  I'm not sure if it's best to just use the gain on the guitar or if I should build in a gain stage in the pedal.
http://www.tcelectronic.com/files/Support/misc/Iphone%20cable%20diagram.jpg

[rschultz]

Hey, there's some more info! 
Just had a look at it, seems very competent. When I get a moment I'll breadboard and check it out.
Thanks a lot.

Definitely interested in your results.  That circuit that Matt posted from instructables.com is hard for me to read.  It would be nice if someone could translate that into a schematic instead of a board layout.

[mattboy2]

TTT

any more progress on this?

[Skruffyhound]

Great there is still interest in this. I am working 80 hour weeks this month so I am not going to get to it until March.
Maybe have a go yourselves. Try to breadboard it from the layout. It takes some concentration the first time but it is do-able, in fact it's the only way to learn this stuff.

[homeroig]

http://a-you.seesaa.net/article/156684704.html

[Skruffyhound]

Interesting thanks. I haven't translated the page yet, is there a schematic link?