Guitar->iPhone/iPad/iPod Touch headphone input

[aron]

I know people have been wondering about this, but this schematic seems the best.

http://forum.harmonicdog.com/viewtopic.php?f=2&t=6&start=50

[karter2000]

I just breadboarded this, and it seems to work.  However, I'm getting distortion when using it with bass and Amplitude's iRig.  The same thing happened when I used a simple adapter.  Reading up on the iRig adapter, there seems to be some impedance matching going on.  Would I have to do the same thing?

[aron]

Try raising the 100K to a higher value???

[karter2000]

I actually substituted a 1M pot in place of the 100k, and it still distorts regardless of value.

[trjones1]

I was just reading reviews this morning, and it seems like distortion is often a problem with the commercial units as well.  Does the type of jfet used make any difference?

[therecordingart]

I was just reading reviews this morning, and it seems like distortion is often a problem with the commercial units as well.  Does the type of jfet used make any difference?

Yup. I modded my Peterson iphone tuner cable and added it to a TRRS. I have to dial back my guitar volume.

[murrayatuptown]

I bought an iRig that was supposedly 'trade show overstock, comes with box, no other items'...initially I had problems getting an iPhone 4 to recognize it, discussed it with IK, who offered many suggestions, then a list of details that indicate whether an iRig is 'real' or 'counterfeit'.

I then realized the lack of a registration card was another warning sign.

I cracked it open to study, then reviewed IK's suggestions and realized that it didn't matter that I had assured the 4-conductor plug went in as far as it could on the iPhone 4...it still didn't completely go in due to the thickness of the plug and my having a Ballistic HC case...so I carved the vinyl molded plug on three sides so it fits, and the phone now recognizes the "iRig".

I sent photos to IK for them to weigh in on; they say it was a counterfeit and I should return it. I had already opened it up thinking I was up the creek and had nothing to lose, and returning it is not an option.

I have most of it drawn out but there is one aspect that doesn't make sense to me, so I'm not done. I also don't feel right publishing it too graphically out of respect for IK's IP rights, and their helpfulness in diagnosing my difficulties, which ended up being operator error.

So, what  I am comfortable discussing is a verbal description. I also spoke to Peterson Tuners as my goal was to get line-level input into the iStrobosoft app. I did not realize Peterson had a cable accessory at the time.

Peterson says it is not the signal level that tells the phone an external device is plugged; it's the impedance it presents to the phone. Their cable has only passive components; no JFET.

The 'counterfeit' I took apart has, to the extent I have figured out so far, IIRC, from the 1/4" input, a coupling capacitor, then a 220k resistor to the gate of an MMBFJ201, and a 10k resistor in parallel with a capacitor from gate to ground. The JFET has a 4.7k drain resistor, and a 1.2 k source resistor. IIRC, there is a coupling capacitor from the JFET, I think drain, to the 4-conductor 3.5 mm plug.

For my purposes, I don't care what the capacitor values are...I just wanted to know what the wiring was.

I see the input circuit as essentially a voltage divider...10k/(10k+220k)...presents approximately a 230k load to the guitar or other line-level input (neglecting the input capacitor impedance), and roughly -27 dB drop in signal level to the mic input. The JFET 4.7k/1.2k suggests a JFET gain of roughly 4, about 12 dB above the -27 dB divider, so -15 dB from guitar level to mic input. This seems potentially reasonable, but I don't really know if it's 'obviously' correct either. Maybe there are other factors I'm ignoring due to the capacitors.

Another reason I'm not bothering with the capacitors is that they are not marked (common with SMT caps), and an in-circuit measurement is kind of untrustworthy. I don't need to know badly enough to remove them at risk of damaging or losing them.

The part I haven't figured out is the drain wiring...the JFET is powered from the electret mic capsule bias the iPhone provides for headsets, integrated-mic earbuds, etc. There may be something else I didn't figure out like the power to the JFET being switched by insertion of a plug...where I left off was puzzling over what appeared to be the drain resistor being grounded, rather than going to a VDD source. I just now had the thought that maybe the drain resistor really DOES go to ground so the operating bias comes from some sort of 'gate leak bias' analogous to grid leak with tubes, or similar idea.

Peterson also saved me a lot of headache regarding the iOS7 'update' removing access to the analog input on the 30-pin docking connector. They do not use it, and they avoid the iOS7 250 Hz 24 dB/octave HPF on the mic input which provides wind noise filtering through their own app code. Their approach is what gives the app the speed it has. Since they do not use it, it is pointless for me to pursue a 30-pin analog input solution. Maybe figure out a USB interface to the iPhone someday...

IK also indicated they 'correct' or otherwise handle in some fashion, the mic input HPF nuisance.

The L&R audio outputs are passed through to the headphone jack, and there is only one mic input available on the iPhone 4-pin plug, despite there being two internal mics.

I will probably bypass the JFET for my purposes (no long cable runs), and for anyone else's curiosity, the input 'impedance matching' appears to be accomplished by the 220k+10k series resistors. The JFET appearing to have approximate gain of 4, at best, is by my estimation equivalent to 220k & 40k instead of 10k. The other thing I shouldn't forget about is the JFET also presents a lower impedance than the 10k from its gate to ground.

Elsewhere I read (maybe Faberacoustical site) that 800 ohms is the iPhone input impedance, so something lower than 10k might be needed to allow the iPhone to recognize insertion of an impedance-appropriate plug. The iPhone GUI for iStrobosoft and Amplitube show an icon that changes from phone to headset when detected.

I hope this is useful or interesting to someone.

I haven't seen a Peterson cable, and the person I spoke with at Peterson didn't know the exact resistance threshold value at which the iPhone recognizes an external input, but his gut feeling was somewhere around 600 ohms.

Regarding the poster's observation of distortion regardless of effort to change input resistance, and my observation of something weird with the drain resistance. I will take a wild guess the fake iRig's 'self-bias' allows the JFET to operate with variations in mic-jack ECM bias, and trying to run the whole circuit on less than 3 VDC and variation in (MMBF)J201 threshold voltage results in clipping in some cases. Maybe the distortion is considered tolerable since the product is sold essentially as a guitar effects box interface where most people don't look for a really clean signal.

I may have to figure out what resistance divider ratio to use that still allows the phone to detect the external input. I think I'm not done figuring this out. I will likely be feeding it with a buffer or preamp or mixer, so I won't need such a large series input resistor a passive guitar pickup would need.

Murray

[Jdansti]

Thanks for the info on the iRig. If you do a search on the forum, you'll find several threads for it. Here's my build: http://www.diystompboxes.com/smfforum/index.php?topic=95461.msg871662#msg871662

I've used it now with five different iPhone apps and it works fine. I've used it with an iPhone 4S, but I currently have a 5S running iOS7.1, so I can't say how a 4 would behave running a recent version of iOS.