Impedance matching

funjunkie27

I'm wondering about the difference in instrument level impedance vs the output impedance of an iOS headphone jack. Is there a difference? If so, is there a DI box that would match it up?

Telefunky

the head phone jack has 1 input line for a special microphone and 2 output lines (stereo) for headphones
you probably refer to the input part only, as the output is independant from it
and that's a tough one: it's not a straight connection for there's a detection circuit before the input amplifier
this circuit checks for a resistance of about 2kOhm (iirc), typical for a headset mic and provides current in that case
any other value larger or smaller will deactivate the input
so you can't directly connect a regular microphone or an instrument - unless they present the proper resistance
(but a symmetric DI wouldn't be of much use anyway because the input circuit is asymetric)
the easiest way is to match input electronically, which is what the iRig devices do
get one of those... 2nd generation (iRig2 and Pre) are excellent value for money
the converters of current IOS devices are surprisingly good (made by Cirrus Logic as Apple OEM stuff)

cheers, Tom

funjunkie27

Thanks Tom, but I was referring to the output. Sorry that that wasn't clear. What I want to do is take the output of my iPhone into an Akai EIE 1/4" jack, which is then connected to an iPad.

Specifically, I have an electronic drum kit that I want to send midi out to my iPhone and drive Gladstone or Bilbao, within Gadget, then send audio to my iPad, via the interface, for recording. That would allow our drummer to change patches at the drums, which currently has fallen on me and proves to be a pita.

Good info on the input side though.

Telefunky

ok, that's a different story ...
you probably have a mini-stereo to 2 x 1/4" cable and already checked the front input with setting mic/line (?)

the 'Insert' jacks on the backside (which have a much lower impedance) can be used as input, too
if there's a difference in sound/quality between front/back, then it reacts to impedance
(which is an individual feature - some circuits are sensitive to this, some not...)
according to specs the backside connector is preferable, but may be less convenient
(due to it's location and the lack of gain adjustment, which you'd have to do on the IOS device)

an 'Insert' jack has a split signal path: the send (to external gear) and return (from external fx unit)
one path is on 'tip', the other one on 'ring' of the TRS plug
you need only the 'Return' path, which usually in on 'ring' (iirc)
in that case use the same cable as before, but plug it in only half way (just after the first resistance point)

cheers, Tom

mrufino1

Yes, there are di boxes with rca inputs so you can take an 1/8" to rca cable and plug in. I know radial makes one, I think whirlwind does also.

funjunkie27

Thanks guys. I'm gonna need a re-read of Tom's post to digest it fully. I think we're on the same page though. If the Z (Impedence) doesn't match, I'll hear the power drop, and would need a DI box. Once the Z matches, then it's just a matter of what part of the jack presents the signal....am I understanding that right?

I did run across this, which might be useful if I can't find a cheaper/effective solution. And yes, XLR would be fine too.

http://www.sweetwater.com/store/detail/LTIBLOX

A different approach to what you suggested @mrufino1 , but I didn't mention that XLR would work. Plus this has a cabled input with an 1/8 plug. Anyone see a problem with something like this?

u0421793

Are you sure you actually want to match the impedance? That’s not necessarily a normal thing to do. In audio, you’d present a lower impedance output to a higher impedance input. As long as the direction is going generally from a lower Ω output -> higher Ω input, things should be as required. Exact matching isn’t normal.

• https://en.wikipedia.org/wiki/Impedance_matching#Loudspeaker_amplifiers
• http://electronicdesign.com/communications/back-basics-impedance-matching-part-1
• http://electronics.stackexchange.com/questions/6846/how-important-is-impedance-matching-in-audio-applications
• https://www.quora.com/Why-is-impedance-matching-important
• http://forum.allaboutcircuits.com/threads/impedance-matching.2126/

On the surface, it would seem that exact matching increases the power transfer efficiency, but really, this is only important if you’re dealing with radar or extremely high frequency signals. At the audio range, the reactance and other physical characteristics of loads such as speakers and similar transducers (microphones also) take over and work against the benefit you’d get from an exact match. The general approach in audio is for a source generator to offer a lower Ω than the thing it is going into, which will present a higher Ω input, then everything will be fine.

Telefunky

that's correct - in audio small signal processing 'power' isn't an issue
my post above was a bit long because I didn't know if you're familiar with the 'insert' procedure

before I had an ioDock I tracked an iPad 1 with a 1/8 stereo to 2 x 1/4 straight into a line-input
(which had similiar impedance figures as your Akai)
what's called DI in this context is often about line isolation against buzz/hum (ground issues)

whatever purpose the LTIBLOX from Sweetwater may serve - it's the wrong device
you have an unbalanced stereo output (1/8 jack)
if you balance it propoerly, you'll have either 2 x XLR or 2 x TRS (1/4) outputs

cheers, Tom

funjunkie27

Great info guys. I have a background in satellite communications, where matching was always stressed, hence the confusion. I believe I have some y cables and adapters to give this a go.

I'm guessing that LTIBLOX just picks up the l or r channel on the 1/8 side to give a mono output.

Processaurus

@u0421793 said:
Are you sure you actually want to match the impedance? That’s not necessarily a normal thing to do. In audio, you’d present a lower impedance output to a higher impedance input.

This is correct. Impedance is not an issue plugging an ipad into an instrument input, like a guitar amp or audio interface.

Guitars are a special case for watching impedance in audio, they have a very wimpy output from their pickups, and need a special, high impedance input, that doesn't load the pickups by demanding a lot of current, to sound good. If you plug them into a low impedance input, like a line level input on a mixer, or stereo, or audio interface that wasn't designed for guitar, the tone loses its treble and sounds bad.

Going the other way is never a problem, plugging line level electronics, which have a low impedance output (strong drive), into a high impedance input on a guitar amp, doesn't have any ill effect.

A note about using a DI box for ipads, you only need one if you are going through a snake. They convert unbalanced instrument signals into balanced signals. The point of balanced signals is for mixers and studio gear to use a noise rejection scheme to get rid of noise that comes from long cable runs. Short cables (less than 10 meters) should not present a noise problem, for running unbalanced signals (such as your headphone output) between your ipad and other equipment.

You normally don't need a DI to plug an instrument into a mixer or audio interface, in a normal sized room. The only scenario for doing that is if you are having buzzing from a ground loop, but that's a different conversation.