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iPhone, Impedance, and You

April 16th, 2010 by Aaron

We’ve had a lot of interest in our recently-released iPhone® application Amps & Cabs™, and also in our up-and-coming audio input solution for iPhone®. Of course, since our solution isn’t yet available, everyone who is using Amps & Cabs™ or any other audio app is either using their headset or some other solution to get audio in and out of iPhone®.

We’ve had a lot of requests like, “Will Product X work with App Y?”, or “Does Product Z have the right impedance to use with a guitar?”.  In an effort to bring the best possible experience to users of our applications, we’ve done a fair amount of research to inform ourselves about these topics.

It’s this research that motivated us to develop our own solution–we’ve found that the solutions available at the moment, even some specifically made for instruments, are not actually designed to connect instruments.

I’m going to take a few paragraphs here to share some of Ground Up Audio’s research with you, in the hope that it will help you in your quest for ultimate tone (and of course in the hope that I might interest you in GUA’s new product once it is available).

Impedance

Impedance is a hot topic these days for whatever reason.  For those unfamiliar, impedance is (very) simply put, resistance; and for our specific needs, it is a property of signal inputs and outputs.  There are plenty of articles on the web that describe and define impedance, so I’m going to skip over the basics to the details.

In relation to iPhone® input, devices that have inputs are generally line level.  I have not found one yet that actually specifies the input impedance, though many will say “line level”.  Signal voltage itself does not imply any particular impedance, but “line level” does imply a particular impedance range.

There are many details that are fun to know, but won’t necessarily help you get good tone. For most active devices, impedance mismatches will cause only level (volume) issues, either too soft or overdriving or somewhere in-between.  However for passive devices, like instruments with pickups, an impedance mismatch will also introduce tone problems.

This is because if the impedance of an input that a pickup is connected to is too low, the input will load the pickup, which causes it to suck the tone out of it. This is why guitar amp inputs are very high impedance, typically in the range of a or so.  This is also why it’s a bad idea to plug a guitar into anything that’s not made to receive a guitar signal, even though it won’t probably wreck anything (except your tone).

All of that is to say, that most input solutions available today aren’t designed with this impedance in mind, and so they will all suck tone out of your guitar or passive instrument.  Due to the way the iPhone® input is designed, some of these solutions will cause popping and crackling issues as well as tone loss and input overdrive (the bad kind).

So, GUA has developed an audio interface for the iPhone® that properly buffers the low-impedance input of the iPhone® into a very high impedance input that won’t suck the tone out of a guitar.  It’s not quite ready yet, but it’s coming soon! Check out our Hardware page for more information or to subscribe to our mailing list.

Below is some information about various signal types and their levels and impedances.

Source Level Input Impedance
Professional Line Level +4 dBv 100-1000 Ω
Consumer Line Level -10 dBu 100-1000 Ω
Typical Guitar Instrument Level -30 dBu 1000000 Ω (1)
Microphone Level -70 dBu 600 Ω
Values are approximate. Specific values also vary from manufacturer to manufacturer.

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