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Old 30th March 2012, 06:25 AM   #1
Fonic is offline Fonic  Canada
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Join Date: Jun 2009
Question iPod guitar dock

I want to build a dock for an iPod Touch that works like a guitar pedal, so that I can program my own digital effects using the iOS SDK. It would need to take a high-impedance instrument-level signal and convert it to line level to go into the iPod, then take the line-level signal from the iPod and convert it back to instrument-level (I would also add a footswitch eventually, but how to protect the iPod from being stepped on is something I can figure out later).

Anyway, I can figure out the iPod pinout and such, but I'm not entirely sure what to do about level conversion. I would like to do this on the cheap, so that means active rather than transformers. I'm a bit of a noob at designing things from the ground up, but I've come up with this general schematic (mainly by looking at gain stages from various guitar pedals). It's a pretty straight-forward circuit, but as you can see, I have no idea what component values to use.

So yeah... I have a lot of questions about this:
  • What sort of gain do I want on the preamp to get from guitar level to line level? Also, how much attenuation on the output buffer - just the inverse of the preamp gain?
  • Wikipedia tells me that line level typically has a 10k input & 100R output. Did I do this properly through R5 & R6?
  • What values of decoupling caps do I use? I've noticed guitar pedals vary wildly in their cap values, so does it really matter? I'm assuming this affects bass response...
  • Is 5V (with 2.5V bias) enough? I'm assuming that would be adequate, as the iPod only takes 5V in the first place and would presumably clip around the same point as a 5V opamp.
  • How do I decide what value of resistor to use on Vb (R3 & R8)? Similarly, what values for the opamp? (I know the ratio R4/R2 will determine the gain, but that ratio says nothing about what the absolute values should be.)
  • I'm also considering adding a pot (or maybe trimpot) for input gain. Should this be necessary? Should it be a voltage divider before the preamp, or a pot inside the opamp feedback loop to adjust gain?

I'm a 3rd-year EE student, so I understand a lot of the theory but I don't have much experience yet. So feel free to use technical jargon and/or math, but excuse my naivety when it comes to application.
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Old 30th March 2012, 07:13 AM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Well, this is not a project, but you might be interested in this product from Peavey, the Ampkit Link. It is a guitar to iPhone interface, loaded with pedal sims and other stuff. Not expensive, and it looks like exactly what you want to make.

Peavey.com: News - Peavey and Agile Partners Launch AmpKit iPhone App & AmpKit LiNK Audio Interface


At the moment, their web site is off for maintenance, but Peavey.com, under products, then recording.

They make a similar thing which is a USB guitar interface for your computer.

Even if you have no plans to buy such a product, download and study the owner manual. Might give you some insight into your project.


As to your design. May I suggest instead of looking at pedals, try looking at preamp stages of actual amps. The input stage of any guitar amp is made with a guitar signal in mind, and the output will be suitable for any line level devices. Floor toys are not generally made to convert between levels, just convert the sound or tone.

I think you wil also find there is no hard and fast exactly this amount sort of rules. Around here at least, we think of line level signal as about a volt, while a guitar puts out something in the 1/10 to 1/2 volt area most of the time. But there is plenty room for overlap.

I would also suggest when discussing guitar amps and overdrives and such that gain and volume or level are not the same thing. We might informally interchange them, but gain refers to amount of amplification, as opposed to volume, which refers to the signal level. And someone else may have even more pedantic thoughts on that than I, but you get the idea, I hope.
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Old 30th March 2012, 02:58 PM   #3
Fonic is offline Fonic  Canada
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Join Date: Jun 2009
Yes, I've seen the Ampkit. However, it's not quite what I'm looking for. For one, it has a headphone jack out, not a 1/4" high-Z instrument level out. And it connects through the headphone jack, which means you can get feedback since the input & output are on the same jack. Plus, connecting through the dock connector is just easier, because it supplies power and everything else. I also can't find a manual for it on that page, but I'm not sure it would be useful anyway as I doubt it has schematics.

I'm also not sure the pre stages of actual amps would help much either, as they are designed to shape the tone quite a bit, and they are much more complicated then necessary for this purpose.
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Old 30th March 2012, 05:03 PM   #4
Fonic is offline Fonic  Canada
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Join Date: Jun 2009
I fixed the schematic a bit - I realized the noninverting inputs need 2.5 volts, not ground. I also cleaned it up a little just so it's more obvious how the power supply is shared.

That raises one question though - do I need a resistor between Vb and the noninverting inputs? Since opamps have a very high input impedance, I would assume not, but it would be good to hear this from someone who knows more than I.
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Old 30th March 2012, 10:45 PM   #5
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
You dont nescessarilly need a High output impedance , there is no problem running a low impedance output into a high impedance input ...... Most guitars with active pickups/electronics have a low output impedance and most effects pedals and effects processors have a low impedance output and have no problem running into a high impedance amp ......

Generally you would use a Voltage divider to get your 1/2 supply and you would use another resistor from 1/2 supply to the noninverting input and this resistor would set the input impedance , these resistors are R3 and R8 but you need 2 more to make 2.5v from your 5v......

And r1 and r6 serve little purpose , they do not set the input inpedance as they are isolated from the opamps input by the DC blocking cap (c1 c3) , if you were useing a true bypass switching system then r1 and r6 would help supress pops but other than that they serve little purpose ......

R4 and R9 also are serving no purpose because the rest of the feedback loop is ommited , the way it is set up now it is a unity gain buffer with R4 and R9 doing nothing ......
Gain is set by a combination of 2 resistors and you are missing one of them , you need another resistor from the inverting input to ground perferably through a capacitor ........

Another thing is that most audio opamps won"t run at 5v but there are a lot of others that can use 5v but I am not familiiar with them ......

Good luck
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