ipod-induced noise in GR electronics...

[Neil_N]

Hi, and thanks for reading.

I have a heavily modified guitar which includes a synth controller, about a dozen capacitive & resistive controls, and microcontroller. The synth controller is older Roland(24 pin), uses bipolar 15vdc, and consists, among other things, of a six pole magnetic pickup feeding six high gain op-amp circuits. There is also a six position Ghost piezo pickup feeding its own preamps, etc. This requires 9vdc. I've picked off 9vdc for the latter, and 5vdc for the microcontroller from the Roland +15vdc line w/ regulators. This much is mostly well behaved.

The problem occurs when I try to power an ipod touch ( +5vdc) from any source in the guitar. It induces a strange, pink noise-type of hissing sound, along with a separate very low freq (20-25hz?) rumble. EDIT: The iPod has no other electrically conductive physical connection to the guitar besides +5v & Gnd. Not using audio.

What I've tried(aside from various Electrolytic caps):
Powering from the 5vdc regulator mentioned above.
Powering from a 5v regulator, applied to various pickoff points on the 15vdv/Ground traces on the main pcb.
Powering from a 5v regulator downstream of the 9v regulator mentioned above.
placing ferrite toroid coils in series w/ the ipod supply lines.

I can make the noise worse, but no better. If I power it w/ a 5v regulator attached to a 9v battery, the noise is gone, even if I ground the neg lead to the guitar, so I assume the ipod is inducing noise in the guitar's ps rails, which is picked up by the fairly high impedance amps. What perplexes me is how this propagates back through the regulators...

I'm not knowledgeable about power supply design, and I apologize if this post sounds ignorant. I'm at the limit of my ability to troubleshoot this, and if anyone could shed any light on a sure-fire way to decouple this ipod touch, I'd be pretty grateful. It's unusable as it is now.
Thanks.

[Martin Pedder]

It sounds like you have checked most of the obvious things. I suspect you have more electronics knowledge than you give yourself credit for.

Where is the audio from the iPod going? Are you using a dock to extract the audio or using the headphone socket. Perhaps you can sketch a plan of what you are doing?

Your solution is probably going to be either to power the iPod from a dedicated supply, using a power pack that is floating, ie it has no connection to earth, or to transformer balance the audio output to isolate that.

I suspect that your problem is actually caused by an earth loop on the audio side. I have had very similar issues using iPods used to source background music systems giving the same noise as soon as they are put on charge or connected to a computer. Depending on where the audio is going, and on whether mono or stereo iPod audio is required, a DI box or a 1:1 transformer is probably going to be the solution.

I have used these where a mic level output is required.

SOUNDLAB SDB20 > Problem Solvers > Accessories/Spares/Misc > Shop > Boomerang Sounds

If you need a line level output, this is one of my favourite boxes as it can interface anything to anything. Every live sound engineer should have one.

ART DTI > DI Boxes, Distribution Amps > Effects/Outboard/PreAmps > Shop > Boomerang Sounds

[Neil_N]

Hey Martin,
Thanks for the detailed reply. After I posted, I realize I should have mentioned (I'll edit it in a minute...) that the only connection to the ipod is the power! That's what's so frustrating. It is being used only as an OSC over WiFi touchscreen controller for outboard synths/processors. Ground loop would have been my first guess if there was anything more than a +5v & Gnd connection. Heck, the ipod chassis isn't even grounded at the mount point (plastic), so I'm not sure where to look. The problem w/ a dedicated supply is that I've used up all 24 conductors in my "guitar cord", so my only option is to find a way to power it from the available sources in the guitar, or run it on battery, which is a major nuisance, not to mention defeat. I still have one trick up my sleeve. Since the ONLY electrical connection to the real world is two wires for 5v, I'm gonna try to pull -5v off the negative 15v supply & offset the ipod below guitar ground. Unless someone can point me in a better direction. I can usually suss this stuff through brute force & persistance, but this one's eluding me. That, and some of my microcontroller firmware bugs. I'm not much of a C programmer...
Thanks again.

[Steve Conner]

The problem is that the current draw of the iPod varies dramatically at an audio frequency rate. This is very common with modern systems that use aggressive power management. The micro can be popping in and out of sleep hundreds of times a second.

Since the problem is audio frequency from the start, no amount of RF filtering will help. And a regulator won't help because it just passes the varying current draw back to its input.

What you need is a really big RC filter, effective down to subsonic frequencies, but with low enough series resistance that the iPod supply doesn't droop unduly. Off the top of my head, something like 2.2 ohms and 10,000uF.

Also verify how much current the pod draws and that it isn't overloading your power supply. (You need to know the current anyway to choose your series resistor.) Apple are notorious for this. The iPad likes to draw 2 amps from your USB port, a violation of the USB spec which states 1 amp max.

[Martin Pedder]

Certainly an odd one with just power connected. It would seem that you have eliminated airborne radiation of the interference with your battery test, assuming that the iPod was in the the same location as before. As connecting the battery power ground to the guitar systems ground was still noise free, I have to agree with your diagnosis and Steve that the noise is getting back on the power rail. I assume your regulator (7805 family I assume) had the usual caps around it. Those caps need to be placed very close to the regulator to be really effective.

You could up the regulator to 6 or 7 volts, by using a 7806 or even just a diode or two in the common to ground lead. Then you would have a volt or so in hand to charge a reservoir cap on the regulated side, and close to the iPod if possible. You could use a circuit along these lines.

The 2 resistors act as a potential divider to drop the 7 volts (or whatever) to the 5 you need. Choose a value for the top one to allow the cap (say 470uF to 2200u) to charge at a sensible rate. You don't want to overwork the regulator charging up the cap on switch-on. Then calculate the lower one to give the right voltage on the output.

A starting point could be 1000uF, 1k and 5k1 resistors with a 7806 reg. The small cap across the electrolytic is to filter higher frequency rubbish coming back from the iPod. say 0.01uF.

The idea is to drive the iPod from the reservoir cap with the regulator keeping the cap topped up and controlling the voltage. Hopefully, the cap will smother the interference and the top resistor will attenuate any remaining noise that gets back to the regulator.

[Neil_N]

Martin, Steve, thanks for the excellent suggestions. I got home from work, & just spent some time w/ variations on both themes. No Go. Here's why: Apple's rabid anti-consumerism strikes again. God forbid they should lose a penny of their obscene profits, or their strangelhold on hardware by someone using a third party usb charger.
See here for details:
MintyBoost - The mysteries of Apple device charging
The short version is that an exact voltage needs to be present on the usb data lines to trick the device into charging. Any more or less, and it won't charge. That's half the problem. With the correct voltage dividers on the data pins, as long as I feed it tightly regulated 5v, it charges, albeit w/ horrendous noise in the guitar audio.

The real issue is that the current draw is all over the map. When the device senses it need charging, 250-400ma, depending on battery level. When not charging, it draws around 80ma, but even this jumps around. (It's interesting that when it switches out of charging mode, the noise vanishes). So any solution which depends on a steady current draw(i.e. series resistor) to deliver a steady voltage seems doomed, in that the initial (or subsequent) charge surge will pull the voltage below 5v, and vice versa. I wish my circuit design chops were better, this is really aggravating, more than it needs to be...

[Steve Conner]

You realise all your troubles would be gone if you ditched the electronics and just plugged a Telecaster straight into a tweed Deluxe? ;-)

If you insist though, here are some ideas.

Use that big RC filter on the +15v rail and put a 7805 downstream of it to regulate the output.

Bust the iturd open and remove its battery. Nothing to charge, no worries about ID voltages, no charging noise! How's that for "think different". Disclaimer: iturd may refuse to work at all without its battery. But what if you then fed 3.0 to 4.2V into the battery terminals? It might think it was running off a battery.

Redesign the voltage dividers to give the correct voltage when fed with 3.3 or 4.1V instead of 5. Then make a tiny regulated supply just for the dividers. The LM4040 is a 4.1V shunt regulator.

Modify your guitar so it takes a 48V supply and derives all the other rails from that with onboard DC-DC converters. This will free up some wires in the Roland synth lead too.

If that isn't enough, use 115V, 400Hz, like they do on board aircraft.

[tedmich]

I built a nice linear power supply for my parents iPod, the correct voltage divider (which Apple changes with each new device to keep charger sales up) is easily found online. I believe a software "handshake" is required before the high charge rate is initiated by the device, otherwise it could short out a weak USB power bus. It will happily charge at the low rate though as long as the usb data pins get the right voltages. Your home made "instrument" likely has all kinds of SMPS hash and ground loops, easier to design out initially but often a pain to "fix in post" good luck!

[Martin Pedder]

I watched the video and read a bit more as well as Steve and Tedmich's posts of course. All interesting stuff. But, where is all the interference coming from, and why is so much of it going back on to Neil's power source that it can't be filtered? This has got me intrigued now. Unless you are throwing in the towel, I will stick with you on this.

I know high impedance guitar inputs will pick up any interference going, but between us, powering an iPod should not be an insurmountable problem. Let's face it, those iPod docking hi-fi boxes and products like Numark's iDJ presumably charge their iPod and run their audio from the same supply. Perhaps we can find a schematic of how they do that to borrow a few ideas. Do they not get a problem "because" they are using the audio? Is it actually Bluetooth or WiFi causing the interference?

Do we assume that if you use a separate mains charger on the iPod that all is well? Have you had a scope on the power lines before and after your regulator to see what is there? Do you have a circuit of the regulator you built and the rest of the system. I must admit I still haven't fully grasped what you are doing. A photo might help too.

[Neil_N]

Thanks again guys, for the encouragement/interest.

A schematic would be tough. I've been working on this guitar (74 Les Paul Custom) literally since it was new & I was 15 years old, and had no idea what it might have been worth today if I hadn't "fixed" it:-). It is a frankenstein, a rat's nest, and it's held together & working by sheer force of will. My "schematics" consist of 30 years worth of handwritten notebook pages, heavily marked up Roland service manuals, etc. I've often had to substitute relentless persistence for talent/knowledge, but until now, it's worked, pretty well, actually. Considering the complexity of the design and the number or analog & digital signals running around inside it, it works amazingly well, and, by 74 Les Paul standards, is acceptably noise/hum free. Until the ipod...

By way of review:

The purpose of the ipod is to act as a WiFi connected touchscreen interface to generate MIDI cc's to other gear, no audio at all.

Eliminated RF/Wifi as the culprit. Noise persists even w/ radio off.

Noise disappears if ipod is powered from external battery.

Noise appears proportional to current draw.

Can't power from mains, since the ipod is mounted in such a way that there is no room for a 30-pin dock connector. In the guitar, it is connected with a permanently mounted 30-pin connector harvested from an apple dock/charging cable, with the casing removed (for space considerations). But my belief is that mains power would be the same as external battery.

The only pins on the 30 pin connector in use are:
USB+
USB-
USB Data (2 pins, for the charge fakeout)

Other than the +5 & Gnd, there are no other electrically conductive connections to the ipod, ground loop would be unlikely.

There is a 7805 w/ the minimum requisite caps, hanging off the +15v line, which powers a 100pin Cypress PSoC controller. If I power the ipod from this, I get noise.

There is a 7809 w/ the minimum requisite caps, also hanging off the +15v line, powering a preamp for the peizo pickups. If I power the ipod from a separate 7805 w/ the minimum requisite caps after this regulator, I still get noise.

If I place a 7805 just for the ipod w/ the minimum requisite caps on the 15v line, I still get noise. By moving the supply connection points of the regulator, I can change the character of the noise, but not eliminate it.

********NOTE**********

I tried Steve's suggestion of placing a 7805 downstream of an RC network consisting of a 470ohm series resistor & a 3300uf cap (biggest I have on hand). This seems to be a step in the right direction. The low frequency rumble component & the hiss are absent, leaving a "buzz" that sounds like a clock. I'll experiment w/ some different caps on the load side tomorrow. Also will try providing the 2v (USB Data fakeout) from a separate source.

I haven't put a scope on it yet, but tomorrow, time permitting.

Martin, thanks for hanging in there, and I wholeheartedly agree, this should NOT be rocket science. The ipod certainly does not, for example, bork the pc's internal audio when it's powered from USB. I know it will be something simple/stupid. But until I figure it out, I'm stupid.

Steve,
Yeah, three or four times a day I think about a stock Les Paul, and a 50 watt Marshall (1969). It's very tempting when I feel I'll never lick whatever the challenge of the day is. But, then again, when it's all working, it's pretty sick:


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