Build Your Own Clone Message Board

I’ve been using my recently built OD2 for about a week now (excellent results with mod’s for bass, BTW) and it works perfectly with 9V battery power.

With an unregulated “9V wall-wart”, the overdrive is fine, but there’s lots of AC hum when the boost is enabled. It’s built with MOSFET boost and I’m using the OP2134 so over-voltage shouldn’t be a problem, up to 18V, if I understood the instructions. I’ve tried two different wall-warts – same result. I’m guessing that a regulated power supply might solve the problem.

Is it worth dead-bugging a regulated power supply using an LM7809 (or LM317)? Is there something else you could recommend? What’s the approximate maximum current draw of the OD2?

Thanks,
Mike Andrew

Trouble is a lot of wall wart supplies are poorly regulated and smoothed. The OD2 has a fair amount of gain so any power supply ripple is likely to get through and be heard as hum.
If you felt like it you could lash on an external 100 ohm half watt resistor in series with the power feed and a largish electrolytic cap, say 220uF, to ground on the pedal side of the resistor (the pedal already has a 100uF across the power rails inside). See if that cuts the hum enough.

A standard Tube Screamer apparently draws 7mA. I wouldn't expect the OD2 to draw more than 10mA.

I like the OD2 a lot, I have mine totally MOSFET. I never liked distortion pedals until I got it. I also have an OD1 built to the standard specs and I like that too.

My ODII draws 4.8ma

If you want to know the draw of a BYOC kit check this thread. The current drawn for each circuit is given at the bottom of the parts list.

viewtopic.php?f=9&t=4508&st=0&sk=t&sd=a

Many of the newer kits have the current draw in the build instructions.

Have you tried powering the pedal with a battery? If there is no noise with a battery the cause of the noise is the power supply. Poorly filtered un-regulated power supplies should never be used to power audio circuits.

Stephen,

michaelandrew says in his first post - works perfectly with 9V battery power.
In fact he says the overdrive itself is fine but there is loads of hum when the boost is engaged. Well i suppose the boost is adding more gain but I am surprised there is that much difference.

michaelandrew,

Presumably the hum varies as you turn the boost level control up and down and drops in level a lot when you bypass the boost? If so it is almost certain that the wall wart just has too much ripple on its DC out.

One thing I have wondered about is - this is US 110 volt AC mains yes? Some wall wart supplies don't actually plug in to the wall but have mains cables with a 2 pin plug. It may make a difference which way round that 2 pin is plugged in. In fact come to think of it even some of the wall plug supplies can be reversed.

Oh, OK then, I'll re-word it and say that if there is no noise with a battery then the noise is being caused by the PS. Un-regulated and poorly filtered garden variety wall wart type power supplies shouldn't be used with audio circuits. Some circuits are more sensitive to the quality of the incoming supply voltage than others. This to me looks like one of those times.

Just a quick note--the boost stage is very simple and completely separate from the overdrive stage, except for being included on the same PCB. The opamp is part of the overdrive stage and has nothing to do with the boost. Take a look at the diagram on page 34 of the OD2 instructions and you'll see most of the boost stage components outlined in green. There's also a 100nf coupling cap just below them and the boost level pot and 10uf electrolytic cap up near it that are part of the boost circuit, but the rest of the board is the overdrive circuit.

In any case, a good quality regulated P/S should solve your problems.

Yes that's true but I do not see what that has to do with this problem. The boost circuit is not completely separate - as in in a different box with a different power connector.

The boost circuit and the overdrive share the same power rails which are de-coupled by the same 100uF cap and wired to the same power connector. The standard configuration of that pedal is with the boost switched in and out following the overdrive. So in the effect chain with both switched in the gain is cumulative. A little hum out of the overdrive gets amplified by the boost.

OH WAIT - michaelandrew - are you saying that the overdrive on its own is fine with a wall wart but the boost ON ITS OWN is not.
I was assuming you meant you have the overdrive on and then notice the hum when you add the boost to it?

I would not be surprised if you got some hum with both the overdrive and the boost on.

If you get hum with just the boost that is a little unexpected.

Remember that hum is picked up by the pickups, guitar electronics and sometimes even the cable and that power supplies radiate a hum field. It is best to keep power supplies and amplifiers as well away from guitars, effects pedals and reverb pans as possible.

"Completely separate" was a bit of an overstatement, as the two effects circuits do indeed share a common power rail/DC jack/battery hookup, and a common grounding circuit. My point is that they are separate effects which operate independently, and have separate controls. You can run either by itself or both simultaneously, in which case the boost follows the overdrive. I mentioned this primarily because the OP stated something about the opamp's voltage capability. I just wanted it to be clear that the opamp is part of the OD circuit and has no function within the boost stage.

You raise a good point, though, Tark. Your assumption was that the noise occurred when the boost was added to the OD, and I assumed that the boost was noisy on its own. Michaelandrew, can you clarify?

Sorry, I haven't been back to this forum in a few days. I still don't know why the hum affected the boost and not the OD, but the trouble was definitely the wall-wart. Here is the/a "fix": I found an LM317 3-terminal regulator in my spare/junk parts bin and "dead-bugged" the basic circuit from the LM317 datasheet (try the National or TI web sites, or Digikey) - it's 2 cap's (0.1 and 1 uF) a 180 or 240 Ohm resistor and a 5K trim pot. Just read the datasheet. No pix, it isn't pretty.

The 9V wall-wart puts out about 13.5V no-load but anything up to 28Vdc input will work and the pot can vary the output from 1.2 V to just less than the input voltage. The regulator gets rid of the hum almost completely - not quite as quiet as the battery, but I did start with a crappy wall-wart.

The LM317 (and 117) are the adjustable versions of the 3-terminal regulators - they come in various fixed voltages as well including 5, 9, 12, 15 etc. They are dirt cheap and if you can BYOC you can BYO regulated P/S.

Next I plan to open up a wall-wart (I have a box full of the things) to see whether I can salvage a transformer and maybe a rectifier circuit - I don't really know what's in them - then add some decent filtering - should be fun.

Thanks for the responses and suggestions - sorry I didn't get back here sooner.