Power Supply Questions!

[Tom Beardsworth]

I've built a DOD 250 clone which after some modding (limited to just chaning cap and resistor values) sound pretty cool but it's REALLY fussy with power supplies in a very strange way. It works perfectly with my two power banks (one a generic 9v 5 output 450mA regulated supply and the other one similar but i think 600mA) but makes this crazy slightly distorted sine wavey mid-range noise when plugged into a battery (and I've made sure the polarity is all kosha). It makes a really loud 'bad earth' noise when plugged into any other supply, similar to the noise you get when you touch the tip of a guitar lead plugged into an amp turned up loud. Does anyone know what could be causing this? The Volume contol on the pedal affects the vol of the noise. It only happens also when the input jack is plugged in as well as the output jack (well certainly when using the battery). Any ideas of what could be causing this at all? My knowledge is somewhat limited.

Also I'm planning on making a power bank which has isolated outputs so that you can plug pedals such as my vintage orange Ross Phaser into it as well as your other pedals - normally the Ross causes the power bank to short out for some reason. It's positively ground I think though I'm not sure how that works! Would putting Electrolytic capacitors in between the ground rail of the bank and the ground of the pedal work at all? Any info / circuit diagrammes would be really gratefully received.

[niftydog]

It makes a really loud 'bad earth' noise when plugged into any other supply, similar to the noise you get when you touch the tip of a guitar lead plugged into an amp turned up loud. Does anyone know what could be causing this?

Are you certain that the "other" power supplies are the correct polarity and are in fully working order? Perhaps the connector size could be an issue too... kinda hard to say! Does jiggling the power connector make any difference? Any change when turning the effect on/off? I suppose the "bad" supplies might also have significant ripple voltages which might be rectified by more filtering.

Would putting Electrolytic capacitors in between the ground rail of the bank and the ground of the pedal work at all?

No, by far the best solution is the isolated outputs power supply. There are other ways, which can be found by searching the forum, but none of them are as ideal as what you intend to do. (BTW, positive ground devices and negative ground devices do not mix - this is because when connected together they short both leads of the power supply to ground. All you can do is use separate supplies for each type.)

[Tom Beardsworth]

Cheers for the infos! I've found the prob - it was a grounding issue caused by C4 (the cap connected to pin 2 of the 741) being blown. In fact what i was doing when using the pedal with my power banks was also plugging it into my other effects pedals which presumably grounded it. The importance of controlled environments in scientific tests eh! Anyway, I think C4 was blown when i did get the polarity wrong.

As far as the power bank goes, I was hoping to use a big 9v battery connected to a load of DC outs (somehow isolated) to materialise and idea I've had. Any ideas on how best to do that? Also can you explain how the positive and negative ground things work please? Thanks!

[niftydog]

a big 9v battery

Lots of people have been down this path, but I'm yet to see a "big" 9V battery up to the task. Some use a 12V lead acid battery and 9V regulators, but to me that's only slightly less hassle than just building a Spyder and being done with it once and for all... and you'll never have to remember to recharge it!

Also can you explain how the positive and negative ground things work please?

Well, this is more than adequately explained on this forum already, so I'm not gonna go into great detail. Suffice to say that the question answers itself if you look at it!

"positive ground" implies a negative supply voltage
"negative ground" implies a positive supply voltage

Now, power supplies are often what we call "floating" (ie; has no fixed ground point) so you can use just about any wall wart to produce either a +ve ground or a -ve ground supply. Power supplies have two wires; one more positive than the other; and you simply decide to impose your ground reference on it by connecting one or the other wire to ground (or by plugging it into a pedal which connects one of the wires to ground for you.) Here's the crunch; you CANNOT use one power supply to create BOTH a +ve ground and a -ve ground at the same time, because by definition that means shorting BOTH wires to ground.