Power supply types

[JohnH]

Im developing an overdrive pedal, with a view to selling on a small scale. I would like to design it to use the most common type of power adapter for pedals in terms of plug/socket polarity, voltage (assume 9v), that buyers may already have. What would the most common socket be? and are the supplies generally smoothed and regulated, or is that generally taken care of in the pedal?

cheers

John

[Mini-Strat_Maine]

Might as well get my disclaimer in at the beginning. :

johnh said:

Im developing an overdrive pedal, with a view to selling on a small scale. I would like to design it to use the most common type of power adapter for pedals in terms of plug/socket polarity, voltage (assume 9v), that buyers may already have. What would the most common socket be?

Pretty much everything in my collection has the 9V, center-negative "barrel plug" jack. I think I've seen them called a 2.5/5mm plug. The only exceptions are the Uni-Vibe (18V, same plug) and JH-2 Fuzz Face, which still has an 1/8" (3.5mm) phono jack, but Analog Mike bypassed that. (PNP geraniums and such. ;D)

and are the supplies generally smoothed and regulated, or is that generally taken care of in the pedal?

I think all the good power supplies are made to be pretty smooth, even some "low-budget" ones like the Danelectro DA-1 (9v) and DA-2 (18V), which they call the "Zero Hum©."
The "default" plug for the Visual Sound "1 Spot" is the center-negative barrel type, but they have 3.5mm adapters, battery clip adapters, and I don't know what-all else for theirs. www.visualsound.net/1spot.htm

Sounds interesting, John.

[JohnH]

Thanks M-S. Ill follow that convention. Interesting that the inner is negative. I would have guessed the outer one would have been, since it is the one that would form the ground

J

[UnklMickey]

John,

for the ultimate in flexibility, you could put a bridge rectifier, filtering, and regulation in your pedal.

then any power adapter could be used regardless of AC or DC, pos or neg tip.

the only constraint would be that it be a high enough voltage, and capable of enough current.

since you like to design low-power stuff, i'm guessing voltage would be the only issue here.


BTW, were the responses from the "musos" (whatever that is) all positive?

unk




EDIT:


"I would have guessed the outer one would have been, since it is the one that would form the ground"

i'm fairly certain those jacks are insulated.

so neither inner nor outer are necessarily ground.

[JohnH]

Once it is partly pushed in, then the neagtive becomes the ground, so the outer barrel is positive. Not a big issue, just wanted to be consistent with convention.

Ive let several musicians have a go on my box. Big smiles all round, and the local music store wants to sell them. I took it down there to test, with the LPmax through it, while I was choosing an amp. It gets very very close to the bluesy tube overfrive sound, or classic rock overdrive, played through a pure clean amp. In fact I cant hear the difference, once a couple of knobs are tweaked. So its quite interesting and encouraging.

John

[sumgai]

John,

unk's right, install a small bridge rectifer in "steering" mode, and be done with it, polarity wise.

Installing a voltage regulator can also be simple, but you need to have at least one volt more on the input side, which from the standpoint of a 9 volt battery, means that your circuit needs to run on 8 volts or less. And at that, batteries run down, so it would be nice if you needed, say, 5 volts for instance, instead of the full 9v. If that's feasible, then 7805's are available in just about any form factor, including SMD.

If some constraint (cost, available room, etc.) prevents you from installing a regulator, then you need to evaluate what your device sounds like with low power, unfiltered AC on the line, spurious noise like switches opening and closing, etc. If your new toy can handle all this with aplomb, then perhaps a regulator would be overkill. Food for thought, that's all.

Good luck on this venture!


sumgai

[JohnH]

Sumgai and Unk

Good ideas. I like the thought of the bridge rectifier. Ill probably try it just on the external power input, and try to configure it so that the battery does not go through it, or maybe just through one diode of it, to maintain voltage available when on battery mode. That should make the device bomb-proof. There a lots of generic power supplies which have either a negative outer barrel, or a reversible one, hence someone will one day get it wrong. I dont think the device will be damaged by moderate overvoltage.

Ive thought about the regulation. I think in this case, if I accept that unfiltered, unsmoothed signals may enter the box, I'm going to risk more gremlins than I care to deal with. It is naturally a very high gain circuit in a small box, so even if I catch that unregulated ripple and stabilise it, Id be lucky if some does not radiate in some way and get picked up on the input side.

cheers

John

[sumgai]

John,

One diode as a protection against a 'reverse' input voltage works, but as a "courtesy" to your customer, why not let him/her use any supply, positive or negative? The cost and size requirements of a bridge rectifier setup can't be so much higher as to become 'too much', can they?

Overvoltage is a bit less problem these days, true. But even a cheap zener will give you 90% of everything you could ask for in protection, so that's a thought too. But I'd prefer the 7805 trick, that lets just about anything be used for power.

Internal shielding. Electro-Harmonix invented it in stomboxes way back when. Don't be afraid to bend up a small piece of copper or aluminum and solder it down where required. That should keep irresolute radiation from leaking back into the input circuitry.


sumgai