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Vintage threads from the first ten years

Adding external power to FX?

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5/17/1999 12:21 AM
nek	Adding external power to FX? Looking for best known methods to add external power to a Vox 847 wah pedal. Is it a simple matter of hooking the appropriate external DC 9V jack in parallel with the battery? Maybe a diode to keep current going in the right direction? Perhaps there is a schematic or DIY resource? Thanks in advance. -nek

5/17/1999 8:40 AM

The problem is purely mechanical, not electrical.

You get a power jack that accepts the common 9V power adapter plugs, drill and shape the hole and mount the jack into the case. These jacks all have three lugs. One is ground, which is pretty simple to connect up. The other two are connected together inside the jack whenever there is no plug in in. You wire the contact that is connected to the plug when the plug is inserted to the effects board, and the battery power lead to the one that opens when the plug is inserted.

That way, the battery is connected to the effects board whenever there is no adapter plug inserted, and when you insert a plug, the battery is disconnected. No diodes needed, and the adapter is not paralleled with the battery - which can cause some batteries to explode, spraying corrosive gook all over the inside of your effect.

5/17/1999 11:01 AM
JD Sleep	Doesn't the DC jack need to be isolated from the chassis? (assuming you're using the standard neg. tip wiring) JD

5/17/1999 11:51 AM

Depends on the jack. Some jacks connect one lug to the chassis, some don't.

I like the Mouser 163-4303 (US$1.38) that has a plastic body - none of the lugs connect to the chassis through
the mounting. Can't get it wrong.

Yeah, if you're using a jack that connects to the chassis through the mounting of the jack, you either have to be sure that you want that lug grounded or insulate the thing from the box.

5/17/1999 6:19 PM
nek	I fabricated insulation out of a plastic bushing and washers that were nearly the right size and thickness I will keep in mind the plastic body jack in the future. Save me a whole bunch of time and effort. Thanks! -nek

5/17/1999 6:47 PM
R.G.	That'll work. I just hate fabbing insulators. Go for it!

5/18/1999 11:44 AM

In principle, yes.

The odds are pretty good, however, that somewhere along the way someone will attempt to insert an adaptor that has the same plug and voltage, but different connector polarity ("That's alright, man, *I'VE* got an adaptor in my case here."). When that day comes, it is good to have a protecting diode in series. Any old 1N4001 in series with the V+ line is fine (stripe going to the effect).

Probably the day after that, someone will stick in an adaptor made for a calculator or something that has minimal regulation requirements, and the hum will drive you nuts. When THAT day comes, it is good to have a moderately large electrolytic capcitor (220uf-1000uf, 16V) across the power lines, just to keep the hum to a minimum.

That being said, if you KNOW that neither of these days will ever arrive, then RG is absolutely correct in his recommendations.

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