Need help choosing the right AC Transformer for pedal power

[gators81]

I am building the Tonepad Pedal Power project (http://www.tonepad.com/getFile.asp?id=111) and am wondering what transformer I should use. I usually shop at Small Bear Electonics for my parts.

[Ben N]

Not an answer to your question, just a note: Besides the safety issue Francisco notes, another reason to use an external wall wart rather than have mains voltage right in your pedalboard is that the AC transformer has the potential to introduce hum, especially if you have an inductor-type wah on board. It can be dealt wiith, it's just one more thing to think about.

[gators81]

Considered that, but I am building it into a rack mount unit with 4 fx circuits i built for use with my GCX switcher and was hoping to avoid the wall wart to save rack space. Francisco doesn't indicate what the amps should be on the transformer so I'm not sure what a safe amperage for the fx circuits.

[armdnrdy]

Francisco doesn't indicate what the amps should be on the transformer so I'm not sure what a safe amperage for the fx circuits.

Electrical equipment only draws what it needs. If you look at Francisco's design he has the output of the power supply split into five outputs.

The size of the transformer depends on the load you are putting on it. You stated that you are powering 4 FX circuits. What is the draw of these circuits? Which circuits are they?

The va sizing of the transformer is solely dependent on the load.

[gators81]

Got it.... 2 of the circuits are Tonepad's Tube screamer, which draw 4.8mA..... A Tonepad Rat (not sure of the draw. Didn't see it listed)... And the Ross modified compressor circuit from guitarpcb.com.

[armdnrdy]

It looks like you're not powering any large draw circuits.

If I were picking a transformer from Small Bear, I believe that this one would be a good choice.

http://www.smallbearelec.com/servlet/Detail?no=339

It's rated at 2.5va which will deliver 200ma @ 12 volts to your power supply circuit. You want to oversize the transformer a bit as with anything to add a little wiggle room in case you decide to power any other effects from the same supply.

The primary input is 115/220 volt and the secondary is center tapped 6/0/6. So you'll have to connect the primary in parallel and the secondary in series.

Here are some examples:
http://www.hammondmfg.com/5CHook.htm

[gators81]

Awesome. Thank for the help.

[gators81]

So I finally had the opportunity to try and finish this project. I purchased the recommended transformer, wired it as instructed, and connected it to the power supply circuit. I'm reading 9V on the output of the transformer, but 18V on each of the circuit outputs (5 total). The trim pot will only lower the voltage about 1V.

I've double-checked the circuit itself to make sure all the components are correct and in the proper place/position on the board. Everything is as listed on the Tonepad circuit PDF.

Is there another circuit I could build to achieve the same purpose. 

[PRR]

> Is there another circuit

That's a very simple common circuit that works for millions of things.

Something's not wired right.

Reversed diode (either D5 or D6) is one possibility. Bad LM317 is a remote possibility. Bad solder joints can cause any kind of trouble.

[vigilante397]

I built the tonepad Pedal Power a while back and no longer use it, but I still have a 12V AC wall wart that I'd be willing to part with. PM me if you're interested.

[gators81]

> Is there another circuit

That's a very simple common circuit that works for millions of things.

Something's not wired right.

Reversed diode (either D5 or D6) is one possibility. Bad LM317 is a remote possibility. Bad solder joints can cause any kind of trouble.

I will check and make sure nothing is reversed and test all the solder joints on the circuit again. I etched the board myself...the traces looked fine. Thanx.

[gators81]

I built the tonepad Pedal Power a while back and no longer use it, but I still have a 12V AC wall wart that I'd be willing to part with. PM me if you're interested.

Thanx. I actually have a wall wart but since this is for a rack mount unit, I'm trying to stay away from wall warts do to power strip limitations and room in my rack.

I use 2 GCX loop switchers and have my pedals on a rack mount drawer that takes up 3 spaces. I'm attempting to build a 4 fx rack mount unit...no switches... Just wired straight to the in/out jacks, that fits in 1 rack space. As soon as I figure the power issue out, I'm good to go...