old external drive power supply for my pedals

[cox]

So, i'm back to soldering, after a false start while ago. Here's the thing: I've built a valvecaster and tried to power it with this -->
https://docs.google.com/file/d/0B2AC_KAJsuzndHVyMTVkTDhHV0U/edit

The great problem now it's that i think that it's not regulated, cause of the huge noise coming from my amp. I tried the valvecaster with the a 9v battery, and it's dead quiet, so it must be from the PS, I think.

I was wondering how to filter this thing without losing to much current and voltage, since i was expecting using it to power another valvecaster and another bunch of pedals. Tonepad project would be a good idea? What do you think i should do?

Thanks in advance

[Jdansti]

The manufacturer's website doesn't say whether it's regulated or not:  http://flypower88.en.alibaba.com/product/225662642-200716078/power_adapter_12V5V_2A_dual_output.html

First, are you sure that you've connected to the 12V output and not true 5V, and second, what is the voltage when it's powering the valvecaster?

It might just have lousy filtering, and if it's a switching PS, it could have inherent noise. I think it would be worth trying a simple filter similar to this:

http://www.olcircuits.com/documents/olc_powerfiltering_guide.pdf

[PRR]

Use an audio pedal supply. They are not that expensive.

The external drive supply must be regulated; it has a w-i-d-e input range and some drives shut-down at 4.7V or 5.3V. However drives really don't mind if a supply jiggles 4.9-5.1V (or 11.9V-12.2V), but some audio circuits will take the least power jiggle and amplify it big.

Also drive supplies might not start, or might not be stable, with the very lower power demand of many audio circuits.

If there is NO other supply available, try the filtering John suggests. Good power filtering is essential to audio design/implementation. Has worked for me. But generally a suitable supply is better than trying to wrestle an unsuitable supply.

[cox]

so,

i made the filtering sugested by john but i got it wrong, i think.... with no connections to valvecaster, it reads 12.something volts, and with the tube heaters connected, it drops to 3.something volts, with the resistor getting very, very hot... i don't think this is normal.

I'm trying again tonight, if somebody have some idea of what went wrong, or have some suggestion, feel free to say it.

I got no background in electronics, and i'm trying to learn something with these tiny projects.

Thanks in advance,
Luís

[Mike Burgundy]

What is the current drawn by the circuit without the extra R and C?
Make sure the supply can churn this out (shouldn't be a problem at all with 2A available). It does draw a fair bit of current compared to more common stompbox circuits, due to the filaments.
Then make sure you wired it up correctly.
You'll need to take the current drawn into account when choosing the resistor (call it "R"). It acts a s a voltage divider with the circuit ("Rcircuit"), so the circuit no longer sees the full supply voltage but V=Vsupply * (Rcircuit/(Rcircuit + R). It also needs to dissipate the power from its own voltage drop and the current running through it. 100Ohms might be very, very large compared to the circuits DC resistance, so it will drop a LOT of voltage, and dissipate all that power as heat. A 100Ohm resistor on its own would allow (Ohms law, I=V/R) 120mA ( which would *just* be enough for the filaments, but it won't work - you're losing voltage over the resistor, remember?). The resistor now dissipates P=V*I=120mA*12V=1.44W.
Use a lower value resistor (try 10Ohms) and make sure it can take the power.
Better yet, take PRR's advice. While stuff like this can be very educational, computer supplies are very finicky and can produce problems even when it "worked fine before".

Edit: corrected for 12V instead of 9.

[Mike Burgundy]

Oh, and if it's wired correctly, Ohms law tells us what the circuits resistance is (at least at that low voltage - it will change with supply voltage since the filaments react differently).
If there's 12V out of the supply and the *circuit's* voltage drops to 3V with the resistor (I'm assuming this is what happens - if the supply itself drops that much you have an almost- short)
that means the circuit drops 3V, the resistor drops 9V, therefor the resistor has a resistance 3 times greater than the circuit.
Ckt R is 100/3=33Ohms.

[Jdansti]

so,

i made the filtering sugested by john but i got it wrong, i think.... with no connections to valvecaster, it reads 12.something volts, and with the tube heaters connected, it drops to 3.something volts, with the resistor getting very, very hot... i don't think this is normal.

I'm trying again tonight, if somebody have some idea of what went wrong, or have some suggestion, feel free to say it.

I got no background in electronics, and i'm trying to learn something with these tiny projects.

Thanks in advance,
Luís

If possible, please show us a photo of your wiring including everything connected to the power jack.

[cox]

i've been thinking about removing the resistor, leaving just the cap. bad idea?

[runmikeyrun]

I've built a bunch of these, and I've found that even pedal power supplies will hum.  Pick up a 12V regulator from Radio Shack and build a small regulator/filtering circuit.  You can build it on a tiny piece of perf with two 220uF 16v caps and the regulator, thats it.  Be sure to heat sink it to the case or to a small heat sink.  Your pedal will be dead quiet, guaranteed.

[cox]

haha, i think it works! i swapped the 100r value for 10r and tried right away. it started with some noise, so i decided to compare it with a fresh 9v battery. the amount of noise was basically the same... I guess that now it's just some bad isolation from being experimenting, i think. i'll let konw if something is wrong in the future when i get everything put into a box!

Thanks a lot everyone that helped me get all this thing to work!

Cheers,
Luís

[PRR]

> removing the resistor, leaving just the cap. bad idea?

You are in a car on a rough road.

You are isolated from the bumps by two factors: car-springs and car-mass.

The car mass tends to follow a steady path, and the car springs allow the wheels to bump up and down without full jolts to the car mass (and you).

Replace the springs with solid bars. The ride will be VERY rough.

Replace the car spring with pen-springs. The car will sag WAY low.

Heavy trucks use springs which sag maybe 2 inches from loose to loaded. Sports car springs maybe 4 inches. Sedans maybe 6 inches. (You can't fully un-load car springs unless you disconnect the shocks; on some cars the springs will fly out.) Knowing the sag from no-load-to full-load tells a lot about how a vehicle will ride.

You need "a good amount" of sag. Not too much, not too little.

Resistors are NOT quite like springs. Nevertheless, the "sag" tells a lot about how it will work. Less than 2% sag is like a heavy truck: all the bumps come through. And 50% (or 75%; 12v to 3V) sag is usually far more than you want. 10% is often a good starting balance between isolation and excess drop.

[cox]

i tried with no resistor at all last night, and didn't worked at all.... Noise was coming through the speaker, so i decided to hook up a 10r resistor like mike sugested and at second time it was working! On the first time i got a burnt resistor.... Not cool.
anyway, it's working and i'm pretty happy with the result.

Cheers,
Luís