[sdiy] [AH] Want to build an AC power supply

[Tristan]

I wouldn't use resistors in this application unless the AC waveform needed to remain pure for some 
reason. Most likely a pedal would only use AC if it was internally rectifying it to produce a positive 
and negative supply rail. In this case it would be better to use a string of five pairs of back to back 
1N4001 diodes in series with the transformer output. The voltage drop across each diode is 
approximately 0.7 volts (so a total of ~3.5V) which would get the output close 9VAC. The main 
downsides of using multiple diodes like this are that they take up a little more space than one or two 
resistors and there will be some waveform distortion around the zero crossing of the AC waveform. 
This could potentially bleed harmonics of the AC frequency through if used directly for the heater on 
a valve but should be fine if directly feeding a rectifier inside the pedal. The voltage drop across the 
diodes is relatively constant with load variation so it provides a more consistent AC voltage than 
using resistors and because the power is dissipated across the string of diodes there are less issues 
with power dissipation in each device.

/Tristan


On Tue, Jan 15th, 2013 at 8:43 AM, Logan Mitchell Sr <prowlerraven32 at hotmail.com> wrote:

> 
> I want to build an AC power supply to work with some effects pedals that use
> 9 volts AC, so would that be just a matter of me taking a 12.6 VAC
> transformer & putting some resistors on both sides of the secondary to reduce
> the voltage down to 9 volts AC or is it more complicated than that ?
>  
> Logan