Audio Circuit – Supply Noise problem
i’m designing an Effects pedal, (for guitar, mic, bass, etc.) to be used on stage.
The effect has several stages, which none of them really matter to this question except for a high gain stage.
I’m deciding the kind of supply i’ll use for this pedal. My idea is to keep it simple and small so i want to do it with an external DC supply, but here comes the trap.
There’s a tube involved in the circuit, typical 12AX7 double triode, so the heater already needs 12V – 150mA.
Ok, so i think of a typical 12V – 500mA supply to be sure BUT! i want to use 20 to 30Vdc for the rest of the circuitry. I wanted to avoid using a transformer with several windings, so i came up with this:
I though of a simple solution with a step-up built with a 555 timer to double those 12Vdc, o even go an get a simple switcher IC. Found this:
I chose the 555 circuit because of its simplicity, its cheap dont need to supply a lot of current (<20mA), and because i don’t have a switcher IC at hand at the moment.
The circuit worked like a charm (changed C2 and C3 to 470uF, and changed R1,R2 and C1 to 4k7, 22k and 390pF to work at around 70kHZ, way above audible frequency and low enough to use simple components like 4148 diodes. (besides, if R1 was 100ohms like circuit says it will probably be set on fire). I went ahead and built the pedal on a breadboard.
It worked pretty good overall except for (what every one here already guessed) … NOISE. Half way Gain stage pot, amazing amplified supply noise.
Measures i took to solve the problem:
Right next to the output of the supply, a Fe Bead and then a passive double stage low-pass filter (with a cutoff freq. of 0.3Hz).
On every IC Vcc pin, a small 100pF cap to ground.
I still get noise with the gain set to 1/2 and above (a lot less than before, you could actually use the pedal now, but i HATE noise).
After all this explanation i want to ask two questions:
First: Can i possibly eliminate this kind of noise when it comes to a switching supply for audio, maybe with a better filter or something with simple components, or do i have to go running back to the old transformer+linear regulator for this kind of application? (which i really don’t want to do)
Second: When it comes to a using this kind of devices on stage, usually there are a lot of sources from noise (electromagnetic, ground loops, etc..). Is a Transformer+Regulator more or less immune to this kind of noise than a switching supply? Please explain
Thank you, regards.
EDIT: I lowered the switching frequency as adviced by @Jhon D. Its working at 30kHz. The noise came down just a little bit, bit still it appears. To clearify some comments, the diodes are 1N4148(datasheet: https://www.vishay.com/docs/81857/1n4148.pdf), and transistors are BC337(http://www.kynix.com/Detail/1159090/BC337.html) and its complement. So they couldn’t be the ones causing the noise, at least not in regards to the working frequency.
I give you two pictures, first the 24Vdc rail. Hellish
Now the Output of the pedal, with NO input (input is grounded), Volume = Max, and Gain = max.