High pitch from PSU (I'm sure about it ;-) ), how do I get rid of it?

[arma61]

Hi there

so I've built a couple of pedals for a friend, an overdrive and a delay, they worked fine with my PSU, now he has mounted them on his pedalboard that is powered by a switching power supply, sold for guitar effects, and indeed it's working fine with all commercial pedals... though not with mine! I get a high pitch surround noise that stay the same whatever knob I turn.
I've recorded few samples of it, leave that big hum... my guitar is hung beside my PC monitor, and in this case I didn't need to play  .

This is my "PSU"
an AC/DC adapter (13,5V - 1A - 13.5VA)  I used to use in late '80s to charge my mobile phone (a Motorola Micto Tac     )
a voltage regulator built around an LM317, kinda Ultra clean power supplier.... can't recall that was yeaaars ago.

This is the switching PSU
Variable output voltages 3,4.5,6,7.5,9,12,1200mA,14.4VA

Only one pedal and the PSU, no other pedals in between
So this is the overdrive on, LED on, with my PSU
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/My PSU.mp3.html

Same condition as above but with the switching PSU
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/The Other PSU.mp3.html

And finally this is the one with the LED removed from the path,  + side cut away
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/The Other PSU without LED.mp3.html


I've read and I've been told to get a better PSU, but first I cannot force my friend to buy another PSU, second that PSU is working fine with other pedals.

I had the same problem with a multieffect box I've built powered with the same switching PSU , and got rid of that noise powering the board with 12V and put an easy voltage regulator, just 7809 and 4 caps!!

So, is there a solution to this problem ?
How is possible the switching PSU works with all the other pedal than mine, and mine PSU works with both kind


Thanks!!!!
Armando

[R O Tiree]

Have you got schematics and layouts for the pedals you made for your friend?

[R.G.]

What he said.

If the pedal schematics and layouts are secrets, then the next question is how much power supply decoupling and what kind of decoupling is inside your pedals?

[arma61]

the delay is the Rebote 2.5 from Tonepad and the overdrive is the HighWay89  by DoughH.

Even if in the multieffects box I've built there were TS,  Little Angel Chorus,  Guvnor, Rebote and DynaComp.

Don't think this pitch can be rleated to the type of pedal.. but my knowledge is very low....  

Thanks

[R.G.]

OK - now the next question is how much and what kind of power supply decoupling is inside your pedals?

[arma61]

Hi

the pedal tested in the above tests is the HW 89

here's a link to the schematic from the Gallery
http://www.aronnelson.com/gallery/main.php/v/DougH/distortion/hwy89/Hwy+89.gif.html

Suppose you're talking about that 47uF top left, isn't it? the Rebote has the same value. Though I've soldered a 220uF across + and - on the PCB to no vail.

Thanks

[R.G.]

Suppose you're talking about that 47uF top left, isn't it? the Rebote has the same value. Though I've soldered a 220uF across + and - on the PCB to no vail.

Try paralleling that 47uF with a 0.1uF monolithic ceramic.

Are you using the input jack for power switching?

[arma61]

Thx RG no jack power switching , will report about the cap

Thanks

[arma61]

Hi

So, I've soldered the .1uF parallel to the 47uF, no change at all, not a bit, though it let me think about that 100R resistor in series with positive we've seen in many circuits, and it seems this did the trick!!, high pitch totally vanished  wow just wow .... a 100R resistor (and probably that paralleled cap): Wednesday will test this pedal again, and fix also the delay, and see if this RC filter solves also that issue.
Will report back.

Thanks everybody,

just for a little bit of theory... RG can you explain what's going on with the 100R resistor ? was it all about it ?

Thanks!!

[R.G.]

Sure. The whine you heard is either a very high pitched audio tone from the power supply switching, or more likely the rate at which the power supply wobbles its clock to help suppress the readings of any radio interference that it creates. In either case, the switching noise gets in through the power cables, and your circuit acts like a crude radio detector and injects the noise into your audio.

The bypass caps are a very low impedance at radio frequencies - except that big caps, 10uF on up, can't be very good at RF because of the construction details that make them big capacitances. So putting a small, monolithic cap in parallel is less capacitance, but it suffers less from the construction issues and is better at RF. Capacitors to ground act like the low-side of a voltage divider, shunting high frequencies to ground.

But that's not good enough, as you know. Some issues with RF are caused by the distributed inductance and capacitance, and adding a resistor eats energy out of any ringing. And on top of that, the resistance of the wires going into the effect from the power supply is milliohms. Adding a resistor makes the filtering/bypass caps much more effective by increasing the high side of the divider. The resistor greatly increases the "high side" of the divider, and drops high frequencies much more than low ones - so it works more on the whine.

[arma61]

Thx RG!  it's clear now.

As I don't think it will eat much power (tomorrow I'll misure the voltage drop) I think this resistor must be a "default" when building pedals! will be the first thing I'll solder in!

Again thank,
Armando