Filtering Power Supply Noise

[CZfreqmod]

I have three Digitech Jamman pedals that I want to run synced up. This is fine but my problem is the power supply. I only have a Dunlop DC-Brick which does not have isolated outputs. The Jamman pedals are very sensitive to noisy power supplys and there is an unacceptable about of background noise. There are some very nice looking pedal board supplies by Voodoo lab that I'm sure would fix this problem immediately, but they are outside of my budget at the moment. I was wondering if it might work putting a simple voltage regulator circuit in between the PSU and the pedal. I gather the voltage regulator ICs reduce the voltage as well as stabilise it , but the DC brick has three 18V outputs . Do you think if i found an IC that took 18v in and put 9v out and had some filtering capacitors in the circuit, that I could clean up excess noise?

[Rob Strand]

Try putting a 10ohm resistor in the negative lead to *each* pedal.

That should work fine unless  the pedals are pulling over 100mA.

[CZfreqmod]

They are pulling more than 100ma. The Solo XT is pulling 350ma and the two Express' 150ma each.

[antonis]

Do you think if i found an IC that took 18v in and put 9v out and had some filtering capacitors in the circuit, that I could clean up excess noise?

Unfortunately not..

So far as all outputs share the same common ground, your issue will persist..

[CZfreqmod]

Oh well, worth a shout. cheers!

[Bunkey]

This is what I'm working on for a small amplifier. It also runs on an unregulated 18v DC wall supply.

If you swap the 2.4k resistor for a 1.5k it will give you a 9.7v output.

The 100mOhm resistor in series with the 220u output cap is there because the LM317 regulator is stable with an ESR greater than 0.1ohms on its output capacitor. 150mOhm is ideal. I've used a series resistor (100mOhm 1W wirewound) with a low ESR cap here to good effect but a higher ESR cap on its own is better. Nichicon FG is working well on its own.

You can omit the 180u capacitor before the regulator but keep the 1u for noise filtering. The 4700u is an additional smoothing/reservoir cap, you might want to increase this depending on the current demand and the size of the cap in your power brick (mine is 1000u).

I'm drawing around 500mA from this supply atm & could maybe use a slightly bigger reservoir at the peaks.

As this is running off a DC wall supply there is not true 'earth', just a common earth relative to the audio circuit - Adding the 100ohm resistors in line on the +ve and -ve between the DC input jack and the common earth point reduced noise considerably. These are 2W metal oxide resistors with good surge handling for use in power applications. *See newer posts*

The 22u cap on the regulator also reduces noise considerably in comparison to any other values I tried here.

As you're dropping a lot of volts (an 18v supply is typically 25v unloaded) at a high current, you should use a good heatsink on the regulator.


This is a work in progress but it works well; it's not perfect but far better than an unregulated supply and well worth implementing.

If you're interested you can find everything I've documented about it here:

https://www.diystompboxes.com/smfforum/index.php?topic=126474.0
https://www.diystompboxes.com/smfforum/index.php?topic=126501.0

[Bunkey]

I should note: This doesn't give you seperate isolated outputs as such but it will remove the power supply noise. If the noise is being created by one pedal affecting the others then it might not do much and I think that's what Antonis is getting at.

As long as you're careful with grounding and 'star' each pedal back to the central point it should help get rid of the supply noise at least - You could then run a ground wire from this point which clips onto a metal radiator pipe or something in your house and that will isolate everything - but again it might not be necessary. Don't hurt yourself.

[Rob Strand]

They are pulling more than 100ma. The Solo XT is pulling 350ma and the two Express' 150ma each.

You might be able to use 1 ohm resistors but they become less effective.    When your options are low it's sometimes useful to try these things.

[Bunkey]

They are pulling more than 100ma. The Solo XT is pulling 350ma and the two Express' 150ma each.

You might be able to use 1 ohm resistors but they become less effective.    When your options are low it's sometimes useful to try these things.

Rob - does this mean my 100r resistors are trying are trying to drop 50v across themselves @ 500mA and are therefore limiting the supply instead?


A mere page ahead of the class I'm trying to teach. I am a fool 

[Rob Strand]

A mere page ahead of the class I'm trying to teach.

It's just an oversight.  The 100 ohms resistors had good intentions.

[Bunkey]

 

The 1ohm here with the 4700uf cap should in theory form a LPF with a corner frequency of 33hz so with any luck that'll still filter out some mains hum. I'll give it a bash tomorrow.
At least now I know why it's unhappy above 1/3 volume.

Thanks man