Power Filtering Advice

[sirdavy]

I've built a (Deacy-style) guitar amp from the output stage of an 18v 1.5w portable radio (Hacker Democrat Rp34). I'm powering it using a 5v/2.4a powerbank and USB buck booster module set to 18v. There's an unwanted sympathetic fizzing, fuzzing buzz when I play notes on the guitar and I believe this might be caused by noise from the buck booster.

1. I've read that a Pi filter - capacitor-inductor-capacitor - might be what I need but despite some research my very limited knowledge of electronics is holding me back. Could anybody recommend a very general starting point for the values of these components? [powerbank is 5v/2.4a, buck booster is 3w max output set to 18v, amp is 18v 1.5w going into a 16ohm speaker]

2. Also I've read somewhere that Pi filters are "not suitable for varying loads" - an amp is a varying load, isn't it? The same website recommends an L section filter "as its output voltage do not vary largely with load current." Is that correct, should I actually be making one of these instead?


(More info: I also have a Range Master treble booster in the guitar signal line that can be switched in or bypassed. See attached diagram.

The amp is quite noise free when the guitar is silent but when you play notes on the strings behind every note you hear the sympathetic fizzing, fuzzing buzz. This buzz decreases in volume when I touch the inductors on the buck booster while playing. And it goes completely when the guitar strings are silenced.

With the treble booster engaged a loud squealing noise is added in, which seems to be triggered by the lightest of touches on the guitar string and sometimes it will continue to squeal even when the strings are silent.

I've tried powering the amp using a 12v DC wall supply the sound is a lot cleaner (compared to the buck booster also at 12v). I am fairly certain that the buck booster module is responsible for the noise and I think I need to filter the power coming from it.)

[antonis]

Hi & Welcome.. 

>I am fairly certain that the buck booster module is responsible for the noise and I think I need to filter the power coming from it.<

True and Correct..

[sirdavy]

Hi & Welcome.. 

>I am fairly certain that the buck booster module is responsible for the noise and I think I need to filter the power coming from it.<

True and Correct..

Thanks Antonis, this is my first post here. Just looking for some advice on how to achieve the filtering. Any ideas?

[PRR]

> I've read somewhere that Pi filters are "not suitable for varying loads"

"Read"? Where? I just read that Parsons opened the gates of hell, also known as Pasadena. A lot of people chew a lot of mushrooms.

I don't see a large capacitor across the power supply, our first defense against supply crap. Try 100uFd 25V.

BTW: sirdavy is also posting on other forums which may increase confusion.

[sirdavy]

> I've read somewhere that Pi filters are "not suitable for varying loads"

"Read"? Where? I just read that Parsons opened the gates of hell, also known as Pasadena. A lot of people chew a lot of mushrooms.

I don't see a large capacitor across the power supply, our first defense against supply crap. Try 100uFd 25V.

BTW: sirdavy is also posting on other forums which may increase confusion.

- This is where I read it, apologies for sounding coy.
https://electronicscoach.com/pi-filter.html (see heading: Disadvantage of Pi filter)

- OK, I'll try a simple capacitor across the buck booster output. Thank you.

- Yes, I have posted to other forums/communities as I'm struggling to understand Pi Filters/filters.

[amptramp]

You may need a lot of capacitance across the line between the power bank and the buck/booster.

I assume the buck/booster is a feedback regulated power converter meaning it attempts to keep its output at 18 volts.  It will attempt to maintain the 18 volt output regardless of the input from the power bank.  But if you are getting a constant voltage and current out of a converter, when the input to the converter drops in voltage, the current at the input has to increase and if the input rises, the current drops because the power demand is constant.  The power bank therefore sees a load that is a negative resistance - current goes up as voltage goes down and vice versa.  If you vary the current drain, the buck/booster becomes the granddaddy of all oscillators, a tuned circuit with inductance and capacitance fed into a negative resistance.

Try using a lot of capacitance across the line going into the buck/booster and no inductance in this connection.  To a rough approximation, the impedance of an L-C filter is SQRT(L/C) although you can get more precise results from simulation.  The negative resistance of the buck/booster being fed 5 volts at three watts, taking inefficiency into account, 5 volts at maybe 4 watts or 0.8 amps input is -6.25 ohms with the minus sign indicating negative resistance.  If you feed it with a source using an L-C filter, the filter impedance is in parallel with the negative resistance of the power converter and if the parallel combination goes negative, it will oscillate.  Even if it is marginally positive, there will be current drains from the amp that may force it into instability.  If the magnitude of the filter impedance (a positive value) exceeds the magnitude of the negative resistance of the converter, oscillation is guaranteed.

[sirdavy]

Thank you very much, Amptramp. I won't pretend to understand all of that but I'm getting the gist.

This is the buck booster module I'm using.
https://www.amazon.co.uk/gp/product/B091TGTG7N/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Think I'm going to get hold of some of caps of varying values - 100uf (as PRR suggests), 200uf, 470uf - and a few inductors - 1mH, 2.2uH - and try a few filter arrangements before and after the buck booster.

Sorry for spamming different forums.

[PRR]

> This is where I read it

Yes. What he says is really applicable to the capacitor-input filter (because a Pi is a cap-input plus an L-C filter, and the L-C has little effect on "varying load"). But in his buried essay on cap-input (THE most used type) he does not say "not suitable for varying loads".