low pass and high pass filtering questions

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low pass and high pass filtering questions

Postby pinkjimiphoton » 19 Apr 2017, 20:42

hi all
i gotta question.

i built a circuit for my bro as a simple overdrive/boost. he loves it. but he lives on top of a mountain and is getting bombarded by rfi noise and almost motorboating sounds. it doesn't happen here at all, it's quiet but up there it's like REALLY bad.

so i had already added more power supply filtering etc.. which is probably redundant, but it got me wondering and i need a couple questions answered.

does low and high pass filtering in the power supply itself have an effect on the tone of the circuit in terms of audible noise etc?

like... can you filter out hum with a high pass filter and hiss with a low pass one in the power supply part of the circuit, or is it better to do it in the audio part of the circuit?

since he's having problems with low freqency interference, i figure adding a high pass filter before the transistor will help roll off some of the unneeded lows and eliminate sub harmonic crud.

at the same time, i wanna add a lowpass filter to roll off most of the hiss and background noise of the pedal.
i DID add a small snubber to it for him between b and c, but it's still pretty hissy. turning the treble on his amp off makes it pretty much go away, so i'm figuring this will let me roll the hiss off.

i looked at an online filter calculator, and trying to work with nearest standard values, i came up with the following:

for my low pass filter i figured roll it off at 6khz, as the guitar spectrum pretty much ends around 4k-ish and the hiss seems to be prevalent above that..so figuring if i cut it off from there it should help him with the noise.

the calculations i got ended up suggesting a 56.44k resistor, so 100k pot as a variable resistance into a 470p cap should do the trick?

for my highpass filter i figure i'll roll it off at 60hz to help keep it tight.
the calculations i ended up with said 27n cap into a 98.24k resistor should do the trick. or 47n into a 56.44k resistor.... so should i try a 100k pot here too?

or am i better off to go down, and use 50k pots for both? i must admit a little confusion. a lot even. ;)

this is my first time trying to design filtering like this, so do those numbers look about right?

not looking for tone control, looking to try and wipe troublesome frequencies he doesn't need so he can use it..

thanks guys

calcs i used:

http://www.ekswai.com/en_highpass.htm

http://www.ekswai.com/en_lowpass.htm
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Re: low pass and high pass filtering questions

Postby quantum » 20 Apr 2017, 02:01

Have you tried employing a "ferrite bead" at the input. If not then you should check them out. Google "ferrite bead "and knock yourself out. I think you'll find that is the solution along with a rf bypass cap at the input. Placing a ferrite bead in the power supply acts like a rf choke also. Hope this helps.

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Re: low pass and high pass filtering questions

Postby pinkjimiphoton » 20 Apr 2017, 02:28

i have in the past without much luck, i'm afraid.

i think this is just a fluke cuzza where he lives. when ya add in the light dimmers, and the dehumidifiers and all the other crap running, it just puts too much hash in the ac line.

it's pretty much dead quiet HERE other than a little hiss
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Re: low pass and high pass filtering questions

Postby lcv » 20 Apr 2017, 10:26

Hi Jimi
What is usually implemented as PSUs in tons of commercial stompboxes MUST be enough.
Anyway you can't get anything quieter than a simple battery.
So it must be picked up noise of some sort, but I don't believe that any rf noise can be so disruptive.
My guess is that your friend simply has a very poor electrical grounding in his house.
Or maybe the circuit is not properly grounded ( connection mistake) ?.
Just a guess though...

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Re: low pass and high pass filtering questions

Postby phatt » 20 Apr 2017, 12:36

I agree with* lcv*, also adding that you may need to take the Exact same equipment to the noisy house to rule out other more obvious problems such as cheap/faulty guitar leads. If your Brother has even one extra gain stage in his amp then that will exaggerate the noise.
His amp may have a much wider bandwidth than the rig you use and will accentuate the hiss.
I've learned from experience that a lot of modern guitar gear has much wider bandwidth than the old stuff. There is often a lot more circuitry in the signal path now and hence noise issues are common place.

As for PSU filtering yes more is better up to a point but keep in mind that good design of the audio path can overcome a lot of noise issues.

The problem often surfaces because everyone is told that Hi Z input is important to capture all the mojo of your uber tone monster guitar PU's and so 1 meg resistors get hung off every stage of a circuit and hence the cumulative effect is twice the Fuzz but 10 times the hiss and fizz,, :twisted: Then add in a ground loop hum problem and the whole rig is utterly useless. :slap:

Also Switchmode supplies are now common place and they can wreak havoc as they can bleed a lot of hi freq switching hash onto the audio signal.

But you are on the right path about bandwidth playing a part I've spent a lot of time reducing the bandwidth in my circuits as well as some filtering tricks to overcome the number one Enemy,,, Noisy circuits.
Phil.

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Re: low pass and high pass filtering questions

Postby pinkjimiphoton » 20 Apr 2017, 17:06

i made him try it with a different power supply, move it to a different circuit, no difference.

i added snubber caps to roll off noise. no difference.

he's not much help cuz he's not one of us. he calls hiss hum ffs ;)

the house is grounded well enough, but i think the problem at his place is a combo of the faders for all the lights, the dehumidifier and all the computers his girl has running.

here, the pedal is quiet. there, it's like a fuckin antennae.

that said tho, he's playing old gibson melody makers with single coil pickups into an ampeg vt40 under neon lights. i mean, HELLOOOOO???

i know what the problem is. it's definitely cuz he didn't wire shit quietly and has a lot of noisy shit he doesn't understand.

being a half mile plus up on top of a mountain doesn't help... he gets ham interference, police interference, it may as well be spinal tap in that respect.

but nothing high gain, nothing modern. he only uses low gain paf's usually, 8 k or higher and he'll reject it as too hot.

he like CLEAN. old school clean. but he wants a hint of dirt. the pedal nails the tone he wants. but the noise thing is insurmountable.

its still noisy with the commercial pedals i've given him even.. a boss super phaser and a dano chorus. put dirt in front of them, and it gets worse of course.

so i'm kinda at a quandary. i told him easiest way would be to get a noise gate. not perfect, but for him, the no brainer approach may be the only way to fly.

thanks for the advice guys.

ok... so i know a 47n/56k resistance will shave off the highs beginning about 6k, and a 56k/47p will shave off the lows..

my question is ok, will it work like regular tone controls if i just use 50k pots? or would i be better off with 100k ones for more range of adjustment?
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Re: low pass and high pass filtering questions

Postby smackoj » 22 Apr 2017, 08:07

"running old school gear with single coil Pups" Maybe It is the guitar itself that is the noise antennae? There are several products available like shielding foil that is used to insulate the guitar's wiring pocket from rf, noisy neighbors and even garbage truck squeaking brakes. :horsey: How about adding shielded wire for all the wires that carry signal? I don't remember if you said he already tried that.
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Re: low pass and high pass filtering questions

Postby pinkjimiphoton » 22 Apr 2017, 18:15

hahahah. yeah, i think its likely the guitar and a lot of other things.

after spraying laquer for 40 years, ya can't argue with the guy. it's impossible ... lol

he will never "get" that the issues are with his guitar or amp or whatever..."there was no noise until i tried your pedal"... but i've been there and played thru the stuff, the power is definitely an issue. i mean, he was having issues even with a boss phaser making noise.

melody makers used shitty pickups with no shielding, and the electronics have none either. i'd bet there's a ground problem IN the guitar itself, but he'll never let me get in there to see, he's too proud of what he did himself... so....

i was just looking for options.

i took the same damn pedal out in a noisy bar with shit power and lights on faders, a/c going, coolers and god knows how many tv's, and quiet as a mouse in terms of noise.

since i doubt he'll let me fuck with it too much, i guess he'll have to live with no distortion, which he likes.

let me give ya an example... he brought me an amp a while back,., "oh it sounds terrible, it distorts when i have the bass all the way up".

i'm like simple. turn the fuckin bass down.

he's like... but thats gonna make me lose all my TONE. i'm like fine, turn the bass down, and turn down the mids and treble too, ya can crank the amp more.

it won't sound the same he goes.

i'm like dude, all that much bass is gonna do is make the amp saturate.

well, it sound right on my other one, so i wanna do it here, too.

i'm like.... dude... a fender dual showman and an ampeg vt40 aren't the same amp, you cannot expect the same tone at the same settings...

sure i can, been doing it for years...

ended up with the amp on my bench. went thru every single fucking component (AND the amp had been recently serviced with new electros etc) and could find nothing wrong.
thru my cabs here, even with the bass pegged, no bad distortion. it was an old emc 130 bass amp or something like that. if ya cranked it full, yeah, it sounded a little farty in the low end. turn the bass down slightly and it sounded great, even with the volume on 10.

send it back... and .... nope...
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Re: low pass and high pass filtering questions

Postby smackoj » 23 Apr 2017, 16:24

There is an old saying my grand daddy use to say, "You can lead a horse to water but you can't make him drink." :block:

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Re: low pass and high pass filtering questions

Postby poiureza » 24 Apr 2017, 09:55

You could take his guitar + amp to your house and test it with the pedal down there.
And while you're at it you could sneak some foil inside the PU pockets when he's not looking ...

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