Noisy Distortion

Box_Stuffer · May 27, 2023, 09:18:33 PM

Thanks to you guys I have gotten my first distortion pedal up and running. It sounds great! BUT, since I am a noob I did not build anything into the circuit to deal with noise. I put a 47uf cap across the +/- of the power jack and a 47pf cap across the +/- of the input jack.

It sounds really good except for a strange sound that is always present. It is hard to describe, but maybe like a fly buzzing at a constant rate? Or maybe like a hum you hear when you are near a high voltage junction box. It kind of sounds like electrical interference so I tried moving things away and turning the lights off and nothing makes a difference. It correlates with the level control and gets louder as you turn up the level. The gain control has no effect.

I have read that pull down resistors in the input stage help to minimize the pop from the footswitch. Do they help reduce other noise as well?

Dormammu · May 28, 2023, 12:10:00 AM

Is your amp\audiochain grounded? Wall sockets?

idy · May 28, 2023, 02:51:39 AM

No, pull down is just for the pop.
Are you using a battery or a wall wart? You should try a battery. If you are using a wall wart is it a good one? Off-brand adaptors, even ones sold as for pedals, are often noisy. "One Spot" is a relatively cheap but nearly perfect adaptor.

Making the pedal so it is reasonably quiet with less than perfect power takes a few extra components. Start with a low value R, (100ohms?) in series with the power jack, before you hit that 47uf cap. Then add a film cap, maybe 100n, in parallel with the big cap.

antonis · May 28, 2023, 07:02:27 AM

It should be helpful to post a schematic of your particular distortion circuit as well as some pics of your build..

ElectricDruid · May 28, 2023, 09:57:58 AM

Quote from: antonis on May 28, 2023, 07:02:27 AM
It should be helpful to post a schematic of your particular distortion circuit as well as some pics of your build..

Yes, that would help us see what's going on.

From your description, it sounds like bad grounding.

duck_arse · May 28, 2023, 10:47:54 AM

Quote from: ElectricDruid on May 28, 2023, 09:57:58 AM
Quote from: antonis on May 28, 2023, 07:02:27 AM
It should be helpful to post a schematic of your particular distortion circuit as well as some pics of your build..

Yes, that would help us see what's going on.

From your description, it sounds like bad grounding.

or a charge pump.

Box_Stuffer · May 28, 2023, 11:22:35 AM

Quote from: idy on May 28, 2023, 02:51:39 AM
No, pull down is just for the pop.
Are you using a battery or a wall wart? You should try a battery. If you are using a wall wart is it a good one? Off-brand adaptors, even ones sold as for pedals, are often noisy. "One Spot" is a relatively cheap but nearly perfect adaptor.

Making the pedal so it is reasonably quiet with less than perfect power takes a few extra components. Start with a low value R, (100ohms?) in series with the power jack, before you hit that 47uf cap. Then add a film cap, maybe 100n, in parallel with the big cap.

The power supply I am using is a cheapo generic one off of either ebay or walmart.com. It is 9v 1A. Is 500mA better? It said it was for "guitar pedals" , but who knows. I only set it up for DC, but I can probably find an old plug and make a battery adapter.
I also may get some tiny ferrite beads and try that as well in the future.

Box_Stuffer · May 28, 2023, 11:26:55 AM

Quote from: antonis on May 28, 2023, 07:02:27 AM
It should be helpful to post a schematic of your particular distortion circuit as well as some pics of your build..

Here is my rough sketch - pretty simple. Its mostly based on the "10 minute distortion" that I saw on the diyguitarpedals channel on youtube. The one with the Australian guy.

ElectricDruid · May 28, 2023, 11:40:44 AM

Quote from: Box_Stuffer on May 28, 2023, 11:22:35 AM
I only set it up for DC, but I can probably find an old plug and make a battery adapter.

It's a very useful test. If the pedal's quiet with a battery, but noisy with the power supply, then it's most likely the noise comes from the supply. If you get noise both ways, you need to be looking elsewhere (like the grounding).

antonis · May 28, 2023, 12:13:01 PM

I'd start with a 47pF to 100pF across 1M feedback resistor..

(and deside about 1μF cap discrepancy between plates symbol and polarity marking..)

P.S.
Too many unidentified diode pairs permanently connected in parallel..
(5 of them are practically useless 'cause diodes of lower forward voltage drop dominate all the others..)

idy · May 28, 2023, 12:14:37 PM

Good on you for picking something easy, even elemental!

What you describe is absolutely the kind of power adaptor I was talking about, generic, advertised as "for guitar pedals." The current rating, for your purpose is irrelevant. Buy a One Spot. Keep your cheapy to test pedals to see how they will respond to a cheap power supply.

Make an adaptor to use a battery. Even if you never use pedals that way it is an important test and handy for a builder.

A funny think I have noticed is that these simple one-transistor circuits are often very sensitive to supply noise from bad adaptors. Op amps have a thing called "common-mode rejection" that this humble circuit lacks.

GibsonGM · May 28, 2023, 12:59:33 PM

A great way to make a battery adapter is to take a 9V battery snap and (minding the polarity of the wires!) attach alligator clips to the ends. I like the rubber-jacketed type to prevent shorts. Use a meter to be sure of the polarity, did I mention that

Then use a red and black insulating cover to show which is plus and minus. You will use this thing ALL the time once you have it.

Box_Stuffer · May 28, 2023, 04:22:41 PM

Quote from: antonis on May 28, 2023, 12:13:01 PM
I'd start with a 47pF to 100pF across 1M feedback resistor..

(and deside about 1μF cap discrepancy between plates symbol and polarity marking..)

P.S.
Too many unidentified diode pairs permanently connected in parallel..
(5 of them are practically useless 'cause diodes of lower forward voltage drop dominate all the others..)

Thanks. That may be true about the diodes, but i spent a lot of time trying different pairings and combinations and they all sound different. i went with a specific combination of germanium, silicon, and red LEDS as diodes. The sound does change if you substitute or omit any of them. This was the way it sounded best to me.

antonis · May 28, 2023, 04:44:07 PM

Quote from: Box_Stuffer on May 28, 2023, 04:22:41 PM
This was the way it sounded best to me.

In such a case, it sounded best to you the - not necessarily adjacent - diode pair of the lower forward voltage drop..

(which, of course, is the Ge pair..)

What I' trying to tell you:

P.S.
You could permanently wire LED pair and switch between Ge & Si pairs via a SPDT ON-OFF-ON switch..

GibsonGM · May 28, 2023, 05:28:24 PM

What Antonis is saying is that once the diodes conduct, the signal is all going thru that pair. This becomes the signal path to ground, causing clipping. The others are like closed valves at this point and are not involved - their forward voltage is not exceeded, so they do not conduct. The pair with the lowest forward voltage (Vf) will win.

To use other combinations of diodes, you can put in a switch and switch the PAIRS of diodes in or out (or, 1 diode one way, and 2 the other way and so on...)

Box_Stuffer · May 28, 2023, 07:15:31 PM

Quote from: GibsonGM on May 28, 2023, 05:28:24 PM
What Antonis is saying is that once the diodes conduct, the signal is all going thru that pair. This becomes the signal path to ground, causing clipping. The others are like closed valves at this point and are not involved - their forward voltage is not exceeded, so they do not conduct. The pair with the lowest forward voltage (Vf) will win.

To use other combinations of diodes, you can put in a switch and switch the PAIRS of diodes in or out (or, 1 diode one way, and 2 the other way and so on...)

Well, however it works, I can tell you that there is an audible difference when moving them around on the breadboard - even when placed after the germanium diodes. They still have an effect on the sound. Having only 2 germanium diodes does NOT sound the same. I took a lot of time tuning it by ear. It really sounds great other than the hum.

GibsonGM · May 29, 2023, 06:34:26 AM

If it ain't broke, don't fix it

If you sense a qualitative difference with them in, it's ok to leave them if you want

It's your build.

Did you try the series resistor/cap to ground at the power input to the board? And +1, get a OneSpot or other filtered, quality power supply when you can.

Box_Stuffer · May 30, 2023, 05:53:51 PM

Quote from: GibsonGM on May 29, 2023, 06:34:26 AM

Did you try the series resistor/cap to ground at the power input to the board? And +1, get a OneSpot or other filtered, quality power supply when you can.

So - I think you are right about the power supply. I did not try a battery yet, but I hooked it up to my cheap Mosky power block that powers my pedalboard and it is already much quieter. It may not be good enough for some snobs, but I think it is pretty good for a first attempt from scratch. I am going to do another one and incorporate some of the other suggestions provided. Thanks!

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Noisy Distortion