Hiss from 555 timer cleaning

Thread Starter

willbloody

Joined Jun 22, 2022
2
Hello World.

I am building fuzz guitar pedal and want to include simple fading led(just a visual design idea) to the circuit. Well it works, but there is a lot of unwanted hiss in the signal. So I am struggling with method to reduce it. There were loud pops at first which matched frequency of a 555 timer, but simple ceramic cap between +and- of a power supply made theme gone. I believe that I must somehow isolate power supply, but don`t know how(Just learning).
fuzzface7.gifcircuit-fading-led.gif
 

crutschow

Joined Mar 14, 2008
30,102
Don't think it has anything to do with isolating the supply.

To reduce hiss try adding a small capacitor (e.g. 10nF) from Q2's collector to ground.
 

seanstevens

Joined Sep 22, 2009
144
Does the fuzz pedal work OK without the 555 operating/being connected? If so, then I think you might be missing some power supply decoupling. Try an electrolytic capacitor, anything between 47uF to 100uF and a 100nF both between + & - of your supply, close to 555 chip, that might help.
 

Audioguru again

Joined Oct 21, 2019
4,793
An NE555 or LM555 are very old and use TTL logic high current circuitry.
The datasheet of the Cmos ICM7555 shows that the old 555 draws a current pulse of 400mA (!) from the power supply each time the output switches (it is called shoot through current). But it will not cause hiss unless it is switching at a high audio frequency, not slowly fading an LED.
 

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Thread Starter

willbloody

Joined Jun 22, 2022
2
Thanks for reply. Using decoupling caps does not change the situation. Let me explain the hiss...it is not even the hiss...it is like synthy swirling birds that sing in beat of fading leds)) if I increase the fading rate to max, I receive a good oscillator:D
 

Ian0

Joined Aug 7, 2020
5,504
At 20mA your LED is putting quite a load on a poor old PP3 battery. I suggest you reduce it to 5mA, and find a higher brightness LED.
The fuzz-face circuit is tremendously sensitive to bias, and you will probably find that the battery voltage is changing just a little bit but enough to change the gain and distortion.
Another suggestion - have two LEDs where one fades in as the other fades out. That will keep the current constant. Alternatively, try regulating the fuzz-face voltage to 7.5V with a LDO regulator.
 
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