Tube sound fuzz: Problem with hissing sound

[Rick899]

I built a tube sound fuzz from the Anderton book. The effect works but I am getting a hissing sound that does not go away when you play. Any ideas on the cause?  Thanks.

[Mark Hammer]

1) Individual 4049 chips cary in their noisiness.  I hope you used a socket, so you can try out different ones and use the quietest.

2) The stock feedback capacitor values are much too low to keep hiss out of the way.  If you are using 10pf on the 2nd invertor, raise it to 100pf.  That should reduce the hiss substantially.

[Rick899]

Mark:

Thanks for the suggestions. I changed the 10 pf cap  for what I think is the 2nd invertor, i.e., the cap labelled C2 on the schematic in the Anderton book;    I also tried two different 4049 chips. But there was no change in the hiss. Both chips were from the same batch from the same supplier so I am going to try one from somewhere else, just in case ... but do you have any other suggestions?  Thanks.

[Mark Hammer]

Did you make sure to tie the inputs of the unused invertors to V+?

[Rick899]

By "invertors"  do you mean the unused pins  of the 4049 chip ?   Or are the inverters the six "triangles" in the schematic?   There are about 5 or 6 pins that are unused but I do not understand how to identify  which of the inverters are unused?   Several of the pins in the schematic are labeled as "NC=No Connection".  Could you please explain how to identify the unused invertors?   Thanks.

[artifus]

each triangle represents an inverter. it is common practice to tie unused cmos inputs  to + or - rather than left floating for stability so if you are using the first two, pins  2 - 5, you should connect pins 7, 9, 11 and 14 to v+ or ground. inputs flat side, outputs pointy.

[Rick899]

Artifus:  Thanks for the explanation. Clarified it for me.

According to the data sheet the inputs of the 4049 are pins 3, 5, 7, 9, 11, 14;  All of these pins are connected as required by the schematic;

The unused pins on my pedal are:  2, 4, 6, 12, 13, and 16;   According to the data sheet 2, 4, 6, and 12 are outputs and 13 and 16 are NC.

So all the inputs are connected.  Are the outputs left unconnected ?  Or should they be connected to something?

[Mark Hammer]

The unused outputs can be left unconnected.  It is the unused inputs that need to be tied to V+.

[Rick899]

I was reading the Anderton article on the Tube Sound fuzz and it said that the input lead and some other wires longer than 3" should be shielded. I did not shield any wires.

Could the unshielded wires be causing the hiss?    (BTW: The hiss is on the volume knob; turn the pedal's volume knob down and the hiss goes away); 

I have some questions about using shielded wire but I suppose I should start a separate thread about that specific issue.

[Mark Hammer]

The hiss is not "on" the volume knob; it's just turning down the level of the entire signal...too much of which is hiss.

The circuit provides a lot of gain.  If you feed it a slightly hissy signal, it will make that hiss much louder.  It will contribute a little hiss of its own (and amplify that), but it will also make any incoming hiss that much louder.  Ideally, one attacks both.

the higher value cap I suggested is one way to tackle both.  It will roll off treble, reducing the audibility of both hiss generated internally, and incoming hiss that was amplified.  personally, on mine I always have larger value caps on both the two invertor stages used.