How can i reduce/eliminate the hiss Tonebender MKII?

[gigimarga]

Hello,

I tested yesterday my Tonebender MKII on a professional setup (Budda tube amplifier+Gibson Les Paul) and it sounded amazing...but it has a lot of hiss.
How can i reduce/eliminate this hiss?

Thx a lot!

[Timebutt]

I'm experiencing the same thing even though I used premium components throughout (metal film 1% resistors, WIMA caps and selected AC128's).
I don't know if you replaced the 8K2 resistor with a trimpot as suggested at fuzzcentral to adjust Q3's bias? I haven't had much time to test it but when I lowered the trimpot the output went up by a whole lot, greatly improving the signal to noise ratio but not changing the character of the fuzz as I had expected from a bias trimpot?
Maybe somebody else can comment on this?

[Chawk]

The hiss seems to be in the nature of these pedals. I built one and there is a hiss. If you listen carefully you can hear the same hiss on many of Jimmy Page's recordings that he used a tonebender on like "You Shook Me". If it's an overwhelming hiss you might have some leaky transistors in there.

[gigimarga]

Thx all for your replies!

I replaced the 8.2K resistor with a 10K trimpot and i adjusted the Q3's bias at -4.5V...all the voltages look ok, as i saw in other build reports.

I don't have no ideea about the leakage ot the trannies...i didn't have time to understand how to measure it.

I remember that i've read (but i don't know where...) that it's possible to reduce the hiss if i replace a resistor with the value R with another one which have the value R/2...but that's all that i can remeber

[gigimarga]

 

[JHS]

Hiss can be reduced by using Ge-trannies with app. 60 hfe and leakage <50uA.

JHS

[hellwood]

ive come across transistors that are noisy even though they pass the gain/leakage test.

[gigimarga]

Not a good news

[hellwood]

it sounded amazing...but it has a lot of hiss.
How can i reduce/eliminate this hiss?

since i have no way of knowing what you consider a lot of hiss, this is all i can suggest...
if you walk around while you are plugged in and stand between your guitar and amp just right, you might be able to find a sweet spot  w/ minimal or no hissing(ive done entire recordings like this and have had to do it on stage). you can also pick songs that are really busy. if that sounds too crazy and it might be, you could swap out the transistors just as a test. you could also try shielded wire(although ive never bothered), shorten excessive wiring, and stay away from carbon comp resistors, but at this point you could just be splitting hairs.

[Mich P]

Do you put that 1nF at the input acting like a low pass filter ?
That's very help, look fuzz central shematic.
Mich P.

[gigimarga]

Thx a lot!

I used the Tonepad's PCB and i didn't see the 1nF cap from Fuzzcentral (and i've compared the schematics...)...i will try to add this cap later in this afternoon!

[frankclarke]

Small (100nF) caps collector-to-base, and better transistors help. But with a gain of about a thousand it will be noisy to some extent.

[gigimarga]

Thx for your ideas...by the way, i think that the collector-to-base capacitors must to be around 100pF, not 100nF

[Gus]

Some transistors are just noisy.  What might be a good idea is to make a circuit fragment to check transistors, using things you might have like a guitar and amp

Input and output cap(1uf for each should be a good place to start) and a 100K volume pot
for NPNs
Bias string +9 VDC 150K to base to  22K to ground
10K collector R to +9VDC
1K emitter R to ground  bypassed with a 47uf

This should give a stable bias so you don't need adjust the bias when you change the transistors and max gain, use a socket and check each transistor for relative noise by going guitar, this booster, amp

You could also sub say a 10K resistor at the input cap to ground and place the circuit in a metal box and plug the circuit without a guitar into an amp and check the noise or just use the bias string and no input cap or input at all using the 150K and 22K as a input resistive R to work with the noise of the transistor.

You also could build a very clean low noise buffer to feed a sound card to record and look at the noise.