Hiss with LaVache

[sauerkraut]

Hi,

my LaVache hisses quite loud when the gain pot is not maxed. I had to lower R4 to 100R (otherwise volume was too low), the rest is stock.
If gain is turned up everything is fine. But if you lower gain just a little bit you get lots of hiss. Definitely too much for such a low gain circuit.
With a 2N5088 there is less hiss than with a 2N3565 but the problem is still there.
Is there anything you can do about it?

Regards
Peter

[RobA]

You could try a BC549C or BC550C. You might end up needing to move R4 back up a bit with them too.

[sauerkraut]

You could try a BC549C or BC550C. You might end up needing to move R4 back up a bit with them too.

I tried a BC549B, BC549C, BC550C, 2N5088 and another 2N3565. They all produce about the same amount of hiss if you dial back the gain pot. So the hiss seems to be part of the design and my first 2N3565 is obviously damaged.

[RobA]

I put this together on the bread board. I've got no hiss with a BC549C. Also, in boost mode, it seems to have plenty of gain and I built it to stock specs. I think there might be something wrong in your build. I'd check all the usual suspects; parts values, solder bridges, continuity, etc. especially around emitter of the transistor. The other thing that could be an issue is the gain pot. Using an audio probe from the input up through the transistor could give you some clues.

[sauerkraut]

Hi, I just did the same, I put this thing together on bread board. It is the same as with my finished LaVache - as soon as you turn back gain it starts to hiss (with a BC550C).
So for me it is clear that it is not an error in my build but a characteristic of the circuit. I just wonder why nobody else has noticed it.

[midwayfair]

Hi, I just did the same, I put this thing together on bread board. It is the same as with my finished LaVache - as soon as you turn back gain it starts to hiss (with a BC550C).
So for me it is clear that it is not an error in my build but a characteristic of the circuit. I just wonder why nobody else has noticed it.

Nah, people have noticed it ... note that the original Les Lius didn't have a gain control at all.

The problem could be fixed with an input buffer. You'd have a Speaker Cranker if you did that. Or you could move the gain control somewhere else, like the emitter or the collector (Trotsky Drive).

[RobA]

The circuit design, with lowering the gain being a an increasing resistance on the input will introduce noise of all the resistor related types, thermal, shot, .... So, it'll depend on the pot that you are using how much noise you're getting and what type of noise.

I just put it on my FFT analyzer. I can't see any noise in a normal setting on my preamp and with the gain on the effect rolled all the way back and volume all the way maxed. If I peg my preamp to the point where playing anything would blow everything up, I can see and hear white noise come in as I roll the gain back. But, even with my preamp totally pegged, the white noise is only at about -80dB.

I'd guess that the pot you have in is a bit more noisy than usual or maybe your amp setup is more sensitive. One thing you could do is convert the gain pot to being a normal voltage divider volume pot setup by taking the third leg to ground instead of tied to the wiper like it is in the original. That will change the way that pot works though. all the way rolled back it'll cut the volume entirely, so it'll be less sensitive in its settings. I just tried it and it does reduce the white noise, but it also makes the gain changes scratchy. You could try it on your breadboard. You might want to test different types of pots too.

Edit: After playing a bit with different pots, I'm less convinced that it mainly has to do with the noise from the resistor. Changing to different pots did change the noise some, but not as much as I guessed it would. Maybe it's from the loading on the input of the transistor? I'll have to experiment more. Any ideas?

[RobA]

Hi, I just did the same, I put this thing together on bread board. It is the same as with my finished LaVache - as soon as you turn back gain it starts to hiss (with a BC550C).
So for me it is clear that it is not an error in my build but a characteristic of the circuit. I just wonder why nobody else has noticed it.

Nah, people have noticed it ... note that the original Les Lius didn't have a gain control at all.
...

Or you could just take the gain pot out entirely . I think I like that idea.

[RobA]

...
The problem could be fixed with an input buffer.
...

Is this because you lower the input impedance into the transistor?

[alanp]

Probably why I never get any hiss from mine -- it sits just after my Pitch Black and Kingslayer. (It's never left my board since I made it.

[RobA]

Probably why I never get any hiss from mine -- it sits just after my Pitch Black and Kingslayer. (It's never left my board since I made it.

I'd be interested to know about where you put the gain setting on the LaVache.

[RobA]

I did some measurements with a bunch of different transistors and fixed resistor values in this circuit this morning. It seems to me that there are two different sources of his/white noise. One changes with the resistance and the other with the type of transistor. The two have a distinct character and you can hear them changing independently with the two different changes. May not very useful info, but I thought it was interesting.

One interesting thing from the transistors switching that maybe useful, the 2N2222A has the least hiss. I didn't expect that. The noise from it seems to be more correlated. It kinda balls up and is less hissy. The other interesting thing is that to me the 2N2222A was the best sounding in the circuit too. I liked it quite a bit more than any of the rest.

I tried a BJT emitter follower buffer in front of the whole thing. It did change when the onset of hiss occurred, but it didn't remove it. And, since you had to pull the gain back more to tame the explosion it kinda offset the reduced hiss. It did sound cool though! At full gain, it kind goes from a totally choked glitchy and blooms out to a really fuzzy nice sustain.

I did find one thing that does significantly reduce hiss. I'll put it in a separate comment.

[RobA]

Putting a 100pF to 470pF cap from collector to base really reduced the hiss level I could hear and measure. Like I said earlier, I don't get much hiss at all and almost nothing audible in more normal playing setup from this circuit. But, with all my preamp settings maxed, there is a very large reduction in audible hiss with a 470pF cap in place. I kinda settled on using a 100pF with the 2N2222A as the setup i liked best.