Prof tweed odd static

[Luke51411]

I built an ROG prof tweed and boxed it up but I'm getting static when master volume is halfway up and higher. The odd thing is that the volume (non master) doesn't effect it even when increasing the signal at all, only the master volume changes the static. Typically I would expect any noise to get amplified by volume or gain type control so I'm a bit baffled. Could this be caused by a bad output cap? I've checked over the solder for the off board wiring and everything seems in order.
http://www.runoffgroove.com/professor.html

[anchovie]

Typically I would expect any noise to get amplified by volume or gain type control so I'm a bit baffled.

Not if the noise is being introduced after the gain control! 

Is the static sound really occurring only when the master volume is over a certain point, or is it just that this is when you notice it? i.e. if you set the master volume low and crank the volume on your amp is it quiet?

See if the noise changes when adjusting the drain voltage of Q2/Q3 too - setting the trimmers for half of supply voltage is a good starting point, but I always give JFETs a final tweak by ear.

[Luke51411]

I swapped the 2nd and 3rd jfet for new ones and rebiased and the static is still present I might try switching out the electro caps next.

[anchovie]

Is the static sound really occurring only when the master volume is over a certain point, or is it just that this is when you notice it? i.e. if you set the master volume low and crank the volume on your amp is it quiet?

[anotherjim]

Is that schematic error free?
Is Q1intended to be biased by leakage? Is there an input cap missing?
What have you got it plugged in to? There's no output cap so if it's something leaking DC the Master vol will be scratchy.

[Luke51411]

The static is occurring on all settings just noticeable when turning the master volume up. Other people have built this layout with no complaints maybe I still have something incorrect in the layout. Could I add an output cap without changing the character of the sound much?

[anotherjim]

Ok, just checking. The input is the so called "Fetzer" tube emulation and it's meant to not have an input cap. This copies tube amps from the earliest times when there were few effects and no active guitars to plug into it.

Plenty of pedals have no DC blocking output cap. But if you fit an electro temporarily (neg towards the vol pot wiper), you'll know if it's made a difference. Without a cap it will only make a noise when you turn the pot ONLY IF DC from the next equipments input is present. If you decide to fit an output Cap, 22uF ought to be plenty. The smaller it is, the more it'll cut bass depending on the input impedance of what you plug into.

Make sure your bias voltages are correct as per the project page and then you need to follow the signal path with an audio probe.
If the tone control doesn't change the tone of the static, your first stage should be good (or the tone control is faulty).

[Luke51411]

Thanks for the suggestions. I will see what I can do. I've biased the drains at as close to 4.5v as I can get though the trim pot for the 3rd stage jfet is VERY touchy but I think I got that to about 5v if I remember correctly. I will break out the old audio probe and see if I can track down the source of the static. Hopefully I can get this sorted soon as this is the first pedal I've built that someone else will be using (of course I didn't notice the static as I basically only play at lower volumes  ).

[Luke51411]

I've been poking around with the audio probe and the static is present at q2drain and getting amplified by q3. I messed around with biasing while the amp was on and when I bias q3 I can get the static to go away but only when q3s drain is almost at full supply around 8v and above...

[anotherjim]

Take Q3 out and see if you can get a good signal at the Source of Q2 thru to main output and at the gate of Q3.

Jim

[Luke51411]

I'm not sure I understand. I removed q3 and there is no signal after it. Upon further probing it seems lik there is almost a ticking sound at the gate of all 3 fets.

[anotherjim]

Well, the ticking could be just noise pickup from lighting or whatever - even your soldering iron.
What I meant was that with Q3 removed any error around it should free Q2 to work properly.
With your guitar or some audio feeding the input, your probe should get a the signal on Drain Q1 - if doesn't you have trouble right there - and I start to think you may have the JFET terminals misidentified as all 3 appear to be misbehaving.
You can meter check JFET's to a certain extent...
http://www.allaboutcircuits.com/vol_3/chpt_5/3.html
In fact, as long as to have the Gate identified, you should have normal function as source and drain are to some extent the same thing and can be swapped.
If it's all good at Q1, Drain Q2 should not only carry the signal, but the tone and first volume controls should have effect and the signal should also (via Q2 source) go right out to the master volume and also the gate of Q3.

[Luke51411]

I swapped out the jfets for smd 5457s and I checked the datasheet for pinout and how it translates to the adapter board and I'm pretty sure they are correct I'll make sure to check again when I get a chance. When I'm not getting any signal in the output with q3 removed, maybe I had the first stage volume turned down? (just thinking of things after the fact, I will check when I get a chance). This has been my most difficult debug yet...

[Luke51411]

I've just built a second prof tweed as I couldn't find the issue with the first one. I'm getting the same background static. Is this just an issue with this design/layout/Jfet choice/me? I've seen another thread or two with similar complaints but I haven't seen a specific solution. I'm assuming people have been able to get this circuit working properly sans hiss, I'm just at a loss for what to try that might fix it.