the nutty professor tweed

[danwalsh]

hi everyone!
i recently breadboarded a professor tweed (love the ROG projects). 
it works, sounds great, voltages on drains look correct.
but the thing is LOUD.  if i dime the volumes, the thing is like 5x as loud as when bypassed. 
the thing is, i like the sound when both volumes (vol and master) are dimed. 
what resistor or cap should i be looking at tweaking to tame it a bit?
i know i could always bring the master down, but the sound is just not the same as when it's up all the way. if i can get the same tone and crunch at a lower volume i will be stoked. 
the schematic:  http://www.runoffgroove.com/professor.html
thanks in advance!
dan

[ralley]

Seems strange that this pedal is so loud.  I've recently built one (still to be boxed up) and don't recall it having this problem.  I think you might have a problem with your build.  Did you do any parts substitutions?  I can post voltages from mine if you want to compare like that.

Rob.

[danwalsh]

voltages look pretty good to me, but i'll post them anyway.  no parts substitutions, using all MPF102s.  i've tried uma floresta's mods and the original, both with the same effect.  i can get it down to normal volumes with the master volume set 1/4 to 1/2 way, but i'd like to get the sound of it up all the way at a more reasonable volume. 

battery, a little weak at 7.72

Q1
4.28
1.65
0

Q2
4.44
1.555
0

Q3
4.28
.791
0

[ralley]

I'll check my build tonight and compare voltages with yours.

Rob.

[danwalsh]

that would be great, thanks!  i'm curious if anyone else who has built the prof tweed has the same experience with the volume settings. 
also, does the speaker emulation at the end of the circuit effect volume at all, or just a tone modeled after speakers? 

[Uma Floresta]

I've actually never tried mine at full volume. The "speaker emulation" is actually just a low pass filter, and it does cut a little volume.

[ralley]

Okay, the plot thickens!  Last night I started checking my voltages and realised that the source for Q3 wasn't connected (I used sockets and the leg had bent when inserting the FET).  I fixed Q3 and went to re-bias and now mine is acting like Dan's - suddenly it's very loud and very distorted (and not in a nice way).  This weekend I will try to get it biased properly - it's very touchy.  I will also try some MPF102's to calm it down.

Before I started fiddling Q1 was biased at ~4.3V, Q2 and Q3 were both ~8.7V (but Q3 was not working) and it sounded as I expected it too.

Rob.

[frank_p]

I did a quick measure on mine and I get a  x 17.5 gain (not dB) at 0.1V -1KHz sin wave at input.  Every knobs maxed.  So yes it can become loud depending on the properties of your fets.  I am surprised too, hope I did not make errors while scoping.

My negative feedback was modified: I putted it back like supposed to be (like on the shem.).

Ask if you want me to take other measures around before I rebox it.

Every fets are soldered (no sockets) so I can't try other ones.

Oh, and there is a moderate amount of distortion on the output signal with this input signal.

See ya.

[danwalsh]

thanks for checking it out frank, that's helpful.  i think i'll just swap some fets and do some experimentation.  rob - i also found the biasing to be touchy -  there seems to be a very narrow range on the trimpots that will give you 4.5V on the drains. 

[frank_p]

Yes, sometimes the biasing is on a very short length of the trimpot, can be tricky to set up conveniently.
I like this effect a lot, I don't know if I just picked up the right FETs but mine became my favorite overdrive.  It was the first effect I built.  When I first played with it, I fell in love with it instantly.

[MikeH]

My guess would be that you like the sound of the pedal with both the volume and master dimed because you like the way that it makes your amp react.  If you do anything to decrease the output, you'll probably lose the sound you like.