Hey all,
ive finally designed a Jfet OD i like, and it sounds great, but it cuts highs dramatically when i clean up from the volume control. Any one know why? heres the schematic: http://www.geocities.com/alphaeffects. Its not those caps in the feedback of the Fet, so what is it?

THe Volume pot on the cct ?

Um, the volume control on the Guitar...

Add a 250PF cap on the guitar volume control.
between the wiper and one of the sides of the pot that should fix it

Nice job Alpha!

Hmmm... let's see.....

How are you cleaning things up? I didn't see any pots in there. Is this with the volume control on your guitar? If so.... does your guitar have a small bypass cap on it's volume control? That could explain it. Glancing at your schematic, I don't see anything that would dull things.

You might want to try adding an input cap to roll off some of the lows. Just a suggestion.

What guitar/pickup combo are you feeding it with?

Take care,
-Peter

Thanx all for such quick input!
Im using a chea Squier strat, No, my guitar doesnt have a cap on the Pot, but i dont really want to have to mod every guitar i use it with...
Peter-i tried putting an input cap on it, no difference. strange...

Add a volume pot to the CCT otherwise it will never be right with other guitars

:lol:

You guys are quick! In the time it took to look and type I think Tim got it.

You'll find that any decent single coil already has that cap as standard equipment. My first electric was a Squire and I was very happy after adding that cap to it. That guitar will have the same issue with all other amps/distortions. The lower the input impedance.... the lower the highs will be.

What cap did you try? If you lower it enough, you'll have a treble boostrer.

Take care,
-Peter

Thanx! Ill put a cap between the outer lugs of the pot tonight i hope. Will 220pf do? And what does CCT stand for?

220pf should be fine
cct stands for circut

You might want to try something larger too.... it depends on the impedance of your pickups and the value of your volume control. Try values between 220pF and maybe 0.0015 at the maximum. I have a 0.0012 in my Yamaha Strat knock-off. It's a bit too crisp... got to lower it some day. Fender used to use (still does?) a 0.001uF on the teles and several hot-rodders have commented about that being too big for that setup.

My guess is that somewhere between 220 and 470pF you'll find what you're looking for.

You'll want to place the cap between the CW end and the wiper.

take care,
-Peter

QuotePeter-i tried putting an input cap on it, no difference. strange...

With almost 1M input resistance (ie effectively the pull-down resistor) you need to go to quite a small input cap to taper any bass.  I generally use a 0.01uF cap for *full-range* input to FET buffers, etc.  To roll off a little bass, try an 560pF (yes, 0.00056 uF!) input cap.  With Fcutoff of 280Hz, that'll tame the open A string and most of the bottom E string.  For a bit more bass cut, try 470 or as low as 220pF  :shock: .

I'm wondering if all those 47pF caps source-to-gate might be doing this.

While the cap added to the guitar may have a effect, it doesn't seem like it's really the cure to this problem.

Hi,

Quote from: Alpha579Hey all,
ive finally designed a Jfet OD i like, and it sounds great, but it cuts highs dramatically when i clean up from the volume control. Any one know why? heres the schematic: http://www.geocities.com/alphaeffects. Its not those caps in the feedback of the Fet, so what is it?

I do believe it is the 47pF caps in the feedback of the FETS. You have created a low pass filter around 3K for each of the 4 gain stages. When a stage is clipping it is boosting the high frequencies, with lots of harmonic distortion, the filter is rolling them off at 6 db/octave above 3Khz. It kind of balances out. As you lower the volume and clean up the signal you are creating a combined 24db/octive filter where the 3db down point will be shifted lower to around 2.5Khz (not exactly sure of the value). This is because there is no or considerably less harmonic distortion boosting the highs. This is a very audible treble cut.

You might try removing C1 & C3 and change the value of C5 and C7 to 22pF. This will cut highs above 6K at 12 db/octave, which should sound great for guitar.

Why is the collector resistor R11 for Q4 different than the other collectors. Is the FET from a different batch?

Good luck with your design.

Regards,
Will

Thanx will!
That sounds about right, so ill try that on breadboard tonight...

Will, just so i know before i try it out, how did you work out the cut off?

Hi,

Quote from: Alpha579Will, just so i know before i try it out, how did you work out the cut off?

It looks like the 47pF feedback caps and 1M resistors are making a low pass filter which works out to 3388Hz (plus or minus 20%) due to cap tolerance. For example C1 & R1.

You could just try removing one leg of each of the 47pF caps just to try it out.

Alternately you can add a filter at the end of the circuit just like Doug Hammond's Meteor.

Your design is interesting as you set the major gain in the 1st stage. The following stages are only doubling the gain. Usually effects have much greater subsequent gain.

Regards,
Will