Jfet buffer question.

[ubaid88]

Previously my buffer circuit working on 9v supply voltage but now I changed the supply to 24v without any mod. The result of this is that output loudness is greatly reduced. Previously on 9v my output loudness was insane i cannot even use one tenth of vol control but now even set to full is barely louder than normal human speech.

Wat I should do? The last stage fet of this circuit is a buffer. After changing regular fet stages to cascodes i had to change the supply voltage to 24V. Why long story.

But i need to know that loudness of output of effected by increase voltage to buffer stage. Any need of changes? P

[brett]

Hi
JFETs have fairly particular biasing needs for high gains.  If the JFET is biased too far towards "off", then gain will suffer. Too far "on" and clipping occurs.  I suspect that there's too much voltage drop across the source resistor.  For J201s there should be about 1 V.  For most other JFETs there should be 2 or 3 V, and for MPF102s there should be 4 or 5 V.  These correspond to drain voltages around 1/2 of the supply voltage. The easiest remedy is to wire a 5k or 10k trimpot on the source (or 25k on the drain) as a variable resistor and move it  until you get lots of gain but minimal distortion.  A drain voltage of supply/2 does fairly much the same thing.
cheers

[ubaid88]

thank for reply,

you got me wrong. I am asking buffer stage not gain stages. If the voltage buffer stage is increase 9v to 24v. Can this effect the output loudness or not??

[alanlan]

Swap R22 for a 27K and this should restore the balance.

[liquids]

Swap R22 for a 27K and this should restore the balance.

Does that really matter?  I'm asking.  

ubaid88 - is it breadboarded or built?  Either way, audio probe at the wiper of the master volume, but if it's bread boarded, disconnect everything following the volume when you probe, for a more accurate audio.

Switch between 24v and 9v and hear the difference minus the buffer.  See,  I doubt it's the buffer.  But you may have already tried this...if not, be sure by checking that out and seeing if it's the buffer or not.

MY experience is, you should absolutely need to reset any trimmers on the gain stages when you 'simply' up the voltage! You may now be biasing well outside of optimal range if you trimmed them for 9v previously.  Use your  DMM on the g/s/d of them and see if that's the case.

I don't know cascodes (or why in the world you'd use them over SRPP, or mu amps with buffers, etc.) but with straight JFETs on trimmers, you can't just plug in a new voltage and think the same trimmer position will bias you at 1/2 Voltage...you might even need a different trimmer in those cases!  You have to pick one voltage or the other, or compare two circuits at different voltages with trimmers properly set to optimum place for each voltage, in order to accurate compare the difference.  Yikes.  So a simple voltage switch is not possible.

In the end, I think your buffer issues are nil, or, are the least of your problems.  But that is just the sophomoric experience of a DIY pedal hack.  I'm not EE, so take it with a grain of salt.         And, try some SRPPs if you haven't already.  You'll have no need for all that trimmer adjustment...with those, you can just up the voltage and it 'self biases' so such comparison are easier, amnd you can just 'switch' 9v vs other charge pump dervied voltages with no other changes.   Note that you can't even do this in the BSIAB, due to the JFET trimmer needing similar adjustment as the above, in this situation.... There are other advantages, like the the SRPP has better a output impedance than a mu amp, and more gain than a mu-amp/minibooster.  Or, if those are too high gain or not to your liking tonally, the classic mu-amp followed by a buffer is making a comeback thank to Catalinbread...reasonable gain levels and the mu-amp tone, but with true buffer-level output impedance, if you don't mind using 3 jfets per stage.  

[alanlan]

Swap R22 for a 27K and this should restore the balance.

Does that really matter?  I'm asking.  

Maybe not.  I'm assuming it has been narrowed down to the output buffer but as you say, there's a lot to go astray before it.
All I'm suggesting is that the 27k will bring the Id of the buffer somewhere near to where is it when running at 9V.  You could also adjust the bias at the gate for a similar effect.

[liquids]

Better yet, get a dual op amp and use one side as the buffer before the EQ, one after, and you can be sure you are getting maximum clean headroom without issue, and no, if you bias it properly, it should be even less likely to clip than the other two...jfets as buffered, if/when clipped, can be kind of unpleasant and barky.  It should also save some room and reduce parts overall (and save 3 jfets in use of a cheap op amp).

And with the mesa theme, you could wire up an active tone stack to simulate that ever-present Mesa graphic V-curve EQ and make it switchable...or in all the time...or make a 5 band with some "op amp inductors"...   

[ubaid88]

Can u suggest a schem for that. Will Tl072 be ok for this>

[liquids]

There's a bit of learning involved, but some are easier than others....get to know op amps..it's a long, worthwhile journey.  Here's some suggestions and links.

Oh, and most op amps should work just fine as buffers for such things...

LM833 Datasheet has a great start for a simple active bass/treble filter.  Locate that. It's the Baxandall.  Pretty common building block for active bass, or pretty much anything with active bass/treble EQ controls.  

Surf the net for the Boss Fa-1 schematic, it's a commonly used variation on such an idea...

That should get you started having fun.  From there, maybe learn about it....

R.G. - Thank You!  http://www.geofex.com/article_folders/eqs/paramet.htm#bridged_t
"Elliott Sound Products,"  Thank you!  http://sound.westhost.com/dwopa2.htm

Also, get to know that there are chips/circuits designed for such a purpose:
http://buildyourownclone.com/eqinstructions.pdf  
And "IC BA3812L - Rohm 5-Band Graphic EQ on a SIP" -  http://www.smallbearelec.com/Categories.bok?category=ICs+-+Op-Amps+and+Linear


I know, I know, it's overwhelming.  Pace yourself.  Lots of great info here, which means, a lot to digest, best done over time at a pace regulated by expirimenting.

Anyhow, note the elliot sound product page, which has multiple write ups about various active filters, to or the basics of op amps, into using op-amp notch filters...gyrators or op-amp inductors, etc.    Ah, the joys (and frustrations) of filters!  

Wait, don't we just want a nice distortion pedal?  

Again, I'd start with the baxandall.   From there, if you need some mid scoop, this 3 band active filter is just a variation on that that adds a mid control in similar fashion: http://freecircuitdiagram.com/wp-content/uploads/2008/11/3-band-graphic-equalizer-circuit.jpg

Tinkering with values, there is a lot to work with there.    Also, bear in mind that your 3 band passive tone stack can hardly do anything but cut mids as it is, as it is with your amp...so even the 3 band may be overkill.  And that's all most people need (if that) right? A way to cut mids, boost bass, and control treble at specific frequencies, no?  =D  

If you crave more control (which does not always equal more tone), maybe then start messing with the notch filters and/or true graphic EQs, which will invariably take up even more space for 5 pots, etc.   The datasheet for that chip mentioned should prove insightful.   All this should prove plenty as a start for using the search function on this very forum...

Or, you can do what many do, which is stick to good old, classic, passive controls and their joys...