JFET input Caps

[newfish]

I recently built something very similar to the excellent 'Sparkleboost' using a J201.

As per all my boosters, I added a DPDT switch for the input cap (I understand it's not *strictly* neccessary with JFETs, but I was looking for a Top / Full boost option.

Cap values are 47nF (Top) and 2.2uF (Full) - but switching between them makes very little difference.  I've checked the switch with a DMM - and it checks out OK.

The build is a 'transplant' as the case used to house a simple BJT Booster (and the Top / Full difference was significant).

Would I be better moving the cap switching to the output of the circuit?
I've searched through other posts, and this has been suggested.

Either way, the Sparkleboost is an *immense* add-on to even a tiny Valve / Tube amp - so thanks to Dragonfly for providing layouts / schematics.

[alanlan]

You probably ought to try something smaller than 47nF for your "top boost" option.  FET designs usually have high input impedance so the low frequency roll off is achieved with smaller values of input cap.  Try a 10nF or 22nF and see how that goes.

[Gus]

Go to R.G.'s site GEOFEX and read the Austin treble blaster PDF.   What you need to understand this is the input resistance of the jfet stage.

In R.G.s write up the RM input is about 12K ohm and there is a .005uf cap, so if your stage is high input resistance you will need a smaller cap.  RC stuff

What is the input resistance of the jfet stage you used, can you post a link?  You might need to adjust the input resistance down in value as well as adjust the input cap.

Part of some treble boosters is the loading caused by the input resistance as well as the high pass filter.

googled sparkleboost it looks like a 1meg gate to ground.  try a 22Kor 33K or 47K gate to ground for a more old school trebleboost input resistance and a .0047uf input cap to start then maybe a .0033uf, .0022uf, .0068uf.  1/( 2pi x R x C).  You can adjust the input loading and high pass filter

[newfish]

Thanks for your responses.Guys..

Input resistor is indeed 1Meg.  I'll do me some thinking and calculating.  I was aware of the 1/(2* pi RC) formula - just didn;t expect to have to use it quite so soon.

This is the first FET project I've changed from stock, so I'm very much still learning.

Cheers!

<edit>

I thought FETs needed a high (1Meg) impedance to operate properly - hence why the 1Meg resistor is there.  ...or am I missing something here?

[MohiZ]

FETs don't "need" a high input impedance, they HAVE a high input impedance. This is why the pulldown resistor can be large as well. Generally a high input impedance is a good thing, so a large value pulldown resistor is beneficial. The FET would work just as well with a smaller resistor, but a smaller resistor would load down the output of the previous stage or device more. That's why you often see 1 Meg used.

[brett]

Hi

try a 22Kor 33K or 47K gate to ground for a more old school trebleboost input resistance

IMO that makes great sense Gus.

but a smaller resistor would load down the output of the previous stage or device more. That's why you often see 1 Meg used

That's doubtful in my opinion.  I suspect it's just an urban myth (and not a very interesting one).  And there are a couple of reasons for NOT using a 1000k tie-down.  Firstly, it encourages noise to be transmitted down the line.  That's because it allows small non-signal charges (static, RFI, etc) to make a relatively high voltage.  For this reason alone it makes sense to use something no bigger than a 220k resistor (as Gus noted, you need to go down around 33k to get vintage style loading). 

Even if you have good hearing, you won't have to worry about any high frequency losses due to "loading" your pickups with 100k+.  (You CAN hear the high frequency (15kHz?) whistle from your TV, right?). 
cheers

[newfish]

Once again, this list proves to be worth its weight in gold (if web pages have mass?).

Dropped my Gate / Ground resistor to 33K, and with the switch in 'treble' position there's a marked difference in the sound of the pedal.  With the switch in the 'Full' position, it's almost like I have an overdrive pedal.

Lots of brilliance (presence?) from this little beauty - and plenty of grit.

Sparkledrive just became my favourite build so far.

Thanks once again guys!