BAJA jfet high z in - low z out jfet buffer

bajaman · Post by **bajaman** » 30 Oct 2007, 10:22

Hi Guys and Girls
Here is a little circuit snippet that I have been using for some time - it eliminates the need for a triple pole switch and provides a very high input impedance (so you won't lose tone)AND a very low output impedance for driving other effects following.
ALSO - you will never be plagued by that switching pop that you get with most true bypass schems - this baby is virtually NOISELESS and transparent.
The circuit is by no means original - but you can stick it in front of any effect you like and enjoy noiseless non tone sucking nirvana

Just about any n channel jfet will work - I just have a real soft spot for the 2SK117 - call me sentimental but I really like these Toshiba jfets, and I am not the only one who does.

Do not ask for a layout - I usually build it on a very small piece of veroboard - you should give it a try yourself - it is not that difficult to do

Enjoy
cheers
bajaman
BAJA n channel jfet buffer

ps: if your effect does not have a suitable half rail voltage (4.5v with 9v circuits), then just use two 100k resistors in series, strung between the 9v rail and ground. Then connect the 1M input resistor to the junction of these two resistors and voila - all done

MoreCowbell · Post by **MoreCowbell** » 30 Oct 2007, 14:33

Nice useful little circuit snippet...thanks !

goaltoday · Post by **goaltoday** » 30 Oct 2007, 16:21

Thanks Bajaman. I'm going to try on some ultramegahigh gain fuzzes I've never been able to get ride of POPs.

goaltoday · Post by **goaltoday** » 02 Nov 2007, 01:20

I'm using this layout, resitor are mounted vertical and don't forget the cut under R2.
Be sure to check the orientation of the FETs you use, in this case a J201.

Click on it to large size version.

bajaman wrote:... Tech 21 use a 2SK 117 as an input buffer in their Classic Sans Amp pedal ... In my humble opinion, this is why these products sound so nice to my ears ... and strongly urge others here to give it a try

I'm going to try it with a DPDT on the output just like the Sansamp Classic.Thanks again.

bajaman · Post by **bajaman** » 02 Nov 2007, 02:14

Hi goaltoday
Good layout

- but make sure you get the fet in the right way

, the 2SK117 has DGS looking at the writing with the legs pointing down. Your layout shows DSG which is better suited to 2N5457, MPF102 or J201 types - all of which should work okay in this application

.
enjoy
bajaman

goaltoday · Post by **goaltoday** » 02 Nov 2007, 02:35

Yes, you're rigth I'm using a J201. I'm going to advertise it on my post to avoid confusions.

bacaruda · Post by **bacaruda** » 02 Nov 2007, 15:26

very useful, thanks bajaman.

p.eat · Post by **p.eat** » 22 Nov 2007, 14:30

Hi Bajaman,

is the buffer suitable for Bass, or are there any substitutions needed? Also can the buffer run off of 18V?

thanks in advance,

Peter

R.G. · Post by **R.G.** » 22 Nov 2007, 15:39

bajaman wrote:ps: if your effect does not have a suitable half rail voltage (4.5v with 9v circuits), then just use two 100k resistors in series, strung between the 9v rail and ground. Then connect the 1M input resistor to the junction of these two resistors and voila - all done

Some high-output guitars will distort on this because the source of the JFET is going to be 0.5 to as much as 3V above the bias voltage. N-channel JFETs bias with their sources higher than the gate.

Do the same biasing procedure, but put a voltmeter across the source resistor. Tweak the bias resistors to get the source close to half of your supply voltage. It doesn't have to be precise, but you do give yourself maximum headroom if it is.

If you have only single coils, you may not need this. Likewise, it depends on your JFET. The J201 has the lowest Vgs I've ever found, 0.3 to 1.0, so it almost doesn't matter on a J201. The 117 can have up to 5V if I remember right, so if you get a worst-case device, you could be bumping up against the power supply all the time.

That ... might... sound good, as this is what the Dallas Rangemaster does with germanium transistor, but it's not a transparent buffer if it does.

Anyway, something to think about.

bajaman · Post by **bajaman** » 23 Nov 2007, 08:08

is the buffer suitable for Bass, or are there any substitutions needed? Also can the buffer run off of 18V?

Yes to both questions

Thanks RG for your informed comments - perhaps the two bias resitors could be replaced by a single 100k trim pot and the source voltage adjusted to half the supply voltage

cheers
bajaman

p.eat · Post by **p.eat** » 30 Jan 2008, 18:37

Hi,

I've got another question. If I want to permenantly use the stage as a frontend, I can skip the 560R, 100K and 2µ2. Is that correct?

thanks,

Peter

bajaman · Post by **bajaman** » 30 Jan 2008, 20:44

If I want to permenantly use the stage as a frontend, I can skip the 560R, 100K and 2µ2.

Yes and no

You must use some form of capacitive coupling on the output (source pin of fet),so it depends a lot on what you are intending to use as the next stage

bajaman

mantralux · Post by **mantralux** » 30 Sep 2008, 21:33

This project is perfect for someone like me trying to learn the basics of veroboard and electronics...but as a beginner I have some n00b questions:

1: How do I connect the input/output wires to the jacks?

2: I'd like to use a 9V input, not a battery...would the layout change?

3: I'd like to eliminate a switch altogether and just have an "always on" type pedal, what do I change to make this possible?

4: I'm following the veroboard layout posted by goaltoday, but I want to use the 2SK117 transistor instead of the J201. Would I place a 2SK117 differently?

5: Could I use Mallory 150 capacitors (630v) as capacitors? What would this do to the sound?

I'm just looking to build a really transparent high quality buffer to try and understand stompbox building. I need it for my all bypass pedal board with long cable runs.

BennyBoop · Post by **BennyBoop** » 28 Dec 2008, 11:53

Nice little circuit Baja!

How do I adapt it to be a separete buffer pedal? Thinking of building a buffer pedal with two separete buffers and a non buffered loop for fuzzes and wah. I mean, Guitar -> buffer -> FX-loop ->buffer -> amp. (?)

EDIT: With a mute switch, that also switches to tuner out.

moltenmetalburn · Post by **moltenmetalburn** » 28 Dec 2008, 15:28

how would I amend this schematic to use it permanently inside my guitar for low impedance output from my passive pickups?

Güero 2.0 · Post by **Güero 2.0** » 29 Dec 2008, 01:58

bajaman wrote: ALSO - you will never be plagued by that switching pop that you get with most true bypass schems - this baby is virtually NOISELESS and transparent.

Hi Bajaman,

That's a cool device! Thanks!!

I'll try this little circuit in my rig!

So, I decide to provide to you guys a layout for this buffer.

Enjoy!

kaycee · Post by **kaycee** » 30 Jun 2010, 07:48

Sorry if I'm being thick here, but which lug of the switch does the switch connection go to (ie. Effect in, effect out, jack in etc).

dirtybogner · Post by **dirtybogner** » 14 Jul 2020, 12:17

Please teach me.
Why are output 100K and 560R resistors there? Is that for transparency of the tone or electrical reason?
Isn't 2u2 coupling enough?

soulsonic · Post by **soulsonic** » 14 Jul 2020, 19:03

The 100K keeps the output 2.2uF cap pulled down so that it doesn't cause a pop when switching. The 560R resistor protects against if the output were shorted to ground.

dirtybogner · Post by **dirtybogner** » 18 Jul 2020, 11:08

Hi, Soulsonic, thank you very much for the reply.
To protect against shorting... I see.　
I have additional questions.
Why is it not good if the output were shorted? I think the output after 2.2uF cap has no voltage. Is it for jfet behavior?
Sorry if this is basic question.