JFET gain stage DESIGN

[Gus]

I have a sim of how I would DESIGN a JFET gain stage for guitar.

I don't seem to get many posts in the threads that I post sims and graphs of circuits.  Is it people don't understand what I am trying to show?  

Is anyone interested in seeing how I would DESIGN a JFET guitar gain stage?  I posted hints about gain stages like this in the past and even posted the amp name you could find JFET gain stages like in the sim.

I think people would start to use this type stage once they understand it.  This sim uses a J201 because DIYer's SEEM to be stuck on using this JFET.  The J201 model used is from a post Of STM's
http://www.diystompboxes.com/smfforum/index.php?topic=71466.0

[Earthscum]

Even though you don't get much reply to them, I don't think effort is wasted.

The Jfet sim, I would like to see. However, you may do a MPF102 as well, and see how much gain you can tweak out of it, then for fun combine the two in an optimal way to take advantage of the J201's higher gain, and the MPF102's higher current handling/output (low impedance driver).

Personally, I've been enjoying them. I actually have been referencing them when I'm screwing around my breadboard. The pics are nice visual aid, for sure.

[waltk]

I like to see them also.  Remember there may be lots of people viewing them, who don't have anything usefull to add.  Also lots of others who are just in the habit of lurking, and never reply to anything.

For example, your "Simple buffer schematic" thread had 2 replies, but 611 views.  So please don't give up on sharing.

[Gus]

R3 acts like a somewhat constant current device.  R3 sets the current in the circuit so you get a 4.5VDC drop across the 39K drain resistor.
R3 is what you adjust to bias the JFET used

The - supply with R3 is what is different than what I have seen for guitar JFET circuits in this and other forums

Note R2 is a gain control one can adjust this value without affecting the DC bias points.

JFETs in microphone can have up to 3gig gate to ground In this one you have a large range of gate to ground values you can use for R4.  R4 can be lower than 1meg

+ and - 9VDC supply, need the -9VDC for the QCC

You can find a circuit like this in the fender solid state Harvard amp schematic in one of the Groove Tube books.

This is a sim for +-9vdcc supplies and a J201.  If using higher voltages you may need to adjust some values and use different JFETS

[waltk]

Thank you!

[Earthscum]

I see it's biased as if it's running from a single 9V supply. Does this allow for a (positive in) negative swing well below the ground without clipping? It only allows for 4.5V upwards, correct?

[WGTP]

Gus, your posts are usually over my head, but I enjoy looking at them and seeing how you do things.  The visual stuff really helps.  

I have 10 new J201's that I have been waiting for something to do with.  I have had more succcess with mosfets for some reason, but I'll try this one.        

[caspercody]

I also want to thank you for your work! It is over my head also, so I usually do not reply because I have no input. But I do breadboard and enjoy making the different stages.

Thanks
Rob

[deadastronaut]

Gus, your posts are usually over my head, but I enjoy looking at them and seeing how you do things.  

me too, 

example nooby question, what is that 2.5H...is is an inductor or something, just never seen it on typical layouts/schemo's..?

i'm messing around with fets at the mo, so this caught my eye...

[arma61]

^looks like all components before the word "input" are there to simulate guitar signal, pickup impedance, tone, cable capacitance ecc.. as stated on the schematic.

Ciao

[deadastronaut]

oh ok....

cheers armando..

[Gus]

L1, R6 a single coil pickup with 2.5 Henry inductance and 7k ohm resistance
R7 C3 tone control 250K at max treble and a .01uf cap
R8 250K volume control at max volume
C4 10 feet of 37pf a foot guitar cable

Select a Id drain current less than the JFETs IDSS at about 9VDC drain to source 4.5VDC / 39k = .115ma
.115ma x 82k= 9.46VDC this also means the source is a +.46VDC above ground
The current in R1 and R3 have to be the same(more correctly about the same the only other path is gate leakage and that is small)

9.46VD across the 82K acts somewhat like a constant current device to so even with different J201s the voltage drop will change but by how much

some made up numbers to show the way R3 sets the DC current of the circuit.  I have not measured J201s
9.0VDC/82K=.109ma      39k X .109ma=4.28VDC drop across R1  4.72VDC drain to ground
9.8VDC/82k=.119ma      39k X .119ma=4.44VDC drop across R1  4.56VDC drain to ground

OR one can use a constant current device instead of R3

Now to get gain from the stage you need to add C1 source to ground for max gain or C1 and R2 for less than max gain.
Being  JFET stage R4 can be larger 4.7meg and C5 lower value .001uf(1000pf)

Green trace is from the input node C5, R9
Blue trace is from the output node C2,R5

[Gus]

Anyone build it? 

Do people understand how R3 and the - supply work to bias the circuit?

[slacker]

I understand how the biasing works once you know that about 0.1ma of current causes about a 4.5 volt drop across the FET, what I don't understand is where these figures come from.
I had a look but I couldn't see it, is this relationship something you can get off the datasheet? Or do you set R3 by experimentation and once you've found the value for a particular FET it will work for others of the same type?

I'll try and build it if I get time.

[Quackzed]

yeah, gus man, don't be disparaged! i for one have got a real kick out of your posts. thought as you are aware, your approach isn't as simple as 'use this circuit to get x sound'. rather you offer a truly in depth look at what is necesarry and what is extranious in a circuit.
i especially appreciate how your personal approach first identifies what matters in a guitars output. it stands repeating. you rarely overlook the output impedences of standard passive pickups and have gone to some lenghts to appropriately identify important aspects of their load in your designs. cable capacitance,  inductance, etc... imho what you focus on seems to be largely overlooked by many designers.
it isn't always comfortable for a quick fix mentality, however i feel your input places emphasis on whats coming IN and how to handle it with different active components... much appretiated man! i believe at the very least, you shine some light on what a guitars needs are and have some valuable insight in how to handle a guitar signal. anyone can set up an opamp clipper, but not everyone can see the effects of impedence or gain on bandwidth and frequency response, insightful and eye opening for sure. you have definately made me think twice about 'simple' circuits and their interaction with pickup loads and what you need to be aware of when designing gain stages.
really good stuff. not as 'flash' as some stuff, but in depth no b.s. analysis of a circuits 'real world' behavior. Cheers! 

[brett]

Hi Gus
definately don't get discouraged - you are a guru, and your ideas are solid and fresh.

A lot of people take time to understand or re-discover useful information. RG's classic "Technology of the Fuzz Face" is a treasure even though it is 10 years after it was written, as are your threads on the importance of the pickup in the tone of the Fuzz Face.

As for jfet stages. I really like them - they have high intrinsic input impedance (which is easily lowered) and are easily cascaded or mu-amped. IMO the stratoblaster is one of themost versatile and best-sounding boosters available. As for the J201 - it has a high Vgs(on/off), and Vgs is correlated with gain. Hence a J201 at -1V is more attaractive to people seeking lots of gain than an MPF102 at -6V. 

Thanks again for all your contributions Gus.

[DougH]

Those pic links don't work on my phone. Hmmm...

[GFR]

Gus,

Sometimes I don't reply because in a short post you're able to give so much to think about, I'll think and analyze and simulate, play with different scenarios and then when I start to understand a small snippet the thread is already old

I like this because I can learn much more than if the information was "in your face" Thanks!

At the moment I'm going through some simulations of piezo pres you have posted a while ago, lots of cool stuff.

[DougH]

Gus, keep at it. Takes time on a forum for things to sink in. Still waters run deep.

[WGTP]

Almost 750 views, folks just don't know what to say.