Jfet design questions

Stompboxes circuits published in magazines, books or on DIY electronics websites.
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grrrunge
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Post by grrrunge »

Oh! And where did you study? I'm looking into going somewhere else and take some courses for a semester. DTU only have 4 dedicated analog design courses i can take for specialization. The rest is digital stuff, control theory and robotics. Bleh... 8)
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Post by Duckman »

mmolteratx wrote:I've got a handful of great ones saved as PDFs.
Can you upload some books here? Please?
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Post by phatt »

mictester wrote:
I did an AC30 emulator some years ago, using op-amps throughout. It wasn't too difficult to emulate the types of clipping (and other non-linearities) in various parts of the circuit, but getting the frequency responses right was a complete nightmare. I decided to try to emulate the speaker as well, and the whole thing ended up on the second-biggest piece of Veroboard I've ever seen! As I recall, it used fourteen quad op-amp packages and three 4066 CMOS switch chips for "Bright/Normal", "Trem" and an additional "hot" setting. When I finished tweaking it, I tried it against a miked-up AC30.... It was close, but "still not Carling" (as the advert would have it). I was really disappointed! Then some German guy put out an "AC30 simulator" using just 3 ICs - his sounded even less like the real thing, but he sold loads of them.

I went back to using a real AC30 if I wanted an AC30 sound!

Unlike most guitarists, I don't much care for glass amplification - granted you can get some nice rich overdriven sounds and some musical colouration and distortion - but they're big, heavy and expensive and instantly destroyed when some thick roadie throws them into the truck.....

I like the compression, distortion and colouration I can get from a carefully designed range of pedals (a bit like valves!). I like using the Bareknuckle and a few other higher output pickups, and I like the reliability of my little Class D amplifier rig. Compared to valve amps, my gear is indestructible - drop proof, mains-voltage error - proof, waterproof, short and open-circuit proof..... However, it doesn't sound like the valve amps it all replaces. The sound is quite different, but I like it!

Thanks mictester, good to know that I'm not alone.
I'm only the amateur and I struggled to make fet circuits even come close to making magic. (I have no doubt in the right hands they might be great but I had no luck) I spent a year on and off tweaking a simple tone circuit and tried fets at one point in the vein hope some extra mojo might show up.
But a simple A/B test proved the fet ideas were not worth the effort when a simple dual opamp worked better with less noise. [smilie=a_whyme.gif]

@ stole59,
In simple terms caps do 2 main tricks to the signal in basic audio circuits like you describe.
1/ In series; the larger the value the more bass passes. (HPF, hi pass filtering), Smaller values cut more bass.

2/ In parallel (across the signal to common); They do the reverse, the larger the value the more treble gets cut.

3/ That might seem easy to grasp BUT (and you DO need to read the buts :-D )
The kicker is that the result is dependent on the impedance at that point in the circuit. :hmmm: :scratch:

Take the input of a Fet where you often have a 1 Meg resistor from gate to common where a 100nF cap is big enough to pass plenty of low frequency because the Z is high.
Now swap the 1Meg resistor to 100k (or less) and you will note a substantial loss of bass. You need to then make the preceding cap much larger to compensate for the lower Z.

Clues can be found in tone stacks where you can change a Hi Z tone circuit into Low Z circuit by multiplying the cap values by 10 and dividing the resistor values by 10. So a 47n cap becomes 470nF and a 250k pot becomes 25k. The Z has changed but the tone response is much the same result.

So all this AC audio stuff is in the realm of Impedance and it's like a big rubber band,, you tug a bit at one place in a circuit only to find you effected something else, often in a negative way. :blackeye :blackeye

Even if you know all the maths to work this stuff out,, 9 times out of ten you end up just trying a few different values until it sounds right.
Which is why I breadboard everything several times over long before I print a PCB. (been caught far to many times) :block:

Try using simulations as it really helped me come to grips with how and where you win and loose tone mojo. So with the help of sims even a mug like me can pull off some good results without having to go back to sckol. :thumbsup

@ duckman,
See if you can track down *Art of Electronics* a very well presented teck book. Even with my basic understanding it helped a lot to explain the analog stuff,, I never read the the digital stuff at the back. way to deep for me. :oops:
Phil.

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stole59
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Post by stole59 »

Thanks you all guys. You made this process of designing jfet overdrive alot easier for beginner like me.
I came to an end of a circuit and I am very satisfied with the sound I get.

Now I have a problem that I also experienced when building a copy of PHARAOH fuzz. I used 100 k log pot for volume control but funny thing happens, as I turn down the volume I get LESS GAIN and not volume. I start losing volume at about 12 clock and get less and less as I turn it down.

Can anyone please explain to me why is that happening?

I solved the problem 90% by connecting 22nf capacitor between lugs 3 and 2 using the same model as they used in section 1( 500k gain pot and 1nf capacitor) but I don't understand why is this happening.

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Post by Duckman »

phatt wrote:@ duckman,
See if you can track down *Art of Electronics* a very well presented teck book. Even with my basic understanding it helped a lot to explain the analog stuff,, I never read the the digital stuff at the back. way to deep for me. :oops:
Phil.
Thanks, I already have it :thumbsup but, you know... never enough books! :lol:... :| ... :hmmm:... :oops:

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Post by mmolteratx »

grrrunge wrote:Yeah. We didn't do that much JFET, mostly MOSFET ;)
We used Sedra/Smith - Microelectronic Circuits mostly, and then Allan Hambley - Electrical Engineering for supplements.
Any good recommendations?
Sedra and Smith is great, though a bit convoluted at times. We used it as our supplementary text, then Razavi - Fundamentals of Microelectronics for our main textbook. In analog IC, we used Razavi - Analog IC Design and Grey/Hurst/Lewis/Meyer - Analysis and Design of Analog ICs. The same books were used with a few more supplementary texts in advanced analog IC.

I really can't say enough good about Razavi's textbooks though. They're clear and provide a good deal of insight into how a given circuit block functions. I also have his RF IC textbook, and his material is consistently great. He's half the reason I want to go to UCLA for my PhD.
grrrunge wrote:Oh! And where did you study? I'm looking into going somewhere else and take some courses for a semester. DTU only have 4 dedicated analog design courses i can take for specialization. The rest is digital stuff, control theory and robotics. Bleh... 8)
University of Texas at Dallas, though I'm headed to Texas A&M for my master's next year since their department is a bit more developed and well known. We got the Texas Analog Center for Excellence added on a few years ago, so they're still building up the undergrad coursework, but we've got a good number of courses already built around high speed analog circuits and analog circuits for communications devices at the grad level.

And controls/robotics is awesome. :mrgreen: Not my preferred field, but probably would have been my second choice behind high speed analog for grad school. Then energy/power. Then comm systems. And digital is way down the list. :lol:
Duckman wrote:
mmolteratx wrote:I've got a handful of great ones saved as PDFs.
Can you upload some books here? Please?
Sorry, can't upload them in public for professional reasons, but PM me and I'll send you some stuff.

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Post by grrrunge »

Duckman wrote:Can you upload some books here? Please?
All of the books mentioned above can be found on various torrent sites, if you're in a copycat mood ;)
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Post by Duckman »

mmolteratx wrote:Sorry, can't upload them in public for professional reasons, but PM me and I'll send you some stuff.
PM sent :thumbsup

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Post by stole59 »

Hi guys. I guess I will have to repeat my question since you wondered off from the subject. No big deal, here is the question:

Now I have a problem that I also experienced when building a copy of PHARAOH fuzz. I used 100 k log pot for volume control but funny thing happens, as I turn down the volume I get LESS GAIN and not volume. I start losing volume at about 12 clock and get less and less as I turn it down.

Can anyone please explain to me why is that happening?

I solved the problem 50% by connecting 22nf capacitor between lugs 3 and 2 using the same model as they used in section 1( 500k gain pot and 1nf capacitor) but I don't understand why is this happening.

Also, my volume level is just a bit higher than unity level and neither EQ or any boost pedal can increase volume level when I put them after the Jfet overdrive pedal I was working on.

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Post by grrrunge »

stole59 wrote:Hi guys. I guess I will have to repeat my question since you wondered off from the subject. No big deal, here is the question:
FYI the circuit analysis section is for circuit analysis. If you need help debugging your build, there's a dedicated section for that.
stole59 wrote:Now I have a problem that I also experienced when building a copy of PHARAOH fuzz. I used 100 k log pot for volume control but funny thing happens, as I turn down the volume I get LESS GAIN and not volume. I start losing volume at about 12 clock and get less and less as I turn it down.

Can anyone please explain to me why is that happening?

I solved the problem 50% by connecting 22nf capacitor between lugs 3 and 2 using the same model as they used in section 1( 500k gain pot and 1nf capacitor) but I don't understand why is this happening.

Also, my volume level is just a bit higher than unity level and neither EQ or any boost pedal can increase volume level when I put them after the Jfet overdrive pedal I was working on.
Your issue is most certainly not your choice of pot value, but either a mistake in your pot wiring, or something else in your build. If you create a thread in the debugging section with schematics, layout and proper pictures of your build, i'm sure a lot uf us will be happy to help ;)
Meanwhile the rest of us will stay on topic in this thread on JFET based design, if that's okay with you ;)
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Post by stole59 »

Oh, sorry. My mistake.
Next question concerning Jfet design: Has anyone ever tried building Jfet fuzz with 4 or 5 cascaded gain stages? Is it possible?

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Post by Groovenut »

stole59 wrote:Oh, sorry. My mistake.
Next question concerning Jfet design: Has anyone ever tried building Jfet fuzz with 4 or 5 cascaded gain stages? Is it possible?
I've built 4 and 5 stage overdrive/distortions to great success but I haven't seen a 5 stage jfet fuzz design but that's probably because it's has a high parts count for most manufacturers. It's definitely possible and the key to making it a fuzz is to allow full frequency through the circuit and have each stage clip. Though I doubt you would need all 5 stages to make a good sounding fuzz it is possible. You could even start with stacking 5 ROG Fetzer Valves ans see where that takes you. Just remember to use trimmers for proper biasing of the fets.

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Post by stole59 »

Thanks Groovenut. I thought there was a line you can't cross with Jfets.

I made a three stage gain overdrive, it sounded good. But when I wanted more gain by adding another gain stage I had a problem because I didn't get any kind of signal out my fourth Jfet. I couldn't even bias it for some reason. I managed somehow to get 4,5v on drain but trimpot had almost no resistance. I tried putting another jfet but had same result.

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Post by Groovenut »

stole59 wrote:Thanks Groovenut. I thought there was a line you can't cross with Jfets.

I made a three stage gain overdrive, it sounded good. But when I wanted more gain by adding another gain stage I had a problem because I didn't get any kind of signal out my fourth Jfet. I couldn't even bias it for some reason. I managed somehow to get 4,5v on drain but trimpot had almost no resistance. I tried putting another jfet but had same result.
To my knowledge there's no line. As long as you have the power supply to support them, you can have as many as you want. It may not sound good but operationally, it should function. There's a limit to the number that is useful, but that depends on how you filter and attenuate the signal between stages. I did a Trainwreck circuit based on Jfets that modeled the phase inverter and power section. I ended up using 7 fets and it sounded really pretty good. It was a bit of overkill but I didn't know any better at the time :D

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Post by stole59 »

Wow, that's a lot man :)

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Post by stole59 »

Are there some general rules concerning signal attenuation between gain stages?

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Post by Groovenut »

stole59 wrote:Are there some general rules concerning signal attenuation between gain stages?
It all depends on how many stages you're going to try to run. Usually for between 2-4 stages I interstage attenuate between 4/1 and 2/1, but it all depends on your circuit goals, how you bias your fet stages and what type of clipping/harmonic content you're after.

It's theoretically possible to have 10 jfet gain stages and never clip the signal (though I cant think of a reason to do so) due to interstage attenuation. I suppose if you really liked the harmonic content a particular Jfet imposed on the clean signal, you could build said circuit. :wink:

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Post by stole59 »

Groovenut wrote:
stole59 wrote:Are there some general rules concerning signal attenuation between gain stages?
It all depends on how many stages you're going to try to run. Usually for between 2-4 stages I interstage attenuate between 4/1 and 2/1, but it all depends on your circuit goals, how you bias your fet stages and what type of clipping/harmonic content you're after.

It's theoretically possible to have 10 jfet gain stages and never clip the signal (though I cant think of a reason to do so) due to interstage attenuation. I suppose if you really liked the harmonic content a particular Jfet imposed on the clean signal, you could build said circuit. :wink:
Yeah, I understand that.
How can I calculate signal attenuation after a gain stage?
For instance, if I put 100k resistor right before gain stage in series with incoming signal and Jfet's gate ( I see that value frequently in ther people designes) what would be attenuation?

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Post by grrrunge »

If the stages are connected directly with a resistor, the attenuation would be the resulting voltage division from the 100K resistor and the succeeding stages input impedance.
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