Jfet design questions

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stole59
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Post by stole59 »

Hello guys. I am currently on a path of learning jfet based OD design so I have few questions. I will post picture of one of more popular amp in a box design so you could better understand my question. Here are my questions:

A) In section 1 what is the use of capacitor C3? does it create some kind of filter with gain potenciometer? I know that when designing opamp based ovedrives designers conect in parralell small value capacitor with potenciometer in feedback loop and that that creates a high pass filter but I am not sure if this is the samo thing.

B) In section 2 what is the use of capacitor C9? Filter or not?

C)In section 3 what is the use of capacitor C6? If this is a filter what kind is it and how much does it drops sertain frequencies in db?

D) If I a am correct section 4 boost gain but how much connecting el capacitor is boosting gain? Is there a formula?

Thanks in advance!

Here is the picture
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Post by grrrunge »

stole59 wrote:Hello guys. I am currently on a path of learning jfet based OD design so I have few questions. I will post picture of one of more popular amp in a box design so you could better understand my question. Here are my questions:

A) In section 1 what is the use of capacitor C3? does it create some kind of filter with gain potenciometer? I know that when designing opamp based ovedrives designers conect in parralell small value capacitor with potenciometer in feedback loop and that that creates a high pass filter but I am not sure if this is the samo thing.

B) In section 2 what is the use of capacitor C9? Filter or not?

C)In section 3 what is the use of capacitor C6? If this is a filter what kind is it and how much does it drops sertain frequencies in db?

D) If I a am correct section 4 boost gain but how much connecting el capacitor is boosting gain? Is there a formula?

Thanks in advance!

Here is the picture
[ Image ]
If you want to get handy with FET circuits, this is the way to go: http://www.amazon.com/Microelectronic-C ... c+circuits
There is no easy way or handy sohrtcut, but if you get down with the chapters on basic fet amplifiers from this book, you're well on your way.

C3 bypasses the gain pot for high frequencies, making sure that the circuit won't sund dull at lower gain settings.
C4 will let AC signals above a certain frequency pass, and it will enable signal swing beyond the limitations set by the DC biasing network.
C6 will act as a short for high frequencies and as a disconnection for low frequencies. C6 parallel with R15 it will then act as a frequency dependent resistor, with a resistance of 500K below a certain frequency and a 0Ω above that frequency, with a transition point in the middle, where the resistance drops gradually towards 0Ω as the frequency goes up.
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Post by Groovenut »

Actually I think section 4 would change the DC operating point of Q2 to R9//R10 then C4 would set the point at which AC signal frequency were at full gain. 4.75k//4.75k=2.375K with 2.2u would give a HPF of ~30Hz

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Post by Intripped »

I'm as well interested in better understanding how exactly C6||R15 works,
Could someone please explain or link to some resources on-line?

I've already made some research, but with poor results
Thanks

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Post by Groovenut »

C6//R15 is a combination borrowed from tube amps. Here is a link to some of the whys in regard to tube amps. http://www.valvewizard.co.uk/gridstopper.html

Unfortunately, JFets dont have the internal capacitance or resistance levels that tubes like 12AX7s do, so the resulting effect is not very noticable in a JFet based circuit. Most of the datasheets dont even list the GS capacitance nor the GD capacitance, but I believe it to be very small.

Here's another link explaining tube Miller capacitance http://www.aikenamps.com/index.php/what ... apacitance

If you wish to duplicate the effect you'll need to add a cap to ground after C6//R15 of around 150p

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Post by mictester »

Groovenut wrote:C6//R15 is a combination borrowed from tube amps. Here is a link to some of the whys in regard to tube amps. http://www.valvewizard.co.uk/gridstopper.html
No. It's just a crude pre-emphasis stage. At 1kHz, the reactance (the AC analogue of resistance) is 339kΩ, and at 3kHz it's 113kΩ. The reactance is in parallel with the fixed 470kΩ resistor, so the net effect is a frequency - dependant resistor, which reduces as frequency rises.
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Post by grrrunge »

Groovenut wrote:Actually I think section 4 would change the DC operating point of Q2 to R9//R10 then C4 would set the point at which AC signal frequency were at full gain. 4.75k//4.75k=2.375K with 2.2u would give a HPF of ~30Hz
Yes. I was a bit fast there, and didn't notice the fact that section 4 is on a switch ;)
The gain formula for a common source amplifier:
AV = -gm*RD/1+gm*RS
Switching in section 4 will then decrease RS to half its original value, giving you close to twice the amount of gain.
A quick LTspice simulation shows that the high-pass filtering caused by C4 is located at around 55Hz.
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Post by Groovenut »

My point about C6//R15 being that this schematic is a jfet version of a Bogner Ecstacy preamp. The design is copied from a tube amp, look to the tube circuit for some background. Even this particular tube circuit section was lifted from another tube amp. Marshal plexi. The 500k series 500k shunt splitter is the same as the 1M gain pot at half, the 500k//470p is the same as the treble peaking circuit that followed the plexi gain pot on the high channel. The difference is that unlike tubes, jfets dont have substancial internal capacitance or resistance. Therefore the effect of this treble peaking filter is minimal without external components to emulate those tube internal specs. After all it's the sound that you're after. Otherwise why copy the design?

A sim of the circuit section show the filtering in the millidecibel range when using a jfet without the external components.

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Post by Intripped »

mictester wrote:
Groovenut wrote:C6//R15 is a combination borrowed from tube amps. Here is a link to some of the whys in regard to tube amps. http://www.valvewizard.co.uk/gridstopper.html
No. It's just a crude pre-emphasis stage. At 1kHz, the reactance (the AC analogue of resistance) is 339kΩ, and at 3kHz it's 113kΩ. The reactance is in parallel with the fixed 470kΩ resistor, so the net effect is a frequency - dependant resistor, which reduces as frequency rises.
...and another (hopefully not so stupid) question naturally arises:
what is the effect of a simple resistance in series with the audio signal? does it attenuate the audio level? (but for signal attenuation don't you need a resistor shunted to ground?)

i know there are RC filters, but here if i'm not wrong we're talking of a single (frequency dependant) resistance in series with the audio signal

...kind of lost here
[thanks for the answers and please excuse me for the thread-hijacking]

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Post by mictester »

Groovenut wrote:A sim of the circuit section show the filtering in the millidecibel range when using a jfet without the external components.
You're exactly right. It's a very poor design (in all sorts of ways) and those guys at Runoffgroove are much better at doing amplifier simulations! The clown that copied the bogner circuit really hasn't got a clue.

FETs can be made to behave quite like valves in a number of ways, but simulating the internal capacitances of a device and then copying the circuit around the original bottle is unlikely to yield credible results.

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!
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Post by Groovenut »

Intripped wrote:i know there are RC filters, but here if i'm not wrong we're talking of a single (frequency dependant) resistance in series with the audio signal

...kind of lost here
[thanks for the answers and please excuse me for the thread-hijacking]
no thread jack as I think this is the discussion that the OP was aiming at.

A resistor in series with an AC signal will, as Mictester stated, provide a reactance with the AC signal. If there is only this resistor in series (and no parallel cap or impedance to ground) then the reactance should cover the entire frequency range.

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Post by Intripped »

I understand that putting a capacitor in parallel with the resistor we get a freq-dependant reactance
What I don't get is what does this reactance actually do to the signal.
Does it attenuate the level?

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Post by induction »

Reactance can be thought of as frequency-dependent resistance. Caps provide more reactance to low frequencies. Inductors provide more reactance to high frequencies.

They also cause frequency-dependent phase shifts. Sometimes that part matters, but often you can ignore it.

Look at standard low-pass and high-pass filters: one cap and one resistor. Think of them as frequency-dependent voltage dividers. That should help you get the gist of it.

To answer your previous question, an inline resistor increases impedance. In other words it reduces the amount of current flowing at a given voltage. It usually won't reduce volume unless the gain of a given subcircuit depends on that particular impedance. Look at the gain formula for an inverting op-amp stage, for example.

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Post by grrrunge »

Intripped wrote:...and another (hopefully not so stupid) question naturally arises:
what is the effect of a simple resistance in series with the audio signal? does it attenuate the audio level? (but for signal attenuation don't you need a resistor shunted to ground?)

i know there are RC filters, but here if i'm not wrong we're talking of a single (frequency dependant) resistance in series with the audio signal
The input impedance of the succeeding stage is your equivalent of a resistor to ground ;) The value of the gate biasing resistors in parallel is a practical approximation of the input impedance.
If you then imagine that resistance to ground after your parallel RC network, you have your passive filter equivalent. I hope that makes sense ;)
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Post by mmolteratx »

1) The low reactance of the capacitor at high frequencies bypasses the gain pot. The exact frequency response will depend on gain setting, but you'll lose less treble as the gain control is rolled down.

2) The low reactance of the capacitor at high frequencies bypasses the drain resistor, sending them to your V+ rail. Since any DC source acts as AC ground, those frequencies are grounded and you wind up with a low pass filter.

3) As previously mentioned, this does nothing really. But if there were a real voltage divider there, or if you were using a tube, it would act exactly like the first capacitor, just at a fixed setting.

4) See below
grrrunge wrote:
Groovenut wrote:Actually I think section 4 would change the DC operating point of Q2 to R9//R10 then C4 would set the point at which AC signal frequency were at full gain. 4.75k//4.75k=2.375K with 2.2u would give a HPF of ~30Hz
Yes. I was a bit fast there, and didn't notice the fact that section 4 is on a switch ;)
The gain formula for a common source amplifier:
AV = -gm*RD/1+gm*RS
Switching in section 4 will then decrease RS to half its original value, giving you close to twice the amount of gain.
A quick LTspice simulation shows that the high-pass filtering caused by C4 is located at around 55Hz.
It should give you more than twice the amount of gain, depending on frequency. Rs is really the two resistors in parallel plus the reactance of the capacitor in parallel. At higher frequencies, the capacitor will effectively drop Rs to a value of 0, giving you the max theoretical gain of -gm*RD, and the gain will decrease as frequency decreases down to the -gm*RD/(1+gm(Rs1||Rs2)) value for low frequencies.

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Post by Intripped »

grrrunge wrote:
Intripped wrote:...and another (hopefully not so stupid) question naturally arises:
what is the effect of a simple resistance in series with the audio signal? does it attenuate the audio level? (but for signal attenuation don't you need a resistor shunted to ground?)

i know there are RC filters, but here if i'm not wrong we're talking of a single (frequency dependant) resistance in series with the audio signal
The input impedance of the succeeding stage is your equivalent of a resistor to ground ;) The value of the gate biasing resistors in parallel is a practical approximation of the input impedance.
If you then imagine that resistance to ground after your parallel RC network, you have your passive filter equivalent. I hope that makes sense ;)
Yes,It does make sense! Thanks

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Post by grrrunge »

Thanks for the clarifying the function of CS Matt!
The guy who taught us FET-stuff last semester wasn't really fit for it. New dude on first semester of his phd with no prior teaching experience, and then he took over the course from an old professor who had it all figured out in a way that made it glue in perfectly between the other electronics courses we're having.
Bam! There's your recipe for total destruction! :D
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Post by stole59 »

Thank you guys for all your replies. All you wrote helped me so much. You were right about this section 4, this combination does apsolutely nothing in jfet circuit. I also didn't notice much difference when I instaled something similar to section 2 in my jfet design, maybe I didn't listen carefully.

How much exactly will there be a volume drop in db for sertain frequencies in classic RC filter? I mean, you can create two filters like this with the same corner frequencies, but will there be a difference in volume drop and how much?

Image

I used this article as a starting point when started creating jfet drive, still experimenting with filters and tone stack
http://www.instructables.com/id/FET-Distortion-Pedal/

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Post by mmolteratx »

grrrunge wrote:Thanks for the clarifying the function of CS Matt!
The guy who taught us FET-stuff last semester wasn't really fit for it. New dude on first semester of his phd with no prior teaching experience, and then he took over the course from an old professor who had it all figured out in a way that made it glue in perfectly between the other electronics courses we're having.
Bam! There's your recipe for total destruction! :D
I'm surprised you did anything with JFETs, to be honest. We never really got past discussing JFET device properties way back in my solid state devices class. Then we designed stuff with MESFETs in RF circuits. All of my microelectronics classes were focused heavily on CMOS, though my school's circuits stuff is all heavily geared towards modern high frequency analog circuits in CMOS.

I feel you on awful teachers though. Luckily all of my circuits teachers were great, mostly because my school now has a giant analog department and tons of funding from the state because of it. What book did you use though? I've got a handful of great ones saved as PDFs.

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Post by grrrunge »

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?
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