Variable input impedance

[Elijah-Baley]

Hi, I have a question about the bias cap in the jfet version, exactly the same schematic posted above.
With the emulator I got something I didn't expect. The gain pot of the first jfet is reverse wired, because I got more boost at zero and viveversa. I can fix that connecting lug 1 and 2 to the source and the lug 3 to the ground. And done.
The thing I find weird is the purpose of the bias cap. It cuts low end (with the cap at 220nF) just at lower setting. At max, when the source is full grounded through the gain pot maxed, it doesn't matter the value of the cap: 47uF, 220nF or 1nF. I got a wide flat boost. Of course is because the cap is bypassed.
Is this how the bias cap supposed to work? Or should I use a "basic" resistor? 220R, 470R...?

Thanks!

[merlinb]

Is this how the bias cap supposed to work?

Yes that's how it works. As the source resistor gets larger, the cut-off frequency of the cap gets higher. With no source resistor there is no cut-off frequancy (cap is shorted out, so a flat response). That's how it works in tube stages too; the frequency response gets shifted as you change the bias R or bias C. Image below shows the effect of changing the cap, but changing the resistance has a similar effect.

Or should I use a "basic" resistor? 220R, 470R...?

If you prefer. It's your circuit, so you can do as you please, whatever sounds best to your ears.

[Elijah-Baley]

Great. Thank you.

[Elijah-Baley]

I'm back.
I called this project Input Box.
Excuse me for the arrangement of the switches in the schematics. It was the easiest way for testing it with the software.

This is the OpAmp version schematic:



- The 1k input resistor could be unecessary. I need it to make the tone pot effective with my software. I'm not sure if an input resistor can give some advantage, I hear it can block some noise. But, my intention is to emulate a Tone pot of a guitar, so if it works without that resistor I'm going to jumper it.
- It has a cap switch for the tone, for different high cut-off.
- It has three cap for low cut-off.
- I placed, in my veroboard layout still not posted, the 10M pull down resistor not there, but immediately after the input. Someone confirmed, I guess, it's the same.
- Three configuration for the bias cap. Those cut-off low end at higher gain setting.
- C12 is 10uF, but even a 1uF doesn't cut low end, so there' no much difference using a bit smaller cap.

And this is the Jfet version schematic:



It's the same schematic, but it use a stage with two JFET suggested me by merlinb.
- About the 1k resistor is the same thing of the Op-Amp version.
- Identical cap tone switch.
- So the 10M resistor in the veroboard layout of the JFET version, still doesn't posted.
- The cap for "no low cut-off" is bigger then the Op-Amp version just because this circuit seems to have more low end loss. Not enough for a guitar, probably, but at higher gain setting the loss  could a bit more accentuated.
- Bias cap switch. The "no low cut" cap is bigger.
- The Jfet version has a 4.7nF cap across to the ground in the end of the circuit because it seems doesn't cut treble by itself, and this cap could help it a little. Maybe this cap doesn't make anything, or maybe it could be cut some high frequenciy noise, or oscillating. The opamp doesn't have it because it seems to have a natural high end roll-off.

What do you think about this project?

[Kipper4]

I like it.
You started with an idea of what you wanted and look like you've achieved a great deal.
I can't explain the jfet 47nf output shunt cap.
Except to say maybe it's forming a CC filter if there's such a thing.
I remember reading long ago every componant has resistance, capacitance and induction.
IIRC electrical engineering 101 was the book.
Still haven't finished it in 7 yrs.

Can't help you with the cap thing sorry maybe someone else has an answer to it.
Great project. Great add on for a few other projects too. Thanks for sharing EB.

[Elijah-Baley]

Thanks Kipper4, I think is prerry fair to share this project, I even received some help for this, so...
Now all this is just theory.

This is the veroboard layout of the Op-Amp version. Still UNVERIFIED.



The Tone and Input Cap toggle switches are peculiar. The blue jumpers is a jumper indeed, and they needs of that. Ignore the black tracks, instead, those are just to show how the switch works in central position.
I hope it's all clear and there aren't mistake. Else, I can give some help to understand it or correct it.

[Kipper4]

I can't get on with vero. Blind to it.
A quick look says D1 is back to front if it's a protection diode. Or else it's gonna short the power supply.

[Elijah-Baley]

I can't make PCB, so I work on veroboard.

D1 is a 1N5817 diode for protection, yes, and it's in line to the 9v, not across the ground.
So, I think it's correct like that. 9v goes in the positive side of the diodes and come out a bit dropped from the negative side. It is like so in the layout as well in my schematic. In the Jfet version this diode is missing, but take it on count.

But thanks, tell me if you see something wrong.

[Elijah-Baley]

Hello, almost one year from my last post here, finally I'll back on this circuit.
Actually, I worked on it building the board, but I stopped it for some months because I used the parts for other my projects, and I didn't order anything for long.
The boards was ready for the first test, but always did other things, and today I decided to test it... and it works.
I'll update the layout with some minor corrections of the label, but the is VERIFIED.

How it works?
Tone pot works, but I noticed a very little bit of treble cut even with the pot completely open, probably the 10nF and especially 2.2nF have less problem with it.
Gain pot is ok, it boost volume, it give to me just at max setting a bit of opamp clipping when I hit hard the strings. I think it's not a real issue, but I still have to test it with a more aggressive guitar.
Bias Cap works fine, too. The 22uF seems to cut no low end at all, but I have to compare the circuit with bypass, it was just a quickly test. In case a 47uF or a 100uF should make the trick perfectly. The 1uF cut the low end, and the 220nF cut a even more for a very sparkly and clean treble booster style sound. Of course the amout of the low end cut depends from the Gain pot setting.
Input Cap. With the 100nF I got no low end cut, the 10nF is almost the same, the difference is very small. The 1nF cut more low end. I think I'll change the 10nF with a 4.7nF or so for a better middle choice.
Impedance. It acts a bit like the tone pot, but it is interactive with the Input Cap. With the 1nF a low impedance cut the treble, but I got a volume loss, too. I'll test it better next time.

So, I have to improve something, but it's done. There's some parts that seems make the same job, but each tone or switch interactive differently with the overall setting. I thinks this could be a nice tone shaping machine.
The only thing I'd like to do, but I still don't know how I can do it, is to get a more overdriven sound at high gain setting. Changing the pot value? Adding a pair of LEDs somewhere?
Of course the clipping have to be natural and amplike or so.

I have a question. The Impedance pot decreases the input inpedance, but this will mean that this circuit can work in front of a fuzz face type of pedal?

[mac]

So, I have to improve something, but it's done

In the Fet version you can avoid Vref and its parts.
You can bias the Fet by first choosing a pot at the Source and then finding a resistor to set the Drain voltage where you want it, say 4.5v.
The Impedance pot can be tied to ground.
Same beer in a different glass.

The only thing I'd like to do, but I still don't know how I can do it, is to get a more overdriven sound at high gain setting. Changing the pot value? Adding a pair of LEDs somewhere?
Of course the clipping have to be natural and amplike or so.

Try Leds or Silicons across the Volume pot. Expect some volume drop.

I have a question. The Impedance pot decreases the input inpedance, but this will mean that this circuit can work in front of a fuzz face type of pedal?

The input impedance Z-in interacts with your guitar, and the output impedance Z-out with a FF or whatever.
The Fet version has a buffer at the output, and the opamp has very low Z-out. It can work.
If not, a series pot or transformer can do it.
My FF has a series pot at the input in case I put something before it.
Search here about it.

mac
 

[Elijah-Baley]

Thanks for the suggestion about the FET version. For the moment, I worked harder on the Op-Amp version, because I prefer it. And, looking at my old layout (still not posted here) of the FET version, indeed, the Impedance 2 goes to the ground. I don't remeber, maybe someone else suggested to me early. 

I wanted to try to add some LEDs, indeed, but I hope to used it in a way to go from a very clean sound to a mild overdrive. It's not so bad getting something more gritty, but the mainly thing I want is a clean sound, so it's important keep it clean at least in a certain setting without add any perceptible clipping or compression. If it's possible.
Looking the Op-Amp version schematic I'm wondering if I can add two LEDs directly between the pin 2 and 6, or lug 1 and lug 2 of the Gain pot. Is it the same thing? This make me a bit confused.
On the layout this will be a bit harder to do.

[mac]

Looking the Op-Amp version schematic I'm wondering if I can add two LEDs directly between the pin 2 and 6, or lug 1 and lug 2 of the Gain pot. Is it the same thing? This make me a bit confused.

Not the same.
LEDs between pins 2 and 6 make the opamp clip softly. (Tube Screamer)
In the second case the opamp hard clips and the diodes across the pot round the square wave. (MXR Dist+)

mac

[Elijah-Baley]

Yes, thank you. I know the difference between soft and hard clipping.
My (stupid) doubt was about the spot where I should I have to place the diodes for the soft clipping mode.

Try to look at my schematic: I'd say that place the diodes between the pin 2 and 6 of the op-amp is the better way to get the soft clipping.
I was wondering what is if I put the diodes between the pin 2 of the opamp (or lug 2 of the Gain pot) and lug 3 of the Gain pot, "after" the pots. (Probably in my early post I typed wrong.) This way seems to me a bit different, schematically.

Meanwhile, I drawing a new layout to simplify the caps switching that will allow to use the normal switch and not Z-type switch. And of course I included the soft or hard clipping.