Variable input impedance

[Elijah-Baley]

Thanks, PRR.

I don't know if someone watched the video I linked above. Maybe it could be useful to figure it out a way to do what I want. Of course, it's very hard find out what there is inside that box, also because is a tube circuit. I don't know, maybe I JFET schematic could be another solution?

I still have to find a right and easy approch to create this impedance control.

How would you do it?

[mac]

Fet and Mosfet Master,

https://www.diystompboxes.com/smfforum/index.php?topic=112798.msg1042276#msg1042276

mac

[merlinb]

I don't know if someone watched the video I linked above. Maybe it could be useful to figure it out a way to do what I want. Of course, it's very hard find out what there is inside that box, also because is a tube circuit. I don't know, maybe I JFET schematic could be another solution?
I still have to find a right and easy approch to create this impedance control.
How would you do it?

Based on his description, the circuit below will emulate 90% of his box. It does all the impedance stuff you asked about, plus an approximation of his tube cathode bypass cap switch. I had to guess at his ICP and BC cap values of course; you can always change them, and use two-way switches instead of three-way if you prefer. But if you want to emulate his bias control aswell then you'll have to find someone who's willing to devote more time to it than I have, presumably using a couple of JFETs.

[PRR]

I don't know if someone watched the video I linked above....

I found the popcorn. I can't find any video in this thread.

However my pot+r is the basic idea. Merlin has kindly fleshed it out as a full-working opamp plan.

[Elijah-Baley]

https://youtu.be/pmUq8uK_AL8?t=1222

Hey, PRR. Do you really miss the video? It was just a link. I quoted myself to link it again. I found it very interesting.

[Elijah-Baley]

I like very much this schematic!

I still have just some doubt, some or these question are just little details.
1. I can put the pulldown resistor after the input, a 10M resistor, to not influence the impedance pot value, right?
2. Do I really need R5 1k?
3. The T 500k pot could be the tone pot, I'm not sure probably a log should work better, but I wonder if I can move it at the very begin of the circuit.
4. I assume V 10k pot should be the gain/volume, but 10k seems to me very little.
5. BC caps should be Bass Cap, not Bias Cap. Right?

The bias thing, as merlinb said, it will be done using a JFET type circuit.
Honestly I guess I prefer op-amp type, I don't know I find it more transparent. But I would like to see how can be the JFET version, just for curiosity.

[merlinb]

1. I can put the pulldown resistor after the input, a 10M resistor, to not influence the impedance pot value, right?

Yes 10M will have minimal effect.

2. Do I really need R5 1k?

It's good to put a small resistor in series with the opamp input to help protect against RF and ESD spikes, but is doesn't have to be 1k. Anything from 100R to 4.7k would work.

3. The T 500k pot could be the tone pot, I'm not sure probably a log should work better, but I wonder if I can move it at the very begin of the circuit.

Yes you can move it

4. I assume V 10k pot should be the gain/volume, but 10k seems to me very little.

You could use 100k if you want more gain. I guess his tube stage has a gain of up to x50, so yeah.

5. BC caps should be Bass Cap, not Bias Cap. Right?

It approximates his cathode Bypass Cap. But yes, it affects the low end, so you could call it a bass cap if you like.

[Elijah-Baley]

Thank you. It is a schematic I can work on it for sure!

Probably I will take a while of time before to work on it. Meanwhile, I'll try to put the basic of a jfet version.

[Elijah-Baley]

I draw about the same schematic in jfet version.
(For the low cut option and for the tone caps I used two switches in that way because the emulator software I use doesn't have a three way switch, in this way I can emulate it well. But a normal three way switch can be used normally.



The pulldown resistor is after the Tone Pot Section, then there's the low cut cap. The impedance pot across the 4.5v. And finally the JFET gain stage.
Helping with the emulator software I found a combination of the resistors from the jfet across the 9v and the one across the ground that give me about 10db of boost. This combination give me an enough linear boost controlled simply by the Volume at the end. I didn't find a good value for gain control using the resistor R3.
I hope this gain level doesn't cause distortion.
The 47uF C7 give me almost no bass cut. A 470nF, for example, cut bass under 1kHz.



Is that jfet version schematic good for my purpose?

[PRR]

The 4.5V ref kinda throws-away half your battery voltage and limits the maximum output.

The Vref should be more than your expected Vgs (0.3V-1.5V) but much less than the supply voltage. I'd throw a dart at "2V", re-figure the source resistor, not obsess about gain, and see how that works. If you are willing to cast-out 10% of your J201 then 1V Vref may work. (The higher Vgs parts may work better in switch, trem, or phaser apps.)

[Elijah-Baley]

Thanks, PRR.
I'm not sure I got what you mean. You said Vgs, that is for? The 9v, maybe?
I try to be more specific about my own schematic.
The voltage is the tipical 9v, so the Vref is 4.5v.

I guess you were talking about the voltagse around the J201, in case it can be useful I written the voltages according by the emulator. (I still didn't test the cricuit in the real world).

My first version:



And, as I said early, I'm not sure I understood your suggestion, but I messed around the resistors R6 and R3 and I adjusted the value of the resistors to get that voltages taking into account anyway the boost keeping it around 10db.



I don't know if I am on the right way.

[merlinb]

The voltage is the tipical 9v, so the Vref is 4.5v.

PRR is advising that you change the 4.5V into 2V. e.g. change R5 into 27k.
Then adjust R3 until you get about 3V on the drain of the JFET.

[Elijah-Baley]

Thank you, merlinb, for the clarification!

I tried to do that.
I changed in the emulator software R5 into 27k getting a Vref of 1.91v.
It was impossible to me getting 3v to the drain of the jfet just changing R3. I necessarily change even R6 until I got this voltages:



I tried to reach voltage closer to the recommended ones:



In both the schematic the gain raise up until about 21db (the old one was 10db), I can tame it a bit with a 470k in line to the output before the Volume pot getting a boost of "only" 16db. I expect that resistor could smooth the highs, but the emulator didn't tell me this, or probably the high cut is so far that I can't see any changes under 100kHz.

[merlinb]

It was impossible to me getting 3v to the drain of the jfet just changing R3. I necessarily change even R6 until I got this voltages:

Yes that looks good. In real life you will probably need to adjust R3 again. Real JFETs will not exactly match your simulation.

In both the schematic the gain raise up until about 21db (the old one was 10db), I can tame it a bit with a 470k in line to the output before the Volume pot

Why bother? That's what the volume pot is for.

If you want to emulate the circuit from the video you can add the bias pot and switchable bypass cap. Then you don't need the Vref anymore.

[Kipper4]

You got this sorted yet EB?
I want to see how it is in practice with yer fuzz face et all.

Merlin can I ask ? Why make R3 variable rather than R6 ?
I only ask since many  jfet od designs do it the other way.
Ref Dr Boogie.
Just reinforcing my learning.

[Kipper4]

And is it worth putting a 50 Ohm stopper resister in?

[merlinb]

Merlin can I ask ? Why make R3 variable rather than R6 ?

Because that's how the guy in the video did it, and you said you wanted to emulate that. Similar (but not the same) effect either way.

And is it worth putting a 50 Ohm stopper resister in?

Yes, sure.

[Kipper4]

Gottya thanks.

[PRR]

> Why make R3 variable rather than R6 ?

R6 is picked for the load you need to *drive*.

R3 only affects the DC bias, not driving ability, and not directly gain.

Why do some projects fiddle the drain resistor? Maybe because, being drain-voltage focused, it is "obvious"? And it can work. IMHO it is less elegant.

[Elijah-Baley]

[...]
If you want to emulate the circuit from the video you can add the bias pot and switchable bypass cap. Then you don't need the Vref anymore.

Thank you merlinb for your last suggestion.

I started to work on some veroboard layouts, but I can't work on it in the real world yet.

Thanks to all, I have much materials about this project. I hope it will be interesting how it is in my mind.