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DIY Stompboxes => Building your own stompbox => Topic started by: Elijah-Baley on November 03, 2019, 03:48:10 AM

Title: Variable input impedance
Post by: Elijah-Baley on November 03, 2019, 03:48:10 AM
Hello. I want to do some experiment and I would like to build a sort of input box.
I'm searching an amp stage circuit using a TL071 or a JFET for example with a variable input impedance control. The range I would is from 1M, a standard high input impedance of a buffer guitar perdal until a low impedance, around 12-10k like the Dallas Rangemaster Treble Booster or even a bit less, 5-8k like the Fuzz Face.
Often the input impedance is setted by two resistors in parallel and I can't find a way to use a pot, maybe a dual pot, to change the value.

Can someone give me some ideas, please? :)

Thank you!
Title: Re: Variable input impedance
Post by: Kipper4 on November 03, 2019, 04:45:55 AM
Reply 46 here

https://www.diystompboxes.com/smfforum/index.php?topic=112900.40
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 03, 2019, 06:13:46 AM
Thank you! ;)
I link the reply here:
https://www.diystompboxes.com/smfforum/index.php?topic=112900.msg1124739#msg1124739

Schematic:
(https://i0.wp.com/samdump.files.wordpress.com/2018/04/tilt_preamp_schematic.jpg)

First question: I need just the first stage, I assume the output can be after C5, or I need R5, too?

Second question: is R1 just a pulldown resistor? Eventually has to be 10M or even a 1M? And I really need it?

Third question: how I can decrease the minimum of the impedance until 8-10k?
Title: Re: Variable input impedance
Post by: amptramp on November 03, 2019, 07:29:08 AM
You could always use a pot for R1 in the tilt preamp schematic.  Audio / log taper would give you better control of the low resistance end.  Since it is referenced to ground, it does not affect the Vb network.  Change the 10K Zin pot to a 10K resistor with the negative side of C2 going to the emitter of Q1.

Since the effect is tone sucking, I would label the control "SUCKAGE". 
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 03, 2019, 09:35:58 AM
1M log for R1 resistor? Ok, but...
In this schematic I can't get how is determinated the impedance, how it works Z(IN) pot or the rule of R1 resistor. So I can't imagine how I can control the impedance resistance from 1M to about 8-10k.
Title: Re: Variable input impedance
Post by: samhay on November 03, 2019, 01:54:37 PM
The Z(in) pot in the Tilt Preamp sets the input impedance. No need to play with R1.
The actual impedance is a little tricky to calculate, but varies from ~100k to ~1M. If you want lower input impedance, reduce the values of R3 and R4.

If you want something simpler with variable input impedance, I would suggest you build a non-inverting op-amp stage with a variable bias resistor. I would use a FET input op-amp like the TL07x.

p.s. Kipper - thanks for pointing out this thread.
Title: Re: Variable input impedance
Post by: PRR on November 03, 2019, 07:07:47 PM
> I can't find a way to use a pot

It is not that complicated. A pot can be wired as a variable resistor. Replace any convenient input resistor with a pot. Use one end and center. (We typically tie the other end to center so when the wiper gets dirty, Z goes to max not infinity.)

Because you are hoping for more than 10:1 range, use Audio taper.

This silly sim computes the input impedance V/I for pot turned to several values. (Note that I had to bunch-up the low values to get decent spread above 10k, this is why you want a Audio taper pot.)

In a real audio amp you may have a jack-side bleeder and an amp-side bias resistor. With TL072, these can be 10MEG, not really hurting the 1Meg intended max.
(https://i.postimg.cc/4YwNg292/Elijah-Baley-42.gif) (https://postimg.cc/4YwNg292)
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 04, 2019, 03:22:59 AM
The Z(in) pot in the Tilt Preamp sets the input impedance. No need to play with R1.
The actual impedance is a little tricky to calculate, but varies from ~100k to ~1M. If you want lower input impedance, reduce the values of R3 and R4.

If you want something simpler with variable input impedance, I would suggest you build a non-inverting op-amp stage with a variable bias resistor. I would use a FET input op-amp like the TL07x.

p.s. Kipper - thanks for pointing out this thread.

If I want to use that preamp input stage I could reduce R3 and R4 to 10k, and I'll get a range ~10k to ~1M?
I used and saw a lot of time a TL071 basic gain stage, like this:

(http://beavisaudio.com/techpages/buffers/Buffer_OrmanOpamp.gif)

But I know that the input impedance depends from R1 and R2 in parallel. Using two 2M I get exactly an input impedance of 1M. But considering the "parallelism" of the two resistors and the pot (even dual) I can use is not bigger than 1M, I can't find a way to set the impedance. This is my issue. Probably I didn't know the right schematic I need.

> I can't find a way to use a pot

It is not that complicated. A pot can be wired as a variable resistor. Replace any convenient input resistor with a pot. Use one end and center. (We typically tie the other end to center so when the wiper gets dirty, Z goes to max not infinity.)

Because you are hoping for more than 10:1 range, use Audio taper.

This silly sim computes the input impedance V/I for pot turned to several values. (Note that I had to bunch-up the low values to get decent spread above 10k, this is why you want a Audio taper pot.)

In a real audio amp you may have a jack-side bleeder and an amp-side bias resistor. With TL072, these can be 10MEG, not really hurting the 1Meg intended max.
(https://i.postimg.cc/4YwNg292/Elijah-Baley-42.gif) (https://postimg.cc/4YwNg292)


Thanks, PRR. Your explanation could be useful, but it's still too much generic fro me. I really don't know how use the TL071 gain stage, how I explain just in this post.

To clearl my target better I link this video: https://youtu.be/pmUq8uK_AL8?t=1222
It's a very long video, but that device is really interesting. Around 20 minutes Simon Jarret starts to talk about the device.
This pedal have a passive tone like on the guitars, with a three caps selectable, and a variable input impedance. For now this is what I would like to start.
Then there's a sort of clean boost tube circuit, maybe at high voltage, where you can choose two different input caps and set the bias, etc... but this is too much, for the moment. :)
Title: Re: Variable input impedance
Post by: Kipper4 on November 04, 2019, 03:42:53 AM
Like this? cant remember......



(https://i.postimg.cc/wRzs071p/Buffer-impedance-setting.png) (https://postimg.cc/wRzs071p)
Title: Re: Variable input impedance
Post by: diffeq on November 04, 2019, 03:54:49 AM
I can use is not bigger than 1M, I can't find a way to set the impedance. This is my issue.
Just liek that?
(https://i.postimg.cc/XBd1sWJc/varimp.png) (https://postimg.cc/XBd1sWJc)
Minimum is R4 + (R1||R2). For 10K min, I'd try 9K and 1.5K (wastes a few mA' of current though).

Edit: Kipper beat me to it. On his scheme, inverting variant also changes gain. Non-inverting does not.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 04, 2019, 04:06:20 AM
Like this? cant remember......

(https://i.postimg.cc/wRzs071p/Buffer-impedance-setting.png) (https://postimg.cc/wRzs071p)


In the non-inverting type maybe I can control the impedance like that, but where's the resistor to the ground, maybe I'll need it at least as pulldown resistor anti-pop. I'm asking it, I'm not sure.
In the inverting input type I guess it could work, I guess the resistance in the inverting input and the one in the feeback control even the gain.


I can use is not bigger than 1M, I can't find a way to set the impedance. This is my issue.
Just liek that?
(https://i.postimg.cc/XBd1sWJc/varimp.png) (https://postimg.cc/XBd1sWJc)
Minimum is R4 + (R1||R2). For 10K min, I'd try 9K and 1.5K (wastes a few mA' of current though).

Edit: Kipper beat me to it. On his scheme, inverting variant also changes gain. Non-inverting does not.

This makes me a bit confused, are inverting input and non-inverting input reversed? Because it's like the non-inverting input version of Kipper 4, except C1 100uf, but it should be a noise filter.
Title: Re: Variable input impedance
Post by: diffeq on November 04, 2019, 05:13:58 AM
Like this? cant remember......

(https://i.postimg.cc/wRzs071p/Buffer-impedance-setting.png) (https://postimg.cc/wRzs071p)


In the non-inverting type maybe I can control the impedance like that, but where's the resistor to the ground, maybe I'll need it at least as pulldown resistor anti-pop. I'm asking it, I'm not sure.
In the inverting input type I guess it could work, I guess the resistance in the inverting input and the one in the feeback control even the gain.


I can use is not bigger than 1M, I can't find a way to set the impedance. This is my issue.
Just liek that?
(https://i.postimg.cc/XBd1sWJc/varimp.png) (https://postimg.cc/XBd1sWJc)
Minimum is R4 + (R1||R2). For 10K min, I'd try 9K and 1.5K (wastes a few mA' of current though).

Edit: Kipper beat me to it. On his scheme, inverting variant also changes gain. Non-inverting does not.

This makes me a bit confused, are inverting input and non-inverting input reversed? Because it's like the non-inverting input version of Kipper 4, except C1 100uf, but it should be a noise filter.
Good catch, sorry! here's corrected schematic:
(https://i.postimg.cc/MvDhcgPM/varimp.png) (https://postimg.cc/MvDhcgPM)
This is a buffer, so gain is unity. To make it into a gain stage, you'd have to insert a divider in the feedback (between "-" and output pins):
(https://i.postimg.cc/PpfQLFPy/varimp.png) (https://postimg.cc/PpfQLFPy)
Gain formula for non-inverting configuration is: 1 + Rf / Rdiv (in the example, it is 2).
For inverting configuration, it is: Rf / Rin, and Rin sets the input impedance. R4 is added for a minimum value (so, Zin is Rin+R4):

(https://i.postimg.cc/NKyR4NY8/varimp.png) (https://postimg.cc/NKyR4NY8)
Changing Rin changes gain ratio. You can set Rf lower that maximum Rin value to make it negligible (say, 10K in this example).

In both configs, input capacitor and impedance after it form a high-pass filter, so roll off low frequencies.
Pull-downs are placed outside the input cap. Values must be larger than your input impedance or they will affect it.

Title: Re: Variable input impedance
Post by: Kipper4 on November 04, 2019, 05:32:25 AM



In the non-inverting type maybe I can control the impedance like that, but where's the resistor to the ground, maybe I'll need it at least as pulldown resistor anti-pop. I'm asking it, I'm not sure.
In the inverting input type I guess it could work, I guess the resistance in the inverting input and the one in the feeback control even the gain.

Forgot it trying to hurry to get a pic up. oops . good catch
Title: Re: Variable input impedance
Post by: Kipper4 on November 04, 2019, 05:45:43 AM
Think this is better. Again quick drawing



(https://i.postimg.cc/nXphV2cK/Buffer-input-impedance.png) (https://postimg.cc/nXphV2cK)
Title: Re: Variable input impedance
Post by: antonis on November 04, 2019, 05:51:39 AM
Unless you use op-amp as a buffer, dual gang pot with different values should be a bit hard to be purchased..

But, a 1M dual gang pot should be convenient for Vbias setting..
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 04, 2019, 06:34:24 AM
Good catch, sorry! here's corrected schematic:
(https://i.postimg.cc/MvDhcgPM/varimp.png) (https://postimg.cc/MvDhcgPM)
This is a buffer, so gain is unity. To make it into a gain stage, you'd have to insert a divider in the feedback (between "-" and output pins):
(https://i.postimg.cc/PpfQLFPy/varimp.png) (https://postimg.cc/PpfQLFPy)
Gain formula for non-inverting configuration is: 1 + Rf / Rdiv (in the example, it is 2).
For inverting configuration, it is: Rf / Rin, and Rin sets the input impedance. R4 is added for a minimum value (so, Zin is Rin+R4):

(https://i.postimg.cc/NKyR4NY8/varimp.png) (https://postimg.cc/NKyR4NY8)
Changing Rin changes gain ratio. You can set Rf lower that maximum Rin value to make it negligible (say, 10K in this example).

In both configs, input capacitor and impedance after it form a high-pass filter, so roll off low frequencies.
Pull-downs are placed outside the input cap. Values must be larger than your input impedance or they will affect it.

Usually I see the resistance in the negative feedback going to the ground, not in the VB. But I don't want to open too much topics! ;D
Thank for your schematic! ;)

Think this is better. Again quick drawing

(https://i.postimg.cc/nXphV2cK/Buffer-input-impedance.png) (https://postimg.cc/nXphV2cK)


Thanks for your corrections! ;)

I feel a bit of messy, anyway.
I'm finding different theory about the calculations of the impedance of the two type of schematic inverting input and non-inverting input.

INVERTING INPUT
Looking at the last schematic posted by Kipper4, I can see the inverting input with a dual pot. Usualy the non-inverting input is not used for higher input impedance, but let's see it enyway.
If the impedance is setted just by the "input resistor" I can use a 10k resistor + 1M pot. And to use a fixed resistor in the negative feedback of the op-amp. I guess the input resistor controls even the gain and the 1M pot at max will give me maybe too much attenuation of the volume?
If even the resistor in the negative feedback controls, with the input resistor, the impedance I could use a dual pot, but I guess it's no so easy to get the right range input impedance 10k-1M, because I assume the resistors will work in parallel, etc...

NON-INVERTING INPUT
About the non-inverting input the input impedance is setted by the resistor to the VB, and it should be pretty easy to add a 10k reesistor + 1M pot.
The input impedance could be influenced by a resistor to the ground, in this case the input impedance will be the value between these two resistors in parallel.
I probably need a resistor to the ground as pulldown resistor. Can I put in before the cap to avoid the pop of the switch and avoid the interaction with the resistor in the VB?
Title: Re: Variable input impedance
Post by: antonis on November 04, 2019, 06:57:08 AM
I feel a bit of messy, anyway.
I'm finding different theory about the calculations of the impedance of the two type of schematic inverting input and non-inverting input.

No reason to get confused..

Without refering on any particular scheme, just follow signal flow..!!
It goes to any kind of ground (DC, AC, Virtual) and stops to any infinite impedance (like non-inverting input)

So, ALL resistances leading to any kind of ground should be considered in parallel and ALL resistances leading to "infinite" impedance should be ignored..
Title: Re: Variable input impedance
Post by: samhay on November 04, 2019, 11:22:33 AM
>NON-INVERTING INPUT
About the non-inverting input the input impedance is setted by the resistor to the VB, and it should be pretty easy to add a 10k reesistor + 1M pot.
The input impedance could be influenced by a resistor to the ground, in this case the input impedance will be the value between these two resistors in parallel.
I probably need a resistor to the ground as pulldown resistor. Can I put in before the cap to avoid the pop of the switch and avoid the interaction with the resistor in the VB?

Yes, you can still add pull down resistor. As you figured, the input impedance will be the parallel resistance between this and the variable bias resistor. So, make the pull down resistor really big - e.g. 10M.

One other thing to note is that the input cap will form a high pass filter with the bias resistor. You thus need to make this cap large enough to work with the minimum (10k) bias resistance or else your control will be dominated by this high pass filter rather than the intended effect(s) of input impedance.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 05, 2019, 06:30:02 AM
No reason to get confused..

[...]

My confusion is about how determinated the input impedance and get an easy variable control.

Yes, you can still add pull down resistor. As you figured, the input impedance will be the parallel resistance between this and the variable bias resistor. So, make the pull down resistor really big - e.g. 10M.

One other thing to note is that the input cap will form a high pass filter with the bias resistor. You thus need to make this cap large enough to work with the minimum (10k) bias resistance or else your control will be dominated by this high pass filter rather than the intended effect(s) of input impedance.

Ok, I'll remember the 10M pulldown resistor.
About the input cap, I hope that the thing you told me will be not a real issue when I use a switchable input cap to cut the low-end. Just for experimentation, again. ;)
Title: Re: Variable input impedance
Post by: PRR on November 05, 2019, 03:35:31 PM
> the input impedance depends from R1 and R2 in parallel. Using two 2M I get exactly an input impedance of 1M.

You are mixing two jobs. 1) Getting a half-supply reference. 2) Setting an input resistor.

This is a too-cheap approach. It mixes half the supply crap into the input. Review R.G.'s "noiseless biasing". You make half-supply, *and* filter the supply crap with a capacitor. THEN run this to the amplifier input to bias and set input impedance. Just ONE resistor to vary. You can get 5Meg single pots.

Also dual pots do not track perfectly so your "half supply" will vary a part-volt as you turn it. Probably not a real problem but just a sign of an inelegant plan.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 07, 2019, 11:55:50 AM
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?
Title: Re: Variable input impedance
Post by: mac on November 08, 2019, 06:01:49 AM
Fet and Mosfet Master,

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

mac
Title: Re: Variable input impedance
Post by: merlinb on November 08, 2019, 09:19:53 AM
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.

(https://i.postimg.cc/PLtbW7KW/66627457-226365828340836-692280541073236292-n.jpg) (https://postimg.cc/PLtbW7KW)
Title: Re: Variable input impedance
Post by: PRR on November 09, 2019, 01:44:20 AM
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 09, 2019, 03:01:59 AM
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. ;)
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 09, 2019, 10:10:47 AM
(https://i.postimg.cc/PLtbW7KW/66627457-226365828340836-692280541073236292-n.jpg) (https://postimg.cc/PLtbW7KW)

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.
Title: Re: Variable input impedance
Post by: merlinb on November 10, 2019, 05:05:02 PM
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.

Quote
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.

Quote
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

Quote
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.

Quote
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 12, 2019, 05:27:55 AM
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 16, 2019, 08:30:29 AM
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.

(https://i.postimg.cc/Ln46Wb3X/Jfet-Input-Box-schematic.jpg) (https://postimg.cc/Ln46Wb3X)

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?
Title: Re: Variable input impedance
Post by: PRR on November 16, 2019, 02:07:42 PM
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.)
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 17, 2019, 04:35:50 AM
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:

(https://i.imgur.com/6LhFN4q.jpg)

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.

(https://i.imgur.com/jKIim8I.jpg)

I don't know if I am on the right way.
Title: Re: Variable input impedance
Post by: merlinb on November 17, 2019, 05:30:51 PM
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 18, 2019, 04:00:44 AM
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:

(https://i.imgur.com/CIzsMT3.jpg)

I tried to reach voltage closer to the recommended ones:

(https://i.imgur.com/ElywNUR.jpg)

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.
Title: Re: Variable input impedance
Post by: merlinb on November 18, 2019, 04:24:11 AM
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.

Quote
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.

(https://i.postimg.cc/rzy9YPM0/1db6c59b7.jpg) (https://postimg.cc/rzy9YPM0)
Title: Re: Variable input impedance
Post by: Kipper4 on November 18, 2019, 01:26:14 PM
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.


Title: Re: Variable input impedance
Post by: Kipper4 on November 18, 2019, 01:31:09 PM
And is it worth putting a 50 Ohm stopper resister in?
Title: Re: Variable input impedance
Post by: merlinb on November 18, 2019, 03:51:19 PM
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.

Quote
And is it worth putting a 50 Ohm stopper resister in?
Yes, sure.
Title: Re: Variable input impedance
Post by: Kipper4 on November 18, 2019, 04:03:55 PM
Gottya thanks. :)
Title: Re: Variable input impedance
Post by: PRR on November 18, 2019, 11:25:17 PM
> 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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 23, 2019, 06:08:50 AM
[...]
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.

(https://i.postimg.cc/rzy9YPM0/1db6c59b7.jpg) (https://postimg.cc/rzy9YPM0)


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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 24, 2019, 09:33:46 AM
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!
Title: Re: Variable input impedance
Post by: merlinb on November 25, 2019, 05:57:10 AM
Quote
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.
(http://valvewizard.co.uk/PartialBypass.jpg)

Quote
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on November 25, 2019, 06:27:04 AM
Great. Thank you. ;)
Title: Re: Variable input impedance
Post by: Elijah-Baley on January 31, 2020, 11:01:25 AM
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:

(https://i.imgur.com/OYWuJga.jpg)

- 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:

(https://i.imgur.com/I3sPw1y.jpg)

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?
Title: Re: Variable input impedance
Post by: Kipper4 on January 31, 2020, 01:16:36 PM
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on February 01, 2020, 01:31:25 PM
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.

(https://i.imgur.com/eLVpLtQ.png)

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. :)
Title: Re: Variable input impedance
Post by: Kipper4 on February 01, 2020, 02:21:25 PM
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.
Title: Re: Variable input impedance
Post by: Elijah-Baley on February 02, 2020, 04:47:05 AM
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. ;)
Title: Re: Variable input impedance
Post by: Elijah-Baley on January 26, 2021, 11:30:02 AM
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?
Title: Re: Variable input impedance
Post by: mac on January 30, 2021, 10:00:11 AM
Quote
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.

Quote
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.

Quote
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
 


Title: Re: Variable input impedance
Post by: Elijah-Baley on January 31, 2021, 04:57:40 AM
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. ;D
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.
Title: Re: Variable input impedance
Post by: mac on February 04, 2021, 08:38:39 AM
Quote
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
Title: Re: Variable input impedance
Post by: Elijah-Baley on February 04, 2021, 01:19:24 PM
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.