Variable input impedance

[Elijah-Baley]

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!

[Kipper4]

Reply 46 here

https://www.diystompboxes.com/smfforum/index.php?topic=112900.40

[Elijah-Baley]

Thank you!
I link the reply here:
https://www.diystompboxes.com/smfforum/index.php?topic=112900.msg1124739#msg1124739

Schematic:


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?

[amptramp]

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". 

[Elijah-Baley]

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.

[samhay]

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.

[PRR]

> 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.

[Elijah-Baley]

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:



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.

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.

[Kipper4]

Like this? cant remember......

[diffeq]

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?

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.

[Elijah-Baley]

Like this? cant remember......

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?

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.

[diffeq]

Like this? cant remember......

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?

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:

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):

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):


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.

[Kipper4]

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

[Kipper4]

Think this is better. Again quick drawing

[antonis]

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..

[Elijah-Baley]

Good catch, sorry! here's corrected schematic:

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):

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):


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!
Thank for your schematic!

Think this is better. Again quick drawing

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?

[antonis]

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..

[samhay]

>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.

[Elijah-Baley]

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.

[PRR]

> 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.