### Author Topic: Passive Components in Guitar Pedal Input Stage  (Read 817 times)

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#### eev_carl

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##### Passive Components in Guitar Pedal Input Stage
« on: August 17, 2018, 10:13:22 pm »
Hi,

I've been reading a lot of schematics on guitar pedal circuits and had a few questions about the input stages that I'm seeing.  One such schematic is attached below.  Most of the schematics have the following components. I'm assuming the function; please correct if it's wrong.

1. An AC coupling cap (C1)
2. A direct coupling resistor (R1)
3. A pull-up resistor (R2) - biasing?
4. A large path-to-ground resistor (not used by this particular schematic, but on others to prevent a "pop" on power off)

I've seen many different combinations of 1+2 and was wondering how to select the values.  I'm most interested in bass frequencies for my application.

The full schematic of the attachment is here at Electrosmash: https://www.electrosmash.com/tube-screamer-analysis

Thanks,
Carl
« Last Edit: August 18, 2018, 09:50:47 am by eev_carl »

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#### Zero999

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #1 on: August 17, 2018, 10:50:59 pm »
Yes, that's correct.

The problem with that circuit is its bias point is dependant on the transistors Hfe, which is extremely variable. The impedance seen at the base is equal to the emitter resistor, multiplied by the transistor's Hfe and forms the lower half of the potential divider. The solution is to add a potential divider, with a lower impedance, than the emitter resistor multiplied by Hfe, to the input stage. This will reduce the input impedance considerably, but bootstrapping can be used to increase it.

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#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #2 on: August 17, 2018, 11:06:21 pm »
Is the input stage using both direct coupling and AC coupling?  If so, are they trying to mitigate the downsides of each by selecting middle-of-the-road values?

#### JS

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #3 on: August 17, 2018, 11:21:44 pm »
No, ac coupled as you don't care about DC. The resistor there would limit the signal to the base of the transistor, but in this case not so much as it's pretty small compared to the surrounding resistors, including the emmiter resistor reflected at the base of the transsistor.

JS

If I don't know how it works, I prefer not to turn it on.

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#### Zero999

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #4 on: August 17, 2018, 11:32:51 pm »
Here's an example of a buffer with a high input impedance. It uses a low impedance potential divider to bias the output to near half the supply voltage and bootstrapping to increase its impedance to around 1M.

LTSpice Simulation
https://www.eevblog.com/forum/projects/resistor-and-pot-in-parallel-not-working-as-i-expected/msg1401083/#msg1401083

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#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #5 on: August 17, 2018, 11:51:59 pm »
Quote
It uses a low impedance potential divider to bias the output to near half the supply voltage and bootstrapping to increase its impedance to around 1M.

Thanks for the schematic.  I understand the biasing R1R2 resistor divider and the coupling C1 and C3. They're in a lot of the tutorial schematics I've found.

Is R3 used to stabilized the circuit?  I've seen that listed as doing so but only in C-E rather than C-C schematics.

What are the roles of R4 and C2?  Does R4 just tune the resistor divider based on what you have on hand?

#### Zero999

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #6 on: August 18, 2018, 12:15:10 am »
R3 is required for a DC return path through the transistor. Without it, there will be no current and the transistor won't work.

R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance. See links below:

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#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #7 on: August 18, 2018, 12:23:22 am »
Thanks for the links!

#### Audioguru

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #8 on: August 18, 2018, 01:39:31 am »
The link to the Tube Screamer doesn't work but the horrible transistor circuit is probably designed to give the bad distortion producing the screaming sounds from an electric guitar.

#### darrellg

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #9 on: August 18, 2018, 02:21:07 am »
The link to the Tube Screamer doesn't work but the horrible transistor circuit is probably designed to give the bad distortion producing the screaming sounds from an electric guitar.

Remove the trailing period from the URL.
https://www.electrosmash.com/images/tech/tube-screamer/tube-screamer-block-diagram.png

#### Audioguru

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #10 on: August 18, 2018, 04:14:15 am »
I see on the larger schematic that the bias resistor for the transistor is fed from a half-the-supply-voltage divider so it should work with low distortion.

#### floobydust

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #11 on: August 18, 2018, 04:34:39 am »
... R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance.

Bootstrapping with positive feedback I would not use, a guitar has a complex output impedance and there would be a (tone/pickup) potentiometer setting that causes oscillation.

Old tube guitar amps were a 1MEG input impedance and here 500k would be fine. Lower just shifts the guitar's passive tone control up.

You must have ESD protection diodes at the input jack. I repair a lot of gear that got zapped from the 1/4" phone jack tip.

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#### Old Printer

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #12 on: August 18, 2018, 04:40:56 am »
R3 is required for a DC return path through the transistor. Without it, there will be no current and the transistor won't work.

R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance. See links below:

Also thanks for the links. I have been investigating guitar effects and pedals as a practical use for the electronics knowledge I am gaining from this hobby. I really enjoy learning electronics, but without a practical outlet to use that knowledge it gets boring at times. Also thanks eev_carl for startiing the thread.
« Last Edit: August 18, 2018, 05:21:14 am by Old Printer »

#### Zero999

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #13 on: August 18, 2018, 06:22:40 am »
... R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance.

Bootstrapping with positive feedback I would not use, a guitar has a complex output impedance and there would be a (tone/pickup) potentiometer setting that causes oscillation.

Old tube guitar amps were a 1MEG input impedance and here 500k would be fine. Lower just shifts the guitar's passive tone control up.

You must have ESD protection diodes at the input jack. I repair a lot of gear that got zapped from the 1/4" phone jack tip.
Why do you think it would oscillate? At low frequencies it just looks like a 1M resistor, in series with an AC coupling capacitor. At higher frequencies, the impedance does drop, but it remains capacitive, just with a lower resistance and the Q would far too low to oscillate with anything, even at 100kHz.

I've simulated the difference in impedance between, the bootstrapped amplifier and a 1M resistor and AC coupling capacitor. There isn't much difference between the two, especially below a couple of kHz. The red is the amplifier's input impedance and the green, is the impedance of a 10nF capacitor  (yes, I realised 4.7nF was a bit small), in series with a 1M resistor.

The only potential problem is bad design. If C2 is too small and C1 overly large, the bootstrapping wouldn't work and the input impedance would become R1|R2+R4, but I've mitigated that by making C2 big and C1 small.
« Last Edit: August 18, 2018, 06:24:20 am by Hero999 »

#### floobydust

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #14 on: August 18, 2018, 07:48:54 am »
As a signal source, a guitar has cable capacitance, pickup inductance and RC tone controls. Many variables that are unique to each instrument and setup. Add that to the Spice model and I think it would be risky. I've never seen bootstrapping used on a guitar preamp front-end. Sure it could work. But across permutations of guitars and settings, not my style.

I think OP would be better off using the JFET input stage instead of this emitter-follower.
Electrosmash has this J201 front-end with ESD protection diodes. You get some low-noise voltage gain instead of zero voltage gain with the emitter-follower.

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#### Zero999

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #15 on: August 18, 2018, 09:22:25 am »
As a signal source, a guitar has cable capacitance, pickup inductance and RC tone controls. Many variables that are unique to each instrument and setup. Add that to the Spice model and I think it would be risky. I've never seen bootstrapping used on a guitar preamp front-end. Sure it could work. But across permutations of guitars and settings, not my style.

I think OP would be better off using the JFET input stage instead of this emitter-follower.
Electrosmash has this J201 front-end with ESD protection diodes. You get some low-noise voltage gain instead of zero voltage gain with the emitter-follower.
I agree on the J-FET input stage. It might have a higher voltage noise but the current noise should be much lower, which will matter more in this case.

#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #16 on: August 18, 2018, 09:51:22 am »
Quote
The link to the Tube Screamer doesn't work

The link is fixed.

#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #17 on: August 18, 2018, 09:53:48 am »
Quote
Also thanks eev_carl for startiing the thread.

Look for upcoming questions on diodes and clipping, filters, and the output stage!

#### eev_carl

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #18 on: August 18, 2018, 09:58:25 am »
Quote
I think OP would be better off using the JFET input stage instead of this emitter-follower.

This came in the mail today!

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##### Re: Passive Components in Guitar Pedal Input Stage
« Reply #19 on: August 18, 2018, 10:29:16 am »
make sense to use a better model of a pickup, first hit on google.

http://i.imgur.com/LHI8XmF.png

Smf