line level input (switchable)

[JP19]

Hello,

I could do with some help if anyone is able to..

I wanted to put a switch at the input of a stereo pedal, to let me choose either line level or guitar level input. I figured just using a 10:1 voltage divider for the line level input would do the job, but it's a stereo pedal and I only have a DPDT switch, so I'm trying to figure out a way to do it using only half of the switch for each channel. Please see my picture for what I was thinking.



- With the switch at position A, I can have a 10:1 pad which is what I want
- With the switch at position B I have no pad (which is what I want) BUT with R1+R2 acting as a 'pulldown' resistor. Will this work? I suppose I would need to use high enough values (1M/100k??) that the pulldown resistor doesn't cause problems with the 'unpadded' signal, but will this cause me issues with the 'padded' signal?

Will this work? Please feel free to school me if this is a terrible idea or if you have a better suggestion!

P.S. I should mention that I was planning to put this right after the input jack, and it will be followed by an opamp buffer.

[antonis]

In theory, it will work..

But, there should appear issues like impedance mismatching, loading and noise..
e.g R1 must have a big value for not loading previous output but low value for not injecting Johnson noise when is set in series with A..

For a more precise R1 / R2 values estimation, we need to know IN output impedance as well as A & B input impedances..

[MrStab]

This is an L-pad, and will work.

Position B will give you around 1.1Meg impedance with stated values (...in so far as what the op-amp input sees). Position A will give you pretty much 100k impedance. Perhaps that's not an issue if line-level outputs ae generally low-Z anyway.

Constant impedance? Look up Pi-pads (but like I say - possibly unnecessary).

This will parallel with the bias resistor for your op-amp, if there is one, further reducing impedance.

Like Antonis says, watch for noise as you navigate around these issues.

[PRR]

Another approach is a 1X/10X amplifier stage.

[MrStab]

Unless there's a risk of input signal clipping at unity gain with the max input range afforded by 9V. Significantly rubbish on a TL072.

(I know you know this, Paul, and I make this comment with some degree of anxiety!)

[JP19]

I'd rather not go for the 1X/10X amplifier option. I want to pad it passively before the rest of the pedal to enable me to potentially bring down loud synth levels which may be outside the range of my power supply and would clip an opamp at the input.

So does anyone have any suggestions on what would be the best way to create a line/guitar level switch at the input of a pedal? I have various commercial pedals that do this (off the top of my head I can only think of the Eventide 'factor' series but I'm sure there I have others. I wonder how they do it?

Off the top of my head I suppose I could use a 4 pole switch (as it's a stereo input) and switch before and after the pad, enabling me to use lower value resistors for the voltage divider as it would be completely out of circuit when in 'guitar mode'. Or is there a better way?

[MrStab]

I think you have the right approach already.

You could try doing it on the other side of the input coupling capacitor, to Vref instead of Ground, and have a 10M or so resistor permanently tied between op-amp non-inverting input & Vref to ensure there's always a bias voltage without significantly interfering with impedance (as depending solely on the pad to provide DC bias to the op-amp would mean a split-second of no connection if your switch is break-before-make)..

[PRR]

WWJD?

i.e., what does EVERYBODY, Fender Peavey Rolls Behringer, do for a universal input?

In past it was not uncommon to have a Inst and a Line jack, with a small opamp to boost the Inst level to Line jack's switch-finger.

[garcho]

Are you building a one-off diy thing? Go expensive switch (4PDT) all the way! You'll never miss that extra few dollars, direct and simple.
Are you going to make 100 or 1000 or 10000 of these? Now you know why boss switching is what it is.

[MrStab]

WWJD?

Jesus was a preacher. Mic level was more his game.

[JP19]

Yeah I suppose a 4pdt switch is probably the way to go, I just wondered if there was a better way.

On a side note, I took the back off my eventide pitch factor and I can see that they are using a DPDT switch to do stereo input level switching, but I'll need to dismantle it further to actually see what's going on inside..

[garcho]

^ Same with all their 'factor' pedals. I've always been amused that those little toggles survived their engineering team.