Author Topic: Switch Pop, Pull Down and Current Limiting Resistors  (Read 357 times)

Timko

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Switch Pop, Pull Down and Current Limiting Resistors
« on: October 25, 2019, 02:25:37 PM »
I've been spending some time this year away from building projects and focusing on understanding how circuits are designed.  Since I began pedal building 4 years ago, I've been fascinated with the Rangemaster and germanium based pedals in general.  So the first design I've been working on has been a layout for a Darlington (well, it's really a Sziklai) based layout using 2 transistors for creating Rangemaster tones with an additional output tone control on the end.  The eventual goal is to replace the tone control with some sort of synth style resonant filter, but I'm starting here for the moment.  My circuit has 4 controls.

* Input cap blend control
* Boost control
* Tone control
* Tone bypass switch

After working through the layout and through getting parts onto my breadboard that sounded good, I ended up with this schematic:
https://i.imgur.com/Pp19p55.png

And this layout:
https://i.imgur.com/096MeHK.jpg

However, after building it and connecting it to a footswitch I got an awful pop.  I spent the past week reading about the causes of switch pop, and I definitely have a 50ish mV voltage on the footswitch pin that corresponds to the effect output.  I noticed that my schematic (like most Rangemasters) lacks an output pull down resistor, so I clipped one of those into the circuit.  However, I noticed that the voltage only dropped when adding a smaller (100k-10k) resistor rather than the typical 1M / 2.2M often seen in modern circuits.  I know the Big Muff uses a 100k pull down resistor, and after reading through the diystompboxes archives, this seems to be an acceptable value.  However, I'm still at a loss as to why that's the case.  I'm not sure if my large output cap for a Rangemaster (220nF) has anything to do with it, but perhaps.

One of the other things I read about in my research was a resistor in series with the input / output as a current limiter.  I didn't find a lot of explanation on this (much of the focus is on the pull down resistor), but I have looked through a few schematics and have seen resistors after the tone/volume/buffer/last stage with a series resistor before the output with some sort of capacitor to ground in parallel.  I was curious if there are times you opt for this type of design and times you don't.  One of the other things that I'm not sure about is whether having a tone control after the boost section is a good idea.  Most of the time I see a tone control placed before the volume, but that won't work in this case.

Thanks for helping.  Circuit design seems to be a black hole - when you're looking for answers, you find more questions :)

madbean

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #1 on: October 25, 2019, 03:45:33 PM »
Try putting a 1M resistor on the outer two lugs of each side on the DPDT (tone and tone bypass switch) to see if that prevents the pop.

Here's a good read on putting resistors in series with inputs and output: https://www.mrblackpedals.com/blogs/straight-jive/6629778-what-really-causes-switch-pop
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Timko

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #2 on: October 25, 2019, 09:47:38 PM »
So 4 resistors on DPDT lugs 1,3, 4, and 6 connected to ground? 

madbean

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #3 on: October 26, 2019, 03:24:38 AM »
So 4 resistors on DPDT lugs 1,3, 4, and 6 connected to ground?

No, no. Like shown below. One of these may help or they may not. It's also possible you have a leaky cap somewhere if you are reading DC at the circuit output. Maybe the 220n.
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Timko

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #4 on: October 26, 2019, 07:56:08 PM »
I removed the switch from my circuit and out it on the breadboard. I tried it with your diagram B. When the tone now The highest value I can read is around -1.5mV, which I believe is acceptable.  Iíll have to do some careful re-wiring but Iíll start tonight. One of the other things I noticed is that my voltage at the output jumps up past 100mV when I turn the boost knob but it settles right back down to the same range as before.

Did you post the two diagrams as 2 different suggestions to try?  I the voltage reading using diagram A had similar readings but the circuit didnít quite sound the same.

Thanks for the help!  The notion of passing both the signal and the signal through a resistor in parallel never occurred to me. I attempted to put 1M resistors at the beginning and end of the circuit before connecting to the footswitch and there was considerable volume drop. Is this a topology thatís in other pedals?

Timko

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #5 on: October 27, 2019, 09:13:29 AM »
Last night I wired everything up using option B from madbean's suggestions.  Everything tested out well after testing the voltage and doing some audio testing.  So I put it back into the box, wired it up, and BAM!  The switch popping and the high voltage was back :(.

One of the thing I failed to mention earlier is that the Rangemaster is only half of the circuit.  It runs into a second board for a buffered clean boost.  Here's the schematic for that pedal:
https://imgur.com/V2zh2hW

And the board layout:
https://imgur.com/axaZwRB

This morning, I was unhooking things again, testing more voltages, and I noticed that when I unhooked the output of the rangemaster (which runs directly into the PCB of the boost), the voltage was gone.  So I assume there's some sort of voltage leaking from the clean boost into switch for the rangemaster.  It looks like I've identified the cause now. I'll try some more options for adding a series/parallel resistor at the input of the clean boost and see if I can solve my issue.

The more questions you answer, the more questions that appear  ::).

Timko

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Re: Switch Pop, Pull Down and Current Limiting Resistors
« Reply #6 on: October 28, 2019, 06:31:58 PM »
After some research on the Klon buffer, I lowered the input cap from 1uF to 100nF, and the pop is gone!  Thanks for the help :).