AND/OR Switching Part II: Active and Passive components

One of the goals with this switching system is to make it with as small of a footprint as possible. With the increased access to SMD services the use of the 14 and 16 pin CMOS chips for switching purposes are no longer a barrier which would take up large amounts of real estate. However, they are not without their drawbacks .

The challenge:

The BYP/CH1 and SEL/CH2 switches would need to pass the signal when not engaged. This is where we run into a road block. Its probably already apparent to most, but I struggled with it and figured I'd share it anyway, but the CMOS chips such as the CD4053 are electronically controlled, even when logic levels are low and you would expect that the SPDT would pass a signal, it will not if there is no power connected to the pedal.

This raises a small issue. Normal pedals using mechanical 3PDT bypass switches, are entirely passive. It doesn't matter if power is supplied to the pedal, when the 3PDT has its connections being met correctly it will pass the signal. This is not the case with the 4053.

I don't know how often people are trying to play through their pedals when they have their pedals unplugged. If power fails to your pedal board, I imagine it would be close to an outlet supplying power to your amp and trying to play anything would be pointless. Nevertheless it does present a challenge.

As I alluded to in the last post, this system would be very difficult to implement with a purely mechanical switching system. Instead I will be developing a microcontroller & CMOS system. If you haven't read through it already, I recommend checking out the GEOFEX article on the electronic bypass system. This project will be based on this and an of course expand upon it.

The first step is figuring out the signal paths and number of switches that will be passed through.

After sketching out some ideas, I have a rough draft where ≥10 SPDT/SPST switches would be required to move between the OR and AND switching. This corresponds to Channel 1 OR Channel 2 as well as Channel 1 INTO Channel 2. With this many switches involved at the moment, I think its safe to stop here for debugging purposes. Drawing anything further will get complicated quickly.

Given the digital nature of all of this we can already start to think of how a sort of "TRUTH" table could be set up to switch between modes and what each of the switches would need to do in order to achieve a given outcome.

Word to the frustrated:

When using photoFETs for muting, make sure you use a current limiting resistor. I had burned some out and not re-tested them and kept wondering, why isn't this muting?

  

AND/OR Switching System\\ Part 1

Part 1: The Gestalt

   The ability to control two different effects is one of the joys of having multiple pedals or multiple effects. To date, I have focused most of my attention on switching between channels in a "parallel" manner. However, while doing research for the DBA sound saw and the Kill Kill Filter, it seems that users were interested in the ability to run one filter into another. The challenge ahead is how to develop a system which could accomplish at least 3 modes of operation (1) A into B, (2) B into A, (3) A or B, and (4) A and B.

    Here's a version of a generalized schematic diagramof the switching systems we're interested in with. It should be noted that (1), (2), and (4) could use different grounding methods depending on your preference. 

    I have left off the LED switching systems at the moment for simplicity. Regardless, moving from (1) to (2) can be accomplished in a simple manner using the Geofex Juggler Effects Reverser.

 

    What we can see from this is that in addition to the Switching system to turn each effect ON and OFF, we require an additional 3 (or 4) SPDT switches to change the effect order.

    So the challenge becomes mostly focused on getting between mode (3) and either mode (1) or (2). As well as getting to mode (4) and making sure that there aren't any phase issues. If we can draw out the paths to get between these modes, then its just a matter of a couple more SPDT switches to change the effect order.

    Herein lies four of the starting challenges I've identified thus far:

A. The number of SPDT switches required.

  We are already at 3 or 4 to just move between modes (1) and (2) NOT including the LED indication. Adding in mode (3), and it should be clear that there is probably not going to be an analog toggle switch or rotary switch which will be able to accomplish this switching. In addition, we will need a way to cycle through states in a relatively easy way.

B. Switch life time.

    Part of the fun of the OR configuration is the ability to switch back and forth between effects/channels to quickly switch between different sounds. This will result in the "Channel Select" switch getting pressed more than the "Bypass" switch. Like everything, mechanical switches have a lifetime, can get worn out, and die on us, which present challenges with replacing them. Normally Open (NO) momentary switches have been used to toggle states of relays for pedals and can be used, but we'll need a way to address the toggled status of effects after power is disconnected. This is notably a problem in the OR mode.

C. CMOS switching and pop.

    To accommodate the anticipated large number of switches, we'll likely need to use CMOS chips for switching. In my experience, there has always been a popping issue for me when I have breadboarded this. We'll need to include a form of muting system to prevent popping from being an issue when switching between the AND/OR modes when the effect is powered up.

D. Method of indicating which MODE we are in.

    Switching between 4 modes, pretty much eliminates a single toggle switch (e.g. ON-ON-ON) to select the mode. I believe the choice is between (i) a rotary switch, and (ii) a push-button switch that cycles through the modes with some additional LEDs to indicate which mode is currently selected. I am leaning toward the pushbutton cycle selection since it is a new area for me to explore and doesn't take up as much space as a rotary switch. In my experience with Alpha Rotary switches (both plastic and metal) is that they stick up higher than the potentiometers, and often times require additional washers or an additional nut to get the right spacing/height with the inside of the enclosure. The issue with LEDs is finding the optimal way to communicate with the user which mode is currently active.

    At the moment, the solution seems to be pointing toward a microcontroller-based solution. I'm pretty sure this switching system could be achieved to a degree without a microcontroller. But one of my goals for 2021 was to push myself in a different direction. I think that incorporating microcontrollers into my effects pedals will push me into uncomfortable territory but ultimately improve my creations.

Beginning with The End

 Well 2021 is off to a strange start, and I have finished up a batch of pedals.  I've started a YouTube Channel, posted items to my Reverb Store, and created an Instagram account to keep people posted on different developments.

There are a couple existing projects I want to finish up and others I want to update and expand on. In no particular order other than my thoughts at the moment, here's what I've got planned for the year.

1. The AND/OR switching system is under development, and is somewhat of a priority since I have a fetish for dual channel designs. Moreover, I think my future designs will require the use of micro controllers to accommodate the ideas I have in mind.

If I can get this to work, I'll document the process for those interested. For two-in-one projects it just seems like it would be a cool option to switch between A or B, A->B, and B->A in with a simple interface..

2. I will be revisiting the Solaris this year. Belton has a new brick coming out that should be interesting to work with. Its based on the Cool Audio multifx chip, but at least it doesn't require me to do any SMD soldering.

3. I have been wanting to do a tremolo pedal for so long and I think I have some ideas. Laowiz suggested I try out the Gristleizer, and I think I will start to breadboard that. I've seen some cool videos and of course the VCA/VCF modes fit nicely into my fetish :)

While learning more about microcontrollers, I may try and put out something for a 

4. Modular Interfacing Longterm Fuzz is something I have been toying around with. Mostly to satisfy my crusade for the ideal fuzz for me. This will probably take the longest time to develop and may never be complete, but the basis of it will involve microcontrollers to simplify my life.

I don't want to get too far ahead of myself 4 projects for the year sound doable. The beginning always starts out strong, and right now since I have been drawing schematics and breadboarding most of December.

All along the way I'll put out some updates for the projects; the microcontroller work especially. I think it'd be nice to see simple microcontroller projects pop-up for other guitar pedal hobbyists out there.

Here's to a productive 2021

Best regards,

   Bradford