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How Many Buses?

A project log for Open Source Analog Effects Pedal

A modular platform for developing and trading guitar (and other) audio effects. Focus on, but not limited to, pure analog signal path.

SmerfjSmerfj 08/19/2014 at 22:180 Comments

The Bus Problem

Edited 4/21/15 - Rearranged the bus, mirroring the 4 power channels to put Vccd on the end. The prototype boards will not have this channel and can use 8 pin headers this way.

Now that the bus structure is defined to use stacking headers, how many bus channels are really needed? To answer this is a combination of requirements and flexibility. I have never designed a bus before, and just as a head's up... I'm not even sure if I'm using the correct terminology!

Power

The obvious required bus channels are Power. The majority of guitar pedals use 9V input from either a battery or external power supply. This is halved to get a psudo ground with +/- 4.5V. Since the majority of the topology of these circuits involves transistors, these are 3 of the required bus channels: Vcc (collector - ground), Vbe (base - emitter/ground), Vee (Ground). Next is support for power for a digital circuit requiring a separate regulated supply, Vccd. Since the power consumption for the modules is usually low, a single ground will suffice. Current draw will become the first derived requirement. This brings the total required buses to 4.

Audio Signal

For the signal buses, one will be the main audio signal bus, and presents a dilemma - it is serial for any module that processes signal, so a method to maintain the vertical stacking shield capability must be implemented. The solution I have found is to place a male pin on the top side of the board, adjacent to the required female plug, and insert a female pin jumper with the blank in line with the stacking header. An alternate solution is to use a stacking header spaced off of the board enough to place an offset bend in the signal pin to move it to an adjacent hole, and place a normal male header pin below it, insulated. Because of the two methods, the signal bus will be placed at the edge of the buses. At this moment, I can find no reason to have more than one main audio signal bus of this nature. This brings the total buses to 5.

Feedback, Wet Mixing, Switching, and Expansion

In audio circuitry, there is sometimes a need for a feedback line for reasons such as distortion control and stability. In my searching, I have not yet encountered a circuit with more than one feedback line. On the other hand, some effects require the dry signal to pass through and mix in with the final processed signal, and ome, such as the Phase pedals, require an LFO. Even implementing a bypass switch that occurs after a buffered input, and before a buffered output poses a requirement for a dry signal. And then there is the need for a communication path for any digital circuitry. Tallying these needs without going overboard, we need 4 more buses.

But what about future expansion? Should I add more than 4 more buses for parallel signals? Where is the trade off for unknowns and board real estate? A better method would be to make these "addressable". The 4 parallel signal buses will be called "General Purpose" buses, with the recommendation that they are used in this order: Feedback, LFO, Wet, Dry. This brings the total number of buses to 9.

Implementation

The bus channels are:

Stacking headers come in various sizes, but common are 6, 8, and 10 pin. The 9 bus channels identified here can be implemented using either an 8 pin stacking header and use the jumper method for the signal on the 9th pin. The second method would be to use the offset pin method with the 10 pin stacking header.

Added 4/21/15: making the Vccd channel optional allowed an 8 pin stacking header to be used with the pin offset method on the prototypes.

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