Recent searches
Search options
@awai@fosstodon.orgAmp switcher: high-level design 
This project's goal is to be able to select a single amplifier out of 6 without having to mess with cables, and use them with a single input cable and speaker cabinet.
Let's start with the block diagram: the guitar input is routed to one of the amplifiers' input, and the speaker output from the same amplifier is routed to the cab.
Although this all looks fairly simple, there are a few points that require significant attention.
1/
First problem: the guitar signal is very low power and high impedance, while the speaker signal is high power and very low impedance.
This means the guitar signal is very sensitive to EMI (electro-magnetic interference), while the speaker signal is prone to generate EMI. As a consequence, the input and output section *must* be physically separate.
An active buffer between the guitar input and corresponding relays array might be helpful as it lowers the impedance, reducing EMI sensitivity.
2/
The other issue is related to tube amplifiers: those should never be used "unloaded" (without a speaker or similar load connected to the output).
We must therefore ensure that:
* no signal can get to the input of any amp except the currently selected one
* unused amps' outputs should be connected to dummy loads (plain resistors) able to dissipate the amp's background noise
3/
Regarding input switching, this can be achieved fairly easily with 2RT relays: in the NC position (unpowered), we can short the jack going to the amp input. This will effectively ground the input signal on the amplifier's side.
The powered relay (only one at a time) will then route the actual guitar signal to the selected amplifier.
Note: routing both the "live" and ground wires from the guitar ensures we don't have to provide a common ground, therefore avoiding ground loops.
4/
Output switching is very similar, except for one (major) difference: instead of shorting the outputs of unused amps, we place a small 2W resistor across them, so they're never unloaded for more than a few ms (relay switching time).
Given the similarities, I expect a single PCB design can be used for both the input and output relay arrays 
Note: the schematic shows a Finder 30.22 relay (rated for 125V/2A), but in order to properly handle amp output, I'll be using Omron G2RL (250V/16A).
5/
Finally, the "control section" will be using a single rotary switch with a few LEDs; the 1st position will act as an OFF switch (all amps disconnected, all LEDs off), each other position will select a given amp, lighting the corresponding LED on.
For switching between N amps, we'll therefore need a switch with N+1 positions (or only N if there's no need for an OFF position).
The whole thing will be powered using a 12VDC wall-wart, saving me the hassle of designing a power supply 
6/6
