Today I have spent a short amount of time “Device Hacking” Using my Marshall portable double stack amp to conduct some experiments. This is our “test subject” .
Our unsuspecting test subject
After slightly dismanteling the amp to get in at her goodies I was able to start measuring what sort of signal inputs it received from guitars and various audio devices (This will become vital in later work). Firstly measured at the Audio in Jack (without any amplification) I found out that the device was outputting around 100 milivolts upwards (to about 600 milivolts) depending on what was being played on the guitar.
This also told me that as the “Volume” of the playing increased – as did the voltage of the signal – Although this is acctually very hard to tell using a digital mulitmeter. So to test this theory and to build upon our last sort of basic LED circuit – I have began thinking about building a digital LED Signal Volume meter (this will also allow further itteration into a more complete product).
However I am doubtful that the basic stamp can quantify such low input voltage – So I measured what the post amped signal was inside the Marshall – On average between 0v-6v but sometimes beyond – far too high for the stamp.
This gives two options
- Further dismantle my precious marshall amp and retrieve the delicious amplifier circuitry and then provide it with the stamps 5v power supply to give it a more reasonable input
OR
- Design a simple transistor circuit and variable potentiometer to acheive the signal strength needed (and range)
So far I have designed a circuit for the latter as I love my amp too much to harm it further. A basic outline of the circuit and coding is pictured below
And what documentation would be complete without showing the other tool I am using?
Stay tuned to see the finished volume meter
– Allan
acousticartifacts 