                            SIDprotector README 
                            -------------------

This simple circuit tries to protect the SID soundchip in the Commodore 64. 
These chips went wrong many times probably due to overvoltages coming back from
the output: Phantom powers of mixers, wrong connections, ESD, you name it...

 I tested the circuit in ngspice (SPICE circuit simulator linux implementation),
you can see the related files and screenshots in this folder. The circuit was
also built and tested on real C64 hardware. Stress-test with real 50V AC feed
was not performed for obvious reasons (too many SIDs dying in C64 history), but
the circuit simulation shows that this simple circuit protects SID's output from
overvoltages and reverse (below zero) voltages/currents. If the diodes are of
Schottky type (with enough reverse breakdown voltage for the task), the circuit
can have less impact on the output signal due to the series diode, and have more
protection in the negative voltage region on SID-output. 50V and more breakdown
voltage diodes are preferred because in practice phantom power around 48V is the
biggest that a mixer or other devices can drive to the C64 output by accident. 
 If the diodes are normal-type (e.g. 1N4148), then D2 in parallel with the Zener
doesn't do much and can be left off...

 The circuit only works with DC-shifted signal on its input so either use direct
output of the SID, or even better,the one-transistor current amplifier's output:

-In the former case SIDprotector's biased output should be filtered by a series
capacitor (to remove the DC component of the output signal), or connected to the
input of the transistor current-amplifier in the C64 which expects biased input.

-The latter has the advantage of working the same way with both 8580 and 6581
chip models. (6581 needs an 1kOhm resistor from its output to the ground.)
 So to be compatible with both SID-models I think it's best to connect the input
of this circuit directly on the output of the transistor (emitter of 2SC1815 or 
2SC945) in the C64 sound output stage (before the output decoupling capacitor).

(Of course you need to insert the circuit by breaking the original signal path.)


I hope this can prevent some further SID-deaths for people who try it...



          2016 Hermit (Mihaly Horvath) http://hermit.sidrip.com
 
