A Cartridge Demagnetiser

What on earth is a cartridge demagnetiser? Surely a cartridge requires magnetism in order to work? These were the first questions that came to mind when encountering this thing 5 or so years ago. It does not demagnetise the cartridge but instead the build up of small magnetic effects in other parts of the cartridge such as the core etc. This is only my understanding of it though. The part I don't understand is that people use these devices on their interconnects, power amps, speakers etc. and claim similar results. I have not yet.

The unit puts out a very small, distorted, high frequency wave form - that is all. The clever bit is the signal rises slowly and dies away slowly so as not to leave magnetic residuals. In comparison to the commercial unit the idea was taken from, this unit, once turned on will do one channel, then the other channel and then switch itself off. No chance of leaving it on and wasting the battery, no hassle of changing leads over. A friend of mine designed it and although it could be simpler, the extra complexity costs so little while adding to its ease of operation.

The results to my ears (and reports by others who have one) are amazing. It is as though someone has opened the window in a room full of stale air; like the system has just had a spring clean.

 

Schematic original design by Gerard Cullen
Parts List
Resistors R1,2,14,15 - 10k R3 - 33k R4 - 2M2 R5,10,11,18,19 - 1M R6 - 6k8 R7 - 150k R8,9 - 220k R12 - 100k R13,21,24 - 2k2 R16 - 47k R17 - 3k3 R20, 23 - 180k R22,25 - 10R R26 - 1k all resistors 1/4 watt Capacitors C1 - 47n C2 - 10u C3,9,10 - 100n C4,5 - 22u C6,7,8 - 1n C11,12,13 - 1u C14 - 100u	Semiconductors IC1 - 555 IC2 - 4017 Tl - ZTX502 T2 - ZTX302 73 - ZTX300 T4 - BC239 T5,6 - BC549 D1 - 1N4001 D2 - LED Miscellaneous SW1 - Pushbutton Switch B1 - 9V PP3 SKT1,2 - Phono Sockets PP3 - Battery Clip

Push-button SW1 starts the sequence by turning on T1 which provides power to the circuitry. R3 turns on T2 which latches T1 on.

ICI is configured as a free running astable, with timing components R4,5 and C2. IC1 clocks IC2 via pin 3. IC2 is a decade/divider that sequences the demagnetiser's operation from pins 3,4 and 10. It is reset automatically by R7 and C3 on start up.

T4 is configured as an oscillator running at about 5.5kHz. Its output is fed via C9/10 to output stages T5/6. T5/6 are emitter follower circuits that provide the low output impedance necessary to drive loads such as a moving cod cartridge.

So when the pushbutton is pressed, pin 3 of IC2 goes high, R8 and C4 provide a time constant to turn on the T6 output stage slowly so the signal builds up gradually and on completion of the cycle, when IC2 pin 3 goes low, dies away slowly. Pin 4 then goes high to provide an output from T5. When pin 10 goes high to complete the sequence, T3 turns on, turning off T2, thereby T1, which turns the unit off.

PCB Design - Track View (not to scale)

Testing

DO NOT hook this up to your cartridge to see if it works!!!!

Look at the output wave form on an oscilloscope. Make sure the wave form for each channel:

• comes up slowly
  
• is of the correct magnitude (about 5mV p-p)
  
• is the correct frequency (about 5.5kHz)
  
• lasts about 25 seconds
  
• dies down slowly
  
• switches off; total time about 2 minutes.
  

Internal layout and PCB closeup.