Historical note for youngsters: cassettes are those little plastic boxes full of brown string that your parents keep in cardboard boxes on top of the wardrobe. The de-facto standard of 129 metres of brown string wound on little bobbins is almost but not quite enough to store one whole album. This is what passed for copy-protection in their day. Left in a car for five summers, all cassette recordings of any kind evolved into either Wings or Steeleye Span anyway. This phenomenon has never been fully explained but is thought to be an early experiment in what is now known as consumer subscription expiry.
For example, you may wish to provide some recorded music during the interval between sets at a live gig, or you may be a quizmaster or quizmistress delivering the "music round" at your local pub. Either way, the material is on your personal device and you want it to go front-of-house, through the desk or whatever.
I know what you are thinking. You go to your local electronics store, you get a 3.5mm to 3.5mm stereo extension cable and a 3.5mm stereo to 1/4" mono converter and the job is done. Oh ho ho! No. You risk (at best) distorting your sound or (at worst) frying your kit. That's your decision but here at The Secret Laboratory we like to do things properly.
Here's how to do it properly:
Find a 3.5mm stereo jack plug, a 1/4" mono jack plug, a length of shielded two-core audio cable, two 10K resistors (brown-black-orange) and some insulating tape or heat-shrink tubing.
Solder all the bits together according to the following schematic and following instructions:
On the left, your 3.5mm stereo jack. Source (R) will probably be a red wire - connect it to the "ring" on the jack (you may need a meter to check which tag is the ring but it's usually the tag on the right as you look from the rear of the plug - red ring right is how I remember it). Source (L) might be blue, white or some other colour. Connect it to the "tip" on the jack (usually the left tag as you look from the rear). Connect the shielding (Source G) to the "sleeve" of the jack (that's the BIG tag that also serves to secure the cable when you crimp it). In this particular application it doesn't actually matter if you get the L and the R mixed up but it's good to get it right. Don't mess with the G though, else you'll be pissing signal to ground.
On the right, your 1/4" mono jack going to your PA. Connect the red signal wire (Source R) to a 10K resistor and the other signal wire (Source L) to another 10K resistor, making sure the two wires don't short at this end (that's where the insulating tape or heat-shrink tubing comes in handy). Twist the other ends of the resistors together and connect them to the "tip" (PA Sig) of the mono jack. Connect the shielding (Source G) to the "sleeve" of the jack (PA G) and crimp to secure the cable, just like you did at the other end. Get this right and the whole assembly will fit inside the 1/4" jack housing. Oh, er, now is a REALLY bad time to remind you that you should have made sure the plug housings were already on the cable - I'm sorry I'm sorry I'm sorry...
Right! You have just constructed a simple "audio mixer" - tell your friends! They'll think you're some kind of electronics genius! "Buy an audio mixer? No, I just make my own - it's nothing, really..."