1

I’ve been trying to revive a set of vintage Apple-branded powered desktop speakers. Nothing more than a gunked up potentiometer and a broken solder joint on the input and they’re now working again.

There’s however a constant hum/buzz coming from both speakers when plugged into their 9v DC power supply, which gets worse when I put a finger over some of the caps’ contacts. Example here: https://youtu.be/XfSJliJzJvA

The noise is there whether any inputs are connected or not. There is nothing else plugged into the electrical outlet, and I’ve tried other outlets in the house. As I understand it this rules out ground loop noise, but I’m by no means an expert.

Boards look fine visually and I’ve pulled and tested each of the larger caps.

I suspect that it’s simply the amplifier(s) picking up noise, and have read that a resistor across the input can cut out this noise. The resistor value should differ based on whether the input is amplified or line level, but these speakers should be able to deal with either scenario, I think.

Would this work, and what value of resistor might you recommend?

Edit: Just wanted to add that I tried running the speakers with another power supply I have (one of those cheap AC/DC switching power supplies), and the noise was even worse.

Rob Schmitt
  • 11
  • 2
  • 1
    Can you try a different 9V power supply? Quite possibly the power supply caps have dried out and are causing the hum. – Steve G Nov 19 '19 at 12:46
  • @SteveG I did in fact try it with another 9V supply I have, and the noise was even worse. Of course that might just mean I have two noisy power supplies. – Rob Schmitt Nov 19 '19 at 12:51
  • 9V DV? Do you mean DC? – JYelton Nov 19 '19 at 16:13
  • @JYelton Thanks, corrected. DC, of course. – Rob Schmitt Nov 19 '19 at 16:46
  • Rule out the power supply first. Do you have a regulated power supply, like a good bench supply? Next, turn out the lights in the room. The video suggests that the amp is picking up ambient electrical noise—when you touch the contacts, you are effectively an antenna that is helping inject that noise. – JYelton Nov 19 '19 at 17:43

0 Answers0