mandag den 18. maj 2020

Noisy PC Speakers - and a Solution

My PC-speakers are pretty standard: A pair of Creative A50 powered with 5 V DC from an USB-port.

I noticed noise coming from the speakers, when no audio signal was present. Changing USB-port did not help. I seemed like my USB ports were noise polluted! When I connected a power-bank (lithium-ion battery) to the speakers, the noise disappeared, but during pauses in the audio, the power-bank shut itself down!

I decided to build a noisefree 5 V DC PSU using a transformer with iron core and a 7805 which has 68 dB ripple rejection and low output noise. The 7805 is thermal overload protected and short-circuit protected. I had all parts laying around, so the building cost was nil.

Figur 1. The noisefree PSU.

Figure 2. Noisefree PSU with open lid.

Figure 3. AC Adaptor. 
Figure 3 shows the transformer which is an old AC adapter with iron core. It is heavier than modern AC adapters! The advantage: no switching technology is involved!

Figure 4. Circuit diagram.
The rectifier bridge is an integrated piece, but four discrete 1 A diodes can be used as well. I found C1, C2, and 7805 in the drawer. The circuit board has holes and solder islands, but no tracks. Figure 5 shows how the component wires are connected on the bottom side. The alu-cabinet is Hammond 1590A.

Figure 5. The circuit board seen from the bottom.
The PC speaker's power consumption is low. The noisefree PSU does not get warm at all.

Now I can enjoy music and speech without background noise - and a good feeling of having improved something!

73 from OZ1BXM
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