A Kentwood Subwoofer was found lying around after we retired the Sherwood Receiver. It is a passive Subwoofer, meaning it does not have a built-in amplifier.
After buying a new TV for our sitting room, we decided to add a sound bar. This sound bar made in China was called “万音 JY A9S. It claimed to have 8 speakers but 4 of them are just “dummies”. We also found that the Subwoofer output of the sound bar could not drive the passive woofer.
We have 3 options:
1. To re-refurbish the existing Sherwood Receiver;
2. To buy another Amplifier receiver;
3. To upgrade the Kenwood Subwoofer
We choose to upgrade the Subwoofer because it is the cheapest. We found this Subwoofer amplifier at TaoBao with many good reviews. The amplifier cost about CNY$230/= (SGD$44) including shipping. The delivery took around 2 weeks by sea.
The Installation
The installation took about 2 hours. Cutting the hole at the back board of the Subwoofer took sometime. The Subwoofer produced fantastic bass. It is worth the money spent.
The Problems
Trouble-Shooting
1. The humming would stop under these circumstance
a) The music resumes playing;
b) The input cable is unplugged;
c) The sound bar is switched off
2. The multi-meter showed some ohmic readings when it is connected to the coaxial cable. There was no readings when the music stopped playing. It was as if the sound bar had some sort of mechanical switch "isolating and cutting off" the cable;
3. The humming noise did not go away when the inputs are short-circuited to earth are the Subwoofer;
4. There was no humming sound when the Subwoofer was connected directly to the phone using a 3.5mm to RCA cable
Other Observations
1. The power supply of the sound bar is a 24 volts switching power without any earthing terminal;
2. The sound bar delivers beautiful sound without humming when connected alone.
What could be wrong?
Floating ground or earthing. There will always be a small voltage between the switching power supply terminal and the earth of the Subwoofer amplifier as shown below. This small voltage will cause humming noise. The humming noise was suppressed when the sound bar was playing music.
The Solution
1. Open up the sound bar and solder a 25 V 100 uf capacitor in series with a low resistor around 20 ohms as shown
Precautionary Safety Measures
Always made sure that the Subwoofer amplifer has proper earthing. Also make sure the earth is connected to the main power supply earth.
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