I have this portable "hybrid tube" headphone amplifier. The Little Bear B4-X. I adore it, it sounds just to my liking. It runs on battery power, and I generally use it by plugging it into my phone, ipad or laptop. However, it has a flaw. If it is connected to, let's say, my laptop, and then I plug my laptop in to charge, the headphone amplifier produces a fairly loud 60hz hum through the headphones. When I touch the metal chassis of the laptop, or the metal parts of the 3.5mm audio signal cable leading to the amplifier, or the metal body of the magsafe charging cable, the hum immediately vanishes. Therefore, it is only possible to use the amplifier with a source that is ALSO running on battery power.
I'm not knowledgeable enough to know why this might be, or how I could attempt to fix it. Perhaps there is some modification I can make to the device which will solve this problem?
I have tried reinforcing the chassis ground on the amplifier itself, even adding a second lead from the negative leg of the battery straight to the grounding hole on the PCB. Nothing seems to make any difference. I have some experience building audio electronics from kits. I know my way around a soldering iron. So if I knew WHAT to do, I could certainly do it. I found this sub after a search, and thought maybe someone here could help me.
I welcome any input or suggestions. I've attached a picture of the PCB here, in case it helps. Thanks in advance.
I'm not knowledgeable enough to know why this might be, or how I could attempt to fix it. Perhaps there is some modification I can make to the device which will solve this problem?
I have tried reinforcing the chassis ground on the amplifier itself, even adding a second lead from the negative leg of the battery straight to the grounding hole on the PCB. Nothing seems to make any difference. I have some experience building audio electronics from kits. I know my way around a soldering iron. So if I knew WHAT to do, I could certainly do it. I found this sub after a search, and thought maybe someone here could help me.
I welcome any input or suggestions. I've attached a picture of the PCB here, in case it helps. Thanks in advance.
Some additional findings: The issue isn't just between the B4X and the laptop. It's between the B4X and ANY device that is plugged into power while connected to the B4X. In the example of the laptop, I can confirm that there is no hiss or hum when the headphones are connected directly to the laptop. The issue is also present when using an external dac with the laptop, or any other device.
Touching the case of the B4X to the metal chassis of the laptop (or other device) does not affect the hum. But strangely, when I touch the b4x or the laptop with my hand, the hum seems to vanish. The hum increases in volume when my hand approaches the amp, and vanishes when contact is made. And again, this is only when the source is connected to power. If the source and the amp are both running on battery only, the hum is not present.
Touching the case of the B4X to the metal chassis of the laptop (or other device) does not affect the hum. But strangely, when I touch the b4x or the laptop with my hand, the hum seems to vanish. The hum increases in volume when my hand approaches the amp, and vanishes when contact is made. And again, this is only when the source is connected to power. If the source and the amp are both running on battery only, the hum is not present.