Hi!
About four years ago (yes, four!) one fine day I decided to build an amplifier from ready-made modules based on the LM3886 in a dual-mono configuration. I should say right away that I hadn’t done any DIY since school days, and about 25 years had passed. For several months I was gathering parts, mostly from our Far Eastern friends, and finally the time came to put it all together. Unfortunately, due to lack of time, I couldn’t finish it straight away. I kept putting the project aside and coming back to it every few months.
I thought I’d manage the wiring without any problems, but sadly, after many attempts and hours spent, I have to admit defeat. Strangely enough, the amplifier works. I really like the cleanliness and clarity of the sound. The problem, however, is mains hum, which—as I’ve heard—is a typical issue with this type of design. I should point out that it’s not huge, but it is audible. I test it with the volume potentiometer fully open and with a minimal input signal.
Unfortunately, I don’t have the schematic for these boards, but given how popular this chip is, it shouldn’t be a mystery for an experienced audio hobbyist or service technician. I don’t really know where to start. Maybe with the fact that I talked a bit with ChatGPT, which suggested creating a single ground point connected to the chassis—the 0 V point of the symmetrical power supply (28 and 18)—via a resistor and a capacitor. Unfortunately, that didn’t help.
Interestingly, when I disconnect one of the RCA inputs (left or right, it doesn’t matter), the hum disappears completely. The conclusion is that the input ground loop is closed through the mini-jack terminating the cable. I should add that the RCA sockets, just like the speaker terminals, are isolated from the chassis. I also shorted the RCA inputs in both channels for testing and there was no hum, which confirms that it’s probably not a power supply issue.
Next, I added 1 kΩ and 22 kΩ resistors to ground at the RCA inputs. Partial success—the hum dropped by about 50%, but it’s still there. By accident, I swapped the negative speaker terminals and suddenly the hum stopped—bingo? Not quite. I then lost the ability to control the L-R balance and “stereo,” so this is not the right solution.
I’m writing this post in an act of desperation, because I’d like to close this topic one way or another, i.e.:
– either asking for help from kind people in the community,
– or commissioning someone to finish this project (I’d appreciate contact details),
– or selling it to someone who will take care of it.
Thank you in advance for all comments.
Best regards,
Jerry
About four years ago (yes, four!) one fine day I decided to build an amplifier from ready-made modules based on the LM3886 in a dual-mono configuration. I should say right away that I hadn’t done any DIY since school days, and about 25 years had passed. For several months I was gathering parts, mostly from our Far Eastern friends, and finally the time came to put it all together. Unfortunately, due to lack of time, I couldn’t finish it straight away. I kept putting the project aside and coming back to it every few months.
I thought I’d manage the wiring without any problems, but sadly, after many attempts and hours spent, I have to admit defeat. Strangely enough, the amplifier works. I really like the cleanliness and clarity of the sound. The problem, however, is mains hum, which—as I’ve heard—is a typical issue with this type of design. I should point out that it’s not huge, but it is audible. I test it with the volume potentiometer fully open and with a minimal input signal.
Unfortunately, I don’t have the schematic for these boards, but given how popular this chip is, it shouldn’t be a mystery for an experienced audio hobbyist or service technician. I don’t really know where to start. Maybe with the fact that I talked a bit with ChatGPT, which suggested creating a single ground point connected to the chassis—the 0 V point of the symmetrical power supply (28 and 18)—via a resistor and a capacitor. Unfortunately, that didn’t help.
Interestingly, when I disconnect one of the RCA inputs (left or right, it doesn’t matter), the hum disappears completely. The conclusion is that the input ground loop is closed through the mini-jack terminating the cable. I should add that the RCA sockets, just like the speaker terminals, are isolated from the chassis. I also shorted the RCA inputs in both channels for testing and there was no hum, which confirms that it’s probably not a power supply issue.
Next, I added 1 kΩ and 22 kΩ resistors to ground at the RCA inputs. Partial success—the hum dropped by about 50%, but it’s still there. By accident, I swapped the negative speaker terminals and suddenly the hum stopped—bingo? Not quite. I then lost the ability to control the L-R balance and “stereo,” so this is not the right solution.
I’m writing this post in an act of desperation, because I’d like to close this topic one way or another, i.e.:
– either asking for help from kind people in the community,
– or commissioning someone to finish this project (I’d appreciate contact details),
– or selling it to someone who will take care of it.
Thank you in advance for all comments.
Best regards,
Jerry
