Hello all,
I've been working on an audio amp for a while now based on the TDA7294 amp module; it has three inputs, a mixer, variable low-pass subwoofer output w/ volume control, and a headphone amp with independent volume control. The schematic, as well as images of the completed boards are attached.
The amp and mixing are all working as expected, except that I can't get a persistent hum to go away. I've read many a thread on this seemingly persistent problem, have tried many of the solutions proposed. The hum at this point is small and is not a problem while music is playing, but is sufficiently audible when no sound is coming from the amp that it would be an annoyance when silence is expected.
To isolate the issue, I have bypassed all components of the circuit and tie my source directly into the amp circuit (see label "direct input to amp" on the images). This is normally where the signal from the master volume control would enter the final amp circuit. I have a commercial amp-on-a-pcb that I purchased which is similar to the amp circuit I made, and when I connect my amp's power supply and use similar signal input and output connections to the commercial amp-on-a-pcb, the hum is negligible. So I know the problem is somewhere in my amp circuit and is not coming from the power supply or my signal source. I should also note that when I had this very circuit bread-boarded, the hum was sufficiently negligible.
Assuming that the hum is a grounding issue, I have made ground connections to the amp circuit in a much of a star pattern as is practical. All ground connections come together on a board "ground bus" that consists of a 12 AWG bare solid wire heavily soldered to the board. Each amp circuit, the snubber caps, input + and input - ground connections all have a direct path to the ground bus.
So I am out of ideas now and looking for suggestions. Any thoughts on what I can do to reduce the hum is appreciated. As a reminder, I have bypassed all elements of the circuit except for the amp, so (for now), please ignore the mixing circuit, headphone and subwoofer elements.
Many thanks!
I've been working on an audio amp for a while now based on the TDA7294 amp module; it has three inputs, a mixer, variable low-pass subwoofer output w/ volume control, and a headphone amp with independent volume control. The schematic, as well as images of the completed boards are attached.
The amp and mixing are all working as expected, except that I can't get a persistent hum to go away. I've read many a thread on this seemingly persistent problem, have tried many of the solutions proposed. The hum at this point is small and is not a problem while music is playing, but is sufficiently audible when no sound is coming from the amp that it would be an annoyance when silence is expected.
To isolate the issue, I have bypassed all components of the circuit and tie my source directly into the amp circuit (see label "direct input to amp" on the images). This is normally where the signal from the master volume control would enter the final amp circuit. I have a commercial amp-on-a-pcb that I purchased which is similar to the amp circuit I made, and when I connect my amp's power supply and use similar signal input and output connections to the commercial amp-on-a-pcb, the hum is negligible. So I know the problem is somewhere in my amp circuit and is not coming from the power supply or my signal source. I should also note that when I had this very circuit bread-boarded, the hum was sufficiently negligible.
Assuming that the hum is a grounding issue, I have made ground connections to the amp circuit in a much of a star pattern as is practical. All ground connections come together on a board "ground bus" that consists of a 12 AWG bare solid wire heavily soldered to the board. Each amp circuit, the snubber caps, input + and input - ground connections all have a direct path to the ground bus.
So I am out of ideas now and looking for suggestions. Any thoughts on what I can do to reduce the hum is appreciated. As a reminder, I have bypassed all elements of the circuit except for the amp, so (for now), please ignore the mixing circuit, headphone and subwoofer elements.
Many thanks!