I'm pretty new to DIY audio, so my apologies if these questions are very basic. I've built a headphone amp and equalizer that each use 5V USB power to supply them (as a matter of convenience, I have a ton of USB charger bricks laying around) with a little SMPS board in each that gives me a +/- 15V dual output to drive some op-amps. I added a linear regulator to each rail along with some other filtering to give a cleaner 12V to each. Of course, USB power only allows positive and negative terminals, no grounding. So my circuit is floating - the SMPS has its own ground pin which I've tied my PCB ground to, but no earth connection.
The trouble is that I'm getting a fair bit of hum on the RCA inputs when they aren't connected. When I connect them to my DAC (or to a stereo jack directly from the source), the hum gets much quieter, but is still there to some degree. If I hold the output TRS jack between my fingers (good ol' human ground) the hum is inaudible. This is the only way I've managed to successfully eliminate the hum thus far.
I was hoping to get some ideas on best grounding practices for this particular situation (USB power with no earthing) to hopefully eliminate this noise. Currently the input/output grounds are tied directly to a ground pour in my board, and the pour is connected to the ground pin of the SMPS. As I mentioned, I'm a beginner to DIY - this is my first headphone amp and second build. I've been reading a lot on this and similar topics in the past few months, and I'd really like to learn more here so my future projects (not just this one amp and equalizer) turn out better.
Thank you in advance!
The trouble is that I'm getting a fair bit of hum on the RCA inputs when they aren't connected. When I connect them to my DAC (or to a stereo jack directly from the source), the hum gets much quieter, but is still there to some degree. If I hold the output TRS jack between my fingers (good ol' human ground) the hum is inaudible. This is the only way I've managed to successfully eliminate the hum thus far.
I was hoping to get some ideas on best grounding practices for this particular situation (USB power with no earthing) to hopefully eliminate this noise. Currently the input/output grounds are tied directly to a ground pour in my board, and the pour is connected to the ground pin of the SMPS. As I mentioned, I'm a beginner to DIY - this is my first headphone amp and second build. I've been reading a lot on this and similar topics in the past few months, and I'd really like to learn more here so my future projects (not just this one amp and equalizer) turn out better.
Thank you in advance!
Let me see if I can help you...
To eliminate hum you need a good grounding scheme, and then good power supplies to start.
So you have a headphone amp, and an equalizer that need 5V, I got that. So powering them with USB power works but the problem is that they're switching supplies and low quality at that so hum is all but guaranteed in most cases. To fix that part you'll need a 5V linear supply and a transformer that'll feed them. That should eliminate most of the issues.
My next set of questions are:
1. What are you powering with the +/- 15V supply? The statement above applies to this as well. I can send you a 15V split rail linear supply and transformer if this helps you?
2. What are you using to send music to the headphone amp? For example, is it a computer, phone, CD player, or streaming device?
3. Are you comfortable with building power supplies that use AC line voltage? If not, battery supplies are the way to go. They're definitely possible with the voltages you mention.
To eliminate hum you need a good grounding scheme, and then good power supplies to start.
So you have a headphone amp, and an equalizer that need 5V, I got that. So powering them with USB power works but the problem is that they're switching supplies and low quality at that so hum is all but guaranteed in most cases. To fix that part you'll need a 5V linear supply and a transformer that'll feed them. That should eliminate most of the issues.
My next set of questions are:
1. What are you powering with the +/- 15V supply? The statement above applies to this as well. I can send you a 15V split rail linear supply and transformer if this helps you?
2. What are you using to send music to the headphone amp? For example, is it a computer, phone, CD player, or streaming device?
3. Are you comfortable with building power supplies that use AC line voltage? If not, battery supplies are the way to go. They're definitely possible with the voltages you mention.
Thanks for the reply!
For the equalizer, the supply is powering opamps in multiple feedback filters. In the headphone amp it supplies a single opamp and a set of four transistors (I’m using the Elliot Sound Products amp design with a passive crossfeed before it). I mostly used cheaper SMPS because these were more for my own learning and experience than anything, and I figured I’d start with some cheaper power supplies that wouldn’t set me back much if they didn’t turn out as well as I hoped. I do like the sound, though (aside from the hum, of course) and wouldn’t mind spending a bit more to get a more robust supply at this point.
Regarding that other option - a split rail supply with transformer - how much are those likely to set me back per device? My last project was a Tubelab SSE that had an Edcor power transformer to supply it, and I’d definitely like to stay well under that price point for these smaller devices if at all possible! I don’t mind working with mains voltage, however I also like the idea of experimenting with battery supplies. I’ve found a few circuits relevant to these but haven’t gotten around to trying any yet. I usually use a DAC connected to my laptop as the source on these two devices.
Again, thanks for the reply - much appreciated!
For the equalizer, the supply is powering opamps in multiple feedback filters. In the headphone amp it supplies a single opamp and a set of four transistors (I’m using the Elliot Sound Products amp design with a passive crossfeed before it). I mostly used cheaper SMPS because these were more for my own learning and experience than anything, and I figured I’d start with some cheaper power supplies that wouldn’t set me back much if they didn’t turn out as well as I hoped. I do like the sound, though (aside from the hum, of course) and wouldn’t mind spending a bit more to get a more robust supply at this point.
Regarding that other option - a split rail supply with transformer - how much are those likely to set me back per device? My last project was a Tubelab SSE that had an Edcor power transformer to supply it, and I’d definitely like to stay well under that price point for these smaller devices if at all possible! I don’t mind working with mains voltage, however I also like the idea of experimenting with battery supplies. I’ve found a few circuits relevant to these but haven’t gotten around to trying any yet. I usually use a DAC connected to my laptop as the source on these two devices.
Again, thanks for the reply - much appreciated!
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