Need help with wire routing for audio pre-amp - Page 1
Forum section: electronics / projects

author: hummusdude
author_rank: Regular Contributor
timestamp: May 05, 2021, 01:24:19 am
content: I've been working on an audio pre-amp/power amp project and I'm nearing the end but I'm wondering how to handle a long wire run for the input. I've marked the location of the output of the preamp and the input of the power amp stage in the pic.The designer of the power amp put the ground for the input on the power supply board in order to minimize potential noise getting into the signal path. The way I'm laying out the circuit solves some problems but I've created a rather long run for the left channel especially...nearly 8 inches from preamp to power amp.On the board of the power amp there is a spade connector for the high voltage (+15 Vrms max) of the channel output. It's conveniently located for attaching the input ground. If I used this ground point than I could twist the preamp wire with a ground wire for noise reduction. I'm wondering what is the potential for the high level out signal contaminating the input signal? Seems like there could be a problem with feedback.Could I use a shielded cable from the preamp to the power amp and ground the shield to the signal out connector on the power amp side and ... somewhere else on the preamp side?

author: TimNJ
author_rank: Super Contributor
timestamp: May 05, 2021, 08:07:05 pm
content: Looks like for this LM3886 PCB, they want you to use a star-ground configuration to keep the high currents isolated from low currents. Each GND lug should be connected back to one of the 8 lugs on the power supply board.I don't think 8 inches is that crazy. I would just try with two sets of tightly twisted pair from pre-amp board to power-amp board. Smaller wire with thinner insulation will allow the conductors to be closer to each other. This will give the best magnetic field immunity. If that is not sufficient, then you can move to shielded twisted-pair.You can tie the shield to the SIG_GND. You can experiment by trying two options: 1. Tie shield to GND on both sides of the cable (i.e. to pre-amp ground and to SIG_GND), or 2. just on on one end. For low-frequency audio work, often times the single side grounding technique is better, depending on the ground structure of the total system.

author: hummusdude
author_rank: Regular Contributor
timestamp: May 05, 2021, 10:35:35 pm
content: QuoteThis is very helpful, thanks! And yes, the designer was very emphatic that the reason the for the unusual ground routing is to maintain a star configuration to keep power ground and signal ground as separate as possible.Just to be clear, it sounds like you're suggesting a separate ground for each signal wire, twisted, from pre-amp to power-amp board. Would the grounds then be terminated at the "signal ground" point on the power-amp board? Or is this similar to what you're saying about how to try different combinations with the shielded cable?

author: TimNJ
author_rank: Super Contributor
timestamp: May 06, 2021, 12:57:48 am
content: I know you can often get away with this kind of star grounding scheme at audio frequency, and often times it does work well, but I do get a bit squeamish when people really go nuts with it. While it's no tube amp, circuit impedances tend to be pretty high, and extra loop inductance and stray capacitance can sometimes lead to oscillation.I'm not 100% clear on some of the details of your amp, but this is what I would suggest as a starting point:For the twisted pair to be effective, the ground conductor in the twisted pair should not (inadvertently) shorted out by some other external ground connection. See my third image. I think I would avoid grounding each SIG_GND (L & R) + pre-amp GND to the star-point at the same time. Twisted-pair is great at rejecting magnetic field pickup, but if a parallel ground connection is made inadvertently, then your "tight twisted pair" becomes more of a "loose twisted pair", with a larger effective pickup loop area.To me, seems the best solution would be to make one ground connection (to the star-point) somewhere on the pre-amp board. The SIG_GND connections on the power amp boards would be made through the twisted pair connection.For shielded twisted pair, not much difference, but you can try attaching the shield at one end or both ends and see if you can perceive a difference. In the attached second image, I just tied on one end, to the star-point on the pre-amp board. Depending on the ground loop situation, tying the shield at one end only can keep 50/60Hz noise currents off the shield, but there are some downsides too, so I'd try both approaches.Anyway, I haven't done anything with audio in quite some time now, so hopefully someone else will chime in if I'm sounding idiotic.

author: hummusdude
author_rank: Regular Contributor
timestamp: May 06, 2021, 04:05:07 am
content: A picture really is worth a thousand words! I kept thinking I had to be careful about creating a loop situation but I couldn't "see" it until you shared your drawing. Thanks for the detailed explanation. Everything you said seems consistent with what I've read about grounding and avoiding high-inductance loops.btw...in case you're wondering... the hard drive looking piece in the center is an actual hard drive, albeit somewhat cut down to size. This project is a combination of another designer's hifi power amp and my own pre-amp board. There has been a significant amount of project creep and at one point I thought it would be fun to add an HDD trigger for the microcontroller. there's a rotary encoder as well but the HDD looks way cool.

author: MIS42N
author_rank: Frequent Contributor
timestamp: May 06, 2021, 04:57:24 am
content: If you use shielded cable, the shield should ground at one end only. If it is terminated both ends, a magnetic field can induce a current in it and that is not desirable. IIRC best to attach to the power end.