Running power over speaker wires? - Page 1
Forum section: electronics / projects

author: dkonigs
author_rank: Regular Contributor
timestamp: December 01, 2017, 10:37:15 pm
content: My house has in-ceiling speakers installed in several rooms. Along with these, each room has a wall-box containing a volume control like this:https://www.russound.com/products/connectivity-and-control/volume-controls/indoor/alt-126r-126-watt-impedance-matching-volume-control-1. However, as I now have a bunch of Sonos Connect:AMP boxes on the other end of these speaker connections, these "legacy" volume controls are now completely unnecessary. (Its better to just leave them all cranked up, and adjust the volume on the amplifier side.)In my frustrating quest to come up with an idea for my next project, one thought is to build a "smart" in-wall control panel to replace these. I can likely figure out the digital half of this. The problem, of course, is that I'm not sure exactly how to power said control panel (and don't want to use batteries). Each box basically has two pairs of speaker wire (left and right channels) going in and out.So I've been thinking, how feasible would it be to piggyback DC power on top of that speaker wire?Two models for accomplishing this come to mind, as I search around for ideas:"Phantom power" - common in the audio world for powering microphones (no one has ever apparently described this for speakers). This approach seems to be very low current, and seems to assume a 3-pin cable where one is a common ground. It also doesn't seem to involve much in the way of "protection" circuitry, unless I'm reading something wrong (which I might be)."Power over Ethernet" - this approach seems to involve center-tapped isolation transformers on either end of the run, and control circuitry that keeps the injected power "safe". With this approach, I'd assume the left speaker wire pair might be the "+", and the right speaker wire pair might be the "-".Of course its entirely possible, or even likely, that I'm completely misunderstanding something fundamental here. There may also be other ideas I haven't considered yet.

author: David Hess
author_rank: Super Contributor
timestamp: December 02, 2017, 04:18:39 am
content: The power over ethernet method could work since you have pairs but good audio transformers are not inexpensive.Something like an RF bias-T could be made for audio use with big aluminum electrolytic coupling capacitors and big inductors to add and then remove the DC power.

author: WastelandTek
author_rank: Frequent Contributor
timestamp: December 02, 2017, 04:54:56 am
content: yeah, bias T was my thought, probably be pretty beefy components if its going to pass audio though

author: ajb
author_rank: Super Contributor
timestamp: December 02, 2017, 05:08:29 am
content: Quote from: dkonigs on December 01, 2017, 10:37:15 pmYou wouldn't actually need transformers at both ends, and they wouldn't need to be center tapped either. Since the speakers are isolated from everything else you would only need to isolate the amp outputs from the DC supply, so you could just connect the secondaries in series with the output of your DC supply (say, L+ to DC- and DC+ to R-) at the sending side and your receiving power supply would connect between L+ and R-. The only advantage to using center tapped transformers at both ends is better EMI (not really a concern for speakers, generally), and better current balancing across the available conductors (a big concern with the long runs and small gauge wire in POE, probably not much of a concern for adding a few tens of mA to a pair of speaker wires). That said, putting even a single pair of transformers on all of your speaker outputs isn't a terribly elegant solution.I would guess the amps have full bridge outputs, but if they don't, you could potentially run both speakers over three of the four wires, and supply a DC voltage on the fourth referenced to the speaker common. You'd likely wind up with a lot of speaker content on your control panel's power supply, but a digital system can probably cope with that.The simplest solution, given that you already have the amps and speakers in place and presumably don't want to change them, might be to just pull an additional wire. Depends on how hard it is to pull wire in your house and how the install was done, but since you already have wires where you need them, you might be able to use one of the existing speaker wires to pull the new wire through without having to open up any walls. That way you could even pull a Cat 5 cable and have some conductors available for a serial link as well as power.

author: SeanB
author_rank: Super Contributor
timestamp: December 02, 2017, 05:11:52 am
content: If wired in conduit ( hopefully, though there generally are a lot that have shortcuts taken) then use the existing wiring to draw a fish tape back to the amplifier area, and redraw the speaker cables back again with an extra CAT6 network cable in the conduit. that gives a power and data ability for whatever you install, and is useful for network extension as well.

author: Twoflower
author_rank: Frequent Contributor
timestamp: December 02, 2017, 09:40:38 am
content: I would be concerned about the power-on moment. Yes, your supply might be DC, but the moment you switch on the power is certainly not DC. In best case you'll hear a pop and after it everything is fine. On worst case thePOPis the last thing you'll hear from that speaker. So a soft start might be a good idea (but who knows how the circuit like this slow supply ramp). Another idea might be to add an relais after the power splitter which switches the audio connection to the speaker after a second or two.