Connecting many audio output devices to one input device

[Hitec]

I have active speakers (Genelec 8020) in my living room. They are now connected directly to a USB DAC but I would like to have also at least TV audio ouput to the speakers.

So I'm designing summing preamplifier without any volume control. The problem is that I don't know what kind of inputs should I use. TV and DAC outputs are one-sided RCAs and Genelecs accept balanced input.

Is there any benefits of using balanced inputs with non-balanced output devices? I think at least the signal paths should be high-impedance to cut any ground loops?

I was thinking something like this:



So it is an instrumentation amplifier with a voltage divider that makes the input lines equal in impedance. This would feed the summing stage.

I have seen something like this:



I just don't understand why there is the ground connection after the 10kohm resistors. In my circuit I have 1Mohm resistors to ground to prevent the signal lines from climbing too high in case they are left floating.

I think I'm referring to the case in bottom right in this figure:



So which circuit is better or should I use even simpler connection as I only have one-sided RCA outputs in my sources?

[Hitec]

Just to clarify the whole question: what is the best input circuit if ground or any other noise type is wanted to minimize? Considering that many devices connect to this node, I think it is important to cut the ground loops.

[chrispenycate]

If you have only unbalanced sources, the only advantage to balancing the inputs is elimination of ground loops, which shouldn't be there anyway (but might. Several modern TVs use grounded cables, for safety.

On the other hand, it might be worth balancing your outputs, particularly if you're intending long(ish) cable runs to your speakers, and/or different power strips feeding them. Proper servo balance (complicated) or just a polarity inverter after the mix stage. Of course, you then have to install balanced cabling.

That balanced input is far too sophisticated for what you're planning, anyway. And the 10k to ground is to get the same voltage to the positive and negative inputs; if it weren't there, the -ve would have unity gain, and the +ve times two. The input buffers (which you could ignore in a use like this) are to make sure current rejection is balanced, as well as voltage (no, that's not very clear, is it? Well, just assume that for applications like this the simple, one IC circuit is adequate.

And, without volume controls? Do you have individual controls on the outputs of all your devices? Otherwise, since the Genelecs' pots are on the speakers themselves, you'll be chasing round the room every time you want to change source.

Always add a few extra inputs when designing a system like this. Since everything but the line drive amp can be passive, it costs a couple of RCA connectors and drilling holes. Oh, resistors and time soldering, of course. And you never know when you want to put in something you hadn't planned on, like an iPod. (Says the live engineer).

[Hitec]

Originally Posted by chrispenycate

And, without volume controls? Do you have individual controls on the outputs of all your devices?

Thank you for your answer. Yes I have remote control to TV and a wireless keyboard for the PC. I'm trying to make this as simple as possible.

[indianajo]

I'm using a Peavey mixer to do this. I'm taking all outputs from RCA analog plug and ultimately converting each 1/4 phone plug input to the mixer. As inputs I have HDTV converter, DTV converter (different channel when I am recording something) DVD player, DVD recorder, VHS tape machine. Output is a stereo power amp to speakers. I'm doubling the RCA jacks for each input so I can also send the sound output to the TV direct through it's 6 inputs. TV sound is okay for talking but not high fidelity for music. The input devices all have different levels, which leads to much holding down the buttons on the remote when I switch from one to another using the TV as the sound system. The mixer evens out the volume. The mixer also has XLR differential inputs if I have a hum problem, but I don't. Mixer is under the coffee table under the TV, about 2' from my feet.

[Hitec]

Ok maybe I try first simple mixer circuit and then think about something more complicated if there are problems. From my previous boombox project I noticed the importance of eliminating ground loops

[chrispenycate]

If it's unbalanced inputs, a resistor from each RCA jack to a virtual earth mix amp/line driver. You could build it in a matchbox with an external power supply (well, a screened matchbox, and everything in duplicate for the stereo, but passive mixing is easy) in half an hour; and half that time would be finding the right sized drills

With balanced inputs it gets quite a bit more complicated, and ground links (DIL switches? soldered wires? You're not going to want to change the setup very often once it's installed) add potentially balanced outputs (the Genelecs do have grounded power supplies, and are generally plugged in some distance from the rest; one of the reasons they have balanced inputs).

If you don't need the volume presets, it's a very easy little project.

[Pano]

Do you want a mixer, or just a switcher? I.E., will you listen to one source at a time or mix them together?

[Hitec]

It will be a mixer. I don't know how good idea it is but I will try out.

[indianajo]

I only have one source at a time on of the list above except when I am recording one program and watching another. I use the HDTV tuner with superior sound for the DTV recorder source, so usually I just leave the power amp off and listen to the TV sound on the regular DTV converter when I do this. If the second program was a music program, I could turn the slider down on the mixer for the HDTV converter. Not nearly the nuisance of building a remote control audio selector switch. All the various sorts of inputs are turned on or off by remote device.