Speaker Input Selector Project ???

[sean0118]

Hi everyone,

Just saved a 50inch plasma tv that someone was throwing out. A bit like the one Dave teared down (but this one works perfectly). 

But it does not have built in speakers, but can be connected to speakers through the standard red and black speaker wires. So what I want to do is create a box that has the TV and my audio amplifier as inputs and my (two) stereo speakers as outputs. So I can then switch between the two inputs (see dodgy diagram).

Ive seen a similar project but they used relays, I would rather just make it passive with switches if possible? Audio really isn't my area, I'm a bit lost about what type and rating of switch would be best for this application. Also, is it ok for me to only switch the positive wires and just connect the negative wires inside the box? Also, are any capacitors required to reduce noise?

Hope it's sort of clear what I'm trying to do? 

[flynwill]

You need to be cautious about this as sometimes the "-" leg on speaker outputs is not actually connected to the system ground, but instead the amplifier is "bridged", meaning that both the - and + speaker outputs are driven. So the safest thing will be to switch both lines to the speakers.  So you need a 4PDT switch, which wouldn't be that hard to find.

The other option is that the TV will almost certainly have a line-level output that could be hooked to a spare input on your amplifier and let the amplifier always drive the speakers.

[sean0118]

You need to be cautious about this as sometimes the "-" leg on speaker outputs is not actually connected to the system ground, but instead the amplifier is "bridged", meaning that both the - and + speaker outputs are driven. So the safest thing will be to switch both lines to the speakers.  So you need a 4PDT switch, which wouldn't be that hard to find.

Ok yeah that makes sense, don't want to let that magic smoke out. The 4PDT switches do look good, I'll add one into my next element14 order. With calculating a ball park figure on the switches current rating the speakers are 50W, 8ohm, so maybe >>2.5A? That seems small...   

The other option is that the TV will almost certainly have a line-level output that could be hooked to a spare input on your amplifier and let the amplifier always drive the speakers.

Yeah this is what I was originally thinking, but it's probably less of a hassle to switch the speaker inputs when needed as the amp isn't used a real lot. Plus the amp itself is a pretty old Akai from the 70's or 80's and has no free inputs on the back. Would probably make a good teardown though. 


Edit: While we're talking about the amp. I noticed it doesn't have an earth connection even though it has an aluminum faceplate. Can't see anything about double insulation either. I might have to look into that further.
Edit2: Yeah... just had a close look at it, it's got an all steel frame, definitely not double insulated, I'll be sure to make another thread about it and post some pics before it's used again. 

[poot36]

If the amp has a transformer in it it will already be isolated from the mains (there may be some supression caps between both live an nutral and the case).  This is to prevent a ground loop between different audio devices that causes hum.  My 1970s Kenwood amp has an unpolerised plug on it and no ground that I remember unless there was one for the phono input.  I haven't been zaped by it yet.

[sean0118]

That's a good point, but it's getting to the age where I'm worried about the integrity of the transformer itself and where the mains connects to the primary. Would it actually cause any issues though if I did earth the frame? I think I'll have to tear it down anyway to replace some suspicious looking capacitors.

[McBryce]

Does the TV have Line-Out Audio (Red and White Cinche connectors)? If so you could wire the TV directly into the Aux-In on your amp and let the amp do the switching.

McBryce.

[SL4P]

switch should be 'break before make' to avoid double loading the outputs

[sean0118]

switch should be 'break before make' to avoid double loading the outputs

Its funny I was just reading about this yesterday in the new Art of Electronics, I actually never knew make before break switches existed. I'll double check I get the right ones now. 

Does the TV have Line-Out Audio (Red and White Cinche connectors)? If so you could wire the TV directly into the Aux-In on your amp and let the amp do the switching.
McBryce.

It might but the amp doesn't have any free inputs. It does have a spare phono input but its bridged out with a piece of metal,  I'll try and workout whats up with that.

[flynwill]

It might but the amp doesn't have any free inputs. It does have a spare phono input but its bridged out with a piece of metal,  I'll try and workout whats up with that.

The Phono input won't do you any good.  It's designed for a the much lower signal level and will also have a de-emphisis filter http://en.wikipedia.org/wiki/RIAA_equalization

It was common for receivers to be shipped with shorting plugs on those inputs, to limit noise they might inject into the rest of the receiver if they are not in use.

[Muttley Snickers]

As SL4P pointed out, the make or break is important so that the two source's don't meet, this can be done with a basic passive  4 way speaker selector switch in reverse providing that it does switch both + & - and has no common connection, there is a good chance that the TV is looking for 4 ohm's and the Amp is looking for 8, your speakers will be 8 ohm's.

1. TV
2. Spare
3. Amp
4. Spare

Just make sure when changing source tthat you press 1 then 2 then 3 etc.


Muttley

[sean0118]

...basic passive  4 way speaker selector switch...

Sorry I'm not too sure which switch you mean, is it the commercial ones you use to connect two pairs of speakers to the one amp? Say if you have indoor and outdoor speakers?


I was thinking of just something like this mounted on a box:
http://au.element14.com/multicomp/mcr13-44e-06/switch-toggle-4pdt-on-off-on-sc3/dp/2128064

That type of switch would work ok right?

[Muttley Snickers]

I'm on a TV so I cant give you links, google Radio Parts Group, Altronics or Jaycar, speaker selector, no soldering it's all ready to hook up as is, even a 3 way will do the job, rough as guts but it will do the job, also running the TV audio out's though the Amp means having the Amp turned on just to watch TV, not always practical, fine for movie's or concert's.

Muttley

[McBryce]

It might but the amp doesn't have any free inputs. It does have a spare phono input but its bridged out with a piece of metal,  I'll try and workout whats up with that.

You could make (or buy) a cinche switch to select which source is connected to your amp Aux-In. This won't need any make before break relay, just a simple switch. Phono input won't work, that's an RIAA input.

McBryce.

[sean0118]

Yeah one of these run in reverse right?

http://www.radioparts.com.au/product/39894110/sps4-4-way-speaker-selector-speaker-switch-box#.VTZXf5MWn5s

You could make (or buy) a cinche switch to select which source is connected to your amp Aux-In. This won't need any make before break relay, just a simple switch. Phono input won't work, that's an RIAA input.

McBryce.

But then I have to turn the amp on AND switch the Aux input. That's twice as much work as just switching the speaker inputs. 

[McBryce]

Yes, but you'll probably get a much better sound from the amps output than from the tiny TV amp.

McBryce.

Btw: have you checked that the amps speakers are the right resistance for the TVs speaker outputs?

[sean0118]

That's true, but the amp built into the TV is still pretty good for general use. I'm not sure how small it would be, the TV does weigh close to 40kg. Anyway, the speakers do fall within the required impedance range.

Looks like there is quite a few ways to solve this problem, I'm not placing the next element14 order for awhile so I'll think it over for a bit.

[macboy]

Personally I would use a pair of DPDT relays with 12 V coils.
Then I would open up the receiver/amp and find a 12 V supply (such as the supply to the opamps) and steal power from there to turn on the relays. Turn on the amp ... and automagically, the speakers switch to 'amp' mode. Turn off the amp ... back to 'TV' mode. So simple.