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DIY component (or RGB) switchbox - here's how to

Discussion in 'Home Cinema Building DIY' started by TorAtle, Jan 10, 2003.

  1. TorAtle

    TorAtle
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    Don't know how interesting this is to you folks, but I've searched this forum and can't find any post that has covered it...

    Anyway, I've just finished building my component switchbox. By the nature of the implementation it could be either component or RGB, the switcher doesn't care. Reason for building one is that my plasma has only one input and I had some free time. The easy way would of course be just ordering one from the states, for instance the Inday or the Audio Authority.

    It should be very high quality and the cost is low (approx 30-40 pounds at a guess - I don't live in the UK).

    You need for 3 component inputs and one output -
    12 RCA or BNC input/output jacks
    9 relays (2 way, 1 ampere - ie, a small inexpensive one)
    A 220AC to 12DC transformer
    An input switch
    A box of some sort

    For simplicity I welded point-to-point, so no curcuit boards necessary.

    Every set of relays (3) need ground and +12V (or whatever the relays require). The +12V is routed via the input switch.

    Just solder signal and ground of every input phono jack to the input side of the relay. Output side goes to the output jack from all 3 relays.

    That's it really. Signal path is very short so the quality should be very good.

    I admit it is a bit time consuming but if you know how to solder the job is straight forward.

    Btw, in my case, all 9 relays are stacked very closely so the box could be very small.

    If anyone would like a picture just to visualize how it can be done just let me know.

    Tor Atle
     
  2. Flimber

    Flimber
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    You could just as well have wired your component through a scart and used a scart switcher. But good job all the same ;)

    Mike.
     
  3. TorAtle

    TorAtle
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    Let me put it this way - if a sub 20 ukp scart switcher would do the job just as well as an expensive component switcher then...well, then there wouldn't be any expensive component switchers out there :p

    Tor Atle
     
  4. Nic Rhodes

    Nic Rhodes
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    Tor Atle

    Mike has a point though. I have no doubt that you switcher will be better that a SCART switcher, simply because it is using BNCs (I hope) and not SCARTs but both solutions are only likely to be effective against simple interlaced signals. Neither unit will have the necessary bandwidth to switch say a PS signal let alone a higher resolution signal now becoming increasingly common, without degradation. A good rule of thumb for zero degradation is 5 x signal bandwidth. This isn’t easy to do at all. Commercial ‘receivers’ make a passable attempt at interlaced component switching (low bandwidth) but struggle with higher quality signals just because the haven’t got video engineers designing the switching circuits in the audio receivers. Some better processors are getting better at this now, with Tag offering a 300Mhz switcher but this is far from the norm. This is expensive to do correctly and is the reason commercial component switchers are generally very expensive. They can switch these high bandwidth signals with all the necessary bandwidth / ground planes etc. From the description of your box, I doubt this will do this, so aren’t we talking apples and oranges here?

    I do applaud your work however but you can’t compare it with an expensive component switcher surely?
     
  5. TorAtle

    TorAtle
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    The only progressive source I have is the PS2 - well I think it is anyway. I've spent the last week looking hard and swapping cables to and from and I can honestly say I see no difference if the switchbox is in the middle or not (the display is a Panasonic plasma).

    Obviously I have no way of measuring that kind of bandwidth you are talking about.

    Do you know what limits bandwidth exactly? If you use a good quality relay with minimum contact resistanse and the cables to and from the input jacks are very short...then what's next? I have a feeling the design of the curciut board is important (other than the length of signal travel), but I'm really not sure.

    FWIW, I had a chat with the distributor of Kramer gear and he told me that their switcher was built exactly the same way. I wasn't very keen on paying the 500 pounds he was asking.

    Apart from the fact that I would have to make of buy specially made cables if I would go for a SCART switcher, I am still not that thrilled about putting a 20 pound box between my cables.

    Criticism is almost always good however so feel free :)
     
  6. Mark Grant

    Mark Grant
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    Hello TorAtle,

    I have an Extron switcher (expensive when new).
    I have had a look inside, and it is just basically a box of relays, on a well designed PCB.

    So your DIY hard wired switch box should be good, if the relays are low contact resistance, cables are the same length for each relay, wire and soldering is good.

    If a metal box is used, isolated BNC's/phono connectors should be used, so that he grounds are not all commoned together.

    An RGBHV switcher could be made the same way, with 5 relays.

    What we need to find are the same relays that extron etc use in expensive switchers.

    I will get the lid of my Extron again and have a look for part numbers, and maybe take some pictures.


    Mark.
     
  7. TorAtle

    TorAtle
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    Mark,

    I used Omron G5V-2. Cheap at 2 ukp (probably less in the UK). Contact resistance is <50 mohm.

    Next up is making a switcher for the speakers. The plasma is on the long wall and the projector is on the short one, so the switcher will 'turn' the soundstage 90 degrees. I used an old CD-player box to house the component switcher relays, so there's plenty of space left.

    After that it's a switcher for SPDIF digital sound (4 sources but sadly only 2 inputs on the AVR).

    Would be interested to see what kind of relays your Extron uses.
     
  8. museumsteve

    museumsteve
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    keep up the good work TorAtle.:) I, for one, am following this with interest...
     
  9. Mark Grant

    Mark Grant
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    Here is a picture of an Omron relay (heavily compressed jpg for the web) as used in a switcher that cost a lot of money when new.

    They have used 5V relays as these will be driven by IC's.

    This is from a fairly old, approximately 1995 switcher, but they wont have changed much.

    [​IMG]



    You should be OK for your SPDIF switching, but dont forget to use isolated BNC's as used below.



    [​IMG]





    Mark.
     
  10. TorAtle

    TorAtle
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    Thanks Mark. The G6H-2 is a bit more expensive, but it's good all the same to see that they are not *that* much more costly. The price difference could be down to volume anyway.

    I use isolated phono's instead of BNC. I know the latter is better but all my cables use them, so the only place I opted for BNC's were at the plasma end.

    The Extron box looks visually very good though!
     
  11. JSW

    JSW
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    Hi all,

    I'd like to make a 6 channel audio switcher so I can change between SACD and DVD A players.

    What would I need ?

    Many thanks
    Bry
     
  12. Nic Rhodes

    Nic Rhodes
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    Tor Atle

    Had a play in the lab on Friday to see if we can find something out here to help. Your basic set up will work surprizingly well but we have found a few things that might help you / others even further in the future.

    I did a single input knock up and fed a series of high bandwidth signals into the system and looked for ‘degredations’ in the system with a digital storage scope. This is what we found.

    Decent coax is not the limiting factor here. Solder joints seem to have the most effect. The more joints from plug to wire to relay to plug, well they all take their tole. Crimped connectors work much better and no connectors better again.

    Thoughts. captive output leads and relays soldered direct to plugs? I suspect you have neat rows of relays in your box, they might not be the best arrangement!

    Hook up wire as opposed to coax also had an effect but coax was difficult to use in these short runs. Even with coax the degreadations were at the joint and screen removal and not in the cable (1GHz bandwidth stuff). It seemed to able to pass DVD type PS signals fine but started to fall off with higher bandwidth signals (better CRTs).

    If I was doing this professionally I think I would have BNCs directly connected to a multi layer board, where both the screening and joints would have less an effect. I would use relays and not silicon to switch and would have an external PSU to make it easier to design but is not essential if do it correctly.

    Hope this helps. Keep up the good work. Certainly much better than the SCART switcher discussed but I couldn’t get the bandwidth professional units claim either. For interest I had a ‘company’ owned video switcher to play with as well. It was no where near it’s claimed specification, not even close! So well done. Any piccyies of your device?

    I won’t mention BNCs!
     

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