Japanese RGB amp circuit-help to make it simpler/easier to make

Discussion in 'The Projects Forum' started by bigsanta11, Feb 10, 2008.

  1. bigsanta11

    Thread Starter Member

    Feb 6, 2008
    14
    0
    Hello all,i'm a beginner to all this,but i have learnt a few things thanks to this and other sites guides/lessons,and made a component tester,with an added push switch,( and very proud i am , of both my circuit and the soldering job )

    I am now ( for the second time) going to make an rgb amp, for a games console,and the circuit diagram as you can see below,is from a japanese made diagram.

    What i really need help with,is can it (the circuit diagram(ignore the r & l audio,as it's not needed))be made more easier to make/follow?

    Maybe turned into a ,is it called a block diagram,with the components drawn on,aswell as a seperate circuit diagram.

    And for now and future projects, i'm using a breadboard first,to test my projects,as my first attemt at making the amp destroyed the previous consoles av ,and the machine is now scrap(that was before i found the "all about circuits site")!:(

    thanks all
    fraser
    UK

    [​IMG]
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Your post is hard to interpret. Please consider writing it in a way that doesn't require high level deciphering.

    Steve

    What I understood...

    - You made a tester with an extra switch, and was proud of your soldering and the circuit design

    - You want to make a "rgb" for a games console from a foreign schematic

    - Can the circuit be simplified and/or be made clearer?

    - You like breadboarding
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi Fraser,
    Welcome to the forums. :) Sorry about your smoke-letting session. :( Sometimes, lessons can be painful, but they can strengthen our resolve to not allow such disasters to reoccur.

    The 2SC1815 transistors may not be readily available by you, but BC182 or BC182L transistors seem to be a reasonably close match, and would likely be easier to find in the UK.

    The 75 Ohm resistors in the emitter circuit are essential; that is what establishes the impedance of the output. If those were changed to some other resistance, you would have terrible video signal distortion. Video cables and video circuitry all have a 75 Ohm impedance.

    The 10 and 300 Ohm resistors on the collectors are there to limit collector current, and adjust output signal level. They also must remain, otherwise the transistors would be operating too closely to their maximum design limits.

    The pair of 10k resistors on the inputs are necessary to establish the bias voltage on the base of the transistors. Were either of those omitted, the circuit would not function properly.

    The input and output capacitors are necessary to pass the effects of the AC signal, but to block DC voltages. Without them, external DC voltages would interfere with the operation of the amplifiers. The input capacitors are much smaller than the output capacitors, because the output signal will be much larger than the input signal.

    I don't see any components that are "extra" in these circuits, or that could be changed or removed without seriously affecting it's operation.

    But basically, you have four repetitions of the exact same circuit, save the one 300 Ohm resistor in the sync amp rather than a 10 Ohm.

    So, you could simplify it to four blocks, the schematic shown for just one instance, with a notation of the different resistor value between RGB or Sync channels.
     
  4. bigsanta11

    Thread Starter Member

    Feb 6, 2008
    14
    0
    Thanks for the warm and friendly welcome scubasteve_911, i'm guessing the "RGB amp" in the topic title and lack of bullet points, left you a bit confused,sorry!

    Rather fitting user name by the way,both of them conjour up thoughts of sad events that i've witnessed,and hope never to see again!

    Cest la vie

    fraser

    .....................................................................

    SgtWookie,your advice was greatfully appreciated!

    fraser
     
  5. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I really wasn't that harsh, take it easy. I really did not know what a RGB amplifier was and thought you were unclear and was encouraging clean communication.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Am I retarded, or whut? I click on the imageshack link and get a schematic that's about 1 inch X 1.5 inches. What's up with that?
     
  7. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Now after reading it, I didn't mean to be so harsh, I'm sorry. I've been routing a 900+ component board all day and it has been a very frustrating experience, then I think your slightly enigmatic question gave me a meltdown.

    Steve
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I see an ImageShack schematic that is pastel blue on a blue background. Surrounded by Imageshack ads. Why wasn't the schematic posted here as a clear black and white???
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    There are no ads on what I see. Just this:
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Audioguru - you need Spybot ver 1.5 to keep the ads off. The print is probably a scan on a blueprint that got run through the Ozalid machine too fast, so the blue background is due to underexposure.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Do you see a readable schematic?
    I'm using Firefox, but I tried it on IE and got the same result.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Hmm - when I single-left click on the image in the first post, I wind up at Imageshack with a large version of the same image. I have to scroll down on that page to see links to post, etc - but no advertisements.

    I think it's small on here because the OP may not have omitted the URL/URL brackets from around ImageShack's given how-to links.
    Let's see what happens when posting without the URL/URL stuff:
    --------------------------------------------------------------------------
    [​IMG]
    --------------------------------------------------------------------------
    Hmm - winds up a tiny photo.
    Going to try something else...
    --------------------------------------------------------------------------
    [​IMG]
    --------------------------------------------------------------------------
    Ahh, ImageShack puts a thumbnail up as bait to link to their site. Actually, that's very helpful for reducing bandwidth consumption for folks on dial-up; they can choose whether or not to see a larger image.

    I don't see the ads on ImageShack's site because they've all been blocked by a combination of:
    Spyware Search & Destroy's "Immunize" feature
    and
    Javacool's Spywareblaster
    which also blocks spyware/potentially harmful sites; but a different set than SS&D's.
     
  13. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    I got this, it seems the be the right size to me

    Steve

    Edit: Wookie beat me to it :(
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    It establishes the impedance, but not at 75 ohms. Assuming 1V DC on the emitter (could vary a lot, because Beta has a lot of slop), the emitter current will be about 13mA, this makes the output resistance about 2 ohms, plus the sum of the source resistance and the intrinsic base resistance divided by beta (approximately). So the output resistance might be on the order of 3 to 6 ohms. (Actually, it will vary with signal level, but will still be low). Not exactly a good match to coax. Also, there is no 75 ohm termination on the input of the emitter follower.
    Ideally, there would be 75 ohms to GND on each input, and 75 ohms in series with each output. Unfortunately, this means you need a gain-of-two buffer, instead of unity gain, as we have here. The good news is, if the source terminations are 75 ohms resistive, reflections from the input of this buffer will be absorbed by the source. If the loads on the receiving end are also 75 ohms resistive, they will not reflect back to the outputs of this buffer.
    There are lots of triple RGB gain-of-2 buffers available, and quad switched-input RGB gain-of-2 buffers, but I couldn't find a quad RGB gain-of-2 buffer without the switches, but they may be available.
     
  15. deadalus

    New Member

    Feb 28, 2008
    2
    0
    I have been thinking of building this circuit as well. How would using a tripple video buffer such as the LMH6739 compare to building this RGB amp...it would be easier for me to build this as I still have a few left over from when I made a YPbPr (component video) to RGBs (analog, composite sync) transcoder using the LMH1251. This also included the LMH6739 tripple video buffer. Any thoughts on how this would turn out without amping the sync?
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Why do you need to amplify RGB?
     
  17. deadalus

    New Member

    Feb 28, 2008
    2
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    Ron H,

    Purely for self interest, and something to play with, I opened up a Playstation one (PSOne) LCD and tapped into the RGB input to feed my older game consoles like the Sega Genesis, Saturn, and Dreamcast etc, and any YPbPr source such as from a DVD player being transcoded to analog RGBs. Anyway, every game system or component video source I feed the PSone LCD looks great...sharp and strong. However, the Dreamcast is horrible. It is so dark I can barely see anything, even with the LCD's contrast maxed out. The picture I do see is stable, just very dark. This is also reflected in the TV hookup. I have all the sega systems rgb output being transcoded to component video (YPbPr) for the TV...all are great, even the Dreamcast, but the dreamcast is darker and must be adjusted in the video menu on the TV. But, there is not enough "tweaking" options on the PSone lcd.

    All of Sega's systems seem to have different RGB output levels...the Genesis (model2) is really strong, saturn is medium, and the dreamcast appears to be low...to put it a non-descript way...
     
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