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2741Re: CDP1861 resistor question

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  • sbirdasn
    Jan 1, 2005
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      --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...> wrote:
      >
      > --- In cosmacelf@yahoogroups.com, "sbirdasn" <sbirdasn@y...> wrote:
      > > --- In cosmacelf@yahoogroups.com, "erd_6502" <erd_6502@y...>
      wrote:
      > > > I'd love to see a decent amp, even if it's more expensive than a
      > > > simple transistor one....
      > >
      > > Are you willing to provide +/- 12 or 15 Volt power supplies?
      >
      > Yes... especially since I have a quantity of +5V -> -12V
      encapsulated
      > charge pumps - we used them at work for providing -12V for 1488/1489
      > sets on a board that was placed into a situation where -12V wasn't
      > available (and it was 20 years ago, so MAX232s weren't a viable
      > solution)...

      But switching supplies (charge pumps included) often inject horrid
      levels of ground noise spikes into the rest of the circuits, while
      charge pumps have lousy regulation and/or poor current delivery
      capabilities. It just adds to the problems in many cases when you are
      talking video and audio. Digital/serial communications are rather
      forgiving in this respect.

      You really need to use linear power supplies when dealing with video
      and audio applications unless you are *very* careful, and have things
      like multi-layer boards with ground/power planes, inductors and caps
      galore all over the layout. It is not at all trivial to control
      switching noise from power supplies getting into everything else.
      Trust me on this.

      > one can also tap off charge pump caps on a MAX232 to get
      > the same effect, and those aren't horribly expensive now.

      But the current delivered is rather small. Just the standby current
      for a 318 would probably load down the switch cap supplies by itself,
      and that ignores the current required to drive 1V into a 75 Ohm load.

      > > If so then sure, use a LM318 and a 2N3904 as a drive transistor
      on
      > > the 318's output. If you really want to get fancy, then changing
      to a
      > > more complicated push-pull output transistor pair will be even
      > > better. There are some LM series op-amps that are simple buffers
      that
      > > can drive 50 Ohms directly, without too much GBW. Once again, you
      > > need the +/- supplies at about 12-15 Volts.

      Fer instance, LH0002 buffer amp, obsoleted by National, but others
      still make something like it.

      >
      > OK... nice suggestions.
      >
      > > Now if you add up all of the parts including decoupling, feedback
      > > compensation (if required), etc., then that cheap transistor
      solution
      > > is looking better all the time.
      >
      > Indeed.
      >
      > > The real issue with your problems was trying to use the resistor
      > > summing node to drive a low impedance video input. The video
      level
      > > got squashed. Just a buffer transistor is all you need (in most
      > > cases) to isolate the high impedance summing node from the
      > > termination in the monitor/TV's 75 Ohms to ground.
      >
      > So I expect, but I'm not enough of a wire-head to be able to figure
      > out a solution. I understand the problem, but that's about it.
      >
      > -ethan

      Now with all of the replies with links, anyone having monitor
      problems should be able to add the buffer transistor, and get it
      working OK. If one has the skills to build an ELF, then the skills to
      add this minor change shouldn't be too tough. If still not able to,
      then some one-on-one face time with someone who has the expertise
      would do the trick.

      Tony.
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