2741Re: CDP1861 resistor question
- Jan 1, 2005--- In email@example.com, "erd_6502" <erd_6502@y...> wrote:
> --- In firstname.lastname@example.org, "sbirdasn" <sbirdasn@y...> wrote:
> > --- In email@example.com, "erd_6502" <erd_6502@y...>
> > > I'd love to see a decent amp, even if it's more expensive than aencapsulated
> > > simple transistor one....
> > Are you willing to provide +/- 12 or 15 Volt power supplies?
> Yes... especially since I have a quantity of +5V -> -12V
> charge pumps - we used them at work for providing -12V for 1488/1489But switching supplies (charge pumps included) often inject horrid
> 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
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 getBut the current delivered is rather small. Just the standby current
> the same effect, and those aren't horribly expensive now.
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 transistoron
> > the 318's output. If you really want to get fancy, then changingto a
> > more complicated push-pull output transistor pair will be eventhat
> > better. There are some LM series op-amps that are simple buffers
> > can drive 50 Ohms directly, without too much GBW. Once again, youFer instance, LH0002 buffer amp, obsoleted by National, but others
> > need the +/- supplies at about 12-15 Volts.
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
> > is looking better all the time.level
> > The real issue with your problems was trying to use the resistor
> > summing node to drive a low impedance video input. The video
> > got squashed. Just a buffer transistor is all you need (in mostNow with all of the replies with links, anyone having monitor
> > 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.
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.
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