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From: "John Jardine"
Subject: Re: LMH6643 Video Driver Question
Date: Sat, 30 Nov 2002 13:05:47 -0000
NNTP-Posting-Date: 30 Nov 2002 12:53:50 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
M.K <[spam]firstname.lastname@example.org> wrote in message
> Your make'n complete sence....
> just one thought though....
> for the audio.... could i use a cheaper easy to get audio driver? or
> i stick to the LMH6643's ?
> will the good ole 741 do the trick ?
> and any idea what i should modify the load resistors in the circut diagram
> for the audio ? video is 75 Ohms... what should i put in place of them
> cheers :)
Technically a '741 would work OK but personally I wouldn't use a '741 or
'324. Not really up to it (nowadays!).
For the audio, cheap audio type op-amps are perfectly OK. (It's still the
same balanced system, just running at lower frequency) If you are sticking
with +/-5v supplies make sure the audio amps will run OK at that voltage.
(most can). Can't offhand suggest a suitable audio op-amp, as there's *cart
loads* of them. It's just a question of looking for something in a wanted
Myself, for similar jobs I use the 'ST' TSH941 quad, video op-amp (European
though). This would be ideal for your circuit, is a lot cheaper than the NSC
part and is also *very* suitable for audio use as well.
For audio I'd change the 50ohm's to 150ohm's and the 100ohm to 300ohm. The
nominal R.F. impedance of CAT3,4,5 cable is from 75ohms to 125ohms but this
only starts to kick in up at the multi MHz areas. For audio it is of no
consequence and impedance matching is not a problem.
Just keep the op-amp 'drive/receive' resistor values as low as you can, to
help prevent excessive noise pickup along the cable run. (The setup is
'balanced' so will naturally reject noise, never though, tempt the devil!)
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