Re: [LAD] [ot] rme fireface: weird balanced output measurements

On Saturday, February 18, 2012 07:02:22 AM Fons Adriaensen did opine: Something I didn't consider in my first generation design since I was still learning myself, which although it had an output impedance at the TLO84 pin of under an ohm, then used 300 ohm per leg build outs. The next generation card, the main design change was to change that 300 ohm to 30 ohms. This then worked much better out in the newsroom, but was even more susceptible to the emp spikes the longer runs picked up. Since the card had to fit the cage, it never did grow the schotkey power diodes to the supply rails that would have absorbed those spikes, no room left on the the card for diodes that at the time (1984-85) were about 1/2" long & 1/4" in diameter. It would have taken 12 of them to protect the inputs as well as the outputs. These cards all had their own rail regulators (78-7915's) for +-15 volts as the cage supply was about 22 volts +-, filtered some but not regulated, good for about 10 amps a rail. 22 cards with 44 pin edge connectors in one 3 space high rack, it did have noticeable but tolerable heat output. The only place with enough farads available to dump the emp was the main rails, and a quick test of that idea using a bench supply showed that wasn't a cure as the output stage of the TLO84 was still destroyed when the output was pulled above or below the cards internal +-15 volts by about 2 volts, long before the schotkeys to the +-22 volt rail would turn on. Since they were very transparent & distortion measurements were difficult because it was well under .01% at 28v p-p output, and even with the emp problems, were 10x as dependable as another rack of Ramco DA's we paid 5 grand for before I came in the door, I made sure we had sticks of the chips, and bags of the capacitors on hand in case mother nature heard somebody call that stuff butter, and ran with it. At least the TLO84's were socketed, those quad 741's (4432's?) in the Ramco weren't. Those eventually were pulled out, they weren't as good sounding by a long ways. Those 4432's were so slew rated they weren't really any good above about a volt p-p out. Pure crap to my ears. With that slew rate clipping, the intermod turned your teeth on edge worse that fingernails on a slate blackboard. Downright fugly was how I described it at the time. Sounds like. In those cases I always figured the cable ohmage if otherwise impedance matched at the src, was a std factor of the load currents flowing, and with a nominally 10k load on the far end, a generally ignorable. Effectively un-terminated, it of course dependent on the src terms to absorb any 'vswr'. In any event, by then I had managed to obtain a function generator, and a 10 to 60 khz sweep actually looked pretty good at the far end, easily within about 5% of flat on the scope. Technically incorrect?, yes. Was better by magnitudes than what was there when I walked in the door?, hell yes. Which also had the effect of making a long 15 kilohertz rated line into one of the best 'brick wall' filters you've ever measured. Maybe a db down at 15 khz when they were done, a twenty mile circuit was down 90 db by 17 khz. And they never did get the S/SN below 48 db, so we eventually bought a subcarrier radio, which made 75db rather nicely. ;-) Yep, sure is today Fons, even for a 10 foot run between racks in our new digital control room. So now the problem is levels, digital doesn't come with a knob. The std, if there even is one, is ignored, which explains the commercials that are 20db louder than the program, nobody cares and we catch hell from the listeners. I don't believe that will be solved until the commish puts some real teeth into it, traces the violating material back to the production facility & then figures out a way to make them legally responsible so they can fine their ass & make it stick in court. Sure, we can re-encode if we have to, but there goes the lip sync unless we bucket brigade the video too. That also takes truly competent manpower most stations don't have near enough of. In many cases, it isn't even available to hire! This is one business where we often have to grow our own technical people. Then, hanging onto them becomes the next problem because its an industry wide problem and the head hunter business is alive and well. We are often caught in the "damned if we do, and damned if we don't" world, so we wind up playing it by ear, letting the ratings tell us how we are doing. IMO, that isn't always optimum. When I first started back in the '60's, where I worked often helped the 'other guy' stay on the air, based on the premise that if he went down and somebody else got that license, they might come in & spend some real money on physical plant and wipe us out of the market. A case of preserving the status quo was good business sense for us. Obviously to protect since this list covers the planet, and its largely historical anyway, that small market won't be named. :) Cheers Fons, Gene -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) My web page: <http://coyoteden.dyndns-free.com:85/gene> You mean you didn't *know* she was off making lots of little phone companies?