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

[Fons Adriaensen]

On Sat, Feb 18, 2012 at 08:28:03AM -0500, gene heskett wrote:
 
> 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.

Should I feel sorry for all those poor TL084s that died while being forced
to work well above their physical limits, abused and expendible like Egyptian
slaves building the pyramids ? :-):-)
 
> 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.

You mean there's *no* volume control between whatever is used to 
play commercials and the transmitter feed ? No continuity A/V mixer
at all ?

Ciao,

-- 
FA

Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.