[net.audio] Power supply in-rush surge suppression w/o relays...

shep@avsdS.UUCP (10/26/83)

While "hot-roding" some thirty phase linear 700's years ago, I too had
to confront the problem of surge suppression.

The modified amplifier drew less than an amp quiescently and up to 15 amps 
full tilt off of the bipolar 100 volt rails. The power supply was simply a 
massive transformer feeding an fwb rectifier feeding equally massive filter 
caps. The original phase linear power transformer was left alone; but a new 
bridge and caps were fitted. As I recall, about half a farad was used on each
rail (~50 joules/rail!). The in-rush current, as you might expect, was very 
large and a "relay charge `em up" scheme was unacceptable, mostly because
of chassis space limitations. Enter NTC (negative temperature coefficient)
resistors, little widgets whose resistance decreases as the temperature
increases. Some firm (the name escapes me) offered them to us in a variety
of flavors. A network was built that would handle the power required, and
it worked fine. When the amplifier was off, the devices were cool and at
at their greatest resistance. At turn on, the caps looked like a short,
the resistors got warm, the caps charged. Once charged, the bias on the
many transistors kept the quiescent draw on the rails high, and thus the
ntc resistors warm and at a low resistance. The low, yet finite, resistance
was never a problem since the caps were so large.

			Shep Siegel, Ampex Corp. 
			s:shep