[sdiy] RE: Forbidden Planet /HP audio osc/negative feedback

[Don Tillman]

   > From: Tim Parkhurst <tparkhurst at siliconbandwidth.com>
   > Date: Fri, 9 May 2003 09:18:48 -0700 
   > 
   > This was one of HP's early breaks and got them a lot of
   > publicity. H and P were literally working out of a garage at the
   > time. The place is a Silicon Valley landmark now.

Yep.  I live a couple miles from the actual HP garage.  It
looks... well.. like a garage.  There's a large rock outside with a
plaque that describes its historical significance, but other than
that, it looks like any other garage.

Last I heard, the house and the garage were rented out by the owner.
I always thought it would be pretty cool to live in that house and
have a workshop set up in the Dave Packard's garage.

   > "In simple terms, Bill's (Hewlett) unique contribution to the
   > oscillator design was to achieve excellent performance at a low
   > cost by adding a small light bulb to act as a "negative feedback"
   > element in the oscillator circuit.
   > 
   > The small light bulb is set to be partially on. If the
   > oscillator's signal strength becomes stronger or weaker, the
   > light bulb turns on more or less to cancel the unwanted
   > variations. This allows the oscillator to maintain a nearly
   > constant output over its designed operating range. "

Close enough.  See also this:

http://www.hp.com/hpinfo/abouthp/histnfacts/museum/earlyinstruments/0002/0002history.html

The resistance of the filament changes with heat, so the light bulb
acts like a compressor to stabilize the amplitude of the Wien bridge
oscillator.

The interesting thing is that the light bulb has a very recognizable
timing characteristic.  When you change change the "range" switch, for
instance, you can hear the light bulb controlled Wien bridge
oscillator settling down.

I can't prove it, but it sounds exactly like one of these HP200
oscillators was used on Frank Zappa's "Return of the Son of Monster
Magnet" on his first album "Freak Out".  30 seconds into the piece you
can hear the oscillator, you can hear him vary it over a 10-to-1
frequency range, and you can hear him switch ranges, with that funny
light-bulb transient.  Of course the best part is imagining Frank
doing all this, attacking an HP oscillator like a mad scientist.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com