Re: gEDA-user: A little puzzled about the purpose of gschem

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Thursday 29 April 2010, Dave McGuire wrote:
>On Apr 29, 2010, at 12:48 AM, Gene Heskett wrote:
>>>> Well, you started out complaining about a 741 model. I'd call that
>>>> a very rare, obsolete part: I haven't actually seen one in a
>>>> circuit in over 30 years. I guess it's still in textbooks (read
>>>> Stephen J. Gould's rants about textbook authors' tendency to copy
>>>> from previous textbooks sometime), but why would anyone use it in a
>>>> new design?
>>>
>>>   Very rare?!  I see 741s everywhere.  WTF?
>>
>> Sorry to bust the bubble, but he's right.  The 741 is well over 40
>> years old,
>> and its open loop first response pole, where the 6db per octave
>> rolloff
>> begins, is a measly 10 hertz.  Today there are $1.00 opamps with a
>> working
>> gain of 20 when feedback is applied, with output slew rates of several
>> thousand volts per second.  Thats working bandwidth to several hundred
>> megahertz at the sort of levels found in either a modern broadcast
>> audio
>> mixer, or a production video switcher, and either of those are
>> driving 60
>> ohms for audio, or 75 for video.
>>
>> Slew rate limits alone in the 741 means you can't honestly ask it
>> for more
>> than a volt of output at full audio bandwidth.  At 3 volts the slew
>> rate
>> distortion is so bad even these 75 year old ears can hear it.  Even
>> a TLO-72
>> or 74 can mop the floor with a 741, and output a +- 15 volt rail to
>> rail
>> signal doing it, but into the old 600 ohm std load.
>
>   No bubbles to bust, I'm not particularly fond of the 741...yes
>there are definitely better opamps out there (I usually use OP07s as
>my general-purpose opamp) but that doesn't change the fact that I see
>741s everywhere.  They are far (VERY far) from rare.
>
>              -Dave
>
At one point I had to replace some custom made on ceramic plates, op-amps in 
a Grass Valley 300-3A/B switcher, and GVG were being asses, wanting $1700 for 
one of them.  I went to the catalogs & found a to5 can that looked good, and 
put them into 3 failed channels of that production video switcher.  They were 
so much faster, for $1.32 each, that it threw it out of color phase by about 
10 degrees.  If the removal and changeover hadn't been at least an hours work 
per channel, and I'd have had to replace about 48 of them all told, I would 
have.  But we were then on notice that digital was coming, so that, 
originally $175,000 switcher was effectively in maintenance mode only.

-- 
Cheers, 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)
Your happiness is intertwined with your outlook on life.


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