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

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Thursday 29 April 2010, Russell Shaw wrote:
>Gene Heskett wrote:
>> On Wednesday 28 April 2010, Dave McGuire wrote:
>>> On Apr 28, 2010, at 3:40 PM, John Doty 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?
>>>
>>>              -Dave
>>
>> 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.
>
>The opamp is 1MHz unity BW. The higher the gain, the lower the first pole.
>An even better opamp would roll off at 1Hz.
>
>> 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.
>
>Those are video buffers. They have much less closed-loop gain and inferior
>offset voltages. They're also noisy and are very prone to oscillation with
>any stray capacitance or with certain feedback resistors.
>
I believe that to be an artifact of the GB product not being high enough in 
what was available, say back in LM-357 days.  When I replaced some custom 
made discreet op-amps in that grass switcher with some fairly modern 
internally compensated ones with a GB of about 10Ghz, it was absolutely not a 
problem.  They were the ideal block of gain & dead stable despite a layout 
when being used to sub for something else, that would give a modern video 
engineer recurring nightmares.  Flying leads up to an inch long just to reach 
the original plates mounting and connecting holes in the PCB.  That had 
"kludge" written all over it, but it technically kicked ass compared to the 
much slower discreet versions grass wanted $1700/copy for.

>> 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.
>
>dV/dt = 2.pi.Vm
>
>at 20kHz and 1V/us, Vm=8Vpk
>
>quite ok for most apps below 5Vpk.
>
>> 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.
>
>LM741 has 1mV OS typical. TL072 is 3mV

Can you hear 3mv dc?

>LM741 would be better than TL072 for control apps, and cheaper.

Maybe so, but with 4 of then in a dip, and room for 22 cards in the cage, I 
used them in multi-tube quantities (5 per card, 22 cards) for utility audio 
DA's at WDTV-5 for nearly 20 years.  Most failures were on longer output run 
circuits, and lightening related.  When you have a 255 foot tower 30 feet out 
the back door, the emp pulse from a lightning strike is considerable, and 
tends to knock out the output stages.  So I designed one with some to5 
outputs to buffer the chip output, and they had an even shorter life plus 
they crowbared the whole cage supply when they failed, much more catastrophic 
in effect as that didn't just cost us one audio src, it took us off the air.  
The old favorite burn your fingers power hog op-amp, 5532 would fail at 20x 
that rate under the same conditions.

In broadcast, you learn to use what gets the job done with audio performance 
that is adequate, and is the _most_ dependable.  Getting rid of that last 
.001% of distortion is not a priority that even makes the list.  However, 
25volts p-p at 20 khz with no slew rate or cross-over discernible on a 100mhz 
scope, or at lower frequencies my ears could hear well was "good enough for 
the girls I went with".

-- 
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)
<dracus> Ctrl+Option+Command + P + R
<Knghtbrd> dracus - YE GODS!  That's worse than EMACS!
<LauraDax> hehehehe
<dracus> don't ask what that does :P


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