Re: gEDA-user: lost newbie

[Gene Heskett <gene.heskett@xxxxxxxxxxx>]

On Sunday 08 January 2006 21:45, Dan McMahill wrote:
>Marc wrote:
>> Dan you forgot to mention its cheaper to run a sim before you
>> actually build anything im not currently in college as everyone
>> suggests here but i might go do Physics next year im currently tired
>> of doing college courses i just finished Cisco.
>>
>> Marc :)
>
>For IC design I agree completely and in that area I simulate things
>almost every day.  For many analog board level designs, I don't agree.
>There are many circuits for which a hand analysis is completely
> adequate and a spice simulation is the wrong tool for the job.  If
> it's a commercial environment, you can quite easily spend more paying
> for someone to try and come up with a simulation which is even close
> to reality than having them just build the thing.
>
>What you very quickly run into unless it is something like a filter or
> a transistor amplifier is that the models either don't exist or
> aren't good enough to be worth anything.  Take a switching power
> supply.  You can simulate the power stage (with some modeling
> effort), but the controller chips are complex and you can spend more
> time trying to come up with a behavioural model which is even 0th
> order correct than actually building something.  A lot of the
> transistor models which are available for discretes aren't very good
> either.  Sometimes it's due to a large process variation, sometimes
> because of poor extraction of the model, sometimes because the model
> used isn't capable of predicting the sort of behaviour of interest. 
> I came across an example of this recently where someone had some
> discrete FET models which are not capable of subthreshold modeling
> but the designer was designing a subthreshold circuit.
>
>I've designed a fairly large number of analog board level circuits and
>essentially none of the problems I've run into when testing them in
> the lab would have been predicted by a simulation.  About the only
> board level simulation I've ever found to be useful is for some
> filters where I wanted to look at some statistical analysis and in
> cases where I wanted to model some finite Q's and board parasitics on
> some RF filters. Certainly other cases can be found, but by in large
> I have not felt that board level analog simulators have been
> important. I'm sure others will disagree with me on this.
>
>-Dan

No, in the real world, I'd concur.

Example: I needed to build an audio distribution amplifier for the tv 
station to replace one designed by an employee, and which had a higher 
than desirable failure rate in its LM7x15 regulators as the whole card 
cage ran pretty warm from them and the output stages.  So rather than 
build an output stage good for 5 or more watts of audio, with its huge 
TO-5 transistors & heat sinks, I simply used the handiest cheap by X4 
opamp at the time, a TLO84.  Under testing after the first etched board 
was populated, it worked great, with distortion into a 600 ohm load at 
<.01% up to about 27 volts peak across the load.  So we built about 30 
of them, and are still using them.

The achilles heel of such a design?  Long I/O lines scattered out over 
several hundred feet of building makes a hell of a good EMP pickup, and 
with a 255 foot tower outside the back door getting tapped by mother 
nature fairly regularly, the failure rate of the TLO84's has been 
rather high.  But the transistors used in the previous version were 
similarly aflicted too, so that was a wash.  They were soldered, and 
the TLO84's were socketed, so repairs were quicker for the TLO84 based 
design.  I had 10x more bandwidth and 5% of the power disappation of a 
5532 design, which 99% of the commercial DA makers were useing, at 
nearly 10x a chip greater cost & zero benefits as their output stages 
are pretty fragile in the real world too.  5532's need clipon heat 
sinks and cooling fans or the failure rate looks like Orvill R's 
Popcorn in the real world.

The end result was better sounding, but no more dependable than the 
older circuit which due to common output amps, didn't have the 
individual output level controls my circuit has.  My circuit also ran 
about 5 watts a card cooler, taking the cage temps down by about 35F 
internallly.  There's 22 of them in the cage, on 3/4" centers, in an 
open frame design, with a 50 lb brute force psu located else where in 
the rack.

To have taken the time to run a spice on it would have been, I think, a 
total waste of time since spice would not have caught that potential 
failure mode.  So we keep a stick or two of TLO84's on hand.  And about 
once a year, we use quite a few of them...

-- 
Cheers, Gene
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