Quad Opamps? (Was Re: ASM-1 PCB availability?)

[Hairy Harry]

Ooohh... I know I know...

Quad op-amps (and duals to a smaller extent) share the same power
supply pins.  Decoupling each section individually is not possible.

So when one section draws a power surge... the others are affected
as well.

IMHO it is OK to use Quad Op-Amps if you are "reasonably skilled in the art" 
(as it goes...).  If you know what each section is doing,
what to expect from them (not too much)... and you do not care about
the possibility of interaction, then go ahead.

Using (let's say) 1/2 of a quad op amp as a high gain, low level audio
preamp, and the other 1/2 for (let's say) a comparator and a power
driver (maybe a couple of LEDs perhaps...)  is a recipe for disaster.

Separate op-amps let you control the power separatly. And change devices 
easily... "oops that 741 is too slow here... time for a TL071..."  or maybe 
"gee the TL071 is too fast... maybe a slower device
would really be better".  Same is true for input impedance, output
drive capability, etc...

Also, two duals may allow a more efficent layout. Cramming all the needed 
resistors around a quad may cause unwanted coupling of signals.

NOTICE: They DO build quads. Sometimes they are the right device for
the job.  I use them, but consider the layout and usage carefully before I 
start... (or I suffer.....) Same goes for device selection...
choose the right part up front and you will be half way there.

H^) harry


>From: leaf at babble.vibrant.org
>To: Jim Patchell <patchell at silcom.com>
>CC: synth-diy at node12b53.a2000.nl
>Subject: Quad Opamps? (Was Re: ASM-1 PCB availability?)
>Date: Tue, 14 Nov 2000 13:24:59 -0800 (PST)
>
>On Tue, 7 Nov 2000, Jim Patchell wrote:
>
> > Rule #3, avoid quad op-amps.  Duals are OK, singles are better.
> > There are some complicated rules I use for choosing an op-amp package, 
>and it
> > appears that other people here have similar rules, but to keep it 
>simple, I
>
>hi... why avoid quad ops?  is this only for audio path, or in general?
>i'm in the middle of building a superseq using tl074's...
>
>thanks,
>Phil
>
>
>

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