[sdiy] Re: Bottom Ten ICs

[phillip m gallo]

And lest we forget the wonderful '7406/'7407/'7438's  part still useful
and in use today.

regards,
p


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of Scott
Gravenhorst
Sent: Thursday, October 07, 2004 10:45 AM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Re: Bottom Ten ICs 


"Cornutt, David K" <david.k.cornutt at boeing.com> wrote:
>From: Harry Bissell Jr [mailto:harrybissell at prodigy.net]
>
>> 74xx
>> 74xxx
>
>What does everyone have against 7400-series logic?
>Admittedly, it's awfully 1980s now, and there are
>better technologies available.  But it was a great
>leap forward at the time.  It it wasn't for the
>minicomputers built out of 7400 logic, we wouldn't
>have the circuits we have now.

No disagreement with this, but there is high supply current requirements
and the noise spike offered to the rails during state switch.  One nasty
thing about totempole outputs that TTL uses is that for an instant, both
bipolar output transistors are on simultaneously.  This has a speed
advantage at the expense of impulse current.  This current can cause
problems with other devices powered by the same supply, especially if
the supply is not up to the task and if bypassing is omitted.  This is
quite apparent in the FatMan design where unmodified, the synth's VCOs
will soft sync easily.  It is cured one of two ways, a better bypassing
arrangement or simply replace the 555 ICs with 7555 CMOS ICs.  The
bipolar 555 uses a standard totempole output for pin 3.


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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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