[Coco] Coco Cassette interface help

Gene Heskett gheskett at shentel.net
Tue Feb 28 16:09:02 EST 2017

On Tuesday 28 February 2017 15:46:32 Dave Philipsen wrote:

> A couple of things to keep in mind. The comparator is set up for
> higher level signals as would drive an 8-ohm speaker in a small tape
> player.  You could tweak it to work on lower amplitude signals
> probably by just fiddling with resistor values.  Also, the comparator
> is basically an op amp. If you need an amplifier *and* a comparator
> you could probably use two of the op amps in a quad op amp package.
> Dave

But the comparator can switch on just a couple millivolts difference, 
having an open loop gain in excess of 1 million to 1. An LM339 is also a 
couple orders of magnitude faster than the universal op-amp like a 741 
look alike.

Today you can get single supply video speed op-amps from TI for less that 
a buck apiece in epoxy-b packaging in onsies or tensies. Double that for 
a tin can like a to-5.  And while fast, are dead stable, running on a 
few micro-amps internally, the major power use being in driving a low 
impedance load.  Audio in the coco world isn't generally low impedance.

> > On Feb 28, 2017, at 1:25 PM, RETRO Innovations 
<go4retro at go4retro.com> wrote:
> >> On 2/28/2017 1:05 PM, Dave Philipsen wrote:
> >> All you want to do is amplify the signal that is coming from the
> >> CoCo cassette out. Use a capacitor to de-couple it from the
> >> comparator circuit. It won't matter if there's DC bias on the
> >> output of the op amp because you will effectively remove it with
> >> the cap.
> >
> > True, and I will give it a shot, but I'll continue to search for a
> > way to make the comparator idea work alone.  Amplifying the source
> > only to get it "over the bias" of the comparator just seems wrong to
> > me :-)
> >
> > Still, I already was thinking of your idea, in the context of using
> > an LM386 to amp the signal brute force to get it to work.  So, I
> > agree it's a workable solution.  Thanks for the suggestion.
> >
> > I am also going to see if I can feed some known signals into the
> > comparator to see if I have messed up the design.
> >
> > Jim
> >
> > --
> > Coco mailing list
> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco

Cheers, Gene Heskett
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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