[Coco] Coco Cassette interface help

[Dave Philipsen]

Sent from my iPad
> On Feb 28, 2017, at 3:09 PM, Gene Heskett <gheskett at shentel.net> wrote:
> 
>> 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.
> 

True, but I have done the task with an op amp before. At the frequencies we're working with on the CoCo with data on tape (less than 5kHz) we're not in need of anything remotely near video frequencies.  I currently am using a circuit with a dual op amp chip (no comparator) and a handful of caps and resistors that reads SMPTE/EBU linear time code which is very similar to the CoCo cassette data.  It uses the first op amp as an amplifier and the second one as a comparator and will work with either balanced or unbalanced audio on the input.  The circuit is very reliable and I'm sure it would work equally as well with the cassette data.  And the amplifier section is already set up for line level audio.

> 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.

The op amp I'm using is a TLC2272 rail-to-rail op amp and while not as fast or low-current as what you're describing, it does the trick for less than a couple of bucks. If I get a chance I'll post a link to a schematic later.

Dave

> 
>>> 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
>>> 
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> 
> 
> Cheers, Gene Heskett
> -- 
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> 
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