Post by Jorge Post by Michael J. Mahon
Jorge, since the speed of the cassette input port is limited by the
unconventional usage of the 741, have you looked at its response to low
level square waves? If it is kept out of saturation, it should be
I think that's very difficult (if not impossible) to do because the band
in which the opamp is in the linear region is very narrow due to the high
gain (84x IIANM), and also because due to the AC coupling the input
signal the 741 sees is AC riding on a variable DC bias thus shifting up
and down, most of the time out of that (very tiny) linear band.
Even if you could make that work (I don't think so), it would be
extremely picky about the volume level which is not good at all.
I've got the demo almost ready... :-)
The input to the 741 is a 2:1 attenuators, so the net gain is about 42. The
output is centered around 0 volts, so the negative swing is comparable to
the positive swing (quite unconventional for a digital input!). The digital
input would like, say, +4v for true, so an input drive at the cassette in
port would be about 100 millivolts for satisfactory operation in the linear
region. 120mv would drive the 741 to saturation and 80mv would be barely
sufficient, so level control within +/-20% is necessary, but achievable.
The time constant of the input circuit is about 2.4 milliseconds, so as
long as the input waveform is symmetrical around zero within about 500
microseconds, DC shift should be manageable.
Since you are set up with a 'scope for the experiment, try decreasing the
input level to the 100 millivolt level and see if the 741 output tracks the
input without any extra delay.
It may be that minimal external circuitry (a Schmitt trigger with an output
attenuator) would allow the data speed to increase significantly.
-michael - NadaNet 3.1 and AppleCrate II: http://michaeljmahon.com