[sdiy] (OT) Doepfer MIDI-CV oddity

[Ullrich Peter]

Hi!

And the 6N139 switching times can further be optimized by used a bias resistor on the 
base of the opto-receiver transistor.  Without this receiver the 6N139 is quite slow.
I normally use a 15k resistor from pin 7 to GND.

The maplin MIDI thrubox has this base resistor in it and works very fine. I had the same 
circuit without the resistor in my own thrubox and had problems.

The problems get bigger and bigger the more MIDI gadgets are connected in series as 
the MIDI pulses get more and more distortions in each thru stage.

Ciao
Peter

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Colin f
Sent: Wednesday, January 19, 2011 12:58 PM
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity

 
> As an aside, this is the kind of thing that makes life hard for
> manufacturers-

Yep, that's why I'm so well acquainted with the range of 6N139s...

Another new factor in the MIDI equation is that some PC interfaces drive the
line with a lower voltage supply.
MIDI is a current loop, so in theory you just need to reduce the series
resistors and 3.3v pull up on pin 4 should be no problem.
But if the forward voltage drop on your opto is higher than usual, you might
find the margins of operation sooner than you'd like.

Cheers,
Colin f


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