[sdiy] (OT) Doepfer MIDI-CV oddity

[Jerry Gray-Eskue]

> 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.

This is true of any communication link that uses an "analog" repeater, if 
you use a device that regenerates the digital pulse train using a system 
clock you get a correction of the distortion at that stage.

----- Original Message ----- 
From: "Ullrich Peter" <Peter.Ullrich at kapsch.net>
To: "Colin f" <colin at colinfraser.com>; <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, January 19, 2011 6:17 AM
Subject: Re: [sdiy] (OT) Doepfer MIDI-CV oddity


> 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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