SS30-M - A Yamaha SS30 in a rack-mount, with MIDI

Wednesday, April 01, 2009

How are you to switch negative voltages?

I've been wrestling with the problem of key switching again. Because the key driver circuits switch a negative voltage to ground it creates a bit of a problem.

When I first looked at the j-Omega MPT8 I thought it coudl switch negative foltages but after a thinking again and e-mailing then it seems not.

What's all the fuss about th though? I can use a solid-state relay, optocoupler or CMOS switch package right? You don't even have to think to hard to get it working. The issue here that I have 49 keys and very limited space. I'd really just like a transistor and maybe one or two resistors per switch. CMOS switches only coem in quad packages at most so I'd need 12 and all the trracking back and across each other to get everything wired. If I must have a PCB at least I'd like it to be simple.

Generally switching negative voltages to ground is not something you get a lot of talk about when looking up these things. Everything is geared to positive voltages and how to bias you trnasistor that way. It's not impossible just less usual and if you wan t o use a simple +nv/0V logic level your options are limited.

The reason for this is not that you can't do it (just switch from n-channel to p-channel FET) but that transistors that switch negative voltages themselves need a negative voltage to switch. which takes you back to square one.

Well, If a CMOS switch can switch a negative oltage with just a +nV power rail how does it do it? I've been wondering.

Google books have a Modern CMOS Circuits Manual online and chapter four has the answers

1 Comments:

  • Hi,
    Where are you with this today ?
    summer 2012 that is ...

    I also got one working SS30 and sadly not enough space for it, so I was thinking that maybe I should build a "triple chorus"-device of it. THis way it could be used with varying soundsources.

    I could do the midi thing as well since I have quite ok code for Arduino backed organ (which the oscillator circuit on ss30 basically is).

    By Blogger psavolai, at July 6, 2012 at 9:25 AM  

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