Cgs synth

Feel free to post my answer - just make sure it has the correction in it
(milliamps not microamps).
Ken

>Hi Ken,
>
>Thank you for the detailed reply!
>
>I'd done a bit of maths, but was mainly looking at the datasheets and

working out safe levels etc... I did try it anyway, and it seems to work,
you can get the dead spot, but not sure how it compares to the BAT48 -
anyway I know it's better than without - a la normal attenuverters/offset
pots...

>
>I posted the same Q on muff's, would you mind if I posted your answer in

case anyone else wanted to read it?

>
>cheers
>
>charlie
>
>http://www.solinahifi.com/
>
>http://thebadproducer.tumblr.com/
>
>--- On Sun, 4/11/12, sasami@... <sasami@...> wrote:
>
>From: sasami@... <sasami@...>
>Subject: Re: [cgs_synth] BAT42 vs BAT48 in CGS 81
>To: cgs_synth@yahoogroups.com
>Date: Sunday, 4 November, 2012, 12:27
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> >Does anyone know if I can sub a BAT42 for the BAT48 in the CGS 81 dual
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>processor?
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>Look at the datasheets:
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>http://www.vishay.com/docs/85660/bat42,%20b.pdf
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>http://www.datasheetcatalog.org/datasheet/vishay/85664.pdf
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>1)Reverse voltage 40 for the BAT48 vs 30. How much signal will it see? As it
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>is back to back with another diode, it will only see the maximum of the
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>other diode's forward drop, which is in the order of half a volt. Well, we
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>ain't getting anywhere near 30 of 40 volts, so that won't make a differece.
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>2)Forward voltage drop. Both are around 400mV for 10mA through them. No
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>difference there. The drop for the is greater at 50ma though. Will this matter?
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>3)Forward current. 350ma for the BAT48. Only 200 for the BAT42. Hmmm....
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>Looking at the circuit we have 270k at one end connected to a summing node
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>on an op-amp. Summing node is always zero volts. Other end of diodes goes to
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>pots that can go between +/- 15. So maximum forward voltage across the 270k
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>is 15 - diode drop, which we will ignore for the moment. Using ohms law
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>V/R=I. 15 volts over 270000 ohms = 0.055555 micro amps. Okay, our current is
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>so tiny we aren't going anywhere near the BAT42's 200mA limit, so that isn't
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>a problem.
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>Going back to 2) datasheet doesn't give forward voltage drop values for the
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>current we are using, but we know it will probably be close enough between
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>the two diode types. We have worked out that we won't damage the part
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>because it is being used well within its specification. The only concern
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>will be its behavior at these undocumented levels.
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>Conclusion. Stick it in and try it. No harm will come of it. It will
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>probably do the job.
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>Reality: All these diodes are doing is providing a tiny dead band in the
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>middle of the pot's rotation, so that zero is easier to select. Any
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>variation in the diodes will only vary the width of this tiny gap. In all
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>likelyhood, you won't be able to tell the difference.
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>Hopefully next time you have a similar problem, you will have an idea of how
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>to find out the answer yourself.
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>Ken
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>>------------------------------------
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>>The CGS Modular Synth home page: http://www.cgs.synth.net/
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>>Yahoo! Groups Links
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>__________________________________________________________
>
>Ken Stone sasami@... otherunicorn@...
>
>Modular Synth PCBs for sale <http://www.cgs.synth.net/>
>
>Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>
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>[Non-text portions of this message have been removed]
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>------------------------------------
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>The CGS Modular Synth home page: http://www.cgs.synth.net/
>Yahoo! Groups Links
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>
>

_______________________________________________________________________
Ken Stone sasami@... otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>