[sdiy] DC offset removal help

[harrybissell at wowway.com]

If you substitute Q2 (2N3904) with an NMOS transistor (such as a VN1206)
and use the shottky diode, you can do even better

With the shottly alone, simulation says about 78mV residual...
the NMOS transistor takes that down to 24mV

H^) harry




On Wed, 13 Aug 2008 08:38:17 -0500, harrybissell wrote
> Looks to me like if you replace D5 and D8 with 1N5819 Shottky
> diodes, the problem will be pretty much gone. The lower drop should
> remove most of the offset and get you much closer to the rail.
> 
> Its not perfect.  Perfect would probably use an analog switch so that
> there is essentially NO voltage drop (just a tiny teeny time error).
> 
> H^) harry
> 
> On Tue, 12 Aug 2008 20:54:57 +0100, Tim Stinchcombe wrote
> > > It's the old EFM Dual ADSR...running off of a 556.  I don't 
> > > know if there are schematics online.  But like I said, there 
> > > is essentially a .2 V offset right after gate, and it 
> > > eventually settles to .1 V.
> > 
> > If the schematic I have for that is the one you have, then I think 
> > that the residual 100mV is the collector-emitter saturation voltage 
> > of Q2, which is hard on with no gate applied (the main cap 
> > discharges through the release pot, a diode and then Q2). Thus if 
> > you could find a nifty way of lowering Q2 emitter, wthout upsetting 
> > the gating action, then that would probably solve it - I actually 
> > have a simulation of that circuit from years ago for another problem,
> >  but at the moment I'm fresh out of 'simple' ideas that might work
> > (brain addled from a long day...)
> > 
> > Tim
> > __________________________________________________________
> > Tim Stinchcombe
> > 
> > Cheltenham, Glos, UK
> > email: tim102 at tstinchcombe.freeserve.co.uk
> > www.timstinchcombe.co.uk
> > 
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> Harry Bissell & Nora Abdullah 4eva
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy


Harry Bissell & Nora Abdullah 4eva