[sdiy] Filter Structure Question

[Magnus Danielson]

From: René Schmitz <uzs159 at uni-bonn.de>
Subject: Re: [sdiy] Filter Structure Question
Date: Sat, 17 Jul 2004 19:15:06 +0200
Message-ID: <40F95E9A.6080309 at uni-bonn.de>

> Hi all,
> 
> harrybissell wrote:
> > The technique of using a spare inverter to estimate the bias point
> > is an old (and useful one).  If I followed correctly, you want to tie the
> > output of that inverter to "real" ground ?  (to the center of a pair
> > of resistors connected to the positive and negative rails).  I don't
> > know about that part. The output of the inverter would have only a
> > very limited ability to move that point to the correct one.
> 
> Besides that, the supply to the 4069 wouldn't be very stiff, and this 
> could lead to "interesting" if not disturbing side effects, since the 
> current consumption of the gates varies with the signal. It would lead 
> to severe crosscoupling between the gates, something which is already 
> present to some extent, since the internal traces and bonding wires were 
> never designed for the amount of current present in a linear application 
> of these chips.

I *did* recommend there to be a cap there, and Scott has included that into
his schematic. Now, a cap isn't regulated power, but it is the local energy
reservoir and will reduce the effect. Also, in this case we should not look at
it as being gates but rather inverting amplifiers. In the case that one wants
isolation, then separating the amplification over several chips, each with its
individual powersupply and capacitor would be possible. 

Cheers,
Magnus - who left the tubes of 4069UB at home, but has another project going