[sdiy] WP LM3900 VCF

[Grant Richter]

That is a very clever design.

There is a application note that shows using the dual PNPs to make a  
voltage input op-amp.
Modulating the emitter currents acts like a VCA into an integrator.
Reminds me of the Serge designs using that 4 transistor gain cell.

On Jan 14, 2006, at 7:35 AM, Paul Perry wrote:

> It's a swindle, really!
> I once spent ages tryng to see how a Norton amp
> could be made into a filter <blush>
> The LM3900 is just used as a current-voltage converter, it's
> matched pairs of 390s's that is doing the actual VCF
> so far as I can see, looking at this schematic
> (thanks again to Don Tillman)
> http://www.till.com/arptech/pdf/4072.pdf
>
> paul perry (up past his bedtime in Melbourne Australia)