[sdiy] Some thoughts about ladder filter circuit

[Simon Brouwer]

Hi,

I have some thoughts about this ladder filter implementation:
http://www.elby-designs.com/asm-2/vcf2/vcf2-filter-asm2-cct.pdf

I have been matching transistors for this filter using a method 
described here: http://simonbr.xs4all.nl/wiki/index.php/Matching_transistors

I get the impression that CA3046 is not being made anymore, Intersil 
lists the part as inactive, and in any case Farnell doesn't have it. So 
I am considering to use the hand-matched transistors for all the pairs 
in the ladder filter. As the CA3046 specified a maximum VBE mismatch of 
5mV, using transistors matched to 1mV could actually be an improvement.

However, I'd like to keep the dissipation in the expo converter 
transistor (Tb in the schematic) as low as possible to minimize it 
heating up compared to the reference transistor Ta.

I'm a bit puzzled by R835. Its only function that I can think of is to 
limit the current into the ladder to about 2 mA. But couldn't you have 
this as well by choosing a value of ca. 5.3k for R836?

If  R835 can be removed (i.e. shorted) then by lowering the voltage on 
C808 to 0.7V (change R814 to 56R and R808 to 510R) the collector voltage 
of Tb would become around 0V, and the maximum dissipation in Tb would be 
more than halved.

I may have to look at keeping C 809 sufficiently biased.

Apart from that, I think it would not change the operation of the filter.

Any comments?

-- 
Vriendelijke groet, 
Simon Brouwer. 

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