[sdiy] Some thoughts about ladder filter circuit

[Pete Swarbrick]

I recently built a version of the Minimoog filter using THAT300s - one for the top/bottom pair and another for the diff amp. All other transistors in the ladder were unselected BC547C [based on some batch sampling I did earlier in the year and comments many of us have made on this list about modern tolerances I figured they were going to be easily close enough without all that fiddling about with tweezers and draught excluders ;<)]
It sounds superb.
I also cloned the Mini VCA using both a THAT300 and a 340 (2NPN/2PNP): - total overkill really but I had the parts at work and wanted to try it!
 
Pete
-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Wiltshire
Sent: 12 December 2008 09:03
To: simon.oo.o at xs4all.nl
Cc: Synth DIY
Subject: Re: [sdiy] Some thoughts about ladder filter circuit

Simon,

Perhaps you could use the THAT Corp matched transistors in your  
filter instead?

http://www.thatcorp.com/300-series_Matched_Transistor_Array_ICs.html

A couple of these chips would deal with the ladder quite nicely.
I've also read of people using the THAT340 matched NPN/PNP chip in  
the discrete SSM2040 designs that are around (Jürgen Haible's/ René  
Schmitz).

Regards,
Tom


On 11 Dec 2008, at 20:48, Simon Brouwer wrote:

> 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.
> | http://nl.openoffice.org | http://www.opentaal.org |
>
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>



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