[sdiy] Some thoughts about ladder filter circuit

[Csaba Zvekan]

> Hi Simon,
>
> When I built a few of Yves Ussons VCO's . I have also tested the  
> version with hand matched transistors to save money. For that I used  
> a design that I saw on the Internet.  If you like to learn how that  
> gets done see my Transistor Matcher.

http://www.csaba.ch/index.php?option=com_content&task=view&id=14&Itemid=30

> Get yourself a batch of transistors and start comparing them and  
> writing the good pairs down. Don't touch them by hand, use pliers .  
> Do it in the morning. Don't open windows (draft ... temperature  
> drop) . Let them sit in the T-Matcher for at least 1 Minute and  
> compare them like in the video .

http://www.youtube.com/watch?v=gdWRWqVRgMw

> . And you will get a few pretty well matched transistors. Bag them  
> up and write the grades from let's say 1 to 10 . Like really good  
> pair is 9.5 or  even 10.
> Have some sort of a system ... you get the point.
> Sure ,we can buy super matched transistors but where would the SDIY  
> fun be there ? :)
> I also want to mention that I don't have any pitch drifts or  
> anything with that particular hand matched VCO module.  See pics at
Yves Ussons website.

http://yusynth.net/Modular/index.html

under VCO the little pictures on near the bottom of the page.

>
> Csaba
>
> PS: Eric ... ping :)
>




On Dec 11, 2008, at 9:48 PM, 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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