New in the list

[Grant Richter]

You will see this in unbuffered filter designs.
A good example is the lowpass filter from the
ARP 2600 envelope follower. I modeled it in SPICE
and it gives a nice sharp 24 Db/Oct rolloff with no
buffering between stages.

Further stages load the previous stage and reduce Q
so the impedance of the stages is modified to end
up with a nice sharp corner at the end. You typically
see the resistor values go up and the cap values
go down for stages later in the chain.

----------
>From: Emmanuel AMADIO <emmanuel at postimpressions.co.uk>
>To: DIY Mailing list <synth-diy at node12b53.a2000.nl>
>Subject: Re: New in the list
>Date: Fri, May 5, 2000, 10:43 AM
>

>>I have a JX10 I want to mod.
>>The filter is a State Variable filter according to the Roland Service
>>Manual.
>>but it seems it is usable only in lp mode (not a problem).
>>the filter seems to be set as a 2 poles filter whereas in the JX8P wich use
>>the same VCF chip it seems to be a 3 poles
>>filter.
>
> The thing which I can't understand is why the circuit has different
> capacitors values.
> Does it mean that they gave the filter response a slope like this ? :
>
>
> ---------
>          ^^^^^
>               ^^^^^
>                    \
>                     \
>
>
>> The block diagram for the chip is like this :
>>             ---------
>> fc  ------->|       |---------------------------
>>             |       |           |              |
>> res ------->|       |-----------|------------- |
>>             ---------         | |            | |
>>                               | |            | |
>>                             -------        -------
>>                             |     |        |     |
>> in ------------------------>| vcf |------->| vcf |----> out
>>                             |     |        |     |
>>                             -------        -------
>>                              |   |          |   |
>>                              -   -          -   -
>>                           C1 -   - C2    C3 -   - C4
>>                              |   |          |   |
>>                ---------------------------------------- GND
>>
>> C1=C2=10 nF
>> C3=C4=220 pF
>
> The other strange thing is that for the JX8P  C1=C2=C3=10 nF and C4=220pF
>
> Regards Emmanuel
>