AW: Can a diode/transistor ladder filter be run from a single-end ed power supply?

Haible Juergen Juergen.Haible at nbgm.siemens.de
Wed Jul 15 10:53:36 CEST 1998


Hi Sean, 

I'll try to give a few short answers:

	>Any ideas how I would go about this? 

Single ended is no problem - you can always create a
reference voltage a few volts above GND. You can either do 
it with an opamp buffer, or you can simply raise the dc level
of the cascade's differential amp with resistor dividers,
like the Moog cascade.
9V total supply voltage should be enough, but not exactly
abundance: the diodes eat approx. 5V, and you want some
space between the top of the ladder and 9V because of
the CMR of your buffer amplifier. The expo converter needs
some dc too.
I'd suggest to fix the low end of the cascade (bases of the
differential pair) to +1.5V, and the top of the cascade to
+7V. Make sure that the differential buffer amplifier after
the cascade works up to this voltage - a discrete circuit
(similar to the original VCS3) is a good idea here, or use
one of these modern rail to rail opamps. (I prefer the first
way.)

	>The tricky part is
	>the transistor(s) that convert the filter voltage control into the
	>exponential current needed for the diode ladder.  I could just copy
the
	>circuitry from the TB-303 

I don't have the 303 circuit in front of me now, but if memory serves
they had more dc "space" for the converter than you wil have.
(wasn't that 12 or even 15V supply, with the reference voltage
at approx. 5V ? )
I suggest a current mirror here, together with a pnp expo converter.

	>Any other issues I would encounter with running the VCS3 filter off
of
	>+9V?  How about the diodes at the "top" of the ladder (i.e. the 3
pairs
	>above the main R/C diode pairs)?  What do they do? 

If you're careful with calculating dc bias points all over your circuit,
a 9V design should be fine.
The 3 diodes at the end is simply a 3R termination of the unbuffered
4-pole RC filter. You can see its influence if you draw a little
simplified circuit: an AC *current* source (the signal current from
the differential pair) feeding a C to GND and an R to the next
stage, another C to GND, R to the next stage, and so on. The last
stage has C and 3R to GND. (You can simulate this with any free
Spice demo version. Add a variable gain stage for the feedback loop,
and you can see why this 4pole filter is called "18dB" filter with good
reason, btw.)
>  
	>Also, could the TB-303 expo stage be used in a Moog ladder filter?
Can
	>the Moog ladder filter run off of a 9V battery?  The schematics for
the
	>discrete Moog filters (Minimoog, 904a) seem to only use the
negative
	>supply for the exponential convertor.  Can these be converted to
run off
	>of a single ended supply?  Advantages, disadvantages?

I don't see much problems here. While the diode ladder has its
dc shift from stage to stage determined by the diode drop voltage,
the transistor ladder's dc shift is determined by the base voltages,
i.e. the resistor divider. Don't remember the original Moog voltage
spacing, but the ladder will surely work if it's just slightly above the 
be voltage drop. And you don't have the 3R termination here.
So, with a little care, no problem. Expo converter same as above.

	>I'm also interested in creating an MS-20 filter section clone that
is
	>battery operated (I LOVE battery operated boxes - my little x0x
boxes
	>are getting a lot of use nowadays, while I wait for the NM to
arrive).
	>Any advice there?  It seems more straightforward - just using a
voltage
	>divider to create a "virtual ground" that is 1/2 of the positive
supply.
	>Any other tricks?

The MS-20 filter 9V adaption looks easy, but be careful with the soft
limiting of the opamps (the diodes ...) You will have to recalculate
this completely. And rail to rail opamps are highly recommended here. 

JH.




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