Can a diode/transistor ladder filter be run from a single-ended power supply?

[Magnus Danielson]

>>>>> "SC" == Sean Costello <costello at seanet.com> writes:

 SC> Hi everyone:
 SC> I want to actually start building some of the circuits I have on my page
 SC> (http://www.seanet.com/~costello), but I would like to make a few
 SC> changes.  Specifically, I would like to create a diode ladder filter (a
 SC> la the VCS3 or TB303) that can be run off of a single ended power supply
 SC> (in this case, a 9V battery).  I just like 9V batteries, OK?

Building for 9V should not be such a big problem, but something in the
back of my head says that it could do well with some more voltage.

You will not need to have all the 0.7 V over each diode even if that
would not be much of a disaster in 9V.

 SC> Any ideas how I would go about this?  I know the TB-303 uses a single
 SC> ended power supply, so I know that it is possible.  The tricky part is
 SC> the transistor(s) that convert the filter voltage control into the
 SC> exponential current needed for the diode ladder.  I could just copy the
 SC> circuitry from the TB-303 (it is pretty darned nifty, with the "gimmick"
 SC> for lowering the cutoff frequency while increasing the envelope
 SC> modulation), but I would really like to know what I am doing before I
 SC> start.  I really don't want to simply clone the 303 filter; I'd like to
 SC> adapt the core of the VCS3 filter (check my page for a reasonable clone
 SC> of the VCS3 filter), and possibly use germanium diodes for the filter.

So 1N914/1N4148 isn't good enought for you?

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

The diodes on the top creates a much more diffrent ending to earth
than just one dioded per leg would. This changes the characteristics
of the sound. The diodes would change their transconductance just as
the other diodes will.

 SC> Also, could the TB-303 expo stage be used in a Moog ladder filter?  Can
 SC> the Moog ladder filter run off of a 9V battery?  The schematics for the
 SC> discrete Moog filters (Minimoog, 904a) seem to only use the negative
 SC> supply for the exponential convertor.  Can these be converted to run off
 SC> of a single ended supply?  Advantages, disadvantages?

There is nothing that really runs on real diffrential powersupply, if
you have single ended supply you can create any intermediate voltage
easy with simple tricks. With some swithcin you can get voltages above
the highest and belowe the lowest also, ever watched TV?

Actually, the Moog ladder has many intermediate voltages generated to
bias the transistors in the ladder.

Also, the input of the diode and Moog ladders is a pair of transistors
that is operating as the input stage of an diffrential amplifier with
variable gain. The first cap is similar to the compensation cap that
is usually available. The tricks done is just some uncommon
termination of this input, so just biasing the signal at about the
right level and the input end will adjust just fine like any op-amp.

 SC> Any advice welcome (answers, directions of where to look for answers,
 SC> etc.). 

One things to consider is that some cursuits may be more voltage
sensitive than others. This must be considered when running of
batteries since they inherently will drop voltage as you go.
It may be worth modifying some cursuits to become less voltage
dependent than they normally would be. Voltage references may be one
way to go.

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

 SC> Thanks,

 SC> Sean Costello (who realizes how this post totally gives away his
 SC> ignorance of all things electronic.  I was reading "The Art of
 SC> Electronics" last night, and learned way too much that I should have
 SC> known before.  Apologies in advance for my ignorance, but I really do
 SC> want to learn this stuff.)

At least you are sitting there with a good book. Strongly recommended.
However, it takes time to fully consume the book so there is still
some usefull tricks to be found every now and then.

Don't use the cursuits with a crossed over light-bulb... :)

Cheers,
Magnus