Transistor/Diode Phase Shift Ladders

Thanks again to Jurgen Haible for publishing his excellent work. Much
additional research could not be carried on without his
generosity.

It is possible to make a variation on the Moog ladder filter that can
be used as a phase shifter. The Aries Model 329 Phase Shifter (Schematic
275K), Tau Systems Model 1190 "The Pipe" (schematic
and manual in pdf 516K) (Thanks to James D. White for pdf) and
ARP Quadra phase shifter (Schematic
100K) use this ladder design.

The Aries unit is a 10 stage phaser (5 notches, 20 transistors?). The
Tau Systems unit is said to be 20 stages (10 notches, 40
transistors?) but has a lot of popcorn noise. I seem to have lost
the Tau schematic, but the Aries unit is much quieter and has more
features. From memory, the current drive sections are identical,
but the Tau unit adds an LFO on the PCB. The Quadra phaser has 14
stages (7 notches, 28 transistors). There is a practical limit to
the total number of stages caused by the intrinsic voltage drop of
the transistor/diode. Assuming 600mv for each diode, there is
probably a practical limit of about 40 stages (20 notches) using
+/-15 volt supply. It is possible noise could become a problem as
the number of stages increases.

The only published information I can find on the actual ladder topology
is contained in Jurgen Haible's phase shifter design located
at:

http://www.oldcrows.net/~jhaible/tonline_stuff/hj_qph_2.gif

As an experiment, I tried simulating a couple of sections from Jurgens
design in Electronics Workbench, a SPICE program variant. This is a
second set of simulations with gain set to unity. Thanks to Don
Tillman for pointing out the capacitor value error in the first
set. I thought Jurgen would be amused and the diode only topology
is rather interesting. Opening the collectors increases the
attenuation of the ladder. Connecting the collectors to the base
make no difference under simulation.

The results seem to confirm differences in ladders built from
transistors and those from diodes only. Presumably, this would
apply to lowpass topologies also.

Comments are welcomed at [email-masked]

Phase and frequency plots from design simulation most similar to
Jurgen's original ladder, All plots are 20Hz to 20Khz

Schematic of simulation

Phase Response

Frequency Response

Phase and frequency plots from design simulation with collectors
disconnected, All plots are 20Hz to 20Khz

Schematic of simulation

Phase Response

Frequency Response

Phase and frequency plots, schematic rearranged to show diode only
topology equivalent to open collector, All plots are 20Hz to
20Khz

Schematic of simulation

Phase Response

Frequency Response

Thanks to Mike I. for the Arp Quadra Phaser schematic and these
spectral plots of the Quadra phaser frequency response

auxnores.gif

mainreg-a.gif

mainreg-b.gif

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