Phaser

[Rick Jansen]

I just built a small phase shifter, from the Elektor 
Formantbook 2. It uses 6 allpass filters as follows:

in ----####---####-
     |      |      |
     |       -|\   |
     |        | >----- out
      ---||---|/
        "C"  |
            #
            #<-  "R"
            #  |
            |  |
           -------
The "C" in the consecutive stages is 1uF, 470n, 330n, 150n, 
100n and 47n. The "R" in this schematic is implemented with 
a FET (BF245C). The gates of all 6 fets are directly 
controlled by a simple LFO, which generates a triangle wave 
from -6V to -4V. The phasing effect can be "automatic", by 
using the LFO or you can do it manually, simply by 
controlling the gates of the fets directly with a potmeter. 
A switch selects between automatic and manual. 

The phaser has 3 potmeters:
- manual 
- automatic (f of the LFO)
- rate (phasing depth)

- input- and output-bus
- switch manual/automatic

The circuit is originally intended for +12V/-12V,
they propose a resistor/zener/elco to convert the Formant 
+/-15V supplies to 12V. Absolutely ridiculous, the opamps 
work fine on 15V, all you have to do is to adjust a few 
resistors to get the right voltage levels at certain points.

The PCB is nice and small, only 12.2 x 8.6 cm, there's also 
a Eurocard PCB listed. I hadn't noticed at the time I ordered
the PCB made from the book though, 
so I have mounted the small PCB onto a eurocard-size piece 
of aluminium to put it in the 19" rack. (Funny that lots of 
electronics stuff is still not metric, 19" racks, ic-pins 
still 0.1 inch apart etc etc...)

Only problem with this circuit is that the FETs specified, 
BF245C, weren't easy to get really. I could get 2 at one 
shop, and 4 at another, exhausting both shops' supplies. You 
cannot substitute BF245A or B, as the A, B, and C denote the 
pinch-off voltage (A: -0.4..-2.2V,  B: -1.6..-3.8V,  C: 
-3.2..-7.5V) and drain saturation current. Of course you 
could choose another FET and adjust the LFO and voltage 
levels accordingly, but I have no FET equivalents guide.

Also I find that with specified values the LFO cannot run 
slow enough, I like really slow phasing, but that can be 
helped easily too by using an 1M potmeter instead of the 
470k one they spec.

This circuit is in the 1982 Formant 2 book, so I'm afraid I 
cannot simply put it on the web or in an archive due to 
copyright etc. Also the text with the circuit is in Dutch, 
though the circuit is easy enough to build and adjust. 

I also have another Elektor phaser circuit in a book 
"Practical filtertechniques", where they use 2 delay lines
with 8 sections each of the folowing:

in ----####--------####-
     |           |      |
     |            -|\   |
     |        /    | >----- out
      -####--o  o--|/
              S   |
                 === C
                  |
                 -------
The "S" in this circuit is a 4066 CMOS switch, driven with
a clock up to 50kHz. This is a much bigger (PCB-wise) circuit 
than the one I described earlier. Anyone who has built a circuit
like this?

Rick Jansen
__
rick at sara.nl   http://www.sara.nl/Rick.Jansen
_____________________________________________
S&H's a module and s&h's looking good