to my homepage
JH. Krautrock Phaser
(a. k. a. "Compact Clone" - insprired
by the Schulte Compact A Phasing)
If you listen
to German records from the 1970's, you often hear a rather unique
phasing that comes from a device called "Compact A Phaser" from the
Berlin company "Gert Schulte Audio Elektronik". It's often referred to
as "Schulte Phaser", and I have heard it being called "Krautrock
Phaser", "Schulze Phaser" (because it's prominent on many early KS
albums), "TD Phaser" (because of Tangerine Dream). In my opinion, this
phaser has been important to the sound of these early electronic
musicians almost as much as the synthesizers the used.
My Compact Clone
In 1999 I have
built a clone of this on veroboard, and put it into the enclosure you
can see in the pictures below.
This has been partially sponsored by the vintage synthesizer shop
Touched by Sound back then, who also have taken my Phaser to NAMM to
test the waters for commercial production. Well, it didn't come to this
for various reasons, but I've been enjoying my Compact Clone ever
since, and mostly use it on my Korg Lambda string ensemble.
view (click on image to enlarge)
cheeks (click on image for greater picture)
The new Compact Clone 2007
I've decided to do
another redesign of that circuit and make a PCB layout.
All who have followed my DIY projects over the years know that I'm
using the term "clone" rather loosely.
I'm not actually "cloning" anything, which would be a 1:1 reproduction
of the original circuit and/or the appearance.
I certainly do strive for a
most perfect reproduction of the sound that made a vintage design
famous, but I also add features of my own that I find usefull, and
tayler the circuit to my own needs and standards.
In case of the Compact Clone this means:
|The incandescent lamp / LDR
combination that is responsible for the unique way of sweeping
|Big incandescent lamp as sweep
indicator on front panel
|The circuit topology: 8 Stages
of Phasing, 2 Stages of Feedback. Negative, lowpass-filtered feedback
|The 741 opamps
|The unique "Osc. Period"
potentiometer that has maximum LFO rate on ccw end. (optional)
|Works with original 7V 100mA
|The possibility to mount
potentiometers direcly on board. (Alps RK11 vertical mount types or
Of course you can connect about every
other potentiometer with wires, too.
|Mains-Powered (optional). For
+/-15V powered, see below.
|Power supply for mains powering
on the PCB, including secondary fuses,
rectifiers, electrolytic caps, voltage regulators and heatsinks.
|It's possible to adapt the
circuit to slightly different lamps and LDRs.
|I've added a lot of coupling
capacitors to keep offset voltages from the outputs
|A Hard-Bypass with a relay that
is controlled by an momentary switch. (Push: Turn on. Push again: Turn
Several of these switches can be wired in parallel, i.e you can have on
on the front panel, and another one connected via jack for remote
|Circuit redesigned for easily
available potentiometers: 50k lin and 50k log (47k is the
It's also possible to adapt it for slightly different pot values (feel
free to ask), if your favorite form factor of potentiometer only comes
in certain values.
|You don't need that special
"staircase" enclosure when using pcb-mounted potentiometers. In my
version, the pots are mounted on the solder side of the board. So the
component side looks down
inside the enclosure, not being in th eway of the front (or rather
|Complete redesign for +/-15V
operation. Option for MOTM-style power connector on the PCB.
|Large heatsinks and stronger
Lamp Driver transistors.
|Mains transformer and primary
fuse not on PCB for safety
|No DIN jack.
Switch for LFO / Manual sweep on front panel
1/4" TRS jacks for remote control of Sweep Rate and Modulation Depth.
The first three
sound samples are from my 2000 Album "Dark
November". The arpeggiated notes from the OB-8 are treated
with the Compact Clone. In the first sample, it's rather tame. At the
beginning of the second sample, the unique resonance Phaser kicks in.
1 (1.172 MB)
Second Loop 2 (1.172
Second Loop 3 (1.172
NEW: PDF document with
schematics of on-board components (and some hints how to cennect
off-board components, too.)
Here's a preview
of what the PCB will look like (preliminary version):
Click on the image for a larger view of the components.
NEW: Component overlay with
reference designators instead of component values. (Very useful for
debugging and modifications)
NEW: Component overlay with component values (as silkscreened on the board)
Bill of Materials
Here's a list of
components that I've used: Krautrock
It only contains the components that are soldered to the board, no
front panel components.
You'll want to have a nice "retro" looking lamp holder on th efront
The incandescent lamp for the front panel is the same type as the two
for illuminating the LDRs.
In Europe, E10 sockets are probably more common than elsewhere in the
world. You can choose whatever socket you want - it just has to fit the
lamp you're using. You can also simply solder the lamps to the PCBs
with short, stiff wires, if you don't find a socket that fits into the
The lamps I'm using are 7V / 100mA types - spec'ed the same as in the
original Compact A Phasing. If you cannot get these, you can try with
6.3V lamps of approximately the same power (0.7 Watt). In that case,
you have to connect two 1N4002 diodes in series with these lamps. The
PCB has the necessary pads and holes for this already, but I have not
tested 6.3V lamps myself.
IMPORTANT: Safety and Thermal
I once had an
original Compact A for testing, and I've been quite shocked about open
mains voltage on the printed circuit board. No, I didn't get an
electrical shock, but I could have. Also, some of the components became
very hot, and the transistors had very small heatsinks. This is in no
way meant for bashing the original design - it may just have been that
particular specimen, and it may have been tampered with (modified) -
What I want to say is this:
We're driving incandescent lamps here, and these need more current than
LEDs, so the transistors which drive them will get hot, the voltage
regulators will get hot, so we have to cool them. There are 4 rather
big heat sinks on the board. Thes will help to dissipate the heat into
the interior of the phaser enclosure. Now, that enclosure should be
sealed against light from the outside, because of the LDRs. This means
it will also be sealed for heat, more or less. A possible solution is
to use a metal enclosure, or at least an enclosure with a large metal
front panel. Keep in mind that the heat transfer then goes like this:
Transistor -> heatsink -> air inside enclosure -> metal
enclosure -> air outside enclosure.
This is why the heatsinks on the board are rather oversized (yet
unexpensive), and why you should have a metal enclosure or large metal
In my first prototype, I have wrapped each Lamp + 4 LDRs combination
with a black adhesive tape, to further improove shielding against light.
With this, you don't need an enclosure that is sealed against light.
But you must be careful about your choice of adhesive tape: It must be
safe against ignition. Ok, the incandescent lamps should only get warm, not hot - the
circuit is designed to run the lamps well below their nominal current
rating, to increase their life-span. But you must make sure that there
are no errors that cause a higher current to flow, before you apply any
tape around the lamps. Run the circuit for a while, and carefully check
the warmth of the lamps with your fingers.
There are many ways to build a phaser with these PCBs, and I cannot
check every possible way how it could be done. In short: I just provide
the boards, but you are responsible
Clone 2007 Prototype Pictures
(to be continued ... )
to my homepage