Updated Version of my FS-1 Frequency
Shifter, complete with PCBs.
About 10 years ago I started to build a frequency shifter with the goal
to have similar sound quality as the Moog / Bode frequency shifter.
I built this on Veroboard, and I was very pleased with the results.
I've chosen a slightly different approach than the famous Moog / Bode
model: Instead of using a beat-frequency oscillator, I implemented a
linear and exponential-controlled Quadrature-Thru-Zero VCO directly.
This covers the whole audio range, and (anlike the Moog / Bode) goes
down far into the sub-audio range, such that you can run it with
something like 1 cycle in 10 seconds, for slow "Barberpole Phasing".
Also, I've chosen a different approach for the noise reduction. Every Frequency shifter has to
battle carrier bleed-thru. In the new version, I have implemented a
second-order trimming which, beyond nulling the carrier in the output,
also allows to suppress the quadratic term, which means cancelling the
bleed-thru at twice the carrier frequency. Even with so much effort
going into the carrier suppression, you want complete silence at the output when
no input signal is present. Moog used a "Squelch" circuit for this,
which is basically a noise gate, and needs manual adjustment of the
threshold, depending on level and nature of the input signal. I am
using a special Compander / Downward-Expander system, similar to the
one found in the Roland VP330 Vocoder Plus (All opamps and OTAs, NE570
etc.!), slightly changed to fit the needs of a frequency shifter.
Now that I've started designing PCBs (Printed
Circuit Boards), and quite flattered by the success of my Tau Phaser PCB
project, I decided the Frequency Shifter would be the next in line.
The whole circuit fits on two 160mm x 100mm
Board 1 contains the Quadrature-Thru-Zero-VCO, the Ring Modulators,
Summing Amps and Compander Circuit.
Board 2 contains the Dome Filter, a Microphone Preamp, an extra 6-pole
All Pass Filter for Barberpole Phasing, an Inverter for CV inversion, a
Power Supply that only needs a transformer and fuses to be connected,
as well as MOTM and Synthesizers.com system connectors for direct
+/-15V power supply.
On the 2nd PCB, you find several components without
component values printed on the board; these are grouped around the
three TL074's and form the "Dome Filter", that performs an
approximation of the Hilbert Transformation, to create a normal and a
quadrature component from the input signal. (Labeled "SigSin" and
"SigCos" on the boards.)
First of all, there are the SIL8 resistor arrays. These contain 4 equal
resistors which are tightly matched to one another. The absolute
resistor value is not important: In fact you can use any value from 10k
to 100k. You need 6 of these arrays. They come in a SIL8 package, and must contain 4 independent
(There are also arrays that countain N-1 resistors for a SIL package
with N pins, with all resistors connected to Pin 1. These are the wrong
Next, there are the resistors and capacitors that set the time
constants. As it's harder to get a certain capacitor value precisely,
the idea is to choose a capacitor with roughly the right capacitance
that is available, and then make a series connection of two resitors to
get the precise resistor value to match the chosen capacitance.
This may sound difficult, but it really isn't. It's just a little time
I have made a Speadsheed to help you calculating the right component
It's pre-filled with reasonable capacitor values, and shows the two
resistors that are calculated fom these.
If you can get these precise capacitor values (1% tolerance), you just
have to read the resistor values like a table and use them, and
If you have different capacitor values, overwrite the pre-filled
numbers and the spreadsheet will do the calculations.
The following picture shows the standard values as a table; if you
click on it, the spreadsheet opens for calculation.
(You may have to download the spreadsheet first, depending on your
system. And you either need MS Office, or the free OpenOffice program.
I've been using the latter. Google for open office if you need the
program; it's free.)
Note: k? means kiloOhms
This project has a lot of trimpots. Frequency
shifting relies on the cancellation of two ring modulation processes,
and therefore high precision is required in many ways. It is
recommended to use a 2-channel oscilloscope for adjustment. Maybe you could also adjust it with
a multimeter and by ear - I'll try to give you some hints in that
direction - but I haven't really tried, and I really recommend using a scope.
Borrow one from a friend if you don't have one (and "borrow" the friend
to help you, too, while you're at it.)
1st Step: Oscillator Level Adjustment
Connect two probes of a 2-channel scope to the two integrator outputs
of the QVCO, as shown in the following picture:
(Clip the probes to the resistor leads of the marked 30k and 200k
resistors. A nearby GND connection is shown at two 10k resistors.)
Set an oscillator frequency of (roughly) 1kHz using the front panel
controls for Range and Frequency.
At Probe 1 and Probe 2 you will see two triangle waves that are 90deg
out of phase with each other, and have different amplitude.
Adjust the multiturn trimpot "Osc_Level_Bal" (see picture above) to get
equal amplitude for the two triangle waves.
2nd Step: Oscillator Waveform
Keep the Oscillator running at approx. 1kHz, as before.
Insert two Jumpers at the positions marked in orange in the picture
Connect Probes at 220k resistors (GND is on neigbouring 220k resistors).
Adjust an approx. Sine waveshape with the "Shape" trimmers. (Top and
Bottom of what previously was a triangle should be equally flattene.)
(You do not need two probes for this: Simply start with adjusting the
first waveshaper, then proceed to the next.
Don't worry about different amplitudes for the two waveshapes at this
point - this will be equalled out in a later step.)
When this adjustment is completed, remove the jumpers.
3rd Step: Oscillator Bleedthru
Keep the Oscillator running at approx. 1kHz, as before.
If you haven't done yet, remove the jumpers left from the previous
Keep the probes where they have been in the previous adjustment.
Make sure that no external audio signals are connected to the PCB.
For each of the two channels, you have to make two adjustments:
First, adjust the 100k multiturn pot ("104", "CarrNull", see picture
below) for minimum signal level at the probe.
Increase the sensitivity of the probe - you will get the level down to
just a few Millivolts. After you have adjusted this
to a minimum, adjust the unmarked 100k single turn trimmer to further
reduce the level.
You have now adjusted the Carrier Bleedthru almost to full cancellation. The
Noise Reduction System will take care of the rest.
(All right - so far I have given adjustment directions without
reference designators, and therefore needed a lot of pictures to
describe where the probes go, and which pots have to be adjusted.
Meanwhile I have a consistent set of drawings - schemos and components
on boards - with reference designators, so instead of drawing a lot of
pictures, I'll just refer to component numbers and pin numbers.)
4th Step: CV Rejection of Compander Adjustment
All referring to Main board (PCB 1).
Without any audio connected to any signal input, we're now feeding a
signal to the CV path of the compander. For this, temporarily attach
one side of a 10k resistor to pin 6 of U14, and feed a square wave of
approx 100Hz, 1Vpp to the other side of the resistor. Exact values are
not critical at all. You can also feed 5Vpp via a 47k resistor instead
- you get the idea.
Now, put a probe to the CmpOut jack and adjust R106 until the signal is
minimal. (It won't disappear completely, which is no problem; just
In a similar way, probe SumOut jack and minimize signal with R127.
Same procedure, probe DiffOut jack and minimize signal with R114.
Remove the resistor and 100Hz feed from pin 6 of U14.
5th Step: Level adjustments
Locate the two 3-pin connectors (for jumpers) near Pin 14 of U10 and U12 (the two 1496 chips).
Plug the jumper onto the two pins that are closer to the 1496 chips.
(These jumpers will be left in permanently for using the frequency shifter - do not remove them after the adjustment.)
With PCB 1 and PCB 2 connected together (signal goes from PCB 1 to PCB
2 via CmpOut connectors, and returns to from PCB2 to PCB 1 via SigSin
and SigCos connectors, and PCB 1 getting its Input from the preamp on
PCB 2), feed a signal of approx 1kHz (sine wave recommended) to the
Frequency Shifter (Mic input or Aux input) and adjust the level until
you get an unclipped signal of 5Vp at the DryOut connector of PCB 1.
Then adjust R99 to get approx. 5Vp at the CmpOut connector.
Probe SumOut and DiffOut connectors and play with the Frequency
settings of the Quadrature Oscillator (Range and Manual
potentiometers); see how the frequency is changed. (There may be some
side effects, i.e. not a pure sine wave, at this point.)
Adjust R123 to get approx. 5Vp on SumOut.
Adjust R110 to get approx. 5Vp on DiffOut.
6th Step: Balance
In the previous step, you may still have some amount of the adjacent
side band in yout frequency shiftet output, which appears as a slight
amplitude modulation in the frequency shifted sine wave. You can
minimize this to some degree with R133 for the SumOut, and with R115
for the DiffOut.
In practice, you probably won't hear much of it, and you cannot cancel
it precisely for every amount of frequency shift, but it's a good idea
to minimize it. Otherwise, just leave R133 and R115 in mod position.