JH. Storm Tide Flanger
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My latest effect device (as I'm writing these lines) is an analogue flanger. There are more Flanger boxes on the market than I could possibly count, some Flanger stomp boxes pretty unexpensive, and I already own a few delay FX boxes (both analogue and digital) which can produce a Flanging effect - so why build another one ?
There's a couple of reasons, because there are some special features you won't find on many Flangers, which I wanted to try, and - if found pleasant - implement them in one device. And indeed I did find pleasant things, many of them coming from the "Instant Flanger", a classic from Eventide Clockworks.
The following circuits and descriptions are provided for personal, educational,
non-commercial use.
I will try and give reference to the commercial products that inspired
me. Most notably that's the Eventide Instant Flanger. Eventide is a company
still in business, and they have filed patents regarding the Instant Flanger
(though I could not find the patent number yet). Everybody who wants to
build this, or a similar circuit should keep that in mind. As for my part,
the combination of features and my own additions, the circuit is presented
for the "DIY community" for personal, non-commercial use.
I'm using the SAD1024 dual BBD chip, which is out of production and,
while not ultra-rare, a bit hard to come by. You could use two single BBD
lines like the TDA1022 or Panasonic "MN" BBDs with a slightly lower limit
of clock frequency. (I'm clocking the SAD1024 up to 1.5MHz, just to test
how far I can go; it's the first time I used the SAD1024.) TDA and MN devices
are no drop in replacements, so you should know how to modify the circuit
before you take that path.
There's a large number of trimpots in my circuit. Many of them are
not necessary, but I used them to test the limits of the circuit. The circuit
is given as I built it, with all the trimmers rather than optimal resistance
values.
Features I wanted to try, and results I got so far:
1. "Theta Processor"
This word was coined by an article in Electronotes for a combination
of Flanging and Phasing.
Flanging, on the one hand, involves constant (i.e. independent
of frequency) delay, i.e. linear phase. Phasing involves frequency dependent
phase shift, which also means frequency dependent delay.
The main shortcoming of Flangers is that the peaks and notches (when
the processed signal is mixed with the dry signal, or when feedback is
applied) are spaced linearly over frequency, which means that you get more
and more of them per octave for higher frequencies. They are are also located
at integer multiples of Hz, so when one peak of the Flanger's response
courve hits a harmonic of an input signal, all the other harmonics will
be hit by other peaks of the flanger at the same time. This is the reason
for the "metallic" sound of flangers.
Phasers, on the other hand, have a more "musical" distribution of peaks
and notches. The spacing can be controlled by design (by detuning individual
all pass fliter stages), but even with several identical stages you get
get very different spacing as with a Flanger. The big drawback is that
for a "deep" sounding Phaser, you need a lot of voltage controlled all
pass filter stages. And even a 14-stage Phaser as found in the ARP Quadra
will only produce 7 notches.
The idea is to combine a delay line (Flanging) with all pass filters
(Phasing), to get the huge number of peaks and notches, but a "musical"
(notches per octave rather than notches per Hz) spacing as with Phasers.
The Electronotes article proposed a combination of a few dozend *fixed*
all pass filters (that is, just an opamp, 3 resistors and one capacitor
each) and a modulated delay line BBD). I never tried this circuit, but
the article showed that with 50 ... 100 all pass filter stages the goal
would be achieved quite perfectly.
The Eventide Instant Flanger uses a similar approach, but with much
less stages. There are two BBD delay lines (clocked at the same rate, so
it's more like a single center-tapped line), and as few as 4 all pass stages.
As it's practice in Phasers and Flangers, the input signal is split into
two paths. One goes into the BBD lines, one goes into the all pass filter
chain. The Eventide creates a stereo output signal like this: One channel
is a mix of the first half of the BBD delay and the first couple of all
pass filters. The other channel uses the full BBD delay and the 4 all pass
filter stages. Now that looks much more sensible than 50 or 100 all pass
filters, and I wondered how effective it could be. I definitely wanted
to try it.
My version has the all pass filter stages switchable (with separate
switches for the first and second pair). I also included a (switchable)
send / return effect loop to try longer all pass filters, or fixed delays,
or frequency shifters, or whatever. (See Thru-Zero-Flanging below).
The result was really stunning. The 4 all pass filters have no big
effect as long as the Flanger runs in mono, and they are certainly not
enough to create the ideal of "musical" spacing of peaks described above.
But as soon as you run it in stereo - BIG effect. Running the Flanger in
stereo *without* the all pass filters is quite nice, but nothing really
special; you have probably heard it many times before. But switching in
the all pass filters sort of "unlocks" the stereo image. Your switching
from an up-front metallic resonating stero effect to a much smoother "space"
sound ("space" as in "room", not as in "outer space"). Well, outer space
is within reach as well. I have only made tests on the veroboard as I'm
typing this, but I think I have a clue about Tomita's drastic Flanging
effects now (;->).
2. Thru Zero Flanging
It is well known that the "original" Flanging (with two tape machines)
can produce an effect that can't be emulated with a single electronic delay.
Electronic flangers can vary the delay time between long and short,
but the delay cannot become negative, i.e. it cannot go "thru zero".
With two tape recorders, any of the two machines can be slower than the
other, so you can manipulate the speed such that one half of the signal
"overtakes" the other. For one moment of overtaking, both signals are in
phase for *all* frequencies, which causes a special effect. To emulate
this with electronic devices, you need two delay lines with their outputs
being mixed (to to be mixed up with the two BBDs in series as in the Eventide),
with one delay time fixed, and the other modulated shorter and longer than
the first. Or with two delay lines modulated in opposite direction.
I have made such experiments with the BBDs of a modified Polysix FX
board, and the results were not quite as desired. A friendly member of
the synth-diy community has told me he had similarly unsatisfactory results.
The problem is aliasing noise here. Delay lines (BBD or digital) are time
discrete devices, and even with the two clock frequencies way above the
audio range, you get interference from the difference frequency of both,
which *will* be in the audio range at some point, when you're modulation
one clock frequency ahainst the other. Worst case is using two identical
BBD chips, as the *fundamentals* of both clocks will interact during the
most interesting thru-zero time.
My choice was not to integrate a second delay line in this box, but
provide an insert path instead. So I can try various external delays, from
Effectron to Dynacord, and see which will give the least interference.
The insert path is inside the compander system of the Storm Tide Flanger,
so the external delay line can always be operated at optimal input level.
3. Noise Reduction
There's a friendly person on the synth-diy list who will remind us
that BBDs sound bad, from time to time.
(Hi Harry !) Compander-less BBD delays have no great SNR indeed, and
require careful adjustment of input levels at least. Many BBD delays use
a NE570 or NE571 for noise reduction. This is a great improovement, especially
when it's done as well as in some of these big Dynacord devices. But these
chips are rather slow (if you design them for low distortion, that is),
so these boxes are still prone to overload from fast attack transients.
I have chosen the simple but effective compander from the Phaser of
the ARP Quadra. This is optimized for use with medium to high level synthesizer
signals. (If you run a readily mixed ballad with voice and e-piano at low
volume thru it, it's not that perfect, but it's great for all kinds of
synthesizer sounds, and for a Wurlitzer at decent level as well.) No more
hairy noise, Harry (;->).
4. Resonance
Resonance can be positive or negative (center zero potentiometer),
and the resonance loop can be chosen short (one BBD) or long (both BBDs).
This provides good variations, but is pretty standard. The interesting
part is the Resonance Limiter, which is also "inside" the Compander system
(after Compressor, before Expanders). Not my invention but ARPs, I cannot
stress enough how sweet this sounds. It's also taken from the Quadra Phaser,
with levels adjusted to fit the BBDs. The idea, as I understand it, is
that you have full control over the "balance" between input signal and
resonating peaks. In many Phasers, Flangers and Synthesizer VCFs the resonance
will either be to "weak", or it will "scream" into your face. Sometimes
this is desired, but sometimes you have to carefully set the resonance
amount to find a "sweet spot" - for one input signal, that is. If your
input signal changes its harmonic contents (not so much a problem with
synth VCOs, but with more complex sonic material), it might suddenly scream
once again, when a new harmonic hits a resonant peak of your device. The
solution is to limit the amplitude, which many circuits do, some better
than the others. Often it's the *combination* of input signal and resonance
loop that is limited, which leads to problems.
Imagine your device clips at 0dB (for ease of description). You may
even have a soft clipper at 0dB to prevent too "harsh" distortion.
Now feed in a signal with variable level, and increase the resonance.
Let's start with a input level of -10dB. Without resonance, that's -10dB
at the output as well. Now increase the resonance (with input level constant
at -10dB) and adjust the delay time until the resonance peak "hits" a harmonic
of the input signal. With increasing resonance, you'll get an increasing
peak (or several peaks) in the frequency response, until your output amplitude
reaches the soft clipping point at 0dB. Increasing the resonance further
will increase distortion (sometimes pleasantly so), but it won't increase
the peak level anymore. So you get a nice resonance of 10dB relative to
the input signal.
Now repeat the very same experiment with an input level of -40dB. Increase
resonance until soft clipping occurs at 0dB, only now your resonance has
become a "scream" 40dB louder than your input signal.
When I was new to electronic music, and my ears were younger, I was
fond of this, but meanwhile I see the benefits of smooth sounding circuits.
With the ARP type resonance scheme, you have full control of the "sweetness"
(or harshness) of resonance. After the peak-limiting type compressor, you
have a pretty constant signal input amplitude, which is then divided down
to fit the BBD's voltage swing. For instance, with the BBD being capable
of 1Vp, you can "partly fill it up" with the input signal to, say, 0.25V,
(or -12dB) *regardless* of the actual signal amplitude before the compressor.
Then you can adjust the soft limiter of the feedback path to 0.75V, so
the sum of both will never exceed 1V. You're effectively limiting the resonance
peaks to 12dB above the input level.
This can be adjusted for other values of course, by choosing a different
divider factor between compressor and BBD, and by adjusting the resonance
limit accordingly. It's just important that both add to the maximum allowed
voltage swing of the BBD.
This is the "big secret" of the ARP Quadra Phaser, IMO. The limiting
factor there is just the level of the all pass filter transistor ladder
rather than a BBD.
5. Bounce
Now this is another Eventide feature. It's not the original circuit,
but I have just used different components to avoid electrolytic capacitors,
so all credit for this goes to Eventide here as well.
The summed control voltage (Manual, Pedal, LFO, Envelope follower)
is fed into a sub audio BPF, then into a nonlinear amplifier with positive
feedback (not a schmitt trigger, but similar), and then filtered by a second
BPF. A variable portion of this is mixed to the straight CV to control
the BBD's clock frequency. The effect is most prominent on single-shot,
non-periodic CV changes, such as a fast single Manual of Pedal sweep. The
single sweep is followed by smooth "echoes" that go in either direction
(faster clock and slower clock). This is to emulate the "bouncing" tape
speed of reel-to-reel tape machines when the friction from a thump against
the reel is suddenly released.
6. Foldback
This is a full wave rectifier in the CV path that prevents the clock frequency from going too low. Rather than just limiting the CV, it folds negative CVs back up, which gives a nice effect with the Bounce circuit and also with LFO waves that would go below zero (in combination with the manual CV, that is). I recon that Eventide has used this to limit the clock rate (sampling theorem ...) and to enhance the Bounce effect. I have changed this such that the foldback threshold is variable, so you can limit the range to higher clock rates if you like. Just bounce between 500us and 300us delay if you want ... (My lowest limit, with the foldback pot fully ccw, is adjusted for 40kHz clock rate.)
7. LFO Waveshapes
With an external CV input available, the internal LFO can be left quite basic. No internal Sample and Hold or SAW waveforms, for instance. I have implemented two options, a linear Triangle, and a EMS-like unsymmetrical Sine wave that produces the famous "roller coaster" or "Dive" effect. The idea is old, but my single-OTA circuit may be not. Input level shifting, nonlinear bending and re-adjusting output DC level in a single 3080 stage.(The Switch performs a brute-force override from the triangle LFO when its closed.)
8. Anti-Aliasing Filter
Built around a single LM301, this has a 24dB/Oct rolloff. Taken from
the Eventide Flanger, does not work with other opamps. (See U2 in Schematics
Part 2.)
Schematics Part 1: Control Voltages
Schematics Part 2: Audio Signal
Supply voltage is +/-15V @100mA