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 Forum index » DIY Hardware and Software » Lunettas - circuits inspired by Stanley Lunetta
Digitally Controlled Phaser
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Sun Aug 06, 2017 7:04 am    Post subject: Digitally Controlled Phaser
Subject description: Set phasers to stun!
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ok, it is not that kind of phaser.


I found this circuit in an old edition of Elektuur (Jul/Aug 1979). It is a pretty straight forward phaser circuit
but instead of using Vactrols or FETs it creates a variable resistance by using a mux and a resistor network.
There were 2 things I noticed about the circuit;
1. CMOS! which is always a good way to grab my attention.
2. A lot of CMOS, which made me wonder what does this big network of logic gates do ?

If you're wondering what network ?, it is a part of the circuit that I simplified. The original circuit had a rather
complex network consisting of 1x binary counter (4024), 12x 2-input NAND gates (4011), 2x 3-input NAND
gates (4023) and 1x flipflop (4024). The whole purpose of this network was to create a 3 bit binary signal that
counts up (000-111) and down (111-000). I simulated it to be sure it didn't do anything peculiar but that's all it
does. Besides the fact that this can be achieved much easier there was something else which made me scratch
my head. The 4th output (Q3) of the counter was connected to its reset input, which by itself isn't necesarry, but
then it used the two 3-input NAND gates and the flipflop to derrive 2 signals from bits Q0..Q2 which was nothing
else than Q3 and Q3 inverted. So 1 inverter would have done the same thing.

As I said counting up/down can be achieved much easier and the way to do it is with XOR gates. XOR gates can
be used to invert a signal and if you invert the outputs of a counter the resulting outputs count in the opposite
direction. Since it has to be toggled between inverting and non inverting at every 8th step you can use Q4 of the
counter to toggle it. So 1 counter and 3 XOR gates is all that's needed. The original circuit was Clocked by a 555
but to reduce the chip count even further I replaced the counter with a 4060 which has an internal network that
can be used to create an oscillator. This brings the total amount of chips for the control circuit down from 7 to 2!

The Clock frequency of the original circuit was variable between 0.01Hz and 10Hz and since I haven't tested the
4060 oscillator I don't know yet what values you would need to get that range. It is possible to calculate it although
I usually find it easier to just breadboard it which I'd have to do anyway to test if the calculations are correct. Also
because the 4060 has more outputs you could make it oscillate at a much higher frequency.

It's a bit more analog than your usually lunetta circuit but I think that because of the digital control that especially the
lunatics here would appreciate it. You could just built the section with the opamps and muxes and then connect the
3 digital control lines to your lunetta synth. no idea what it does but it might give some interesting results.
One thing to pay attention to though is that it uses a DUAL SUPPLY of -/+6V. You might get away with using a
12V supply and create a fake ground with a 6V voltage regulator or a more suitable component. Because of this dual
supply the 0 reference for the CMOS chips is -6V so you have to keep that in mind when connecting it to something
else as the GND connection is used as a reference for the input signal. With some additional circuitry you could make
it so that GND is also used as a reference for the CMOS chips.

note that in the original circuit pins 6 (INH) and 7 (VEE) were not drawn/connected. Pin 6 should definitely be
connected to -6V, pin 7 might be used to change the reference voltage for the control inputs for the muxes but I will
have to look into that. I also think there might be some improvement in changing the values chosen for the resistor
network.

Anyway here's my adaptation of the circuit Cool


Digitally Controlled Phaser.gif
 Description:
Digitally Controlled phaser

original ciruit by G. Duffau [elektuur 189/190 (Jul/Aug 1979)]
adapted by PHOBoS.
 Filesize:  85.27 KB
 Viewed:  697 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

Digitally Controlled Phaser.gif



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Last edited by PHOBoS on Mon Aug 07, 2017 4:41 am; edited 3 times in total
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PHOBoS



Joined: Jan 14, 2010
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Audio files: 705

PostPosted: Sun Aug 06, 2017 7:31 am    Post subject: Re: Digitally Controlled Phaser
Subject description: Set phasers to stun!
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PHOBoS wrote:
.. pin 7 might be used to change the reference voltage for the control inputs for the muxes but I will have to look into that..

I just had a quick look at the datasheet and if I understand it correct than you could connect pin 7 (VEE) to -6V and
pin 8 (VSS) to GND which would make the control inputs accept signals of 0 (GND)/+6V. This also means that if you'd
still want to use the control circuit it would have to be powered from GND and +6V. The max. supply voltage of the
4015 seems to be 18V so you might be able to use a -5V/0/+12V supply. (useful if you already power other lunetta
circuits from +12V)

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piedwagtail



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PostPosted: Sun Aug 06, 2017 7:53 am    Post subject: Reply with quote  Mark this post and the followings unread

Sample! Exclamation

Spelling mistake; think you mean lunetics Smile

Just been putting an audio rate NANDulator through a test bed ARP 4023 filter with provisional VC Resonance. Awfully good!




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PHOBoS



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PostPosted: Sun Aug 06, 2017 8:16 am    Post subject: Reply with quote  Mark this post and the followings unread

ah yes, lunetics! That's the word I was looking for. Laughing

I can't give you any samples as I haven't build it (yet), but I am working on something else at the moment
and this might become a future addition to it. I do want to test the resistor network first though as the ratios
between them are: 1.45/1.76/1.83/1.50/2.06/1.47/1.20 not sure if is trying to approach a sinewave but they
seem a bit of.

Good to hear you are getting so much milage out of the NANDulator. Very Happy

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PHOBoS



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PostPosted: Sun Aug 06, 2017 1:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

In the circuit description it says that the range of the Clock frequency is 0.01Hz to 10Hz which seems to be a
VERY large range (x1000). Also if I calculate the frequency of the 555 oscillator I get a range of 14Hz to 720Hz.
So I don't know what's up with that or what might be a good frequency range for a phaser. I guess it needs some
experimentation.

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blue hell
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PostPosted: Sun Aug 06, 2017 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

0.1 to 10 Hz I'd say .. not sure if longer than 10s would survive my attention span ... but audio rate might be fun :-)
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also .. could someone please turn down the thermostat a bit.
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PostPosted: Sun Aug 06, 2017 2:00 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah high frequencies might be fun and I can see a problem with the slow frequency as it has a very low resolution.
Of course you can experiment with using different frequencies for the control pins Very Happy

Just for fun I made a plot of the resistor values which looks like this. It's a bit sine like but could be better.
edit: using 4K2 instead of 3K3 preserves the overall shape but smooths out one kink. (looks like this)
I don't know how that would translate to the sound though.

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PHOBoS



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PostPosted: Sun Aug 06, 2017 8:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

after doing some calculations I found the following resistors from the E24 series get closest to a sine
(at least when using the same minimum [470] and maximum [12K] values) : 470, 1K, 2K7, 5K1, 7K5,
10K, 11K, 12K. which results in this plot. If you add a series resistor of 510 to the 11K it aleady
looks much nicer.


For more accuracy you could use the following resistors from the E96 series: 470, 1K05, 2K67, 4K99,
7K5, 9K76, 11K5, 12K. which results in this beauty.

I don't know if a sinewave is preferred though.

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PostPosted: Mon Aug 07, 2017 7:19 am    Post subject: Reply with quote  Mark this post and the followings unread

I've been looking at some other existing phaser circuits and so far I came across triangle and sine wave LFO's used as
a modulation source. It does make some sense to me as it is the modulation signal so you will hear it as it produces the
phasing sweep. Different waveforms could create interesting results though and I think a voltage controlled phaser might
be fun. For this phaser circuit it would mean changing the resistor values so that's not very practical. However, because
of the digital control you can change the order of the resistors which of course also changes the shape of the modulation
signal and I think that is the fun part of this circuit.

There is something else I noticed. The allpass filters used in other circuits all seem to use the same capacitor values. So
not a range as it is used in this phaser. This does make sense to me now that I have a bit more understanding of how a
phaser works (thanks jan, chat was helpful Very Happy). So I don't know why this is done and will look at some more phaser
circuits to see what I can find.

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PostPosted: Mon Aug 07, 2017 8:36 am    Post subject: Reply with quote  Mark this post and the followings unread

Re. chat yesterday .. I remarked then that the shape of the resistor curve would not necessarily have to be linear .. with some mumbling about frequencies ... thinking again today it's not the frequencies that are interesting but the phase shift .. or actually the delay time ..

Still .. as the phase shift is not linear with the resistance .. the resistor curve might still look a bit odd when you'd want the delay time to vary as a .. say .. a sine .. it's all a little more complicated than I thought Laughing

There probably is is a dependency of the delay time on frequency as well - as the thing is not perfect .. maybe that would explain the different C values used?

mumble mumble ... Laughing

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also .. could someone please turn down the thermostat a bit.
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PHOBoS



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PostPosted: Mon Aug 07, 2017 10:24 am    Post subject: Reply with quote  Mark this post and the followings unread

[warning! math ahead]

Let's see if I can explain how I calculated the resistor values to get a sinewave as this might be useful for future reference.
I might have used more steps than necessary but I got the result I wanted.

I first had to refresh my memory about how to draw a sinewave and calculate points on it. The following image was very
helpful for that. y= sin(x) is what I needed to know.
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Looking at the waveform you can see that the section between 90º and 270º is the sweep the muxes + resistors create when
counting in one direction. Counting in the other direction mirrors this sweep resulting in a full period. You can also see that
from 90º to 270º y varies between 1 and -1 so a total 'amplitude' of 2, this will come back later on.

The muxes have 8 steps, so the sweep get's divided in 7 sections. An odd number of steps might be better as it would have a
single point at the top and bottom. With an even number of steps they get doubled so there are flat spots. It would be possible
to change the control circuit so it doesn't produce the double 111 and 000 steps but that would probably make it a lot more
complex.

First I divided the range from 90º to 270º into 7 sections. 270º-90º=180º, 180º/7=25,71º (note that when calculating the values I
didn't round the numbers but put them in memory to reduce errors as much as possible) This resulted in the following values for x.
Code:
x1 is at                     90.00º
x2 is at  90º    + 25.71º = 115,71º
x3 is at 115,71º + 25.71º = 141.43º
x4 is at 141.43º + 25.71º = 167.14º
x5 is at 167.14º + 25.71º = 192.86º
x6 is at 192.86º + 25.71º = 218.57º
x7 is at 218.57º + 25.71º = 244.29º
x8 is at 244.29º + 25.71º = 270.00º


next I calculated the values for y by using y = sin(x)
Code:
y1 = sin  90.00º =  1.00
y2 = sin 115,71º =  0.90
y3 = sin 141.43º =  0.62
y4 = sin 167.14º =  0.22
y5 = sin 192.86º = -0.22
y6 = sin 218.57º = -0.62
y7 = sin 244.29º = -0.90
y8 = sin 270.00º = -1.00


The next step was to translate the values of y1..y8 to resistor values. 470 is the minimum value which corresponds to y1 = 1,
12000 is the maximum value which corresponds to y8 = -1. (you could do it the other way around as well).
As mentioned earlier the difference between these values is 2. the difference between the min and max resistor values is
12000 - 470 = 11530. So if you change y by 2 the resistance changes by 11530 ohm and if you change y by 1 the resistance
changes by 11530/2 = 5765. Of course if you look at the values for y it doesn't change by 1 so I first calculated the change in values of y.
Code:
y1 .. Y2 =  1.00 ..  0.90 = 0.10
y2 .. y3 =  0.90 ..  0.62 = 0.28
y3 .. y4 =  0.62 ..  0.22 = 0.40
y4 .. y5 =  0.22 .. -0.22 = 0.44
y5 .. y6 = -0.22 .. -0.62 = 0.40
y6 .. y7 = -0.62 .. -0.90 = 0.28
y7 .. y8 = -0.90 .. -1.00 = 0.10


Then I multiplied the change in y with 5765 to get the corresponding change in resistance and added it to the previous
value to get the total resistance as there is only one resistor used at a time. (note that if you would calculate it using these
numbers the results are different because of rounding errors)
Code:
Ra                            =   470.00
Rb = (0.10 x 5765) +   470.00 =  1040.91
Rc = (0.28 x 5765) +  1040.91 =  2640.58
Rd = (0.40 x 5765) +  2640.58 =  4952.17
Re = (0.44 x 5765) +  4952.17 =  7517.83
Rf = (0.40 x 5765) +  7517.83 =  9827.42
Rg = (0.28 x 5765) +  9827.42 = 11429.09
Rh = (0.10 x 5765) + 11429.09 = 12000.00


And finally I picked the closest values from the E24 and E96 series resulting in the values posted in a previous post.

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PHOBoS



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PostPosted: Mon Aug 07, 2017 10:41 am    Post subject: Reply with quote  Mark this post and the followings unread

I actually did start on calculating frequencies but that does correspond in a linear fashion with the resistor values {f= 1/2πRC}.
The delay time is indeed more complex. A for the different C values it looks a bit like it would function as having different phasers
in parallel except that there would have to be groups of 2 filters with the same value. I still find it confusing as I think that the fact
it has digital control shouldn't really matter. Anyway even if you would calculate some kind of pefect responce, whatever that may
be, there is still another variable; the human ear and the brain connected to it. (apart from speakers/headphones, other gear,
the sound you put through it etc.). I guess the only way to find out is to built it and make recordings using different resistor values
and then compare those. Maybe use the same values for C as well to see how that changes it.

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LFLab



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PostPosted: Mon Aug 07, 2017 12:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Interesting circuit!
You may also want to add some feedback? As it stands it is probably more of a subtle effect.
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PHOBoS



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PostPosted: Mon Aug 07, 2017 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, when looking at some other circuits I came across the feedback to have resonance control so that would be
a nice addition. I'd probably also add another mux with some LEDs to have some nice blinky lights.

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