Saturday, 7 August 2021

ARP 2500 - 1047 Multimode Filter / Resonator

The ARP2500 filter/resonator 1047 module


It's a lovely sounding filter whos design evolved from analog computing.
 
 This module is a combined Low Pass, Bandpass, Highpass and Notch (band reject) filter.
All the filter outputs are available simultaneously in the lower section of the module.
This is why it's called a multimode filter.
 
I think the 1047 was one of the first multimode filters ever produced.
 
Resonance or Q, controls filter shape.  
Low Q settings give wider and smoother filter shapes.
They result in a gentle effect on the sound.
 
As you increase Q, the filter shapes become narrower & sharper. THis helps in focusing on narrower frequency bands.
As Q goes even higher "pinging" may occur, esp if you trigger the filter with a gate.
This is good for percussive sounds.
 The filters start to peak boosting some frequencies into overload territory.
 
 
 
 
 
 
 
 
 
 
These are the outputs and inputs
 
There are two audio inputs, two filter cutoff frequency control voltage inputs and two 
resonance inputs for CV control. The 6 pots are attenuators for these inputs .
 

What's really interesting about this filter is the bandpass section.
The 1047's band pass filter mimics that of a natural acoustic resonator.
Typical examples of natural resonators include strings, pipes, horns, drums.
The bandpass is a single pole 6 dB slope.
This gentle 6dB slop which lies on either side of the frequency cutoff point makes it
ideally suited for replicating things like drums, violins, etc


The filter can self oscillate as the resonance (Q) is turned up.
 

So you can patch a trigger into the filter and make it "ring" at very high resonance levels.
The input for your trigger is in the top right corner of the module.
 
The decay time of the ring is set by the resonance control. 
 
This filter design is how many analog drum machines
emulate sounds like Kicks and Toms.
You use the frequency knob to set the pitch.
I like to use a sequencer to automate varying the pitch while pinging the filter with a trigger/gate signal.

This ringing occurs as it's design is similar to mechanical resonators which ring when struck by an impulse of energy.
 
 
 
 
Turning the "Keyboard Percussion" switch to "on"  will connect the trigger output of your keyboard (or whatever other module is producing the trigger) to the audio input of the filter.

 
The Algorythm by Grayscale is excellent for this task.
 
 

 
Unlike earlier bandpass filters which simply combined LP & HP filters, 
The 1047 filter is an analog computing circuit consisting of summers and integrators.
 
If I'm reading the schematic correctly it looks like a State Variable filter.
 
 The state variable filter is a type of multiple-feedback filter circuit that can produce all filter responses simultaneously from the same single active filter design.
 
In programming a state variable filter/resonator on an analog computer, we'll need two integrators and one inverter connected into a loop.
 
You can see 3 op-amps (A1,A2,A3)
The last two use capacitors in a negative feedback loop -- they are RC integrators.
The first opamp is a summing amp.


Tapping the output of the first amp gives you the HP output.
Tapping the second gives you the BP output.
This BP out is fed back into the 1st op-amp, (non-inverting input).



Tapping the output from the third OP amp gives you the LP output.
This is fed back into the 1st opamp (the Summing amp)... the non-inverting input.
 

Filter frequency can be set both manually & with control voltages. 
The center frequency Fc of the band-pass output is the
cutoff frequency of the high-pass and low-pass outputs.
F c may be set by the coarse and fine frequency knobs
over the range of 16 Hz to 16 kHz.

There is also CV control over resonance.
With the resonance (Q) knob at minimum and the resonance switch set to "norm," the band-pass output has a gain of 0.5, and attenuates 6 dB per octave above and below Fc

 
 
 
 
 
 
 
When triggering, watch the overload light.
It indicates excessive input
 
Note that there is a resonance limit switch.
Setting it to "LIM" prevents signal overload when high levels of resonance are used.
 
 
 
 
Or the INPUT attenuator knob can be turned down
if the input source is the cause of the overload.
 
The switch effectively limits the height of a filter’s resonant peak. 
The LIM setting is preferred for signals which possess a strong harmonic or fundamental frequency.
 
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The Notch Filter

This notch Fc knob is used to offset the notch filter’s center frequency (“fc”) set by the COARSE and FINE frequency controls. 
 
 The filter passes lows and highs, cuts out frequencies somewhere in the middle
 
 
The default setting for the notch filter is 1. 
 
In the pic above, the dial is fully counter clockwise. 
Thus the Notch frequency is shifted significantly below the filter's frequency cutoff (fc).
In effect, the notch filter is a copy of the high-pass filter. 

The reverse occurs if the dial is fully clockwise (ie past 4). 
The Notch frequency is shifted significantly above the filter's frequency cutoff (fc).
In effect, the notch filter is a copy of the low-pass filter. 

Notch filters can be used to create a faux phasing effect by modulating the cutoff with an LFO.
The notch filter is kind of like a high pass & low pass filter in parallel. 
The sound is quite cool, esp when sweeping noise


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Links
 
For more info google:
 
stand-alone Analog Computation equipment used for Electronic Music

North America based companies EG&G (Edgerton, Germeshausen & Grier) and PARC (Princeton Applied Research Corporation) of New Jersey (Princeton roughly between New York and Philadelphia),


EG&G PARC model 121
 
 
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NLC Bongo Fury - Part 2 - Build

 This is part two of my NLC bongo drum build.
 
Part 1 is here:
 
This is a triple version of the original Bongo drum
Bong0 - Bongo - Drum / VCO build notes 
 
 This is where I left off.

Most of the components are on the board.

The signal diodes next



The power rectifiers


 


 
100K pots.. linear





 
Added the zener diodes

 Careful with the orientation




Links
 
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You can find more NLC builds here.
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Wednesday, 4 August 2021

Ait Benhaddou - Morocco

 Since covid is preventing travel these days I'm reliving an old trip.
 
 
These are some pics from Morocco.
 
 
It's a fortified village which along the former caravan route between the Sahara and Marrakech.
 

The village is called  Ait Benhaddou.



It's a UNESCO world heritage site.


It is considered a great example of Moroccan earthen clay architecture
 

  The site of the ksar has been fortified since the 11th century during the Almoravid period
 

  Structures are made entirely out of rammed earth, adobe, clay bricks, and wood.
So they need constant maintenance. They are easily eroded by rain .


Monday, 2 August 2021

Bongo Fury - NLC build notes - part 1

 These are some build notes for the bongo fury.


Three drums in a 8 hp panel.

The circuits are common twin-T driven oscillators, the LEDs work as nonlinear resistors in one section of the twin-T circuit.

 
 This will probably work well with Andrew's Triad - triple envelope generator.
 
This is a triple version of the original Bongo drum
Bong0 - Bongo - Drum / VCO build notes 
 
 
 
 
 
 
 
 
 
 
 
Some pics of the unpopulated board


 Fairly common parts.. except the diodes - 5.1V zener
I do have these, but the PCB may not have the room 

 These are the diodes you want.

 
 
the 3 opamps first.
TL072s


100K resistors

 
the trimpot has been replaced with a 75k resistor (marked 75k* on PCB)

 
Caps
100n, C1, C2, C3
I'm using the same on all.


To Be continued....


Links

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You can find more NLC builds here.
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Sunday, 1 August 2021

Klee Sequencer - Eurorack format - part 1

Finally getting this Klee sequencer built.
The Klee sequencer is an exercise in the unpredictable.
It can be used just like a normal linear step sequencer but that would negate all the complicated circuitry inside.

 
Klee patterns are like "loose clay". That's one of the reasons for its name. 

That, and I suspect it may also be in honor of the artist Paul Klee, who's abstract paintings reflect this unconventional type of programming.

I think I purchased the kit in 2016 but you can still buy these from Synthcube
This is Rev 1.22D
 

These are the virgin PCBs



 It looks like one of the main differences between the Klee and  a standard step sequencers is that the Klee can have more than one step running at a time. 
 
You need to load the steps you wish to use. Loading just 1 step (actually a bit) will approximate a standard sequencer. 
 

Links


The LED driver circuit

















You can load up to 16 steps or bits before you run your sequence.
(You do this by flicking the switch above the step "up" and pressing the LOAD button).
 

The output voltage of each of these bits are added together.
There are 3 CV outputs:
CV A Output: The output voltage of all the steps in the "A" section
CV B Output: the output of all the steps in the "B" section
CV A+B Output: the combined outputs of all the steps together



Getting back to the build:

Need 16 of these to match the sliders (8 on each side)
 

Slider LEDs in sync with bits: Cut/strip the included purple wire into the appropriate lengths and place them into the Step spaces


The board is divided between digital and analog sections.
Start  with the IC sockets



Analogue side first

25 x 100K resistors first.
R1 to R 25
They are all on the left of the board

These 100K resistors have a tolerance of 0.1%
These are involved in mixing the voltages from the programming pots.
Their tolerance will play a role in how accurately this is accomplished.













100ohm next
R49 to 57
















Diodes 
There are three varieties of diodes to deal with on the Analogue Board. The 1N4148 diodes could be substituted with 1N914 diodes. The 1N4001 could be substituted by any other high power rectifier diode (such as 1N4002, etc). The BAT85 could be substituted by any low voltage Schottky diode (such as a BAT42), but be very careful that it does not exceed 400 mV at 10 mA.
 
 
There are 28 x 1n4148 signal diodes
D1-D28



UA78L10ACLP Voltage Regulator
The 1n4001 diode
Cap - Film Capacitors 63volts 0.33uF 5% - C9

 
 
 
 
 
 
 
 
 
 
 
 
 
 
Electrolytic Cap 4.7uF - C11, 12, C10 ??
 

 

The 104 caps - 0.1uF
There are 14 of these
I think these are just for decoupling
C1 - C8 & C13 to C18
 
 
Resistors The default value for R33 is 1K – it is used if the standard 8V fixed range is selected as the function of position 8 of the rotary Range Switch.
R33 is going to determine the maximum voltage that can be “dialed in” with the Variable Range Control.
 
 
 
 
100K OHM TRIMMER POTENTIOMETER CERMET 25 TURNS 3296W
These are marked 104
R42, R43
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Range Switch Position / Maximum Pot Range Voltage/Interval (V/Oct) 
1                                   0.333V, Major 3rd Interval 
2                                   0.4167V, Perfect 4th Interval 
3                                   0.583V, Perfect 5th Interval 
4                                   0.666V, Minor 6th Interval 
5                                   1V, One Octave 
6                                   2V, Two Octaves 
7                                   4V, Four Octaves 
8                                   8V, Eight Octaves
 
Fortunately, these values are adjustable using the trim pots. 
With the standard values, ranges 1 through 3 are adjustable from 0V through 0.625V. 
Range 4 is adjustable through 1.28V with the standard values. 
Ranges 5 through seven are adjustable through 5V 
Range 8 is adjustable through 8.1V. 
If one changes any of the divider resistor values for these ranges (R26 through R33),
it is possible to exceed these ranges. 
 

BAT85 Diode --  D30
D30 provides protection from any negative voltages that may be applied to the External Variable Range jack. This jack directly feeds the input of U7, which is a CD4051. The CD4051 cannot accept a voltage lower than 0.5V below its supply rail. D30 is a BAT85 Schottky diode that will prevent voltages 0.4V below ground from being applied to U7.




These 8 trimmers are marked 103
10K Vertical 20 Turn 3296W
Their designators are R34 to R41
These 10K trimmers are used for calibration of the Range Switch voltages.


The trim pot on the bottom left (R41) in the pic needs to be removed. 
So really you only need to install 7 of these










Some notes re the GATE BUS
As I'm building this kit, im reading the manual..... so this may change as I learn more.
Please shoot me an email if i'm in error.
 

It seems that the gate switches work on a similar principle to the CV section.
 
There are 3 gates/triggers which trigger when the switches for each step are in the up position...
ie when  the steps are illuminated. 
 
Gate bus 1 corresponds to sequence A
Gate Bus 3 corresponds to sequence B
Gate bus 2 is kind of a merge of both sequencers.
 
The centre gate bus 2 is a logical NOR function of Bus 1 and Bus 3.
That is, it will only trigger if bus 1 & bus 3 don't trigger (light up).
 

To be continued .......

 

Links