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

+ EG & G Selective Amplifier model 189

+ Dennis Collins paper

 

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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