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ARP Voltage Controlled Filters
 Overview
ARP produced a number of interesting voltage controlled filters in the 1970's, starting with a "Moog" 24dB LPF and a multi-mode filter in the ARP 2500 Modular. Shortly after this they launched the 2600 Semi-Modular synthesizer with
the same "Moog" filter, but ARP ran into patent problems with Moog, and they were forced to design completely different filters, which is fortunate as they sound very good!
At the Analog Metropolis I have cloned 4 of these famous
designs and produced them as AM format modules, for use within the AM Modular. These designs are also available as downloads, so other Synth DIYers can build these circuits.
ARP 1006
This is the 24dB Low Pass Filter from the ARP 2500 Modular syntheszer of 1970. It is a traditional Moog transistor ladder filter, which must have enfringed Moogs copyright. It uses a temperature compensated exponential converter. All
the transistors are matched pairs. I have recreated this filter as the AM4006 module... more »
ARP 1047 This is the multi-mode 2-pole filter from the ARP
2500 Modular syntheszer, with Low Pass, High Pass, Band Pass and Notch responese. The details are here. more »
ARP 4012 This is a 4-pole 24dB low-pass filter constructed
around a Moog-like transistor ladder. It is temperature compensated and has a wide cutoff range between 0 - 35kHz. It appeared in early ARP 2600 synthesizers. Its a development of the 1006 filter.
The original design uses matched TZ-81 transistor pair, a dual FET and a LM301 Op Amp in the final signal path. I have recreated this filter as the AM4012 module ... more »
ARP 4023 This is a 2-pole 12dB low-pass filter constructed
around operational transconductance amplifiers (OTA's). It is similar to the clasic Oberheim 2-pole low-pass filter that uses the same design idea. LM301 Op Amp's are used both in the
CV summing circuit and the final signal path. The core design was patented by ARP in 1976 and the 4023 appeared in the early white faced Odyssey, manufactured from 1974 - 1975.
This filter has a very smooth behaviour, and unlike most 4-pole filters, does not attenuate the low end with increased resonance settings. Good for bass and lead sounds. I have recreated this filter as the AM4023 module more »
ARP 4034 Exactly the same as the ARP 4035 but used in the ARP Explorer, a 3-octave preset synthesizer made in 1974.
ARP4035
This is a 4-pole low pass-filter based on the Moog transistor ladder. Rather too close a copy, as it had to be dropped due to legal problems with Moog Music. It is tempearture compensated, and it briefly appeared in Mk2 ARP
Odyssey models manufactured from 1975 - 1976. The design is similar to the 4012 but with some updated components.
The original design uses four CA3086 transistor array chips, a
dual FET, and a LM301 Op Amp for the final signal stage. It is easy to replicate this circuit using the same transistor arrays, single FET's and an improved NE5532 Op Amp. I have recreated this filter as the AM4035 module more »
ARP 4072 Exactly the same as the ARP 4075, but used in the late ARP 2600 models.
ARP 4075 This is a 4-pole low pass filter based around a
cascade of integrators. It is temperature compensated and it was the standard ARP filter in 1976 - 1977, appearing in the Mk3 Odyssey, Axxe, Omni, Avatar, Quadra, etc.
Its cutoff range was severely limited to just 12kHz by a design
error, which can now be easily corrected. This is an excellent filter, with a clean low distortion ouput that will oscillate at higher resonance settings.
The original design uses 2N5087 matched and thermally
coupled transistor pairs, as well as the LM3900 Norton amplifier chip. Both of these components are still available. ARP sensibly ditched the old LM301 Op Amp's, and replaced them with LM1458's.
I have recreated this filter as the AM4075 module ... more »
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