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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 24dB LPF and a multimode filter in the ARP2500 Modular. Shortly after this
ARP launched the 2600 Semi-Modular synthesizer with a Moog-like LPF. This design ran into patent problems with Moog and ARP were forced to design completely different filters, which is fortunate as they sound very good!
At the
Analog Metropolis I have cloned 3 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 1006/1047 These are the filters in the ARP 2500 Modular syntheszer, details can be found 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.
The original design uses matched TZ-81 transistor pairs and a dual FET. There is also a LM301 Op Amp in the final signal path. I have followed the ideas of Joachim Verghese, with a CA3086 transistor array and single FET's. Joachim
maintains an excellent ARP web site and he has redesigned many classic ARP filters using modern components. I have replaced the poor performance LM301 with a low noise NE5532 Op Amp.
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 it 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 souns, but a bit noisy.
The original design uses CA3080 OTA's and 2N5172/TZ-581 transistor pairs for the exponential converters. Once again I have followed Joachim's
ideas, keeping the CA3080's and using unmatched 2N3904/2N3906 transistors. I have retained the LM301 Op Amp as a CV summer, but upgraded to a low noise Op Amp in the signal path by using the NE5532.
I have cloned this filter as the
AM4023 module, and the schematic and PCB layout are available as a download.
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 around a Moog-like 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 the Mk2 ARP Odyssey, manufactured from 1975 - 1976. The design is very similar to the 4012 but with some updated components.
The original design uses two 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.
ARP 4072
This is a 4-pole low pass filter based around a cascade of integrators. It is temperature compensated and it appeared in later ARP 2600 models. Its cutoff range was severely limited to just 12kHz by a design error, which can 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.
ARP 4075 This filter is nealy identical to the 4072, and it was the standard ARP filter in 1976 - 1977, appearing in the Mk3 Odyssey, Axxe, Omni, Avatar, Quadra, etc.
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