AM 4006 Module - ARP "Moog" 24dB Low Pass VCF

Overview  This is the 4-pole 24dB low-pass filter from the ARP 2500 Modular Synthesizer of 1970. It is the first filter that ARP every produced and it is based on the traditional "Moog" transistor ladder. It was the first commercial VCF to be temperature compensated. It was used through out the life of the ARP 2500 (1970-1975) and was never replaced as a result of legal action by Moog.

Original Circuit The ARP 1006 module was designed in the spring of 1970 and it contains both a 24dB filter and a VCA. The original filter design uses 8 pairs of matched and thermally coupled TZ-81 and TZ-581 transistor pairs, plus an exponential converter (4001 module) which contains 5 transistors and a 100 ohm tempco resistor. There is also a LM301 Op Amp for summing the CV's, and the VCA acts as the output buffer and contains a Teledyne 1339 Op Amp.

The circuit is very similar to the later ARP 4012 VCF module that appeared in the ARP 2600.

New AM Circuit The AM4006 module includes the "Moog" filter design but leaves out the VCA (a separate module for the future). The original circuit has been re-used except for the use of trimmers rather than having to select resistors. The TZ transistors are obsolete so standard 2N3904/3906's have been used.

The AM4006 uses the original exponential converter circuit with newly manufactured 100 ohm 3500 ppm/C tempcos.

The LM301 Op Amp has been kept for CV summing duties, and a final stage Op Amp added (LF353).

 

The ARP 2500 Owners Manual is here: more »