AM1005 Module - ARP Modulation Amplifier

Overview  This module is a clone of the unusual ARP 1005 module from the mighty ARP 2500 Analog Modular Sytthesizer. uses the SSM2164 Quad VCA chip from Analog Devices, to create a dual VCA with either linear or exponential CV response. The SSM2164 is a high quality VCA design intended for audio control applications such as mixing desks, compressors and limiters. The specifications are:

• 0.02% THD
• No External Trimming
• 120 dB Gain Range
• 0.07 dB Gain Matching (Unity Gain)
• Class A or AB Operation

AM8164 Module In order to use the SSM2164 for voltage controlled synthesizers some characteristics of the chip need to be re-worked. The attentuation increases with increasing voltages (which is fine), but the attentuation CV range of just 2V has to be matched to the typical analog synthesizer +5V CV signals, and the attenuation control response is exponential, whilst a linear response is more suited to analog synthesizers which have exponential ADSR CV signals.

The linear response can be obtained by using Mike Irwins circuit which was published in the July 24, 2002 issue of EDN. This is a simple yet elegant technique for linearizing the SSM2164 VCA, by using two of the VCA's in each chip per channel.

The 4x VCA's are therefore configured into 2 separate channels, each with the following front panel controls:

• Signal Input Level
• Switch for Linear or Exponential Response
• 3mm LED shows overall output level
• VCA Gain
• CV Level

There are two sets of 3x 3.5mm jack sockets mounted on the left hand side of the panel, these are:

• Signal Input
• CV Input
• Signal Output

Build Notes This simple but flexible VCA design goes together easily and there are no hard to find parts. The reference volatges for the design need settuing up initially and these are easy to trim to the right levels with a voltmeter.

ARP1005 Front Panel