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AM8164 Module - Dual Linear/Expo VCA
Overview This module 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:
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.
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