AM1042 Triple VCA

OVERVIEW The AM1042 is a 16HP module that re-imagines the "lost" ARP module of the 2500 modular synthesizer. Prototypes of the original appeared in 1970 but the module never made it into production. The AMSynths module fills in a gap of synthesizer history for Eurorack.

The module has three transistor voltage controlled amplifiers with both AC and DC coupled inputs. 

DESCRIPTION Each of the three channels is based on the discrete transistor VCA sub module (4014) in the original ARP 2500 and 2600. The AC and DC inputs are mixed into each VCA, with an external CV signal and GAIN potentiometer controlling the output signal level.

The AC input of the second and third VCA are normalised to the AC input of the first VCA, to enable parallel VCA operation from one signal.

The response mode of each VCA can be set as linear or exponential. We did investigate whether a continuously variable response rate was useful but it made no significant difference apart from creating a -6dB gain loss in mid position.

The gain of the VCA is adjusted with the main rotary potentiometer or by the modulation input (MOD) and its depth potentiometer. An amber LED shows the signal level for each channel.

High quality Alpha potentiometers, short trimmers and Rogan knobs have been used to ensure control voltages can be easily set and adjusted. 

CONNECTIONS There are six 3.5 mm jack sockets for each VCA channel :

• AC Input - audio signal input (10V p-tp) which is AC coupled
• DC Input - audio signal input (10V p-t-p) which is DC coupled
• Mod Input - control voltage (0 to +10V) to control the gain of the VCA
• Output - Audio signal output