CAPI SumBus Summing Mixer

After years of waiting, we can finally build these again! Lead time is 2-3 Weeks.

Please select the 'Deposit' option if you'd like to pay the balance upon shipment. 

The SumBus is available in various channel configuration, and is expandable if you'd like to add additional channels at a later date.

The SumBus can be configured with a single stereo buss (x2 models) or a 2 stereo outputs (x4 models). We configure the 2nd stereo buss with High Nickel transformers for a different tone. These are high performance transformers with improved distortion and phase shift specs.

Please see the DSUB wiring diagram in the photos. Configurations of 8 channels or 24 channels requires an additional DSUB to connect all channels. For example, an 8 channel configuration connects 4 channels each of DSUB inputs 'A' and 'C'. 

The audio path of the SumBus is quite unique. Each input channel contains a CA2622 input transformer and CAPI's own DTO5 discrete opamp. Each ACA is a 2520 style opamp and a quadfilar 2503 wired 1:3. The sound of this box is like no other. This is largely due to the multiple transformers in each channel of the audio path. It's not just about the actual summing process itself but more about the additional circuitry that comes before and after those little bus resistors. Most summing mixers have transformers on the stereo output ONLY, so you're paying a small fortune for a bunch of $0.02 resistors and a pair of transformers. The SumBus is a completely different animal! 

The SumBus is a semi-modular design meaning the channel count can be expanded as work flow requires or funds permit. The input channels are on individual cards which plug into a PCB backplane. The top cover of the chassis needs to be removed for the input channel cards to be properly inserted, hence "semi-modular". The ACA cards are also inserted into PCB backplane card edge connectors. These can be removed and reinserted without removing the top of the carcass. 

The gain staging of the SumBus is overall based on a vintage API console and the 2-ACA-Bo card. The 2-ACA-Bo is a 2-stage mix bus. This means the ACA (active combing amplifier) stage is followed by an active booster stage. 6dB of gain is provided by the booster stage. The SumBus only contains the first half of the 2-ACA-Bo's signal path so technically the SumBus should be followed by a pair of line amps to makeup this 6dB. The SumBus inputs are at +4dBu nominal and the outputs are -2dBu nominal.