Zander said:
I want to build a new summing box for mixing a small number of channels (min 4-max 16). I have the CAPI ACA-BO and a Neumann V475-2C. What would be the best option for this.
A very simple question that triggers a lengthy answer! First, it depends what is your opinion on "best". Some like spanking clean, others like a dose of "dirt" or "warmth". Both alternatives are definitely on the clean side. However, the CAPI can be fitted with different DOA's, and that may start and endless quest for the ultimate DOA. The Neumann has both input and output xfmrs, which may impart a slightly different colour, although the input xfmr, being under heavy NFB, is essentially transparent. The CAPI output stage, with the 2503 xfmr has a reputation of adding "colour".
You are likely to get as many opinions as there are members in this group about the respective sonic signatures of the proponents.
On a different level, that of purely objective performance, there are mainly two aspects:
Headroom: In this area, there is no doubt the CAPI is the winner, with its bipolar 16v rails, compred to the single 20V rail of the Neumann.
Noise: The Neumann, with its input xfmr, is optimized for the low bus impedance (typically about 200-500 ohms) resulting from a large number of channels being routed via ca. 10k resistors. Otherwise, the CAPI (with the standard 2520) is optimized for much higher bus impedance (typically about 5-10 kohms), which is consistent with the recommended use of 47k bus resistors.
Depending on the bus resistor value and number of channels, the Neumann may be more noisy than the CAPI, if the resulting bus impedance was much higher than the optimum value. Unfortunately, the noise figures (input noise voltage and input noise current) for the 475 seem unavailable.
Please note that there are DOA's that are optimized for lower source impedance than the 2520; they are low noise only if the source impedance is low enough; they may actually be noisier in a low channel-count application.