With multichannel sources your source device will use its own built-in crossover system to send low-frequency information to the subwoofer and spare the front channels from low-bass duties. With two-channel sources, the MCP-18 gives you two channels of output. But what if you have a system that uses smaller front right and left speakers with limited low-frequency capabilities? With two-channel stereo sources, the MCP-18 sends the full frequency signal to your two front channels without any crossover to route bass into your subwoofers.
If you want to use your subwoofer with two-channel material you will have to do some extra work. You will need to set up a way to route your two-channel music through a crossover so that the bass will go to the subwoofer. Most subwoofers have low-pass/ high-pass crossovers built into them that you could use—merely run the line-level output from the MCP-18 into your subwoofer and then use its built-in crossover. But the disadvantage of this arrangement is that when you go to a multichannel source that already has crossed-over low frequencies to a .1 subwoofer circuit, the sound will have too much bass. To go from multichannel to two-channel and back requires changing the circuit path if you want to use your subwoofers for both two-channel and multichannel material.
Many subwoofers have multiple selectable inputs. This allows you to have one input coming from the MCP-18’s sub output as well as a second input—a stereo pair coming from the MCP-18’s front left and right output, connected simultaneously to your subwoofer. The front left and right stereo feed will go through the subwoofer’s internal crossover and then to your power amplifier. When you want to listen to two-channel sources you’ll employ the subwoofer’s crossover. But when you listen to multichannel sources you’ll go directly from the sub output to your subwoofer. To accomplish this you will need to disconnect the stereo feeds from the subwoofer and connect them directly to your front-channel power amplifier. Depending on the physical location of your subwoofer and front-channel power amplifier, the switchover could be less than convenient.
I have two Parasound P7 ($1995) multichannel analog preamps, one in each of my two room-based systems. The Parasound P7 has very similar functionality to the NuForce MCP-18, but includes a built-in crossover for two-channel sources so that you can go seamlessly from two channels to multichannel, using your subwoofer with both kinds of sources. From an ergonomic perspective, it’s unfortunate that NuForce chose not to include a similar crossover scheme in the MCP-18.
When I set up the MCP-18 as a stereo preamplifier in my computer-audio system I used a different wiring arrangement. Since I didn’t have to worry about multichannel sources I connected the balanced XLR front left and right outputs directly to my front-channel amplifier and then connected the single-ended stereo outputs to my subwoofer. After adjusting the subwoofer’s output and crossover points, the setup was done and required no additional adjustments or cable switching.
Reviewing the sound of a preamp used to be easy. All you needed was another reference preamp that had a tape-loop circuit in it. We used to put the preamp under review in the tape loop and then switch it in and out of the circuit and compare the sound. The only preamp that I own that still has a tape loop circuit is an Accuphase C-200, and when I tried the tape-loop test I could not hear any difference when the MCP-18 was part of the circuit. Although the Accuphase was recently refurbished and operating within spec, either the MCP-18 was completely transparent or the Accuphase was not sufficiently high resolution for me to discern the differences when the MCP-18 was in the circuit. I needed to go to plan B.
Plan B was simple—connect more than one USB DAC via its analog outputs to the MCP-18 and compare the sound. Since my next review will be of several small-footprint USB DACs, this method killed two reviews with one “Stone,” so to speak. I connected several DACs to the MCP-18 and began listening.