Have you ever been in a position where you needed to connect a few audio inputs together, perhaps provide phantom power to a condenser microphone, but didn’t want either the size, expense or complexity of a typical small mixer? If so, Radio Design Lab’s RDL-MX4, one of numerous products in RDL’s Rack-Up line, may be the answer you’ve been looking for. Originally designed for the small conference room or boardroom, this four input mixer has just the right features for a small-scale application. But its usefulness extends farther than just a boardroom. At one rack space (RU) in height and 1/3-rack width, this mixer is small enough to be unobtrusive and fits easily in any bag for traveling. However, don’t let its small size fool you. This little unit possesses several professional features that make it stand out. All four inputs possess both balanced XLR microphone inputs and unbalanced RCA line-level inputs. Phantom power is individually switchable for each input. A compressor/limiter is available on the output bus, with a recessed switch to disable it. And last but not least, the power and output signal can be daisy-chained to facilitate multiple units together, increasing the number of input channels available.
To put this unit through its paces, I used it with a variety of inputs, and analyzed the output by running it into an Audigy sound card and capturing the signal with Sound Forge (an audio processing application). I measured a noise floor of -81 dB coming out of the mixer, which ought to be quite sufficient for the typical applications for which this mixer was designed. The spec sheet specifies residual noise levels of between -70 to -79 dB for the microphone inputs. Output signals were clear, and the tonal qualities seemed uncolored. The mixer handled both condenser and dynamic microphones, with the mixer able to power the condenser microphone with its built-in phantom power supply.
The recessed switches on the back of the unit for phantom power and the compressor/limiter were easy to operate with a pointed object such as a small screwdriver or letter opener. A security cover is available for permanent installations that attaches over the back of the unit, concealing the switches for phantom power and the compressor/limiter. This is handy for those situations where uneducated fingers like to wander over any switch they encounter. Front panel controls are self-explanatory, as there are only five of them: four input channel level knobs, and one output attenuation knob. The front panel also possesses signal level and clip indicator LED’s for each channel, an eight-segment LED output level VU meter and an LED indicating when the output signal is being attenuated by the compressor/limiter.
While using the mixer, I noticed two minor issues. When bringing up the level of a channel from its fully counter-clockwise position, the signal level jumped 25 dB instantly. Because of this, if you wanted to smoothly fade in some music at low volume via the level controls, it wouldn’t be possible. However, this mixer is not designed or intended for a full soundreinforcement sound reinforcement environment. Its nitch in the church might be for the small children’s ministry room, controlling perhaps a CD player and a couple microphones. It would also be highly useful for a video/multimedia ministry, where you have a few lavalier microphones on interview subjects and need to provide phantom power and submix them before feeding the audio into the recording equipment. Neither of these situations require a smooth fade from -8, and thus this attribute shouldn’t present a problem. RDL engineering also pointed out that this jump shouldn’t be noticeable in a live sound environment, as the output of a sound system at this output level is still likely to be below the ambient room noise level.
The second issue occurred when I tried to balance the microphone level signal from an SM87 with a line-level signal from a CD player. To get a strong output signal from the mixer with the microphone, both the master volume and microphone level control needed to be turned up to their highest level. With the output level turned up this high, the CD player line-level signal quickly overpowered the microphone signal even with the input controls for the CD player channels set at a low level. In discussing this with the engineers at RDL, they indicated that condenser microphones typically have an output level 12 to 15 dB higher than dynamic microphones. To support the higher output level of these condenser microphones, the RU-MX4 automatically applies a 15-dB pad on any input that has phantom power turned on. Unlike your typical condenser microphone, the Shure SM87 has an output level comparable to dynamic microphones, and thus the application of the pad brought its level down to 15 dB lower than a typical dynamic microphone. When I tried the same application with a Shure SM58 dynamic microphone, turning the phantom power off for that input, there was no problem with getting the microphone to a higher level than the CD player.
Despite these two quirks, the unit would perform well for the situations I have described and is worth looking at. At a MSRP of $340.00 (street prices are lower), this unit can be an excellent cost-effective purchase for a small mixing solution. Combining the RU-MX4 with other products in the Rack-Up line, such as equalizers, video switchers and audio/video distribution units with the same physical profile as the RU-MX4 can enable you to build a highly functional, compact sound system for small room environments.
