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Radial Space Heater Analog Summing Mixer

8 Channels of Tube Flavor for Your DAW

The Radial Space Heater is a box designed to impart warm, tube-based analog flavor across the inputs of cold, sterile, digital interfaces. Without being a compressor or equalizer, it provides tube-based gain or full-on drive, enriching the harmonic content of signals prior to A/D conversion.

While the Space Heater performs this function impeccably, theoretically it could also be used to flavor signals leaving a DAW, heading to a mixing console, and appropriate I/O to serve this purpose is provided. For those preferring to avoid a mixing console but still experience analog summing, the Space Heater provides an integrated analog stem mixer that can combine four stereo stems into a single stereo result. So, tube flavor can be imparted during tracking, and then more tube saturation can enhance the mix stems before they are combined with clean, voltage-based summing.

On the Surface

A wealth of I/O fills the back panel. The eight inputs of the unit can be fed by either a DB-25 connector or a set of eight female ¼-inch TRS balanced connectors. Each channel can be “warmed up” and addressed individually when taking advantage of the DB-25 output connector. Alternatively, the combined signal from all of the inputs can be outputted through a pair of XLR connectors, with oddly numbered inputs summing to the Left output and even-numbered inputs summing to the Right. This summed signal redundantly feeds a front-panel headphone jack with a dedicated volume control. Each channel has an individual ¼-inch TRS insert send and separate ¼-inch TRS return, which sits after the tube gain in the signal path. This allows external processing across each channel before summing, or before being discretely fed to the recorder.

The front panel controls are divided into four sets, corresponding to each of the odd/even stereo input pairs. Though each of the eight channels can treat individual mono sources in a tracking or mixing scenario, each pair of inputs shares a basic set of controls, while still having individual input drive and level controls. For each pair, there is an On button that turns on or bypasses the gain circuit. The circuit can be engaged as a solid-state gain control, effectively working as a line amp. However, a separate On button engages the tube drive circuit, allowing the signal to be selectively saturated with tube gain.

Whether working with the solid-state or tube-based gain, the same set of knobs adjusts the amount of drive. Two pairs of continuously variable, co-centric, black plastic knobs each have a small, slender center knob extending out that controls input gain, while the lower-sitting outer ring controls output gain. To that end, the inner knob controls drive, while the outer ring can back off the level to avoid clipping the input of the next circuit down the line. Accompanying the drive controls is a switch to adjust plate voltage of the single 12AX7 tube, wired in stereo, for each input pair. Thirty-five, 70 or 140 volts can be applied, altering the character of the saturation. As lower voltages seemed to result in a near-immediate breakup, especially in the presence of abundant low frequencies, each input pair is also conveniently outfitted with an optional highpass filter, which is centered at 40 Hz.

Outside of the Box

While the channels can be used completely independently, one of the most exciting features of this eight-channel Space Heater is the summing amplifier. To use it, each stereo pair has a Bus button, which will add the signal to the bus feeding the summing amp. Keeping in mind that summing circuits like this one use a passive resistor network when combining signals, they generally produce a low-level end result, which must be gained back up to line level. There are a number of these types of passive summing amps out there that require an external preamplifier to do the job, allowing the user to impart the flavor of any particular gain structure that they desire.

The Space Heater, on the other hand, includes its own clean-sounding, high-headroom amplifier circuit. If you have heard Radial’s mic preamps, you are familiar with their combination of low noise floor, tight bottom end, low distortion, and clear, detailed top end. Without being too bold, they do offer a welcome touch of personality. The gain circuit of the Space Heater certainly adheres to these expectations with a clean, full sound.

In the Studio

When it came to getting connected, I really appreciated that there was a DB-25 input, so cabling it up for use as a summing amplifier was a snap. That said, when it came to tracking, the DB-25 output and individual TRS inputs couldn’t have been more welcome, so that different preamps could feed each input without the need for a breakout snake.

I started out by recording individual sources through the Space Heater, just to get a sense of its tube drive characteristics. When running a DI’d bass through a preamp and then feeding it through the unit, the sound was fantastic. I have heard a lot of mic preamps with DI inputs and tube circuitry that have added a subtle bit of warmth to electric bass, but haven’t come anywhere near the pleasant distortion of a bass amp or overdrive pedal. By contrast, the Space Heater brought nice crunchy distortion without compromising the intelligibility of the signal, resulting in a signal that benefitted from a little bit of EQ after the fact but required little additional processing.

Whether adding drive to bass and electric guitar or more subtly enriching the sound of acoustic instruments, the Space Heater always sounded fantastic. I spent the most time using it as a tube-driven summing amp for mixing purposes. In general, mixing music through analog components, even if it was just combining stems, awakens a mix and allows certain elements to pop and blend in a way that is just decidedly different than mixing in the box. The Space Heater took this to a whole new level, to the point where its tonality would inspire entirely new creative decisions and lend new insight into a composition. In every case, mixes took on this incredible sound that truly evoked the character of a vintage desk.

Feeding a drum stem through the tube drive circuit with the voltage set to 140V, the result was perfect. Cranking the drive more and more, the bottom end of the kick drum got fuller, the snare and cymbals got crisper, and the whole kit sounded wider and just all around bigger. Incorporating these drums into the track, they went from merely being a solid foundation to something that could cut through anything. When mixing a track with an okay-sounding electronic drum loop, this effect was particularly powerful as it injected new life into the once lackluster component and made it a focal point in the track.

One slight hindrance when dealing with elements like drum stems was the fact that the gain controls could not be linked when doing true-stereo processing. This was compounded by the fact that the controls had no detents, and thus no certain points to match between the left and right controls. Considering that I had the unit in a rack off to the side of the sweet spot, I found myself soloing the drum stem, reaching over to the controls and eyeballing them, while looking back to the stereo mixdown meters and adjusting accordingly. Then, as I would return my head to the center I would have to re-tweak it. After a while, I started to embrace this method, double-checking on headphones and just appreciating that my subjective perception was as good as any other tool for judging whether the sound was balanced. Radial explains that not having detents was no accident, the two elements in the tube are impossible to predict so left right matching must be done by ear (see the “Try This” sidebar for the workaround.) As with all things tube, there is no such thing as perfect, there is only what sounds good.

Building a mix with a drum/bass stem, electric guitar stem, acoustic guitar stem, and a vocal stem, each thrived in different amounts of saturation, but was able to find a comfortable place in the mix created by the Space Heater. For acoustic guitar, the subtlest twist of drive at 140V seemed brighten up the buzz and resonance of the strings, making it pop and enhancing the stereo width of the panned, double-tracked performance. On the distorted electric guitar stem, the signal could be added to the mix with no additional saturation imparted, and this seemed entirely appropriate. At the same time, a subtle bit of extra drive took to the sound as well, with either option being very usable.

Given the option of lower plate voltages, I couldn’t help but toggle through all of the choices before settling into a sound on any source. That said, I never found a really good use for the 35V setting. With the slightest twist of the drive control, the signal entered brutal distortion. There are probably genres of industrial music or punk that would welcome the way that this utterly destroys bass, drums, or guitars, but in every case, it went far beyond subtle coloration or even conventional drive. The 70V setting, on the other hand, was far more practical, and when used in conservative amounts it seemed to clear signals up while adding a little bit of bite on top. For vocals, this was actually a pretty good choice. It seemed to warm the chest without muddying it, and enhance consonants.

The 140V setting was always a safe bet, however. In subtle amounts it could simply make a vocal pop in the track, almost like a frequency-dependent expander, hyping any frequency that made a significant contribution to the overall sound. When throttling the drive control, it could crank out great, gritty, garage-rock vocal sounds. At the end of the day, any direction the track wanted to go, there was a sound to suit it.

A Must-Have?

Aside from buying a large format-console, it is hard to imagine any single piece of gear that could have a more profound influence on all of your recordings and mixes than the Space Heater. If you need to get out of a rut, or take your tracks to the next level, this is a surefire way to breathe new life into your work. If you are shopping for an analog summing mixer to finish off your in-the-box mixes, keep in mind that while the detent-less controls will slow recalls and skew their accuracy, the Space Heater’s drive controls can be completely bypassed, and all four stems can run at unity gain for speedier operation. One other note is that the pairs can only run in stereo with no mono-splitting or summing options. That said, multiple units can be chained together using the “link” connectors to create a larger summing system.

If you are looking to warm up your tracks prior to A/D conversion, the price of eight channels of Space Heater are half the price of two channels of Culture Vulture and only slightly more than a pair of single-channel Space Heater 500 Series units, plus you get the bonus summing amp. To that end, it seems like a smoking deal that you surely won’t regret.

Brandon T. Hickey is an Arizona-based audio professional.


COMPANY: Radial Engineering

PRODUCT: Space Heater

PRICES: $1,699

Pros: Good amount of options and features that sound fantastic.

Cons: Continuously variable controls make for challenging recalls.