The multichannel Radial Engineering Space Heater rackmount mixer and single-channel Space Heater 500 module (for 500-series frames) are analog distortion units that offer tube distortion and transformer saturation from actual tubes and transformers.

The rackmount version is set up as four stereo channels of distortion, with a built-in summing mixer and headphone amp as well. Inside the 1RU-height chassis are four 12AX7 tubes (each dual-triode tube can handle two channels) and eight custom-wound Eclipse ET-LD2 transformers (which are based on Deane Jensen's original designs — Radial Engineering acquired Jensen Transformers in 2014). Jam-packed around these tubes and oversized transformers are standard through-hole components: Burr-Brown line drivers and receivers; Analog Devices op-amps; EPCOS metallized-polyester-film capacitors; and banks of individual metal-film resistors. Like all of Radial's products, the Space Heater has a chassis fashioned from cold-rolled steel, with a thick, durable powder-coat finish. Not surprisingly, the unit is heavy — 8.5 lb according to my postal scale — despite it being only 6'' deep.

The front panel is divided into five sections. There are four sets of stereo channel controls, and one master section for the summing bus and headphone amp. Each stereo channel includes pushbuttons to relay-bypass the distortion circuit entirely, enable a 40 Hz high-pass filter, assign the channel to the summing bus, and switch in Tube Drive. Tube Drive is further controlled with a three-position Heat switch that chooses 35, 70, or 140 V for the plate voltage of the 12AX7 tube, as well as two concentric potentiometers (one each for odd and even sides of the stereo channel), with an inner Drive knob for input level and an outer Level knob for output level of the tube circuit.

The backside is incredibly dense with I/O. There are eight 1/4'' TRS balanced inputs, two balanced XLR outputs for the summing bus, and two 1/4'' TRS stereo jacks for linking multiple units together. Additionally, there are eight pairs of 1/4'' TRS send and receive jacks for balanced inserts. What about the individual channel outputs? These are on a DB-25 socket. The inputs also show up on a second DB-25 socket (in parallel to the TRS jacks).

The Space Heater 500 module, which takes up one space in a 500-series frame, is similar in internal componentry, except for a compact Jensen JT-11-YMPC transformer in place of the larger Eclipse, and Panasonic aluminum electrolytic capacitors. Like its rackmount sibling, it has Drive and Level knobs to vary the input and output levels of the tube circuit, as well as the same three- position Heat switch for plate voltage. But unlike its sibling, the 500 module's HPF is sweepable from 5–500 Hz, and a sweepable 500 Hz – 20 kHz LPF is also included. While the HPF of the rackmount version is post-tube, the filters on the 500 module can be switched pre or post–tube. A single "In" switch bypasses the whole module, but there's no facility to bypass just the tube circuit and leave in the transformer — something the rackmount version can do.

Veteran reviewer Garrett Haines asked to test-drive the rackmount Space Heater in his studio. I also received a rackmount unit to try in my personal studio, alongside the two Space Heater 500 modules that I had previously purchased. Our two perspectives follow.

GH: Understanding the signal flow of the Space Heater is important. The inputs feed 12AX7 tubes, and each channel has a Heat switch for selecting the plate voltage of its tube. In plain- speak, and of course generalizing — lower voltages starve the tube, increasing distortion. The Drive and Level controls allow you to fine-tune the amount of distortion on each channel. The high-pass filter affects the signal coming out of the tube circuit, before its fed into the transformer. You can bypass the tube circuit and drive just the transformer. After the transformer is the send/receive loop for inserting external processors before the Space Heater's summing bus.

When summing, there are no pan pots, so the left side of each stereo channel goes to the left side of the summed output, and the right to the right. If four stereo channels (for four stems) are not enough, the summing matrix can be expanded by adding additional Space Heater units via the link jacks. Four units can be chained for a total of 16 stereo pairs (32 tracks) of summing.

I like that Radial prints the DB-25 pinout on the rear panel, which is a big deal since there is not a single standard for DB-25 connectors. There are analog, digital, and even some more variants — from the likes of TASCAM, Yamaha, and Avid/Digidesign. Anyone who suggests this isn't a big deal is welcome to find out what happens when you plug the wrong DB-25 cable into a powered system. The Space Heater follows the TASCAM analog pinout.

AH: Personally, I'm a big fan of D-sub connectors for multi-pair audio lines, as they're cheaper to manufacturer than individual cables and easier to handle. Plus, there are plenty of multichannel converters and studio patchbays that rely on DB-25 — so wiring can really be a breeze. I haven't had any personal mishaps due to misconnecting DB-25 cables, like Garrett has, but all of my gear with DB-25 analog sockets — from TASCAM, Radial Engineering, SSL, Harrison, Antelope Audio, Dangerous Music, and Switchcraft — follow the same TASCAM pinout, which is now the AES59 standard.

By the way, if you'd rather not use DB-25 snakes for the channel outputs of the Spacer Heater, you can use the 1/4'' TRS send jacks to simply output the individual post-transformer channels to wherever. (The only caveat with this scheme is that the relay-bypass feature won't work; in bypass mode, nothing comes out of the send jacks.)

GH: I only had one unit for this review, so it's difficult to draw comprehensive conclusions about summing. Personally, I prefer a summing mixer that accommodates at least 8 to 12 stereo stems (16 to 24 tracks), so I would need two or more Space Heaters. However, if you plan to use it in a variety of ways — as a front-end processor, for a monitor mix, or in a live sound rig, for example — as well as for summing, then it begins to make more sense. It makes a great preamp "follower" on occasions when the source tracks need just a little more "more."

Because the Space Heater is only 1RU high, putting together a multi-unit mixer is entirely feasible (as long as you leave space between the units for heat dissipation), and there is no doubt that the transformers go a long way towards achieving a "console" sound when summing. Furthermore, the tube drive does range from subtle to slightly overdriven.

For live sound, if you loaned a set of these to the FOH engineer, you wouldn't get them back, because it'd be like taking candy from a baby. Subtle tube distortion plus transformer saturation would make for a happy audience experience. Furthermore, both features would come in handy for monitoring. Whether on stage and in the studio, in-ear monitoring is becoming more common. If you have ever tried IEMs, you can attest that they take some time getting used to, and they often have a dry, almost sterile sound. Being able to add some "drive" to the monitoring mix would be a plus. Thus, a Space Heater would be a killer submixer for foldback. By the way, the rolled-steel chassis is not only durable, but it's also ideal for shielding. This box should certainly withstand the rigors of travel and professional use.

AH: Actually, I wouldn't recommend using the rackmount Space Heater as your only summing mixer. Instead, I'd suggest using it as a stem or track processor in conjunction with another summing system. In my mixing workflow, I found the Space Heater was great at "gluing" things together. Tiny bits of tube distortion and transformer saturation can go a long way in helping you achieve the "console" sound, as Garrett explained already. So you could feed another summing system into the Space Heater, using the latter to add "glue"; or you could take the outputs of the Space Heater and feed a bigger summing system. You could even intermix the two to create routings for parallel processing.

But with that said, you have to be diligent about polarity (what some people erroneously call "phase") when using the rackmount Space Heater for summing multi-mic'd instruments (like drums) or for parallel processing. Each stage of its circuitry inverts the signal. Therefore, if you're bypassing the tube stage and using only the transformer stage, the channel's input and output polarities will be opposite of each other. Or, if you're bypassing both the tube and transformer stages on your channel, but still feeding the channel to the summing mixer, the signal from that channel will be polarity reversed at the summing mixer's output.

GH: To get the most out of the Space Heater, I suggest wiring it to an appropriate patchbay. In addition to facilitating use of the unit with stereo-bus and insert devices, you'll also be more tempted to try it in other situations. As a front-end "tone" box, the Space Heater was impressive on a wide variety of sources, especially keyboards and DI'd bass guitar. Since the controls are laid out in stereo pairs, consider other stereo sources, such as room mics, synths, backing vocals, brass sections, or even virtual instruments. Sometimes compressors don't offer the right coloration (or you might not own enough of them), so the set of tube/transformer channels in the Space Heater could be just the ticket — for almost any musical genre.

AH: I've used both Space Heater models on a variety of sources during tracking, using it as Garrett suggests. Most of the time, I prefer 140 V operation. On snare drum, I like to set up just enough Drive to bring out a tiny bit of "thwack" courtesy of tube-clipping, which is also a nice, musical way of limiting the snare signal before it hits my converters. On floor and rack toms, a bit more Drive adds a little more "thwack," while turning up the Level knob adds transformer resonance. In this manner, toms sound huge, and kick drum mics also respond in a similar way. On electric guitar, I can add "bite" at 140 V Heat, or a ton of harmonics and grit at 35 V. On bass, either mic'd or DI'd, careful placement of the Drive knob can make the performance more lively and dynamic, as the tube-driven tone reacts to the volume. When I do this, I like sandwiching -the Space Heater with compressors on both ends, so I have finer control of the tube-distortion effect. On vocals, I can certainly add lots of "hair" at 35 or 70 V, but for the most part, I'm happy adding very subtle texture and "sparkle" with low Drive settings at 140 V. In all of these instances, the 500 module's sweepable HPF and LPF come in very handy.

GH: Speaking of the Drive and Level controls, these knobs are full range, and they're difficult to recall. Therefore, if you're using the Space Heater as a summing box, printing test tones for each channel might be the only way to match settings at a later time. I found the Drive controls in particular to be very sensitive, with small adjustments resulting in large changes. This could be due to variations in tube performance, the specific pots, or both. Once you find the setting you like, it is easy to move the control a few degrees too far and end up in a different sonic location.

AH: I agree that the Drive knob can be jumpy. In its first quarter turn, it seems to cover a huge range, from zero to "a heck of a lot." But I do like the fact that both knobs offer full range, especially when using the Space Heater as a tracking effect. My vintage API 312 and BAE 312A preamps [Tape Op #45] have input gain controls but lack output level. I almost always have to put some device (like a compressor or EQ) immediately after them so I can be "creative" with my gain structure. A Space Heater channel, especially the 500 version, is a great tool for managing post-preamp levels, as well as for adding character to the signal. For example, I like to track drums "hot" through API-style preamps, so that transients clip and saturate; the resulting sound is usually punchy and huge. If I take the output of the preamp and then immediately feed a Space Heater channel, I can use the Heat and Drive controls to add tube clipping as I explained earlier — sometimes a lot, sometimes a little bit, and sometimes none. Then the Level knob allows me to manage the signal before it's fed to another device, basically using the knob as an output fader. Meanwhile, the HPF allows me to take out some of the unneeded thump that might otherwise cause a downstream compressor to overreact or a converter to clip prematurely. On the rackmount Space Heater, the post-tube Level control is active even when the tube stage is bypassed, so I can still control the channel's output level, and concurrently, the amount of transformer saturation, with tube drive off.

I also love the Space Heater 500 for mixing, especially for parallel processing when it's mounted in my Radial Engineering Workhorse rack and mixer [Tape Op #85]. Using the Space Heater 500's sweepable filters, I'm able to focus what triggers the distortion (with the filters set pre-tube) or focus the distorted output itself (filters set post-tube), and then add that distortion back to the undistorted signal. This affords me very fine control over what portion of the signal I want to distort, while still allowing the full spectrum of the signal to come through. Plus, the immediacy of turning real knobs makes this setup a powerful and creative tone-shaping tool.

GH: During my time with the Space Heater, my only regret was that I did not have time to experiment with different tubes. 12AX7s are affordable and can be found in a variety of "color flavors." (But don't try similar-looking 12AT7 or 12AU7 tubes. They operate at different voltages and could damage the unit.)

AH: Speaking of color, if you're curious about how the Space Heater colors your signal, here's what I learned from taking some measurements:
Frequency and phase response of the rackmount version, channel input to channel output, with just the transformer stage active, is ruler flat from 20 Hz to 20 kHz. In fact, it's only down by −0.2 dB at 12 Hz and 48 kHz. With the tube stage active, the frequency-response graph follows a shallow dropoff in the highs: −1 dB at 11 kHz, −2.5 dB at 20 kHz, and −6.5 dB at 40 kHz. The summing bus (with tubes and transformers bypassed) is down by −0.5 dB at 20 Hz and −0.1 dB at 48 kHz.

The tube stage adds primarily whole-spectrum, second-order harmonic distortion as Drive is turned up, as well as other even-orders (to a lesser degree, as expected). But once you hit a certain point (say in the last 10% turn of the Drive knob with a high-level input), the post-tube buffer can distort, and third-order harmonic distortion swamps the signal, adding a harsh buzz — good or bad, depending on what you're trying to distort.

As the Level is turned up, the transformer stage saturates, and third-order harmonic distortion dominates, with the "knee" of the graph starting at 1 kHz and then sloping up significantly at lower frequencies. The second and fourth-order harmonics start about one octave below at 400 Hz, also climbing as the frequency goes down. I would characterize the resulting sound as low- frequency resonance, versus the full-spectrum buzziness of the overloading post-tube buffer. This resonance can add a rich "bloom" to "heavier" sounds, as well as "density" to mixes and stems.

The Space Heater 500 performed similarly with both of its stages active. I wasn't able to measure its transformer stage separately (since there's no bypass facility for just the tube stage), but with the Drive set very low, I could see that the smaller Jensen transformer generates third- order harmonics starting at roughly an octave lower than the bigger Eclipse, and it exhibits much more odd-order harmonic distortion at lower frequencies. At 15 Hz, transformer resonance leads to more distortion than fundamental.

Although I didn't measure signal-to-noise ratio, I will say that self-noise has yet to be a problem when using either of the Space Heater models, even at extreme settings (like Drive turned way down and Level turned far up — or vice-versa).

Now that I've bored you with these specs, let me end my portion of the review by saying that I love what I can do with my Space Heater 500 modules, and I think the multichannel Space Heater is a very cost-effective way to obtain eight channels of real tube and transformer distortion — for both track and stem processing. Plus, like all of Radial Engineering's products, both models have great secondary features — sweepable filters on the module, summing bus and headphone output on the rackmount — that add real value, truly making them multi-use boxes. What does Garrett think?

GH: They sound good, are built like a tank, and are backed by a respected manufacturer.

Tape Op is a bi-monthly magazine devoted to the art of record making.

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