The Vac Rac modular valve processor is built like a tank and has the colour‑scheme of a battleship — but what can it do for your sound? Hugh Robjohns investigates the marriage of over‑the‑top engineering and traditional tube technology.
The Vac Rac is produced by an American company called Inward Connections, based in California and established in 1987 by Steve Firlotte. His experience with PA companies in the '70s and a major console manufacturer in the '80s left him in the perfect position to form a company which specialises in producing audio equipment with first‑class sonics, total reliability, and a 'no‑compromise' construction technique. The Vac Rac also benefits from the design efforts of two others — Steve Barker (BBAT Productions) and John Hall, and the design team as a whole boasts over a century of experience! Their philosophy is quoted in the manual as "Function dictates form and never the opposite".
The Vac Rac is a very distinctive valve‑based modular signal processor. Four modules can be accommodated in the rack: a mic preamp, a 3‑band equaliser, an instrument interface (DI box), and a limiter, and the purchaser can specify the combination. The Vac Rac reviewed here consists of a mic preamp, an equaliser, and a pair of limiters. That much technology can easily be accommodated in a 2U rackmount box, so it came as a bit of a shock to discover that the Vac Rac is the size of a microwave oven and about three times heavier!
The Vac Rac's power unit itself is surprisingly heavy for a box measuring just 8.5 x 8.25 x 1.75 inches (half‑rack width and 1U high). This module features an IEC mains lead input, a multi‑pole output lead, and a fuse holder and illuminated power switch on the front panel. The enormous torroidal transformer has been generously over‑rated, and the unit emits no detectable audio buzzing, nor strong magnetic fields. It's supplied as a 240V unit; changing the mains supply voltage rating involves internal rewiring of the transformer primary taps. The manual provides instructions in case the user ever needs to do this.
The 18‑pin circular connector which carries the necessary supply voltages to the Vac Rac main frame is made of plastic and looks like a military‑spec design. However, it actually feels rather more flimsy than it looks, which does not inspire confidence in its longevity. This cable carries a range of AC voltages directly from the transformer in the power unit to the Vac Rac proper, the highest being the 325V anode voltage, followed by 48V (phantom), 15V, two 6.3V heater tappings, and two 24V supplies.
The main 19‑inch rackmounting Vac Rac chassis is nothing short of enormous, measuring 19 x 12.25 x 5.25 inches (4U high), and it's supplied with a 1U ventilation panel which must be mounted above the chassis when it's installed in a rack (the main rack does generate a reasonable amount of heat in use). Despite its size, the chassis can accommodate only four signal processing modules, a large section to the right of the chassis being given over to the high‑voltage power supply section (complete with small ventilation fan on the rear panel).
Both the Vac Rac and the separate transformer block are painted battleship grey, and the module front panels feature 'period' window VU meters and octagonal knobs, which give the unit the exact appearance of something salvaged from a 1950s American radio station!
Each signal processing module is wholly independent of the others, and only shares power supply feeds from the main chassis. All operational controls are on the front panel, and almost all connections are at the rear.
The review Vac Rac system was supplied with a TMP1 Tube Microphone Preamplifier as its first unit. There are two versions of this unit: one with a simple output, and the other with insert points and provision for an external fader. I had the latter variety, whose rear panel features XLR connectors for the microphone input and line output, plus three quarter‑inch jack sockets for an insert send and return, and a remote fader. The mic input is transformer balanced, as is the main output (optionally), but the line‑level insert points are unbalanced, as is the signal on the fader jack (basically just another insert point wired as send and return on a single jack!). A solid‑state buffer amplifier follows the fader jack (which can be internally adjusted to give 0, 10, or 20dB of gain, allowing the fader to be operated in an appropriate position) and this drives the balanced output XLR. The idea behind the fader jack is that the unit could be used to record signals direct to tape, but with the added convenience of an external fader for fine level adjustments. Internally, amplification is taken care of by 6072A and 12BH7 valves, with coarse gain adjustments made via a stepped feedback network.
The specifications might look disappointing in comparison with some of the best solid state devices, but they are quite respectable for a valve processor. Maximum gain is restricted to just 53dB, equivalent input noise is quoted as ‑126dBm, and the overall signal/noise ratio is ‑85dB. Distortion and intermodulation figures are actually very good, as is the headroom, with +26dBm acceptable on the input and +30dBm available from the output.
The front panel is dominated by a large VU meter with suitable retro styling (an internal jumper allows calibration for either +4dBm or ‑10dBV, with reference to 0VU). Beneath this are four flat toggle switches, a quarter‑inch jack input socket and two octagonal knobs, one switched and the other continuous. The switches provide phantom power, a 20dB pad, polarity reversal, and mic or line switching. There are no indicators for these functions, other than the position of the toggle bars, and the only sign that the unit is powered is the backlight in the VU meter. The front‑panel jack socket is intended for high‑impedance DI inputs, and works extremely well in that capacity.
The gain trim control is situated before the first valve and sets the input level to the amplifier chain, the gain of which is set by a stepped feedback network providing eight positions, with 2.5dB additional gain per step. With a microphone input the gain trim knob covers a 10dB range, completely swamping the effect of the coarse gain switch, and with a line input the trim covers the full signal range all the way down to silence. This makes setting up the unit a slightly unusual experience, and it's easy to overdrive the input stage if care is not taken. You have to find the best compromise between active gain and input drive, noting that too much of the first can add noise, and too much of the second can add distortion. Unfortunately the manual provides no advice on how to optimise the gain structure of the module.
Overall, the mic stage performs very well, and far better, in terms of noise, than the specifications might suggest, though it's important to set the gain structure properly. There's barely enough gain in the module to raise the level of a decent condenser mic to working line levels with the spoken voice, but with sung vocals close to a mic there's no problem at all. The line input is easily able to accommodate full line levels from a Pro Tools workstation, as well as the outputs from keyboards and guitar pickups. In fact, the sonic quality of the tube mic preamp is extremely effective with most guitars — although overdriving the input stage does not introduce a pleasing form of distortion.
The only other point to mention is that the switched rotary controls on the review unit were very stiff to turn. This may simply have been because the unit was new, and the switch springs may ease off with use, but I found it tiring to use, as the controls have to be gripped firmly to move them at all.
The TEQ1 module is a classic 3‑band mono equaliser. The rear panel carries both XLR and quarter‑inch inputs and outputs, with inputs being electronically balanced and outputs unbalanced (unless the optional output transformer is installed). The input stage uses a 6072A valve feeding into a solid state stage which drives the EQ circuitry. The gain compensation for the loss in the equaliser circuitry is also solid state.
The three bell‑shaped EQ bands have identical facilities, with stepped cut/boost controls (providing gain or attenuation of 2, 4, 6, 9, or 12dB), and five stepped centre frequencies. The unit uses the same octagonal knobs as the mic preamp, but introduces green plastic push‑buttons for the additional switch functions, as a space‑saving measure over toggle switches.
The top EQ band covers the range between 5 and 15kHz, the middle provides 400Hz‑5kHz, and the lowest band covers 50‑400Hz. The upper and lower bands can also be switched into shelving filters via push‑buttons, and a further pair of push‑buttons introduces a 15kHz low‑pass filter and a 50Hz high‑pass filter.
The final operational control is another push‑button which bypasses the equaliser (but not the high‑ or low‑pass filters), and this has an associated LED to indicate when the EQ is in circuit. A second LED shows when the module is powered.
I found the equaliser module extremely useful in shaping the sonic character of musical instruments — it has clearly been optimised for this role. The scaling of the cut/boost controls and the chosen centre frequencies combine to make this a very usable tool. My only concern was with large amounts of HF boost, when hiss and noise become rather more apparent than I was expecting.
The TLM1 is a basic peak limiter module styled to match the mic preamp, with identical VU meter, a row of three toggle switches, and a pair of octagonal knobs. The first of the toggles provides a bypass of the limiter circuitry (but not the input gain control), the second switches the VU meter between showing the output level and the gain reduction, and the third activates stereo linking with another module. The front panel also has two preset trimmers, one for adjusting the zero point when the VU meter is in gain reduction mode, and the other for setting the meter calibration in audio output mode.
The two main operational controls are marked Gain and Reduction. The former provides a 10dB gain range on the input to the first 6072A valve stage (the output drive is taken care of by a 12BH7 valve). The Reduction control determines the amount of input signal which is fed to a solid state amplifier and used to drive an optical attenuator, arranged to reduce the input‑signal level to the unit.
The rear panel accommodates balanced inputs on both an XLR and a quarter‑inch jack. Line‑level outputs are also provided, with XLR and jack socket, but are unbalanced unless the optional output transformer is installed. A third jack socket, marked Link, is provided. This allows you to link two units for stereo operation, via an unbalanced jack‑to‑jack lead.
Setting up this unit is trivially easy. The Reduction control sets the desired level of peak limiting, and the Gain knob is then adjusted to restore the overall signal level, while you monitor the output level on the VU meter. The manual doesn't give any specifications for attack or release times, but in use the limiter works very well. It can apply huge amounts of gain reduction with negligible side‑effects, and the dynamic response is such that 'pumping' is minimal, even when the unit is being severely overdriven with transient signals such as drum tracks.
This limiter can't really be used as a creative tool — its controls don't provide sufficient flexibility for that — but it is a solid and reliable protection limiter that can accommodate serious abuse with remarkable ease, and almost completely inaudibly.
The Vac Rac sounds OK, but I didn't really feel it was anything particularly special.
The Vac Rac is an interesting unit, and one I enjoyed experimenting with, but I'm not sure who it will appeal to. The retro styling is certainly distinctive, but it doesn't appeal to me particularly (my first professional audio experiences in the BBC were with equipment that looked remarkably similar to the Vac Rac, and I don't hanker to return to those times, thank you very much!).
Despite the unit's sonic suitability for keyboard and guitar processing, its size and weight will not appeal to anyone who tours (well, not to their roadies, anyway). As a studio processor I guess it's more appropriate, although it occupies an awful lot of rack space for just four signal processing units, and there are plenty of better specified units occupying much less space — some of them still employing valve circuitry, too.
The Vac Rac sounds OK, but I didn't really feel it was anything particularly special. The equaliser does work well in creatively shaping the sounds of most musical instruments and voices, and the limiter is a very effective level controller, but I can't really commend the Vac Rac beyond that. The hybrid circuitry is quiet enough in most circumstances (aided by the provision of a very modest 53dB of maximum gain in the mic preamp), and certainly adds the expected and characteristic 'valve warmth', although none of the processors sound good when overdriven. It is extremely well built, and (apart from some slight reservations concerning the multi‑pole power cable) I don't think it will ever break!
So what it comes down to is this: the Vac Rac is extremely solid and robust; it provides usable and competent signal processing in a modular form, allowing customisation; and it looks like something salvaged from a wartime battleship. It is also extremely heavy, provides a poor return on processing per 'U' of rack space and, beyond benefiting from the typical sonic signature of well designed valve circuitry, provides nothing that could be described as exceptional. If you like the look of the Vac Rac and space is not an issue, go for it, but if not I would suggest that you consider alternative valve‑based signal processing units which will potentially give better flexibility in more efficient packages.
This unit was not fitted in the review Vac Rac. It provides an optimised interface for such instruments as keyboards and guitars, featuring simple bass and treble controls, with input and output level trims, front‑panel input and amplifier outputs (with three switchable take‑off points), and a main signal output (at mic level) on the rear panel. The rear panel also has an earth‑lift switch.
- Retro styling (possibly).
- Extremely solidly built and well engineered.
- Characteristic valve sound qualities.
- Creative equaliser module.
- Very capable peak limiter.
- Retro styling (possibly).
- Extraordinarily heavy.
- Wasteful of rack space.
- Restricted maximum gain in mic preamp module.
It sounds like a valve processor and works well, but it's rather large and heavy for what it offers. For most people it's the retro styling and rugged construction which will be the main attractions, along with the fact that the combination of processor modules can be customised by the buyer.