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AEA RPQ2

Dual-channel Microphone Preamplifier
Published June 2017
By Hugh Robjohns

AEA RPQ2

With the RPQ2, AEA have improved on what was already an excellent ribbon-mic preamp.

Audio Engineering Associates are American manufacturers who specialise in ribbon microphones and associated hardware. Ribbon mics present a particular challenge for the preamp since they prefer to work into unusually high impedances and need a lot of quiet gain — yet low noise and high impedances don’t normally sit very comfortably together! AEA’s first ribbon preamp was the TRP (‘The Ribbon Preamp’), which I reviewed in April 2007 and was so impressed with that I purchased the review model. However, its desktop format and relatively basic facilities (and absence of phantom power), inherently limited its application.

AEA’s next preamp (joining the TRP, rather than replacing it) was designed deliberately to have a wider and more versatile appeal. Based on the TRP’s core preamp circuitry, the RPQ was built as a 1U 19-inch rackmounting, low-noise, high-impedance, high-gain preamp, but with equalisation facilities and a second set of inputs with phantom power, allowing it to be used with the full range of microphones. I reviewed that model in November 2009 but now, eight years later, the RPQ has received a makeover expanding its facilities even further, and broadening its appeal considerably to a very wide range of potential users.

Overview

Built entirely in Pasadena, California, the new RPQ2 retains the 1U rackmounting format of its forebear. It also retains the same core set of features and controls, as well as a similar panel layout, but it also introduces some subtle but powerful new additions.

The very classy brushed aluminium case extends around 195mm behind the rack ears, and the front panel has crisp white control legends around polished vintage-style aluminium knobs and small round plastic buttons. The first two rotary controls for each channel set the gain structure, with a 12-step switched gain control spanning 13 to 63 dB in 5dB increments, and a continuous output fader ranging from -60 to +19 dB (with a 0dB position marked at the one o’clock position). The output fader control sits between the EQ section and balanced output driver, and with the mic gain and output fader both at maximum, 81dB of gain is available overall. The output level is indicated by a simple trio of LEDs in the familiar traffic light arrangement, with a green light showing for signals above -20dBu, yellow above 0dBu, and red above +24dBu, 4dB below clipping (the maximum output level is +28dBu).

Three more rotary controls relate to the EQ facilities taken directly from the original RPQ. The first knob adjusts a low-cut shelving equaliser’s turnover point from 515Hz down to 22Hz. The maximum amount of attenuation imposed by this equalisation is 20dB, and it is intended to compensate for proximity effect, as well as helping to manage unwanted subsonic signals.

Moving further across the panel, the next two controls operate AEA’s unique ‘CurveShaper’ equaliser, which introduces a presence or ‘air’ boost, and is intended to restore the extreme high end which is typically a little subdued in ribbon mics. In fact, this section has a peaking bell response, but as the centre frequency ranges from about 30 to 120 kHz the audible part of the response below 20kHz appears as a boosting shelf equaliser.

Unusually, the frequency control panel markings actually relate to the point where the shelf boost curve rises to +3dB, and so range from 2.5 to 30 kHz; it’s a rather unconventional way of calibrating a bell equaliser but it makes much more practical sense in this application. Interestingly, the filter slope gets steeper and the bell’s bandwidth reduces as the frequency and gain controls are increased, which translates to a more focused or targeted HF boost. The gain control ranges from zero to a maximum of +20dB, and the review unit exhibited a centre detent, which I didn’t expect.

In addition to these five rotary controls there are also six small round buttons, all with status LEDs. Four carry over from the original RPQ design to activate phantom power (red), a polarity inversion, and to enable the separate low-cut and high-boost shelving EQ sections. The two extra buttons relate to the RPQ2’s brand-new features and both select alternative inputs. The first selects a rear-panel quarter-inch TRS line input (which also doubles as an insert return) in place of the rear-panel XLR mic input, while the second selects a front-panel quarter-inch instrument input socket, which replaces both the mic and line inputs.

Connectivity

At this point, we need to examine the rear panel, which is radically different from the original RPQ. The IEC power inlet accepts a fixed mains voltage, which is configured at the factory for either 240 or 120 V AC operation according to the country of sale. The connector incorporates an integrated fuse holder, and the mains on/off switch is on the right-hand side of the front panel with a status LED.

Whereas the original RPQ featured two mic input XLRs for each channel — one capable of providing phantom power and another that was DC-coupled and completely isolated from phantom — the RPQ2 has a single mic input XLR and phantom is switched from the front panel in the usual way. The idea for the RPQ’s separate inputs was to avoid any risk of damage to delicate ribbon mics from phantom power, and to avoid the LF phase shifts and signal distortion associated with DC-blocking capacitors. In reality, though, all but a very small selection of vintage ribbon mics are quite happy to tolerate phantom power, and DC-coupling is a luxury that few demand, so the RPQ2 has a more conventional mic input connectivity. The output side of the RPQ2 has also been simplified slightly, with the omission of the original unit’s unbalanced output socket; now there is just a balanced XLR for the line output.

However, between these two XLRs are two quarter-inch TRS sockets — one labelled ‘Pre Out’ and the other ‘Line In’ — and these form a balanced insert point for the mic preamp section. Of course, the preamp out can also be used to provide a ‘clean’ mic signal for recording without the EQ processing, while the line input allows external access to the EQ section, if required. The level from the Preamp Out socket is around 6dB lower than that from the main output, and there is around 8dB of gain from the line input socket to the main output (both with the output fader at 0dB), which means that unity gain from line in to main out requires the output fader to be set at about 11 o’clock.

The signal path is configured so that the front-panel mic/line switch routes either the preamp output or the line input into the EQ section via the mic/DI switch, which means that the instrument input takes priority over everything. The output fader is placed after the EQ section and provides the only level control available to the line and DI inputs, but with a range of -60 to +19 dB, it should be more than sufficient!

As well as the usual balanced analogue mic input and line output, the RPQ2 features a balanced line-level insert point, before the onboard EQ.As well as the usual balanced analogue mic input and line output, the RPQ2 features a balanced line-level insert point, before the onboard EQ.

Technology

While the RPQ2’s core circuitry is derived from the original TRP, there are several significant differences. For a start, the microphone gain stage uses Linear Systems LSK389 JFETs rather than the Toshiba LSK170s employed in the TRP, and the balanced output driver is built around a THAT Corporation line driver chip instead of the TRP’s discrete output circuitry. The signal path bandwidth still extends to over 200kHz, though, with -3dB points around 1Hz and 100kHz. The quality of the design is revealed clearly in the fact that even at maximum gain, the total harmonic distortion measures around 0.005 percent right up to the clipping level of +28dBu.

The phantom powering arrangements are also new, and arranged so that the input impedance seen by a microphone is a massive 63kΩ when phantom is turned off, and reduces to a still pretty high 10kΩ when phantom is switched on. Ribbon microphones are quite sensitive to a preamp’s input impedance, not least because their own output impedance typically rises quite substantially at low frequencies. So a high input impedance is important for a passive ribbon microphone to maximise the low frequency response and extension, as well as to preserve the best possible transient response, widest bandwidth, and highest sensitivity. Previous AEA preamps have offered 18 and 30 kΩ input impedances, but the new RPQ2 really resets the benchmark at 63kΩ — most conventional preamps offer input impedances of between 1.5 and 2.5 kΩ, with a few venturing as high as 5kΩ — so this really is quite unusual and special.

Of course, ordinary dynamic and capacitor mics will also work quite happily with the RPQ2’s high input impedances — they all work within a ‘constant voltage’ interface paradigm which requires a low source impedance and a high destination impedance. For some, the fear associated with high impedances is of greater thermal (or Johnson) noise, but AEA claims the RPQ2 has an equivalent input noise (EIN) figure of -130dBu (A-wtd) with a 150Ω source, which is very respectable.

With all this talk of impedances I should also mention that the front-panel instrument input presents an input impedance of over 5MΩ, and the line input’s impedance is 18kΩ. Most instrument inputs offer around 1MΩ so the RPQ2 is, again, a little unusual, but this higher figure replicates the ribbon mic input philosophy, ensuring minimal loading on the guitar pickups to give maximum sensitivity, bandwidth, and transient performance. The instrument input sensitivity is similar to the line input, so an input signal of -8dBu produces 0dBu at the output with the output fader at its unity gain position. The minimum instrument input level for +4dBu at the balanced output is -21dBu, and at that level I measured harmonic distortion at 0.002 percent, rising to 0.02 percent if the input level was increased to produce +24dBu at the output.

In Use

I still remember being surprised by how much better my passive ribbon mics sounded when I first started using the TRP instead of conventional preamps, and that holds just as true for the RPQ2, which maintains the high sonic standards set by its elder sibling. With the EQ bypassed there is no obvious tonal flavour here; this is one of the cleanest and most neutral-sounding preamps I’ve used, with a blissfully quiet noise floor even at high gain and masses of headroom.

The EQ section is also beautifully judged for reigning in the worst and bringing out the best of passive ribbon mics, although perhaps it’s not quite so well adapted for use with capacitor mics. The adjustable low-cut equaliser allows the commonly excessive proximity effect of ribbon mics to be tamed very naturally and easily, but while it is helpful in reducing pops and thumps, and even air-conditioning and traffic noise, its effectiveness is inherently limited in these applications because the filter slope is relatively gentle and the maximum attenuation restricted to 20dB.

Given that the preamp itself is flat almost to DC, and ribbon mics tend to resonate around 10 to 15 Hz, I often found myself having to apply an additional steep (18dB/Octave) high-pass filter in my DAW, tuned to 40Hz or lower, just to remove unwanted subsonic components. When I reviewed the original RPQ I suggested that a fixed high-pass filter would be a useful addition and I’m disappointed that the RPQ2 still doesn’t include such a feature, especially as the TRP benefits from a switchable 12dB/octave filter turning over at 100Hz.

Turning now to the high-frequency region, although ribbon mics are inclined to roll off the high end rather earlier than most capacitor mics, they also have the delightful characteristic of ‘taking’ HF boost EQ rather nicely, without any tendency towards harshness. The RPQ2’s CurveShaper EQ is, once again, expertly judged to allow a lovely sense of ‘air’ and presence to be dialled in to a ribbon mic without anything becoming edgy or harsh. However, I found that some care was needed when trying to add sparkle to capacitor microphones since the resonant frequency of typical capacitor diaphragms tends to be in the 15 to 20 kHz region and it’s easy to make things sound gritty rather than airy, especially with the frequency and gain knobs towards the upper end of their ranges. With up to 20dB of boost on hand, it’s also important to manage the gain structure appropriately.

For anyone who has or is thinking of getting some ribbon mics, the RPQ2 is a very attractive proposition. It actually costs less than the original RPQ model despite adding much more versatile facilities. It’s clearly still optimised for passive ribbon-mic applications, with masses of clean gain, a very high input impedance, and carefully tailored EQ facilities, but it’s also perfectly well suited to working with moving-coil and capacitor mics, and can now also serve as a very impressive instrument input too. The provision of an insert point, to allow external dynamics processing or EQ, as well as the ability to use the EQ processing on a line-level signal, also enhances the RPQ2’s usability and attractiveness. This really is a lovely preamp, retaining all the virtues of its predecessor while adding extra capabilities and flexibility and reducing the price. What’s not to like?

Alternatives

The most obvious alternative is AEA’s other ribbon preamp, the TRP, although that model doesn’t provide phantom power, EQ or instrument inputs. Several other manufacturers offer preamps specifically optimised for use with passive ribbon mics, though, including the excellent Integer Audio RMP2, and the True Systems P-Solo.

Published June 2017