AEA RPQ

Dual-channel Microphone Preamplifier

Published in SOS November 2009
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Reviews : Preamp

AEA have tweaked the design of their TRP ribbon‑mic preamp, already something of a favourite, to create a device that's much more versatile — and without compromising on quality.

Hugh Robjohns

Audio Engineering Associates, or AEA, are an American company best known for their large‑format ribbon microphones, both reincarnated vintage models and their own, more modern designs. To partner that expansive range of ribbon mics, AEA designed and built their own dedicated ribbon‑mic preamp called, logically enough, 'The Ribbon Preamp' or the TRP — and I reviewed it back in SOS April 2007. It's an impressive-sounding preamp, so much so that I ended up buying one for use with my own passive ribbon mics, although it is also very good with other dynamic mics and contact mics, as well as valve capacitor mics: anything that doesn't require phantom power, in fact. Amongst the TRP's strengths are 83dB of clean gain with an extremely low noise floor, vast bandwidth, high input impedance, and a minimal signal path that uses JFET‑based amplifier circuits designed by Fred Forssell, which just sound fabulous.

There are now several active ribbon mics on the market that do require phantom power, though, including one of AEA's own (the A440, reviewed in the June issue earlier this year), and the JFET preamp design is so good that it seems a shame to exclude the world of capacitor mics from its sublime sound. Fortunately, Wes Dooley and his team at AEA have taken pity on us all and launched the new RPQ dual‑channel preamp.

Overview

The underlying technology of the RPQ is derived from the TRP design, and the specifications are similar as a result. The maximum gain on offer is a massive 80dB, the bandwidth extends from 1Hz to 200kHz (‑3dB points), and the input impedance is still well over 10kΩ. However, pretty much every other aspect of the RPQ is different from its sibling, starting with the physical format: instead of a half rack‑width compact unit, this is a full‑sized, 19‑inch, 1U model. The review model came with rackmounting ears in the box, but not actually fitted.

The RPQ also provides switchable phantom power, although there's some peace of mind built in (as we'll see) for those who are squeamish about applying phantom power to ribbon mics. The final new features are an equaliser, which provides both a variable-frequency low‑cut filter (to help tame proximity effect) and a variable high‑boost 'curve shaper' filter (to maximise HF presence and clarity).

A brief tour of the rear panel reveals a fixed‑voltage fused IEC mains power inlet on the left-hand side, with the I/O connectivity for the two channels occupying the rest of the available space. The TRP relied on an external power-supply unit, which I'm not particularly fond of, so I'm glad to see the power supply incorporated in this design.

There are two input XLRs, which are effectively wired in parallel to the electronically balanced input stage, although one is permanently and fully isolated from the phantom power supply (and DC coupled to the mic preamp input), thus providing the promised peace of mind, while the other can have phantom power switched to it via a front‑panel button, if required. The DC‑coupled input shuts down automatically if phantom power is activated for that channel. The input impedance for either input is well over 10kΩ (the TRP is about 18kΩ, and I suspect the RPQ is much the same). This is ideal for ribbon mics, but works remarkably well on all dynamic mics too, and won't upset any capacitor mics I've ever used, either. Although such a high input impedance is unusual in a mic preamp (it is more normally in the range of 1.5 to 2.5kΩ), there's growing support and appreciation of the practical and sonic advantages of higher mic‑input impedances, even with 'ordinary' mics — and in fact many of Rupert Neve's famous preamp designs have presented relatively high input impedances (typically around 5kΩ), with few people being aware of the fact!

Each channel is also equipped with two separate outputs, one actively balanced on an XLR, and the other unbalanced on a TRS quarter‑inch socket. The unbalanced output can manage a maximum output level of a very healthy 22dBu, while the balanced output goes an impressive 6dB louder. Both outputs can be used simultaneously, although because the unbalanced output isn't separately buffered it's not advisable to plug or unplug connections while the balanced output is in use, as the output level could fluctuate.

The front panel is very clean and simple, with clear, white control legends printed on a metallic‑blue powder‑coat finish. The main power switch is a red button on the extreme right‑hand side, with an associated green LED that illuminates when powered up. Each of the two control channels is identically equipped with five rotary controls, fitted with knurled anodised knobs, and four push-button switches. The first button cap is red, and this activates the 48V phantom power for that channel. There's no warning LED for this function, which I find slightly surprising, given that it is notoriously difficult to tell from a distance whether this type of button is depressed or not. The next button is coloured black and this provides an input polarity reversal (again, without an LED indicator).

The first two rotary controls provide the coarse input-gain and fine output-gain adjustments, with a 12‑position Grayhill switch setting the gain from 7 to 55dB in 4dB or 5dB steps, and a continuous output trim control allowing up to a further 19dB of gain to be introduced. There is a subtle centre‑detent notch at the 7dB position on the trim control. The mathematicians amongst you will have already realised the total gain on offer here is only 74dB... but these figures are based on the gain to the unbalanced output; the balanced output is 6dB hotter, providing the full-spec maximum gain of 80dB.

The remaining two (white) push buttons and three continuous rotary controls constitute the equaliser section. The first button activates the low-shelf cut equaliser, which provides a fixed 20dB of attenuation with a 6dB/octave slope, but turning over at a variable frequency between 18 and 360Hz, as determined by the associated rotary control. The second white button activates the high‑boost 'curve shaper' section, which features both a variable turnover frequency (2.1 to 26kHz) and variable gain (up to 18dB). With both white buttons released, the entire EQ section is completely bypassed.

At the right-hand side of each preamp section, a traffic-light arrangement of LEDs indicates signal present at ‑5dBu (the green LED illuminates), and approaching overload at +20dBu (the red LED illuminates). The intensity of the centre, yellow LED varies with signal level from zero to +20dBu. (All these signal levels relate, again, to the unbalanced output.)

The inside isn't the easiest to access for maintenance, but the construction is to very high standards, and I'm confident that this RPQ will enjoy a very long and happy lifetime of amplifying microphone signals. Like the TRP design, the RPQ combines discrete devices in the front‑end architecture with JFET‑based op-amps in the following stages, the entirety bestowing a lovely openness and natural sound character.

In Use

Each of the RPQ's two channels has two separate XLR inputs, one of which is totally isolated from the phantom‑power supply to ensure that sensitive ribbon mics don't get damaged inadvertently.

The RPQ is every bit as impressive-sounding as its elder sibling, but with the added advantage of the equalisation, which is surprisingly useful, and very well judged for bringing the best out of ribbon mics in particular. The preamp is very clean and neutral in character, with a very quiet noise floor, even at high gain settings, and it passes on transients and dynamics with superb clarity and fidelity. These qualities were very apparent in the TRP design and I'm pleased to report they're present here too, even when using the AC‑coupled (phantom‑powered) input connection — although I still think the DC‑coupled input has a slight sonic edge when using a ribbon or a valve capacitor mic.

The low-shelf equaliser compensates for proximity effect wonderfully, both on ribbons and other directional mics, and adjusting the turnover‑frequency control upwards quickly establishes the optimum balance of low‑frequency extension and natural bass energy levels. It is less effective at removing unwanted subsonic rumbles than dedicated high‑pass filters, primarily because the slope is too shallow and the attenuation limited to 20dB, but it helps to a degree and encourages the use of effective shockmounts and wind‑shielding, which is no bad thing, really!

The 'curve shaper' HF equaliser is fabulous for restoring brilliance and air in those situations where it's lacking, and that's not an uncommon situation with the larger ribbon mics. Although ribbons have a lovely smooth character, it can sometimes be a tad too smooth, even bordering on dull and lacking vibrancy. It needs to be used with care when trying to equalise capacitor mics, as I found it all too easy to end up emphasising the nasty HF resonant character of some large‑diaphragm models. The curve shaper is essentially a boosting parametric equaliser bell‑response, and it does a great job of bringing out that HF presence and air in a subtle and natural way, thanks largely to the way the bandwidth and slope vary as the gain and centre-frequency controls are adjusted. The bandwidth is wider, with a lower frequency setting and gets narrower as the turnover frequency is raised or the gain increased. Obviously, adding gain risks overloading the output stage, which is why the output gain control follows the equaliser section, and why AEA recommend setting the unit up with the output trim control fairly well advanced — specifically so that the overall level can be reduced if significant HF equalisation is added.

Opinion

Starting with the negative aspects, the tailoring of the low‑cut filter to tame ribbon proximity effect, and the curve shaper HF equaliser to compensate for ribbon dullness works very well for all ribbon microphones (as you'd expect), but is somewhat less effective and useful for other microphone types. It might have been nice to have an additional proper high‑pass filter to really sort out unwanted subsonics — but this is a very minor criticism in the grand scheme of things. It certainly wouldn't put me off investing in the RPQ or frustrate me during its use.

In fact, the only real down side that I can see is the cost, which, at nearly £1800 in the UK, is twice that of the TRP model upon which it is based. OK, so it boasts the advantages of some useful equalisation, phantom power and a built‑in power supply (which would be a powerful persuader all on its own!), but it still feels like a very high premium just for those facilities.

On the positive side of the equation, the RPQ is a fabulous‑sounding preamp by anyone's standards. While it is obviously particularly well suited to partnering ribbon mics — because the gain, input impedance and equalisation facilities are all obviously optimised in that direction — it is also mightily impressive with less exotic microphones, including conventional capacitor and dynamic mics, helping the latter, in particular, to really excel. If I didn't already have the TRP I'd be very tempted to acquire the RPQ, because it is a far more versatile and useful design that doesn't sacrifice any of the sonic quality.

Verdict

If you're a fan of ribbon mics — and you have this kind of budget available — the RPQ is a must‑buy product. I was astonished at how much better every ribbon I've ever reviewed sounded when it was connected to the TRP preamp, as compared with other preamps (whether budget, professional or high‑end boutique), and this RPQ version matches that all the way. This design just seems to extract the very best from ribbon mics of all kinds, and other dynamic mics too, but it also provides a sublime precision and neutrality for all microphone sources.  .

Alternatives

The obvious alternative is AEA's own TRP, which is another two‑channel design. It costs almost half as much, but it doesn't offer the luxuries of phantom power and equalisation. I'm not aware of any other microphone preamps optimised to such a degree for ribbon microphones, although there are several designs with higher than usual gain specifically to suit ribbon‑mic applications, such as the True Systems P‑Solo ribbon mic preamp. Other high‑gain designs popular for use with ribbon mics include the AMS‑Neve 1073DPA (and the countless 1073 clones), Focusrite ISA One (and other ISA preamp models, most of which provide adjustable input impedance, including a high 6.8kΩ option), and the stunning Grace Design M201 and its siblings.

AEA RPQ £1753$1700
pros
Maintains the TRP's sound quality with ribbons and other mics.
Internal mains power supply.
Both DC‑coupled and AC‑coupled inputs, the latter with switchable phantom power.
Nicely tailored equalisation facilities.
Huge amounts of gain available.
cons
Limited capability to remove subsonic rumbles.
No phantom‑power warning indicator.
Much more expensive than the TRP.
summary
A solid‑state preamp optimised for use with ribbon microphones through its phantom‑protected inputs, high input impedance and carefully tailored equalisation, but more than capable of delivering stunning results with more conventional microphones too. Building on the superb sonic character of the company's TRP model, the RPQ is a true high‑end microphone preamp.
information
£1753 including VAT.
Affinity Audio +44 (0)1923 265400.
$1700.
Audio Engineering Associates +1 800 798 9127.

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