Dual-channel Microphone Preamplifiers

Preamps / Channel Strips

Published March 2024

Designed to get the very best out of passive ribbon mics, AEA’s preamps offer way more gain than most. But they’re not just for ribbons...

AEA's original TRP, which I reviewed in SOS April 2007 (https://sosm.ag/aea-r92-and-trp), was a compact (half‑rack width), no‑frills, two‑channel preamp with an external power supply. Designed by Fred Forsell, it was entirely dedicated to getting the best possible signal from vintage ribbon mics, which are notorious for their low sensitivity. Consequently, it didn’t provide phantom power and featured a DC‑coupled input with an exceptionally high input impedance (18kΩ) to minimise the electrical load on ribbon mic motors. Despite the absence of (free‑gain) input and output transformers, the active circuitry provided a whopping 83dB of gain using a Grayhill rotary switch (6‑63 dB in 12 steps) plus a continuous output level control offering up to 20dB of extra gain (as well as fading down to silence).

Other features included on each channel were a second‑order (12dB/oct) 100Hz high‑pass filter, polarity inversion and simple traffic‑light metering. Internally, the circuitry was based around a combination of discrete JFET gain stages at the front end and op‑amps for the filtering and output sections, fabricated using mostly SMD (surface mount) technology, with sealed relays for switching functions. Despite the enormous gain on offer, the design maintained a huge bandwidth (‑3dB at 6Hz and 300kHz) for a phenomenal transient response, with a generous headroom margin (+28dBu), and incredibly low noise (EIN figure of ‑130dBu A‑wtd).

So impressed was I with the TRP’s sublimely neutral, yet musical sound character, unusually high‑gain, and astonishingly quiet noise‑floor that I bought the review model without hesitation. I’ve used it regularly ever since, both with my best low‑output moving‑coil mics (Beyer M201 and AKG D224E) as well as all manner of vintage and modern mono and stereo passive ribbons. It never disappoints, and always extracts the very best that any of these dynamic mics can deliver.

Of course, being the annoyingly picky soul I am, I identified a few ‘cons’ of the original TRP. The lack a front‑panel power switch was a mild frustration, as was the seven‑pin DIN power socket, and the high‑pass filter design turned over from too high a frequency to be useful at removing rumble, yet was too steep to properly correct for proximity effects. Not that any of these minor grumbles really bothered me in practice, but I’ve taken a keen interest in the development of this excellent design since then. We’re now on version three of the TRP, which is reviewed here, and there have been a few related products too, not least the RPQ and its own revisions.

Take Two

The first successor to the TRP, unsurprisingly named the TRP2, was introduced in 2018 and although this provided essentially the same functionality and specifications as the original, it benefited significantly from various small improvements to both the hardware and circuitry. The most obvious of these were on the back panel, where the DIN power socket was replaced with a robust five‑pin XLR, and the duplicate unbalanced preamp outputs were omitted (the TRP2’s balanced outputs could still be used with unbalanced destinations through appropriate cable wiring). On the front panel, the original black stubby plastic knobs were replaced with more elegant long‑necked, shiny aluminium ones, and a power button was added as well. So, although the high‑pass filter design remained unchanged, two of my petty gripes were vanquished!

On the technical side, the input impedance was raised to 63kΩ and the maximum gain increased slightly to 85dB, with 7‑63dB available on the gain switch while the output level control’s range was altered to span ‑55 to +22 dB. Metering LED levels were also altered slightly, with green coming on at a more helpful ‑20dBu (instead of ‑5dBu) and the red illuminating (less sensibly) at +24dBu instead of +20dBu. Interestingly, the TRP2’s specifications claimed that the preamp’s bandwidth was altered to ‑3dB at 1Hz and over 100kHz, instead of the 6Hz and 300kHz of the original.

Undeniably, though, far and away the most significant difference between the original TRP and TRP2 was the addition of phantom power, with individual front‑panel buttons (and status LEDs). This rather surprising inclusion was to allow the full gamut of capacitor mics to benefit from the TRP’s high gain and low‑noise, too. However, adding phantom power inevitably also forced the introduction of DC‑blocking capacitors at the inputs, which dismayed some potential customers — so it’s worth pointing out that the greater LF extension (‑3dB at 1Hz) suggests these capacitors were carefully chosen to minimise the inherent LF phase shift. For vintage ribbon mic owners of a nervous disposition, quivering at the thought of knocking a phantom power button and accidentally destroying their pride and joy, an internal ‘no‑blow’ kill switch was also included to allow phantom power to be disabled completely, if preferred — a thoughtful and reassuring feature.

Another side‑effect of adding phantom power was that, when engaged, the input impedance inherently falls dramatically — in this case to 10kΩ, purely because of the parallel loading effect of the necessary feed resistors passing +48V into the mic lines. Of course, 10kΩ is still much higher than ‘standard’ mic preamps (which typically present 1.5‑2.5 kΩ), but it’s a difference worth noting all the same.

TRP3 Overview

Moving forward five years, AEA have recently revised the design once again to create the TRP3 and the RPQ3, which is based on the same circuit — I’ll focus on the TRP3 in the main text and you can find out what more the RPQ3 model brings to the table in the separate box. The TRP3 shares the same format and front‑panel layout as the previous models, with some minor aesthetic tweaks and some updates to the core circuitry, which features a discrete JFET front end, a handful of Burr Brown OP1656 dual op‑amps for the main gain stages, and THAT 1606 ‘transformer‑like’ output drivers.

The maximum gain remains 85dB, with 7‑65 dB available on the 12‑position Mic Gain rotary switch, and a further 20dB available via the continuous Output Level control. The latter’s anti‑clockwise position, though, now provides unity gain (0dB) rather than the ‑55dB of the TRP2 or minus‑infinity of the TRP. It’s a worthwhile, practical improvement, greatly improving the control’s resolution when fine‑tuning and matching channel gains. The previous ability to attenuate the output always seemed pointless to me, since such a feature is only really useful in preamps where the front‑end is designed to be overloaded intentionally, for musical effect — not part of the TRP’s ethos!

Various circuitry tweaks have improved the THD figure by an order of magnitude (from 0.02% to 0.0015%, at 30dB gain and a +4dBu output level). The input impedance has been increased slightly to 68kΩ (falling to 11.3kΩ when phantom is switched on), while other specifications show both the LF and HF bandwidth limits have been raised slightly, reaching ‑3dB at 10Hz and 200kHz (at the full 85dB gain). Surprisingly, the TRP2’s phantom ‘No‑Blow’ switch is absent here — nervous ribbon mic owners beware — but, to be fair, the phantom buttons are stiff enough to resist accidental contact, and the gain knobs long enough to keep fingers well away from the buttons.

While the TRP3 has an external switching PSU, the rackmount RPQ3 has a linear one built into the chassis, and connects to mains AC via an IEC inlet. The more fully featured RPQ3 also includes a balanced direct out and line in, the pair of which double up as a pre‑EQ insert point.

A far more significant, pleasing change concerns the high‑pass filter: the buttons engaging the 100Hz second‑order option in the previous two models have been replaced with three‑way toggle switches. These offer two first‑order (6dB) slope options instead, with ‑3dB points at 115Hz or 230Hz (plus off) — a very valuable improvement that addresses the proximity effect issues associated with close miking. The simple trio of level indicating LEDs remains, but the trigger level for the red LED has been reduced to the original TRP’s +20dBu level, which more usefully warns of impending converter overload.

It’s blissfully quiet, even at frightening gain levels, with a distinctly colourless, neutral yet beautifully transparent and effortlessly dynamic sound character.

Impressions

AEA’s third iterations of the TRP and RPQ deliver very nicely optimised feature sets, retaining almost all of the earlier designs’ features and qualities, while addressing their very few minor practical foibles. In terms of sound quality, they remain completely beyond reproach in every respect and I rate them as two of the very best neutral microphone preamps currently available. They’re blissfully quiet, even at frightening gain levels, and offer a distinctly colourless, neutral yet beautifully transparent, effortlessly dynamic character that’s also far from sterile: well balanced, with deep, powerful lows and pristinely detailed transients, yet naturally musical and involving. These preamps extract everything that any moving‑coil or ribbon mic can deliver, with impressive clarity and precision. They’re the perfect means of accessing the real quality of Coles 4038s, Shure SM7Bs, AEA R44s, AKG D224Es, Royer SF12s… and so on! Highly recommended.

RPQ3: A TRP3 On Steroids?

For those who prefer conventional rackmount preamps, AEA offer the 1U 19‑inch rack‑mountable RPQ3. Another two‑channel design, this is based on the TRP3’s circuitry but enhanced with versatile two‑band semi‑parametric equalisation (though omitting the TRP3’s more basic high‑pass filter facilities). A distinct feature of the RPQ3 is its inclusion of impedance‑balanced preamp direct outputs and selectable balanced line inputs (feeding the EQ section), both of which are presented on quarter‑inch TRS sockets. The combination also serves as a practical balanced pre‑EQ insert point to each channel. Buttons on the front panel select the line input/insert return mode independently for each channel.

The EQ sections are much more traditional compared with those in the very first RPQ (which had a tunable high‑pass filter and boosting HF bell section). In the RPQ3, the EQ offers more conventional fully tunable LF and HF bell EQs with a ±20dB gain ranges (this can be reduced to ±10dB using front‑panel buttons). The LF band spans 40‑675 Hz, while the HF band covers 2‑28 kHz, and both bands feature AEA’s bespoke CurveShaper technology, which means the bandwidth (or Q factor) of each bell response varies according to the frequency and gain settings in a musically appropriate way. Further buttons select or bypass each EQ band independently, as well as the entire EQ section. Phantom power and polarity inversion buttons are also present, of course, as is a global power on‑off button. The larger 1U rack case of the RPQ3 allows the power supply to be fully integrated rather than external, and this is accessed through a standard IEC mains inlet, switchable from the rear panel for 115 or 230 Volt mains supplies.

Alternatives

The same core preamp design is also available in the RPQ3 (see box) and the TRP500 500‑series module. There are one or two computer interfaces that offer 80dB (or more) of gain, such as the Metric Halo ULN‑8 (90dB), but I can’t think of any other standalone preamps that go that far.

Pros

Stunning performance with astonishing maximum gain, yet incredibly low noise.
Revised high‑pass filtering options much more useful than earlier models.
Improved metering and output level control range.

Cons

It has phantom power and thus inputs are not DC‑coupled (unlike the original TRP).
Removal of No‑Blow phantom protection option might concern some.

Summary

The latest iteration of AEA’s remarkable TRP dual‑channel preamp design, with an even more polished and honed feature set, utterly sublime performance, and more gain available than you’d ever want or need!