Aphex 207D

Can a two-channel preamp at this price really give you the best aspects of tube and solid-state circuitry?

Unlike many tube-based mic preamps, the Aphex Model 207D is a hybrid design that uses a solid-state front end followed by a tube gain stage — the idea being to offer the best of both worlds. The solid-state circuitry provides low noise with a good transient response, while the tube provides the 'flavouring'. Of course, every American technical innovation has to be given a trademarked name, and for the 207D the power under the hood is 'Tubessence'!

Overview

This 1U, rackmount, dual-channel preamp uses Aphex's established 'reflected plate' tube technology to give the performance of a tube running on a high voltage while actually running on a low-voltage circuit. Essentially they've taken an under-run tube and put solid-state feedback circuitry around it to counter the increase in plate resistance that otherwise occurs when a tube is run at a very low voltage. This approach saves on cost, extends tube life and also enables the unit to be powered from a switch-mode power supply that can adapt to local line voltages automatically — something that has the happy side-effect of keeping manufacturing costs down. It is also safer, because normal tube HT voltages are high enough to be lethal!

A delayed switch-on circuit mutes the audio path until the tube has warmed up, and to indicate the status the power LED at first glows orange, then turns green when the unit is ready for operation. The 48V phantom power source (which is independent for the two channels) also ramps up when activated, to avoid stressing the microphone circuitry.

In addition to its two channels of hybrid solid-state/tube circuitry, the 207D also features a switchable limiter and an inbuilt 24-bit digital converter with word clock connection that can operate at up to 96kHz, with both AES/EBU and S/PDIF digital outputs. The limiter is useful in situations where you can't accurately predict the maximum peak input level, and there's no setting up required: once it is switched in, the limiter automatically operates to prevent clipping. The circuit is Aphex's own optical design and comes from the much more costly Aphex 1100 two-channel and 1788 eight-channel preamps. It detects the level at the preamp output, but works on the mic input line to limit the signal prior to preamplification. Because of this approach, it can increase the effective headroom by up to 20dB — though, of course, for the highest audio quality it is best to have a limiter only step in on occasional peaks. I'm not 100 percent sure if this limiter works fast enough to avoid all clipping, but it is certainly fast enough that you don't notice any.

The 207D features both AES/EBU and S/PIDF digital outputs, on XLR and phono connectors respectively.The 207D is set out quite differently from the earlier 107. Every channel features a phase-invert switch, a 20dB pad switch, and a low-cut filter, as well as an LED display to indicate available headroom.

Each mic input offers 20 to 65dB of gain, and the high-impedance instrument input also allows up to 65dB of gain to be added. For line-level signals you use the mic inputs with the pad switched in and double-check that the phantom power is switched off! The output operating level can be set (using slide-switches) to -10dBv or +4dBu to suit semi-pro or pro systems, and the output level can be trimmed using the trim pots on the front panel. Power comes into the unit via a standard IEC mains cable.

All the audio connections accept balanced connectors — the outputs are available on both XLR and jacks, the mic inputs are XLRs, and the line and instrument inputs (the latter are, for ease of access, located on the front panel) are jacks. Both mic channels also have an unbalanced insert point on a TRS jack, which allows other processing to be patched post-preamp but pre-converters.

The digital section, which comes as standard on this model (the original 207 is no longer available), comprises both AES/EBU (XLR) and S/PDIF (phono) outputs, as well as a word clock input on the usual BNC connector. When the 207D is hooked up digitally, the sample rate can be stepped through using the button between the centrally-mounted bar-graph meters, and the LED indicators display the available choices of 44.1kHz, 48kHz, 88.2kHz, 96kHz or external sync.

Testing Times

Aphex make a number of claims for their circuitry, including greater dynamic range and frequency response than traditional tube circuits, as well as very low noise — so I was keen to put this to the test. As I found with the earlier Aphex 107, when I reviewed it many years ago, the 207D warms things up in a very subtle way. It creates a sense of air around the sound, which makes me think that some subtle high-end enhancement also takes place. The result is a slightly more detailed, crisper sound than you'd expect from a strictly neutral preamp — something that can help the signal survive subsequent processing without becoming dull — but other than that the signal path is very clean and transparent-sounding, and this preamp seemed happy with all of the mics I plugged into it.

Of course, you don't have to use the 207D as a mic preamp: as a simple DI box, it also turns in a classy performance, and here the limiter becomes a very useful ally against more enthusiastic level excursions, especially if you were using the 207D for live recording applications.

Verdict

All in all, this is a very classy-sounding and versatile preamp, that gives you the unmistakably glittery and flattering Aphex sound. It also represents great value at its current price, and there are very few comparable units available for the same budget — see the SSL VHD Pre review on page 146 of this issue for a list of some possible alternatives, though bear in mind that some of these do not offer a digital output as standard.