API Select T25

API have been making excellent solid‑state outboard and consoles for decades, but their latest venture sees them using valves for the very first time.

Most of our readers will know that API’s famous console and outboard gear designs employ the revered 2520 discrete solid‑state op‑amps and related designs, and that these play a huge role in the distinctive sound character associated with that marque. The company’s main range continues to draw on those design traditions but they’ve also been keen to allow their engineers more freedom to experiment with other technologies. Thus, last year, they announced a new API Select brand, which hosts some refreshed designs from the company’s old JDK‑badged sub‑brand along with some entirely new creations, notably the T12 mic preamp and T25 compressor‑limiter. Both of these are hybrid devices which combine conventional solid‑state op‑amps with valve circuitry, and it’s the latter that I’m reviewing here.

Construction & Circuitry

A block diagram included in the user manual reveals how these different technologies are employed in the T25. The audio signal path starts with an electronically balanced solid‑state input stage (presenting a 20kΩ input impedance). This drives a FET gain‑reduction element which is driven by an RMS‑sensing side‑chain connected in a feedback arrangement.

A Class‑A valve amplifier employing a 12AT7WC (ECC81 equivalent) provides the make‑up gain, while a beefier 12BH7 drives a custom API output transformer. Both valves are Electro‑Harmonix dual‑triodes plugged into good‑quality sockets, and I spotted a number of standard NE5532s and TLO82 op‑amps among the solid‑state circuitry, with the former appearing to serve the audio paths and the latter probably in the metering and side‑chain sections.

A single, selected, NTE457 N‑channel J‑FET provides the gain‑reduction element for each channel, each one being mounted on small plug‑in circuit board. Most of the circuitry is carried on a single PCB covering the whole of the chassis base, and surface‑mount components are widely employed. The user controls and LED metering are built onto secondary circuit boards mounted behind the front panel, and the whole construction is tidy, robust and well thought‑out.

Output transformers and the 12BH7 valves towards the bottom. The 12AT7s are up near the control panel. The FET screw‑down circuit boards are just to the right of the colourful meter wiring for each channel. The yellow rectangular blocks (two below the output transformers and the rest amongst the audio circuitry at the top) are the sealed relays.

T25 Operation

The T25 is a dual‑channel compressor that’s encased in a substantial 2U rackmounting chassis with a smart blue front panel. The controls for the two channels are laid out identically, with plenty of space and clear function labelling, although as there are no intermediate scale markings on any of the controls you have to count and note detent clicks to recall settings.

Perforated metal grilles allow each channel’s 12AT7 valve to be seen (the 12BH7s are towards the back of the circuit board and harder to view), and LEDs mounted in the centre of their valve‑holders provide a cosy (if misleading) golden glow.

Moving left to right, the first vertical pair of rotary switches control the attack and release time‑constants, both with 11 settings. The attack time spans an extremely fast 0.02ms (20 microseconds) down to a still very quick 0.8ms, while the release time ranges from 50ms to 1.2 second; there’s no auto‑release mode. The ratio control is a four‑way rotary switch marked 2:1, 4:1, 8:1 and 20:1, although this device is generally rather more assertive than those labels might suggest (as I’ll explain in more detail below) and I noticed that the threshold changed over roughly a 6dB range with different ratio values (lower for the 2:1 setting and higher for 20:1).

The lower part of the panel carries input and output level controls, both having 31 detented actions and covering the range from fully off up to +20dB. Unity gain is with both knobs set around 10 o’clock. The output level knob doubles up as the gain make‑up control, while the input serves as the threshold, both functions being identified with secondary labels. As the T25 has a fixed internal compression threshold level (slight changes introduced by the ratio control excepted), to introduce more compression you turn up the In/Thresh control to push more of the input signal over the threshold. I found this input control to be surprisingly sensitive, and rarely had it above 11 o’clock when working with nominal line levels.

A vertical 10‑segment bar graph indicates applied gain reduction over a 24dB range, while a round, vintage‑style, backlit VU meter displays the output level. An associated peak‑level LED illuminates at +18dBu (6dB below clipping), and although the meter’s 0VU mark is aligned at the factory to +4dBu, an internal jumper allows that reference to be changed to +14dBu for those who prefer to work with hot signal levels.

A plot showing how the Total Harmonic Distortion (THD) varies with input level. Note how the distortion rises with increasing level.

Using an Audio Precision test system, I measured a low THD of 0.05 percent at an output level of ‑10dBu, but that rose progressively to 0.5 percent at +4dBu, 1.2 percent at +10dBu, and 3 percent by +14dBu — so if you’re looking for old‑school character, there’s plenty of it available here. Regarding the metering arrangements, I really like that the output level and amount of gain reduction are indicated at the same time; so many compressors force the user to select one or the other on a shared meter!

Five buttons under the meter activate sealed relays to introduce two different side‑chain filters, soft and hard bypass modes, and link together the side‑chains of both channels for stereo applications. The unit’s on/off button is over on the right‑hand side of the chassis, and I second API’s recommendation to allow 15 minutes for everything to warm up and stabilise before serious use.

Most compressors offer a high‑pass filter to desensitise the side‑chain at low frequencies, making them less likely to ‘pump’ on kick drums and bass guitars, but API’s acclaimed (and patented) Thrust design is rather more ingenious than that — you can find out more about that in the separate box. A second side‑chain filter option labelled D‑S is a de‑esser mode, and the Thrust and D‑S filters can be used together, if desired. The de‑esser employs a narrow 6kHz boost in the side‑chain to increase the compressor’s sensitivity to sibilant frequencies, thereby ducking them a little more assertively to reduce sibilance.

When working with stereo signals the Link button uses the strongest side‑chain control signal from either channel, sharing it with both so that the stereo image doesn’t pull to one side or the other when gain‑reduction is active. As with most such designs, both channels’ controls must be set exactly the same to ensure equal responsiveness on both sides.

A bypass button connects the input sockets directly to the corresponding output sockets via a relay, which means the unit can be powered down to preserve valve life while maintaining a signal path and avoiding the need for re‑patching. However, there’s also a ‘soft bypass’ mode which effectively turns the T25 into a characterful line amp: if the In button is not pressed the side‑chain control signal is disabled, so there’s no gain‑reduction but the signal still passes through the entire circuit path, and the valve and transformer sonics can be used as a signal conditioner. Flipping the In button also makes it practical to compare the sound with and without compression, of course.

On the rear panel, the inputs and outputs are presented on parallel‑wired XLRs and TRS jacks, and each channel’s output can thus feed two separate destinations simultaneously.

Audio connections for all inputs and outputs are presented on rear‑panel XLRs with TRS quarter‑inch sockets wired in parallel, so you can choose which you prefer to use and, although only one input connector format can be used at a time, both output formats can be used simultaneously to feed the same signal to two destinations. Mains power is provided to the hefty internal linear PSU via a grounded IEC inlet complete with a fuse‑holder and voltage selector (115/230 Volts AC), and the unit consumes 45 Watts.

Most FET compressors are inherently fast‑acting and colourful, but the API Select T25 goes further than most in both respects.

In Use

As well as trying the T25 on a range of material, I performed standard bench tests, which revealed that its audio bandwidth (with no gain reduction) has ‑3dB points at 6Hz and around 90kHz, which is impressive. Relative to +4dBu at the output, the signal‑to‑noise ratio measured 80dB, and crosstalk between channels at 10kHz was a healthy ‑101dB.

Driving the input quite hard with the output backed off, intentionally to ‘push’ the thermionics and transformers, I achieved some really nice, musically pleasing saturation, even without engaging the compressor. As I mentioned earlier, there is more than enough ‘warm analogue character’ available here to satisfy the strongest digi‑phobe, and I found myself running all sorts of stereo mixes through it just to fatten and warm them up. Unfortunately, though, I found precise matching of channel gains for stereo tracks a little tricky, because the detented input and output level controls impose relatively large adjustment increments.

This plot shows the transfer curve with different ratio settings. The top red line is with the compression switched out, revealing some valve/transformer saturation creeping in at higher levels. The four almost horizontal lines are from the four ratio settings, clearly demonstrating how the selected ratios all quickly trend towards limiting as the signal rises more than about 6dB over the threshold. The way the threshold also changes with the ratio can also be seen. The lowest curve is the 2:1 setting, which trends to more than 8:1.

Most FET compressors are inherently fast‑acting and colourful, but the API Select T25 goes further than most in both respects. However, I found it very hard to dial in modest amounts of more subtle compression, and when I explored the dynamic behaviour on the test bench I discovered that once the signal exceeds the threshold by about 6dB, all four ratios very quickly become extremely high. Even the 2:1 setting soon tends towards 8:1, and the higher three settings soon reach ratios of at least 15:1. So the ratio control has almost no effect on the nature of compression outside of a fairly small sweet spot: when you have a signal that’s already fairly controlled in terms of dynamic range, you can experiment with the input level to achieve subtly different compression effects in that region, but push it further and the T25 generally behaves more as a limiter than as a compressor.

it certainly is powerful and effective on heavily dynamic instruments such as basses, and on percussive sources like drums... anything that requires both assertive control and character.

With that in mind, I wouldn’t really recommend the T25 for use as a subtle bus compressor to glue things together. Yet, it certainly is powerful and effective on heavily dynamic instruments such as basses, and on percussive sources like drums (both the individual close mics and stereo kit overheads); anything that requires both assertive control and character. The attack range allows precise control of the transients, while the release is fast enough to make the result as dynamically bouncy and rhythmic as you like. The Thrust and D‑S modes are also useful and effective additions, and I found myself using the Thrust mode a lot.

This plot is an expanded version of the previous chart, revealing more clearly an initial soft‑knee character which acts over a few decibels before flattening into a high‑ratio slope.

Verdict

The new API Select range is interesting in that it allows the company space to explore new directions of technology and sound texture, and while there may be a few foibles in the way the T25 works that mean it’s probably not the best choice as anyone’s first compressor (easier access to more subtle compression settings would address that) it could be argued that they add to its character rather than detract from its abilities — it is a powerful tool that can certainly be used to great musical effect in the right circumstances.