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API 7600

Input Module
By Hugh Robjohns

Not only does the API 7600 offer the heritage sound of the company's renowned Legacy consoles in a stand-alone rack unit, but multiple units can also be used to build a complete small mixer for audiophile recording.

Mic preamps and voice channels are in plentiful supply, and feature regularly in the pages of this magazine. However, the complexity of such devices varies enormously, from the most basic level of just providing a gain control (such as on the GML 8304 mic preamps), right through to feature-packed high-end producer units such as those from Focusrite and Millennia Media, providing EQ, dynamics and even A-D facilities. However, the API 7600 Input Module reviewed here takes things a stage further still. Automated Processes Incorporated have taken a complete channel strip from their high-end Legacy analogue mixer, engineered it to fit in a 1U rackmount case, and provided sufficient interfacing to enable multiple units to be combined with the same facilities as a proper mixer! As such, the 7600 incorporates the kinds of facilities that you would only expect to see on a console strip.

API 7600 input module rack.Photo: Mike CameronSo although this unit can be used in a stand-alone mode as a sophisticated mic preamp and processor, it can also form part of a high-quality, full-facility modular mixing console. The company's soon-to-be-released 7800 Master Output Module will provide the back-end facilities in such an application, including channel solo, mute groups, buss masters, talkback and monitoring controls, and I gather that a lot of big-name engineers, producers and artists are eager to acquire systems constructed from these components.

The only other company I am aware of that produces stand-alone input and output units which can be combined to form a mixing console in this kind of way (albeit with fewer routing facilities) is Amek, with its Channel In A Box and Driver In A Box units. However, the API 7600/7800 products more closely match the functionality expected of a conventional high-end console. It should be noted, though, that this review is based entirely on the abilities of the 7600 as a stand-alone unit; I cannot comment on how it performs as part of a multi-module configuration in conjunction with a 7800 Output Module.

API 7600 £2231
  • API's legendary sound quality.
  • Versatile facilities and comprehensive interfacing.
  • Flexible dynamics and EQ sections.
  • Enables construction of a very high-quality modular mixer.
  • Redundant facilities if used as a stand-alone unit.
  • Disappointing handbook with unexplained features.
  • Poor ergonomics as a mixer.
A complete mixer channel strip in a rack box, enabling a modular API Legacy mixer to be constructed when used in conjunction with the forthcoming 7800 Master Output Module. Superb sound quality is a given, and the unit contains a flexible array of components, but much of it is redundant in a stand-alone application.


Modules & Signal Path

The 7600 is a fairly deep 1U rackmount box, extending around 255mm behind its substantial rack ears. The controls are laid out in much the same way that a channel strip would be, starting with the buss assignment and input section on the right, with the channel pan, solo and mute controls on the left.

The principal signal-processing facilities comprise an input preamp, a compressor, and a three-band equaliser — all derived from API's highly regarded stand-alone products — the 212L mic preamp, the 225L compressor, and the 550A equaliser. These facilities are supplemented with further controls to access four aux send busses. Additional features include a main stereo output buss with onboard rotary fader and provision for an external linear fader; facilities to break into the signal path between each signal processing section, plus a switched insert loop; and all the normal console channel features, such as channel mute, solo and solo-safe functions, and so forth.

With such a feature list, the rear panel of this unit is extremely busy, with no less than nineteen connectors, not including the IEC mains inlet! The mic and channel direct outputs are presented on XLRs, while most other inputs and outputs are catered for with balanced TRS sockets. The auxiliary and buss sends are accessed through a D-Sub connector conforming to the ubiquitous Tascam DA88 wiring format, and a very large multi-pin connector is employed to handle all the control data and audio busses through a ribbon cable link when working with multiple 7600s and a 7800 Master Output Module.

Construction is generally to a high standard, with a sturdy chassis, substantial front-panel metalwork, decent metal knobs, and LED indicators associated with every switch (except for the Mute and Solo buttons, which are illuminated types). Internally, there are two main PCBs mounted one above the other, with all the components on the inside of the sandwich. There are an impressive five — yes five! — audio transformers providing galvanic isolation and output balancing/unbalancing for each stage of circuitry: the mic input, mic preamp output, EQ output, compressor output, and channel direct output.

Input Section

The 212L mic preamp circuitry is the same as that employed in the API Legacy consoles and the API 200- and 500-series rack products. It uses the company's all-discrete solid-state 2520 op amp gain block. This amplifier design dates from the mid-'70s and adds a recognisable 'LA' sound character to everything passing through it. It sounds big, with a natural but rich bottom end — always a sign of a really good mic preamp — and a crisp, detailed, and very clean HF. The brochure for the 7600 module claims this preamp design is 'right out of the glory days of rock and roll', and when you hear it you know exactly what is meant — it really does have that sound characteristic of so many well-known American tracks dating back to the '70s and '80s.

Three small buttons select a -20dB pad, phantom power, and mic or line inputs — and each has an associated LED indicator. In addition to the mic and line inputs, there is also a high-impedance DI input on the rear panel. Plugging an instrument lead into the quarter-inch DI socket automatically overrides the mic input, although there is no indication on the front panel that this has happened. A nice feature is that, rather than using the contacts on the DI socket itself to switch between the mic and DI sources as most preamps do, the DI socket contacts are used to operate a sealed relay which switches the appropriate signal into a second relay. This selects either the mic/DI source or the line input, and is controlled via the front-panel switch. The use of relays to switch signals is a good indication of the attention to detail and preservation of a high-quality audio path throughout the design of this unit.

Up to about 60dB of gain is available for the mic input, although there are no calibrations on the rotary control knob. The DI and Line inputs benefit from up to 30dB of gain. Setting the optimum level is made simple thanks to an LED bar-graph VU meter next to the gain control, displaying the output level of the preamp stage. The meter spans -12dB to +18dB in seven steps, with the 0VU point calibrated to +4dBu. The output from the preamp stage can have its polarity inverted courtesy of another button (with an indicator LED). However, this facility is tucked away with the buss assign switches for some strange reason, instead of with the input-stage controls. The panel location of the switch suggests it inverts the polarity at the buss output, but the inversion actually occurs at the output of the mic preamp — which is where it should be.

API 7600The output of the preamp stage is transformer coupled to the Pre Out socket on the rear panel, and this is internally normalled to the FX1 In socket. This 'FX' term is used in relation to the inputs and outputs of the two signal processing stages: the equaliser and compressor. The compressor can be switched before or after the EQ, and the inputs and outputs of both sections are made available on the rear panel to enable further processing to be inserted in the signal path, if required. The normal signal path through the main components of the channel circuitry and connectors is as follows: mic/line/DI input; preamp output; FX1 input; equaliser; FX1 output; FX2 input; compressor; FX2 output; insert send; insert return; fader; direct output. The fader signal is also routed to the pan control and on to the Stereo Out connector, as well as to the aux and buss sends (on the D-Sub port).

However, if the compressor is switched pre-EQ then the FX1 connections apply to the compressor and the FX2 connections access the equaliser. It probably sounds more complex than it really is, but there is a lot of flexibility here — although whether being able to break into the signal path in so many places is of any significant practical benefit is debatable. Essentially, this arrangement provides the channel with three insert points: two permanent and one which can be switched in and out.

EQ & Dynamics

The three-band equaliser is based on the company's 550A design, dating back to the '70s again. However, in this incarnation the original circuitry has been modified with a balanced input stage and seven switched frequencies per band, instead of the original five. The top and bottom bands can be switched individually to either a shelf or bell (peaking) response, and two further buttons select an overall bypass and a band-pass filter. The latter curtails the frequency response below 50Hz and above 15kHz. All the buttons have LED indicators again, so the status of the equaliser is clear from a distance.

Each of the three EQ bands is controlled with a pair of concentric rotary switches. The larger outer knob adjusts the gain over a ±12dB range, starting with 2dB steps and expanding to 3dB increments towards the extreme gain settings. The inner (cyan) knob adjusts the filter centre frequency, with seven options per band. The LF control spans 30Hz to 400Hz, the centre band covers 300Hz to 5kHz, and the HF knob offers 2.5kHz to 20kHz. The frequency increments are well chosen, but the panel markings are perhaps not as clear as they could be. Having said that, in practice most people simply turn the control until they find the desired sound, so the rather cluttered and semi-visible (especially in low light) blue legends don't really pose a problem. The equaliser output is transformer balanced before appearing at the appropriate rear-panel FX output socket.

The compressor section is derived from the standard API 225L module, and is unusually sophisticated. Three rotary controls set the threshold (+10dB to -20dB), ratio (zero to infinity), and release time (0.05s to 3s), while an LED bar-graph displays the amount of gain reduction to a maximum of 30dB. There is no gain make-up control because the system always maintains the peak level, regardless of the ratio and threshold settings — in other words, this is a 'bootstrap' compressor.

Six buttons aligned below the rotary controls (and all with associated LEDs, once again) provide the additional functions. The compressor can be bypassed, linked to a second unit (using a link socket on the rear panel), and routed pre-equaliser. The attack time can also be switched between slow (10ms) and fast (30µS), and the knee can be selected from hard or soft slopes. The side-chain configuration can be chosen as either Old or New, and these quaint terms refer to the way in which the side-chain signal is derived. The New mode employs a feed-forward path which is typical of most modern VCA-based compressors — the side-chain signal is derived from the compressor's input. This approach tends to be produce a fast and accurate dynamic control, but it also tends to sound hard and aggressive — ideal for some material, but not for all. In contrast, the Old mode reconfigures the side-chain to a feed-back design, in which the amount of compression is determined from the compressor's output signal. This is a more old-fashioned approach which is typical of many classic vintage compressors such as the Urei 1176. It tends to have a smoother and softer effect, which is more transparent but less precise.

The various combinations of ratio, hard/soft knee and Old/New mode provide an unusually broad variety of compressor characteristics, ranging from a really strong 'in yer face' sound to an extremely natural and subtle control. There is something here for every occasion! Keeping the compressor link connector company on the rear panel is a pair of sockets providing a side-chain insert. Like the equaliser, the compressor output is also transformer coupled to the relevant FX output socket on the rear panel.

To the left of the EQ knobs is a control section associated with the four auxiliary sends. A quartet of rotary controls determines the send levels (there are no unity-gain markings), and each has an associated on-off button (with LED). Pre/post switching is performed in pairs with further buttons located between the first and second, and third and fourth aux sends. As mentioned earlier, the four aux sends are made available both on the huge multi-pin connector, for linking with the 7800 Master Output module, and also on the D-Sub socket, where they are balanced but at a -2dBu nominal level.

 Output Section 
 The output circuitry is divided across two sections of the control panel. On the extreme left is a section containing a Pan control (with a very light action and no centre detent), three small buttons, and two illuminated ones. The three small switches select solo-safe, enable muting from a mute group, and activate the insert return. The two larger illuminated switches provide channel mute and solo functions. Obviously, the mute group and solo functions are only active if the 7600 unit is linked to a 7800 Master Output Module.

Over at the extreme right of the unit are the remaining output controls, starting with a rotary channel fader, scaled with 10dB of gain above the marked unity position. This control is disabled if an external fader is connected via the appropriate rear-panel socket. A button below the control routes the post-fade, post-pan signal to the main stereo output on the rear panel (unbalanced on a TRS socket at a -2dBu nominal level), and also enables the solo-in-place muting function. Another LED bar-graph meter shows the channel signal level at the direct output, the latter being transformer coupled, once again.

The line input, all the insert sends and returns, and the direct output operate with a nominal +4dBu signal level, whereas the main stereo output, auxiliary busses and mix buss sends operate at a nominal level of -2dBu. The reason that the buss sends are at this lower level is to optimise headroom in the buss mix amps (when used with the 7800 Master Output module). However, it does make life slightly less convenient if these outputs are required as separate independent outputs directly from the 7600. The four aux sends and four mix busses appear on the D-Sub connector as balanced signals at the -2dBu nominal level.

There are six other buttons in this section of the panel. One is the polarity switch mentioned earlier, four route the post-fade signal to each of the four mix busses, and the last activates the pan control way over at the other end of the panel! When switched in, the pan control affects the main stereo output and all four busses (in odd/even pairs) simultaneously, with 3dB attenuation at the centre of its travel.


Classic API Sound

The 7600 sounds fantastic — there is no mistaking the sonic quality and character of this product. It is also very versatile, as a good console channel should be. The preamp retains (but arguably enhances) the detail within any signal fed to it, and it coped easily with the tricky 12-string acoustic guitar test, handling the complex intermodulations with complete transparency. The bottom end was also full and solid, but with a natural quality which is a common feature of the best preamps. The circuitry seems to have plenty of headroom and is pretty quiet, with plenty of gain for most applications. The DI input handles electric guitars and piezo pickups with suitable aplomb, too.

The EQ section is lovely, although I would have preferred the ability to split the high-pass and low-pass filters. I often need to roll off subsonic rubbish while preserving a sense of 'air', but the combined band-pass filter does not permit one without the other. The three adjustable EQ bands work very well, though, and the frequency allocations are sensibly chosen. This is not as flexible as a parametric design, but there are sonic benefits — this is a very musical EQ.

The compressor section is also very well appointed, and careful setting of the controls rewards the user with a very broad palette of sound characters, as already described. The inclusion of the side-chain insert enables frequency-conscious dynamics such as de-essing, for example. With suitable patching on the rear panel it is possible to use the onboard EQ for the task, but a front-panel switch would have been easier.

Given the complexity of the API 7600's internal routing structure and connectivity, the complete block diagram on the top panel (above) is a real help... until you put it in a rack!Given the complexity of the API 7600's internal routing structure and connectivity, the complete block diagram on the top panel (above) is a real help... until you put it in a rack!The handbook for the 7600 is a surprisingly meagre eight-page affair, two pages of which carry over-large diagrams of compressor knee characteristics and side-chain routing options. The text explains the functions of the front-panel knobs and buttons — although most, if not all, are entirely obvious — but nowhere is the overall signal path described, nor the complex array of rear-panel connections. In fact, several aspects of the interfacing are not mentioned anywhere, including how multiple 7600s can be used together or with a 7800 module to realise the modular mixer concept which is such a core element of this design. Technical specifications are also conspicuously absent, although I don't think there is any cause for concern in this regard — the product appears to perform to very high technical standards.

Fortunately, a block diagram is painted on the lid of the unit and I made frequent use of it during the review. However, this should be replicated in the handbook, as it obviously isn't visible if the unit is rackmounted! It also became apparent that there are several functional discrepancies between the block diagram and the product. All of this was a little disappointing, given the attention to detail shown in other aspects of this product.


There can be no doubt that this is an impressively equipped and specified unit, and the individual sections — preamp, EQ and compressor — all sound fabulous. However, I was left wondering just to whom it would appeal, since it is really neither one thing or the other: it's not quite a proper mixing console (even when multiple units are used together), neither is it a conventional recording channel.

My initial reaction when I unpacked the 7600 and scanned the front panel was that its aux sends might be a useful feature. In reality, though, they are not that easy to access and, since they operate at a lower than usual nominal level, they aren't even that easy to use with other equipment. With a sole 7600 unit operating alone, you can't easily use the aux sends to create a headphone cue mix, for example, or to route the signal to a reverb. The multiple insert points are more useful in that regard, but there is no facility to mix in the effects return, so you're back to square one. The buss routing facilities are completely irrelevant in this context, too, hogging panel space that could have been used more productively had these facilities been omitted.

Consequently, if your interest in the 7600 is as a single-channel recording unit, I would suggest you might be better off looking at the core modules in API's modular 200 and 500 ranges, or elsewhere entirely. This is, after all, an expensive product in the UK, and you would be paying for a lot of features that would never be used. On the other hand, these facilities become more relevant if multiple 7600s are combined (with a 7800 Master Output Module) to form a small high-quality mixer, perhaps for tracking purposes. In this context the unit provides better value for money, and the facilities start to make more sense.

However, I remain unconvinced about the balance and usefulness of facilities, and I would have thought you could purchase a fully professional compact mixer (a Calrec M3, for example) with similar facilities but better ergonomics (due to its physical shape) for a comparable cost. Having said that, I appreciate that the 7600/7800 combination is unique in what it offers, and that these products replicate the Legacy console in both facilities and sound quality. I just wonder how many customers require a rackmount modular version of it!

The bottom line is that if you are looking for a really high-quality mixer with a distinctive high-end sound quality and bags of flexibility, but with perhaps just a handful of channels, the 7600 and 7800 system may be just what you are looking for, and is in a class of its own. If you are seeking a conventional package of preamp, EQ and dynamics, there are other options that I would place higher on my auditioning list — although I can't fault the sound quality of the 7600 in any way. To me, its a case of 'nice, but no cigar' — the 7600 doesn't seem to know quite what it wants to be. Perhaps API should consider using these components to create a genuine producer package, for which there would be a far larger market.

infop.gif £2231.33 including VAT.
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Published April 2003