Heritage Audio i73 Pro Edge

Does Heritage Audio’s new range of transformer‑based audio interfaces mean you no longer need separate mic preamps?

From bedroom setups to Brad Pitt’s château, practically every studio nowadays is based around a computer. That means practically every studio needs a computer audio interface. And that, in turn, means the market for computer audio interfaces is fiercely competitive. So it’s strange that, as I’ve observed more than once in these pages, manufacturers have allowed an obvious gap in that market to remain unfilled for many years. The large majority of audio interfaces nowadays have mic preamps built in — but the preamps that get built into audio interfaces are of a type that lots of studio engineers don’t use.

Many engineers want characterful mic preamps that can add pleasing thickness, coloration and saturation to their recordings. What we get in our audio interfaces are transformerless, chip‑based preamps that offer impressive technical specs but add nothing to the sound. That’s slowly beginning to change, courtesy of products like the Neve 88M and 1073 SPX‑D, Steinberg UR‑RT series and the Icon/Harrison 32Ci we reviewed in September. Other manufacturers such as Universal Audio have chosen a different route to the same destination, developing input circuits that can emulate the sound of older console preamps. But it’s fair to say that if you want a good‑quality audio interface with classic transformer‑based preamps, your options are still very limited.

Heritage Audio’s i73 Pro series thus provoked much interest when it was announced in January. It’s billed as “the first ever USB‑C audio interface with 73‑style Class‑A preamps built in”, and as if to push home the point, it has square holes in the top panel through which the input transformers proudly protrude. And, although the preamps are definitely the headline feature, the i73 Pro offers plenty of interesting design choices in other departments too.

Three Times 73

The i73 Pro One is the baby of the range, with just one transformer‑based preamp input.The i73 Pro range contains three models, of which the i73 Pro One is the simplest. On the input side, it has a single channel of transformer‑balanced preamplification that can accommodate mic, line or instrument signals, plus a mono line‑level input. Output‑wise, it sports a pair of line‑level monitor outputs on balanced jacks, and a single front‑panel headphone output.

The next step up is the i73 Pro 2, which replaces the line‑level input with a second transformer‑based preamp; unlike the first, though, this does not have an instrument input option. In other respects, it’s largely identical to the Pro One, with the same output arrangements.

Top of the range is the i73 Pro Edge, as supplied for review. This features two transformer‑balanced input channels that both accept mic, line and instrument signals, plus a second pair of line‑level inputs on balanced rear‑panel jacks that can be switched between ‑10dBV and +4dBu input sensitivities. Output‑wise, the Edge adds a second headphone socket and a second pair of balanced line‑level jacks. And, unlike the other two models, it also has digital audio I/O. A pair of optical sockets accept and transmit up to eight channels of ADAT Lightpipe audio at base sample rates, with no option to switch to stereo S/PDIF.

The use of Class‑A transformer‑based preamps means that bus powering is out of the question, so all three models employ external 12V DC supplies. And in addition to their audio I/O, all three interfaces also have MIDI in and out. However, both MIDI connections are presented on a single six‑pin mini‑DIN socket, so you’ll need an adaptor cable to use them. This is a cost option and is not included as standard. Connection to the host computer is indeed on a single USB‑C socket, so Heritage Audio’s billing remains technically true: the Neve 1073 SPX‑D was available first, but has a Type‑B socket.

With their chunky wedge‑shaped desktop cases, the i73 models have a form factor similar to Universal Audio’s Apollo Twin interfaces, but they’re considerably larger. The Pro Edge version occupies about 20x23cm of desk space and stands a full 9cm high. The colour scheme and Marconi‑style controls make obvious visual reference to the Neve consoles of yore, although the slightly wobbly knobs and wood‑effect plastic end cheeks don’t quite communicate the same sense of permanence and gravitas.

Finally, there’s one more important factor that the i73 series has in common with UA’s Apollos. These are not only audio interfaces: they’re also DSP platforms that come with their own suite of plug‑ins. Where it matters, these exist not only in native formats but also as processors that can be loaded directly into the i73’s DSP chips, allowing their output to be monitored and recorded at very low latency.

73 Or 72?

All three i73 Pro models share the same ‘master section’ features, located on the right side of the top panel. These include mono, mute and dim buttons, along with a dual‑concentric encoder. This has a push‑button action that toggles whether the inner ring controls the volume of the main monitor outs or that of outs 3+4, and which of the two headphone amps is governed by the outer ring. LEDs indicate which of the two are presently under encoder control, but there’s no visual indication of the actual volume level in either case. A five‑segment LED ladder at the top of the ‘master section’ can display either levels at inputs 1+2 or at the monitor outputs. All the controls in the master section are virtual, in the sense that they don’t adjust analogue potentiometers or switches but digital settings that can also be controlled in software.

Settings for the inputs, by contrast, are all analogue. They reside on the left‑hand side of the front panel and begin with ‑20dB pad, polarity and +48V phantom power buttons, followed by switched and continuous gain controls. These look at first glance like the ones you’d find on a ‘full fat’ vintage preamp bearing the number 1073 (which, along with the Neve name, is a registered trademark, hence the abbreviations and euphemisms flying around), but there are some nuances here that are worth teasing out.

On a genuine Neve 1073, the main gain control has 22 switched positions that span almost a full 360‑degree arc, and include two settings marked ‘off’. This is because it has multiple gain stages. Settings in the middle of the gain range engage just one of the two mic preamp boards, and the second stage is brought in only once you go past the second ‘off’ setting. By contrast, the i73’s preamps have just one gain stage, and the switched gain control has only six positions. As on the 1073, these are stepped at approximately 5dB intervals, so the total gain range available from this control is just 25dB. The pad extends this range by a further 20dB at the lower end, so for mics, the minimum gain available is +25dB, with the pad engaged, and the maximum is +70dB. The pad is not available for line inputs, so the total gain range in this case spans ‑20 to +5 dB. There’s also no equivalent of the 1073’s low‑impedance option in the i73.

The flagship i73 Pro Edge features a second pair of line‑level inputs and outputs as well as ADAT I/O. A special adaptor cable is needed to use the MIDI ports.

To put all this in language that will make sense to Neve geeks, the input stage in the i73 is actually not so much a 1073 copy, but much closer to the 1272 transformer‑balanced line amp that was widely used in early ’70s Neve consoles. The 1272 is now often repurposed as a mic preamp, and is seen as being sonically very similar to the 1073 within its more limited gain range.

The key to wringing colour and harmonic saturation from any mic preamp is the availability of an output trim control, because this allows you to crank up the main gain setting and then back things off on the way out to avoid overloading downstream devices. On the i73, the downstream device is always the built‑in A‑D converter, as there are no insert points into which EQs, compressors or attenuators could be placed, and the trim control runs from minus infinity to zero (unity gain). So, no matter how hard you’re hitting the preamp, you can always avoid overloading the A‑D converter by turning the output control down far enough — but this won’t ameliorate any clipping or other non‑linearity that takes place within the analogue stages.

Slotting In

Heritage Audio’s marketing for the i73 Pro models trumpets the Full Analog Experience that they’re said to offer. Slightly counter‑intuitively, quite a lot of this ‘analogue’ experience is actually digital, and is made possible by the built‑in DSP. In order to access this, and indeed to use the i73 at all, you’ll need to install two separate pieces of software. An app called Heritage Sync runs in the background and handles communication with the Heritage website, authorisation, firmware updates and so on, whilst actual operation is carried out using the i73 Mixer utility. Heritage Sync was a little bit touchy on my Mac; it wouldn’t let me use the i73 without updating the firmware, but the update failed several times before going through OK. Once you’ve got everything working and registered your unit, you can download the free plug‑ins that complete the Full Analog Experience and authorise them to your iLok account.

The Mixer software is more than a little reminiscent of UA’s Console, albeit with a slightly more obviously Neve‑esque GUI. The i73 Edge Pro has a total of 12 physical input channels, comprising the two mic/line/instrument inputs, the additional line inputs, and the eight ADAT ins, while its 16 outputs are made up of the main and secondary line outs, the two stereo headphone outs, and the eight ADAT outs. However, your DAW ‘sees’ an additional four inputs and two outputs. Two of each are accounted for by a stereo loopback channel, whilst the final two input sources are related to the i73’s DSP features.

These DSP facilities are available only on the two mic/line/instrument inputs, and are presented as four insert slots. By clicking on these, you can load one of the four DSP plug‑ins currently available, which are described in the boxout. Any processing you use in these insert slots is always in the monitor path through the Mixer utility, but the record path is split, so you can choose to capture either the wet signal (with processing), the dry signal, or both. You’re only allowed to load one instance of any given DSP plug‑in per channel, and in fact four insert slots is currently overkill, as there is no combination of four DSP plug‑ins that can be loaded without maxing out the DSP resources.

All the other mixer channel features are common to every channel, and include a fader, pan pot, mute and solo buttons, and two auxiliary sends with separate pre/post‑fader switches. Channels can be named, and adjacent channels linked for stereo operation.

The i73 Mixer utility. The first two input channels have insert slots where Heritage’s DSP plug‑ins can be loaded, while the two auxiliary channels (right) can each host a single VST plug‑in.

Great Auxes

On some audio interface mixers, the aux sends are used to set up alternate balances that can feed outputs other than the main pair. That’s not the case on the i73 Pro Edge. The main and alternate outputs each have their own separate mixer panels, while the two headphone outputs can separately be switched to pick up either of these mixers, or to be independent and thus have a mixer of their own each. So, in total, the i73’s internal mix engine can run up to four independent mix panels, each routed to a separate physical output.

Any processing applied using the DSP plug‑ins on the insert slots is common to all of the mixers, as you’d expect. However, each mixer has its own auxiliary send and return structure. The sends are hard‑wired to stereo aux channels that are returned into the same mixer, so for example there’s no way to use the sends from the main output mixer to feed the mixer for outputs 3+4. But what these aux channels do have is a single insert slot that can be used to host VST3 plug‑ins. The obvious application is to host reverb and delay plug‑ins for performers’ cue mixes, and the Full Analog Experience includes emulations of the EMT 240 ‘gold foil’ plate and Maestro Echoplex, in case you don’t already have such effects available to you.

Included as native plug‑ins but not in DSP format are Heritage’s emulations of the EMT 240 ‘gold foil’ plate reverb and Maestro Echoplex delay.

The mutual independence of the i73’s mixer panels has some pluses and some minuses. On the plus side, not only can you set up four entirely different mix and effects balances for the different outputs, but they can each have different auxiliary effects loaded. You’re not bound to use a single global reverb for everyone. It also means that soloing a channel in one mixer has no effect on the others, which in my experience is generally what you want. On the down side, the only DAW return channel that appears in any of these i73 mixer panes is the loopback channel. So, if you set the headphones to be independent but leave your DAW addressing outputs 1+2, you won’t hear the DAW output in your headphones. In order to do so, you would either need to create a second DAW output feeding the headphone bus, or route the DAW output to the loopback channel instead of outputs 1+2.

One thing that is missing from the Mac version of i73 Mixer is any reference to ADAT clocking. I eventually tracked this setting down within Audio MIDI Setup on my Mac, but it would be nice if there was some indication elsewhere of what clock source is selected, or indeed if the topic was discussed in the PDF manual. I’m told that clock selection is handled within i73 Mixer on Windows, though on both platforms, changing clock source makes it necessary to close and reopen both i73 Mixer and your DAW project, which is a little annoying.

On balance, though, I think i73 Mixer is pretty well thought out, and feels admirably polished and complete given that this is an entirely new product. There are some unexpected nice touches such as the ability to disable monitor control on the line outputs for situations where you’re using the i73 with an external monitor controller, and although the skeuomorphic graphics are a bit gratuitous, everything is generally pretty clear. In an ideal world it might be nice if the UI could be resized, and the positions of the virtual toggle switches in the master section can be hard to discern at a glance, but those are minor gripes.

Heritage have developed a custom Windows driver, but the i73 uses the built‑in Core Audio macOS driver. Its mixer has some latency of its own, so at 44.1kHz a 32‑sample buffer yields a round‑trip delay of 8ms, while the direct monitor path has 1ms or so. Compared with the dry path, the wet signal path through the i73’s DSP has additional latency of about 85 samples with no plug‑ins loaded, or 91 samples through plug‑ins. At present this needs to be compensated for manually in order to align wet and dry versions of the same signal, but a future firmware version will offer the option to delay the dry signal automatically.

In To Out

From an operational point of view, if you’ve ever used another audio interface with its own mixer utility, I don’t think you’ll take long to get accustomed to the i73. The only slightly unusual factor is the aforementioned absence of DAW return channels in the mixer. On those inevitable occasions when you scratch your head thinking “Why can’t I hear anything?” the answer is either that your DAW is not feeding whatever output you happen to be listening to, or that you’ve forgotten the i73 always powers up with its outputs muted. (Ask me how I know...)

The i73’s claims to uniqueness are mainly to do with its distinctive preamps and its ability to act as a plug‑in host.

The i73’s claims to uniqueness are mainly to do with its distinctive preamps and its ability to act as a plug‑in host. To take the input side of the Full Analog Experience first, it’s perhaps worth pointing out that the input transformer in this sort of circuit precedes all the gain stages apart from the input pad. Any saturation or non‑linearity the transformer itself imparts is strictly related to the content and amplitude of the input signal, and unaffected by the settings of the main or output gain controls. In other words, you can’t “overdrive the input transformer” by turning up the gain. What you can do is overload the active gain stages themselves, then use the output level control to back things off. I suspect that most of what people think of as “transformer saturation” in this type of preamp is actually generated in the active circuitry or in the output transformer, if there is one. (In this case, there isn’t, as the preamp is coupled directly to the A‑D converter.)

Either way, it’s perfectly possible to get clean‑sounding recordings out of the i73’s preamps as long as you can ensure the signal coming in isn’t too hot. For example, when using a pair of beyerdynamic MC930 capacitor mics as drum overheads and with the output control at unity gain, I needed to have the pads on both the mics and the i73 active in order to attenuate the signal enough for the i73 to cope. At this point, the main difference compared with recording the same mics through a modern transformerless preamp was a fairly subtle richness in the low mids.

Switching off either of the pads made it necessary to turn the output control a fair way down in order to avoid overloading the A‑D converter. The recorded signal was then visibly rounded off on peaks, and sounded quite different from the clean version. I liked the extra energy and life that this clipping delivered, though you’d need to be pretty careful to avoid overdoing it. Turning up the gain and turning down the output control further took things into the realms of obvious crunch. Again, there’s a place for this in some circumstances, and it’s nice to have the option, but it would be easy to ruin a good take by going too far.

In my experience, this sort of obvious clipping also works better on drums than it does on many other sources. The i73’s relatively narrow gain range means you may have to take care with vocal recording, for example, if you want the preamps to add warmth without introducing blatant distortion on peaks. But, again, I don’t mean that as a criticism; if you want the sound of vintage‑style preamps, you have to be prepared to put the effort into optimising the gain structure to find the sweet spot. Incidentally, audio specifications for the i73 were unavailable at the time of writing, but those aspects of its sound that are meant to be clean seemed good to me, and I’d particularly highlight the headphone amps, which have a lot more oomph than those found in most audio interfaces.

The Complete Package?

Although previous products have incorporated digital elements, Heritage Audio are known mainly as manufacturers of analogue gear. So the i73 Pro series represents an ambitious move, incorporating as it does not only USB interfacing but also a new DSP plug‑in platform. Feature‑wise, its obvious rival is Universal Audio’s Apollo Twin range. These are slightly more affordable and offer more DSP power and an enormous catalogue of plug‑ins, plus the ability to emulate a wide variety of vintage preamps — but they don’t actually have transformers, or the same hands‑on approach to setting gain structure for maximum impact. Neve’s 88M, meanwhile, brings the transformers, the insert points and the Neve name, but doesn’t offer an easy way to get a ‘distressed’ sound from the preamps, and has no built‑in DSP.

Are there ways in which the i73 could be better? Of course: the DSP is underpowered, to the point where you can’t run amp simulators on both inputs at once, and the benefits of the preamp design would be even greater if it had insert points. But I think Heritage have got the major calls right here. Their suite of plug‑ins is surprisingly great, the software mixer is slick and feels mature, and the balance of features is well judged. And above all, this is the first desktop interface I’ve ever encountered that offers really obvious colour from its built‑in mic preamps.