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Boss Tube Amp Expander

Boss Tube Amp Expander

Widely touted as Boss's response to UA's popular OX, the Tube Amp Expander actually offers a contrasting approach. Will one of them be a clear winner?

When the Boss division of Roland announced their Waza Craft Tube Amp Expander (let's call it the TAE) at Winter NAMM 2019, many people saw it as a direct challenger to Universal Audio's OX tube-amp load box and speaker simulator. And in some ways, it is: it undoubtedly targets a similar user and fundamentally seeks to address the same issues arising out of the use of tube guitar amps, both on stage and in the studio.

At the same time, however, it's a very different solution, based on some fundamentally different design choices. Where the OX uses a dummy load with a single, fixed impedance curve, the TAE has a choice of 16, able to be derived from its Resonance and Presence switches. Where the OX uses dynamically modelled speaker simulation, the TAE exclusively employs speaker impulse responses (IRs). Whilst the OX's footswitch facility is, "as yet", almost entirely unimplemented, the TAE has a comprehensive footswitch option available from day one. Where the OX's software editor requires a wireless connection, the TAE's app connects via USB. Finally, and perhaps most significantly, whilst the OX offers an attenuated speaker output, the TAE feeds its post-load signal to an integral solid-state power amp to derive a speaker-level output. Anyone still thinking they are "basically the same"?

Physically, the TAE is a 2U black box, slightly narrower than the normal 19-inch format, but rackmountable nonetheless via the included adaptor rack ears. The reactive load that receives the speaker output of your tube amp is switchable between 4, 8 and 16 Ω settings, with a three-position power-matching switch offering a choice of 10, 50 and 100 Watts, for internal level optimisation. Maximum power input is spec'ed at 150 Watts, and there is a 16Ω 'safety load' in place if you forget to power up the TAE before switching on a connected amp.

If you were just using the TAE to get a big tube amp into its sweet spot at a lower volume, you'd plug your amp's speaker output into the TAE and then take your actual speaker signal from one of the two speaker outputs, using the front-panel Speaker Out knob to give you a continuously variable level control from nothing to everything, and all points in between. So far, so simple, but we are not done yet...

Loading Zone

There are two reactive load controls on the front panel — Resonance Z and Presence Z — that between them can be used to modify the impedance curve of the load presented to the amplifier's output transformer. We tend to think of speakers purely in terms of their nominal impedance — usually 4, 8 or 16 Ω in the guitar world — but the actual impedance will vary significantly across the whole frequency range of the speaker, with a big peak at the bottom, around the resonant frequency of the unit, somewhere between about 55 and 95 Hz, followed by a slow rise through the average value, with a gentle tip-up at the end. The resulting impedance curve is fundamental to the way cone drivers are made and it's not surprising, therefore, that all guitar speakers exhibit a similar basic shape in their free-field impedance plots. There are differences between designs, however, and even more differences when the effects of different enclosures are also factored in. The impedance curve that the output stage of the amplifier 'sees' affects the power transfer (along with the phase angle, but let's not go there) at different frequencies and, in conjunction with the basic frequency response of the speaker, determines how the speaker, cabinet and amp combination will actually sound.

Early designs of dummy load for tube amps were usually purely resistive, which means their impedance did not vary with frequency like a real speaker. Players often reported needing to make significant changes to their amps' tone-control settings to try to compensate, and also that they made the guitar feel 'stiff' and less responsive under the fingers. The 'reactive' dummy load — ie. a load exhibiting an impedance curve — was developed to present a more speaker-like load to the output stages of amps, and what we have here in the TAE is a further development of that in the form of a 'tunable' reactive load that allows you to approximate the impedance curve of a number of different speakers and enclosure types.

With the tube output stage driving the dummy load, the speaker is fed from the TAE’s own power amp.With the tube output stage driving the dummy load, the speaker is fed from the TAE’s own power amp.

The Resonance Z control tweaks the bottom end of the impedance curve where the speaker's resonant frequency lies, while Presence Z shapes the top, with both four-position switches simply labelled Lo, L-Mid, H-Mid, Hi. For anyone not sure how to set these, there's a helpful matrix printed on the top panel of the TAE (and in the documentation, of course) that gives a recommended combination for 10 popular cabinet types.

These aren't EQ controls, although they will appear to function as subtle ones, but rather the means to make the connected amplifier behave as if it is driving not just 'a speaker' but a specific speaker or cabinet. The logical goal therefore is to match the settings to either the physical speaker that you've connected to the on-board power-amp output, or the virtual speaker whose IR you are using via the line output. What happens when you 'mismatch' the impedance curve setting ranges from the very subtle to 'something's not quite right'. To put this into some perspective, however, you can just set Resonance to L-Mid and Presence to H-Mid, or vice versa, and the majority of internal cabs and decent cab IRs will sound OK with some EQ tweaking. It's only when you want to make something quite specific sound as 'real' as possible, such as emulating a thunderous 'Dual Recto into a deep 4x12' perhaps, that absolutely optimising the impedance curve setting becomes crucial.

The TAE (top) was compared with the Universal Audio OX — each has its relative strengths that will likely hold appeal for different players.The TAE (top) was compared with the Universal Audio OX — each has its relative strengths that will likely hold appeal for different players.The OX's single, fixed impedance curve is somewhat akin to leaving the TAE's reactive load controls set on L-Mid and H-Mid, and perhaps explains why some people feel there are a few amps that don't quite seem to give of their best with it using their normal settings. But the OX uses a dynamic modelling process for its speaker simulation, rather than the fixed transfer characteristic of IRs, and given that the load is a constant, the modelling can seek to create an appropriate response for whatever virtual cabinet type is selected. TAE users browsing through cab libraries should ideally remember to check the impedance curve setting for each significantly different type to be sure they are hearing it at its best. On the plus side, however, with the right settings, they will know that they have an optimised impedance curve and that their amp should definitely be performing very similarly to how it would with a real speaker/cab of the selected type.

The TAE's attenuator is always at full attenuation because there is no 'speaker through' signal being used to drive a real speaker. This contrasts with the OX's stepped attenuation of the amp's output. With a varying amount of the 'real speaker' load involved as well, maintaining consistent tonality across all the steps of an attenuator is a real technical challenge, and most systems that attempt this usually invoke some form of compensating EQ for the lower output settings. The tonality of the TAE's attenuated signal will be constant, regardless of where the integral power amp's output is set, and this configuration also has the benefit of allowing you to make a low-powered amp louder, perhaps allowing you to use a favourite studio amp on stage — a hundred Watts of Fender Champ, anyone? The power amp is described as Class–A/B Tube Logic, which is Roland/Boss's flagship analogue solid-state emulation of a tube amp, and features thermal protection and fan cooling when needed.

Direct Recording

The Boss TAE, just like the OX, also facilitates direct recording, with no real speaker involved at all. In this configuration, the signal derived from the dummy load passes through an A-D converter and feeds the integral impulse-response host, where you can access both the integral cabinet IRs and any that you may have added to the library yourself. These are managed via the TAE's Mac- and Windows-compatible software app over a USB 2.0 connection. Don't expect Universal Audio-style photo-realistic graphics: the TAE's editor is... let's agree to call it 'functional'. It's not an efficient interface, in my view, but you can get at everything you need, and on the plus side, it didn't crash once, or drop its Wi-Fi connection, because it doesn't require one.

Though it’s effective enough in
a utilitarian sense, the software editor’s designers haven’t really made the most of the GUI’s screen real estate.Though it’s effective enough in a utilitarian sense, the software editor’s designers haven’t really made the most of the GUI’s screen real estate.

The editor allows you to store your own 'Rigs' — whole setups of routing, IR, EQ and effects — with 10 of these being able to reside within the TAE hardware for instant recall via the front-panel control or footswitch. Apart from the Speaker Out level and reactive load settings, the only other control knobs on the front are the Rig selector, Reverb, Line Out and Headphones. The reverb control is always live and can therefore override whatever has been recalled within a Rig. Also recalled within a Rig are the status of the back-panel analogue effects loop, the Solo/EQ parameter and 'Amp Control' setting — the latter being an external amp-channel switching system that can be set to operate as simple contact closure or electronic switching.

What's not recalled is the reactive-load setting — understandably, as it is on hardware rotary switches — so, whilst you can footswitch from a 4x12 IR on your 'big solo' sound, to a 1x12 combo speaker IR for your cleans, one or the other of them won't necessarily have an optimised impedance-curve setting. Of course, you can change the settings manually, which is perhaps worth doing for a whole song but not so much for flipping from verse to chorus and back again. It makes sense, therefore, to set up your on-stage Rigs with the impedance curve that suits the sound you use most and then compensate for any significantly different IRs that you'll be using with a bit of EQ. Having the exact impedance curve of a particular speaker or cab seems to matter only in a few very specific instances, and any combination of the middle settings will get you to the point where the other variables — virtual miking, the room, the guitar, and you as a player — are making more of a difference to the final output.

Round the back, there are two paralleled speaker outputs. Anyone who has ever used a stage rig with real stereo delay and reverb will rue the fact that the power stage of the TAE is mono — an opportunity missed, perhaps, although I can see that the design choice is one that makes sense for the majority of users.

There's a headphone output, with independent level control, one use for which would surely be silent personal practice using a real amp without having to annoy anyone else. There's no 'clean' analogue line input for connecting a backing-track player so, as with the OX, you'd have to use a small mixer to create that sort of setup. The effects loop Return might look like a candidate, for a mono source at least, but it returns before the speaker-sim stage, and you wouldn't want your backing track playing back through that. What you can do, however, is input a digital signal via the USB port — the TAE can actually be instantiated as an audio interface within suitable software, such as a DAW, which means you can also record a digital signal from it. I found setting it up for recording and stereo playback in Logic Pro entirely painless, with just the initial defaults working exactly as you'd want.

Among the array of rear-panel connectors are two parallel speaker output jacks and a stereo pair of main line-output XLRs. Although some of the on-board effects are stereo, the power stage itself is mono.Among the array of rear-panel connectors are two parallel speaker output jacks and a stereo pair of main line-output XLRs. Although some of the on-board effects are stereo, the power stage itself is mono.

The TAE offers two USB audio output streams: 'Line' and 'Pre Effect': the latter taking a mono signal from immediately after the external effects loop and thus before the internal effects and speaker sim. You can record from both simultaneously, so if you like the performance but not the sound, you can send the Pre Effect signal back to the TAE for 're-speakering', or treat it with an IR of your choice in your DAW. It's not 're-amping', of course, as the signal already includes the sound of the amp.

The USB port is also the connection to the software editor and IR loader apps (I'm not sure why the latter isn't just part of the editing software). The functional part of the editor is organised as three panels: Rigs to the left, block diagram at the top, with parameters for the selected process below. A full snapshot of (nearly) everything is a Rig, and you can make groupings of up to 15 Rigs as a Live Set, with the editor software allowing you to create up to 50 Live Sets. Rigs and Live Sets can be named, saved and backed up, along with system settings.

The analogue line outputs, on XLRs, appear as a stereo pair, plus a summed mono — the latter, a good, practical inclusion for an FOH feed, is independent of the Line Out level, but perhaps a bit hot for something that will often be plugged into a mic amp. The only 'stereo' here, however, is the delays and reverb/room effects. There's no pan facility or left/right assign to allow you to take advantage of the Direct Mix control in the software: the direct signal offers a pre-cab output that could be usefully recorded simultaneously for the same purpose as the USB clean-feed facility, but only if it could be assigned its own output. You can get an independent, pre-cab output from the effects send, however, if you are not using the loop.

Cabs, Mics & IRs

Designing a Rig, you have a choice of 22 cabinet types, plus four user impulse-response slots — I am sure I am not alone in wishing there were a whole lot more of the latter. There are five microphone options, one of which is 'flat', so in terms of 'character' mics we actually have only four. The Shure SM57 and Sennheiser 421 are 'standards', particularly for stage use, but AKG's C451 and a Neumann U87 seem odd choices for alternative mics, to me. Personally, I'd much rather have something like a U67 and, ideally, a Royer R-121 ribbon. Personally, I'd never choose a 451 or a U87 as a close mic for a guitar speaker. Certainly, the latter is a popular choice for a second mic, sometimes at a bit of a distance — but that's not an option here, as you only get access to one mic at a time with the integral cabs. The user-IR slots, of course, make it possible to use double-miked IRs. Some of the integral cab collection are listed as specifically 'FOH' versions. In these, the characteristic peaks and dips of the response have been slightly smoothed out, as this was found by the designers to be preferable for live amplification, compared to the 'full-detail' versions used in a recording situation.

The Mic Position parameters are a choice of Short, Medium or Long, combined with a nine-step Distance parameter, from 2 to 10 cm, representing distance sideways from the centre of the cone. The final parameter is a virtual Room Mic with three settings: Anechoic, Small Room and Big Room. With a fixed level at each setting and no ability to record this as a separate source, I found this to be a rather limited implementation. In a live setting, you will be in a real 'small room' or possibly a 'large room', and may not want the sound of another 'virtual' one of either size imposed on your signal. In a recording context, I can't think of any reason why you'd want a fixed amount of room sound 'baked in' to your recorded signal, when you can apply precisely the right amount with a software process in your live monitoring, or in the mix, or indeed both.

The Position and Distance parameters are disabled when you activate user-IRs. I initially thought that might be because the various settings for the internal cabs were calling different IRs, but they were subsequently described to me as "filter" processes, which begs the question, why couldn't they also work with user IRs? Perhaps the format of the integral IRs has some additional data to interact with. Precisely what that format is remains invisible, as does the eventual length of the IR used, as the separate IR loader app can assimilate a variety of formats — 44.1, 48 and 96 kHz, 16- or 24-bit — and make them its own. That's a big plus if you've got a substantial collection of IRs in different formats that you'd like to try with the TAE's reactive load.

If in doubt, there’s a helpful block diagram of the signal path printed on the top panel.If in doubt, there’s a helpful block diagram of the signal path printed on the top panel.Moving up to the front of the processing chain, we have the analogue effects loop. Other than On/Off, all the settings here are in hardware: series, parallel, +4dBu/-10dBV operation, and independent ground lifts on send and return. It seems odd to me that it is mono, when most external rack effects that would use the +4 setting will have stereo return signals. I know you could patch such a processor in-line after the TAE, but then you'd have to adjust levels in two places and lose the convenient fold-down to mono for FOH.

Next up, we have a compressor stage, offering a choice of 'Rack 160D' or 'VTG Rack U', emulating a dbx 160 or a UREI 1178 (according to the documentation). Parameters are the same for both: Threshold, Ratio, Level, Attack, and Release. There's obviously no attempt to emulate the actual controls on either unit here, but both processors are perfectly functional. In fact, the '1178' is very nice, and I used a hint of it on every Rig I designed for myself. I'm not sure why it is an '1178' — the stereo version of the FET-based 1176 — as there seems to be no way for it to receive a stereo input. There's no gain-reduction meter or indicator of any kind, and in my opinion that's a regrettable omission; it wouldn't have to be a fancy VU graphic — a line of virtual LEDs would do the same job — but it really should be there in some form. The amount of compression can be difficult to judge by ear alone with a single-source signal, especially one that is already compressed to some extent, such as distorted guitar.

Delay & Reverb

The delay section offers Single, Pan, Stereo, Analog and SDE-3000. Single is obviously mono, but Pan gives you a variable sub-division of the delay for the other side of the L/R pair, while Stereo is actually one channel dry and the other delayed. Analog is a bandwidth-limited mono delay, while SDE-3000 mimics Roland's flagship, mono delay unit of the mid-1980s — its 12-bit conversion and integral modulation facilities made it a firm favourite with the 'big rack' guys of that era, and you'll still find some of them in use today. The parameters vary a little between the delay types, but all have modulation. The delay-time control scaling makes it very hard to set low values using the on-screen control. A double–click gets you a numeric keypad on screen to enter a precise time, but that is not the same as a continuous adjustment made by ear. I was similarly frustrated by the scaling of the delay level and feedback controls: most users, most of the time, will be using a delay mix between five and 15 percent, with a similar or lower setting on the feedback. Yet the entirety of that range lies with the bottom 30 degrees of the available 270-degree arc of the control, while the rest of it will almost never be used other than for the odd special effect. If this were hardware, you'd choose a log pot to put more resolution at the bottom of the scale, and it is not hard to get this right in software. Just to be fair, though, I'll reiterate that I made precisely the same criticism of the OX's software app. And, in contrast, the scaling on the front-panel Speaker Out control is just perfect, with lots of fine control where you need it most.

The reverb block offers a choice of Hall 1, Hall 2, Plate, Room 1, Room 2 and Spring. Sensibly, there are separate levels available for the speaker output and the line outputs, so you don't have to send the same mix to both. High- and Low-Damping controls and up to 200ms of pre-delay form part of a comprehensive array of controls but, like the delay section, the reverb's decay time crams its most usable, 'up to three seconds' part of the scale in a quarter of the control. Not many guitar players will use a 5-10s reverb decay time, but they may well want to adjust in the range between two and three seconds by ear, using a continuous control rather than successive approximations via numeric entry.

It allowed me to greatly enjoy some of the amps I own that are now considered anti-social — and equally joyful, at the other extreme, was hearing a diminutive H&K Tubemeister 5 at stage-viable volume!

For an interface of more than just a very few controls to be used efficiently, I believe there needs to be some degree of visual or layout hierarchy. The TAE editor screens have none. The controls are all the same size, the same colour and use the same small, all-upper-case text. Some differentiation of the most frequently used controls by size, position or colour would make a difference. In hardware design, the user interface is often compromised by the needs of the circuit and limited front-panel space, but this is software. There is no reason why it can't be better at allowing the user to locate the most important and frequently used controls more efficiently.

The final stage, before we arrive back at the speaker sim, splits the signal into two paths, with an EQ in each. The two EQs can be activated separately, or ganged for joint operation. There is a choice of 10-band graphic, or a parametric with two 'true parametric' (with Q) bands, plus Low Cut and shelving Low Gain, and High Cut and shelving High Gain. There's no graphic representation of the response curve in the editor, however — something regular DAW plug-in users are now very familiar and comfortable with. Like many who grew up using studio hardware, I am happy to interpret the virtual knob positions on the TAE's parametric, but I find those who have only ever used software EQs often don't have the same instinctive sense of what the parametric numbers mean in terms of a response shape. It's not as if there's no unused screen space to do it in.

The EQ stage can also be used as a 'solo boost' facility, with up to 20dB of footswitchable, post-EQ gain available to both line and speaker outputs, depending on whether or not the EQs are ganged. This is a post-distortion boost that will result in a genuine level change, rather than just more drive, and attaching it to an EQ can make it more usable — trimming off ultra-highs and lows can help your boosted signal to still sit in a mix properly even though it is louder. However, I found I wanted to use at least some EQ to fine-tune the basic sound of every Rig that I created for myself, which meant the solo EQ option was then no longer available as a solo boost. I would urge the addition of a second EQ stage, independent of the solo option, for any future software update.

In Action

The Boss Waza Craft Tube Amp Expander almost seems designed to a brief of addressing some of the perceived limitations of competing products, specifically: overcoming the concerns of those who feel that a single impedance curve is not sufficient in a reactive load; offering an open system that can accommodate third-party impulse responses; facilitating continuous, 'un-stepped' volume control when used as a 'virtual attenuator'; remote switchable, and with extensive MIDI implementation for live use; and offering bi-directional digital audio paths that allow it to operate as a DAW interface.

It does indeed do all of those things, many of them very well. In its 'virtual attenuator' mode (it actually fully loads and then re-amps) you can get a tube amp into its 'happy place' and listen to it at any volume you like. There's even the option to switch in a compensating Fletcher-Munson correction to boost low and high frequencies when the 're-amping' volume is set very low. The reactive load preserves the dynamic behaviour of the output stage — the 'feel' experienced by the player — and that is translated via the neutral, solid-state power amp through to whatever real speaker(s) you have it hooked up to. The key issue is, how well does the dynamic behaviour of the amp survive the trip? Two things are crucial here in properly assessing and understanding the outcome, I find: one is that you use 'normal' amp settings — if you don't normally listen to your amp on 10, now is not a good time to start — and the second is that you do your initial listening test at something close to performance volume from the speaker. There is no process that compensates for "the magic of loud", as I've heard it described. Experience it loud first, and then you should be able to know if any adjustments you are making when you turn it down are just to compensate for 'lack of loud'.

Does it sound exactly the same as the amp straight into the speaker? I don't quite think it does, any more than the OX does. But does it sound and feel good in its own way? Yes, very much so. It allowed me to greatly enjoy some of the amps I have that are now considered anti-social — and equally joyful, at the other extreme was hearing a diminutive H&K Tubemeister 5 at stage-viable volume! The integral power amp claims 100W into 8Ω, and can get very loud even into a single, 8Ω 12-inch driver!

“It allowed me to greatly enjoy some of the amps I have that are now considered anti-social — and equally joyful, at the other extreme was hearing a diminutive H&K Tubemeister 5 at stage-viable volume!”“It allowed me to greatly enjoy some of the amps I have that are now considered anti-social — and equally joyful, at the other extreme was hearing a diminutive H&K Tubemeister 5 at stage-viable volume!”To keep things as clean and neutral as possible, I performed all my initial testing of the load and power-amp combination with all internal processors disabled. This is still not an all-analogue path — you are passing through the 32-bit/96kHz converters — but I can't say I perceived any tonal artifacts or latency attributable to a very well-spec'ed A–D stage. Starting to work with the effects and processors, the compressor proves useful for putting back some 'bounce' when playing more quietly than is ideal, and reverb and delay are all the better for being post-amp, but the absence of a dedicated modulation block feels like an omission. I know there is an effects loop and integrated MIDI switching available to work with an additional effects unit, but that is another thing to carry and rig for a gigging player. When you've already incorporated a sophisticated, multi-capability unit into your system, you surely want it to at least cover all the basics?

The 10 on-board stored Rigs accessible from the front-panel knob are footswitchable as two banks of five, and change fast enough to be used in live performance. A short step on the Rig footswitch allows you to select 1 to 5, with a longer press accessing 6 to 10. Stepping on the Rig switch again allows you to see the status of the effects loop, delay, tap tempo and solo/EQ, but no longer a Rig indication. Bearing in mind that the front-panel knob may be displaying something not current (it will update the footswitch, but not the other way round), and that either a quick or a long press of the Rig footswitch may display nothing if you happen to be set to one of the Rigs in the other bank, I think this is slightly sub-optimal. You might be using the Amp Control feature to select amp channel with your Rig selection, too, making certainty about which Rig you are in even more important. Another row of LEDs on the footswitch wouldn't have gone amiss.

A likely TAE-based live-performance scenario is an on-stage speaker for the player fed from the power amp, while front-of-house gets a speaker–sim feed from the mono line out. Although you have separate level settings for Line and Speaker outputs in the reverb, there are no such independent mixes for the delay — what you dial up for your speaker is also what FOH will get. This means you can't really use the TAE to make a wet-dry-wet stereo rig by setting a completely dry effects mix to the speaker out and connecting a pair of FRFR speakers to the line outs for the 'wet' signal. Well, you can, with just the reverb, but that's not quite the full deal.

The software also doesn't allow you to deploy the IR stage with the power-amp outputs. I guess the assumption is that you will only want to use the power stage with conventional guitar speakers, but with products like Celestion's excellent F12-X200 'Full Range, Live Response' speaker now on the market [see SOS August 2019 review], and the fact that you can get an analogue signal into the TAE pre cab-sim via the effects return, I think that's missing an open goal.

The Sum Of Its Parts

The Boss Waza Craft Tube Amp Expander is capable of some stellar direct recording tones. It shines particularly brightly with 4x12 emulations where, in the real thing, the thump and messy bottom-end resonance works so well to balance out the brittleness of the upper mids. With the right amp, the TAE can do a great version of 'big-cab feel' — the sort that lets you use less distortion and yet somehow sound more powerful. Smaller speaker emulations too are supported by some nice low-end resonance, which you can move up the spectrum with the Resonance Z control, or push away below audibility. Comparing directly to the OX, to my ears, there's always a slightly more lively and resonant bottom end in the TAE that I think lends a degree of playing comfort for people used to the interaction of amps and real speakers. That said, I found I usually had to get rid of a lot of that resonant bottom end from TAE recordings, just as I would with the mic signal from real cabs. If, like me, you tend to filter that stuff at source, seeking to record as 'finished' a sound as you can, its absence from the OX is of no consequence.

Playing live, or 'amp in the room' solo practice is a different matter: there is clearly greater volume-setting flexibility in the TAE's re-amping configuration, and the 'feel' seems to survive the process better at low volume. I find the OX is fine for a couple of notches down on the attenuator, but then I use a Deluxe Reverb most of the time, so there's not too much to attenuate. For bigger amps and any need for much quieter volumes, the TAE offers more control. Compared to the OX's instant gratification provided by the easy GUI, I find the TAE takes a bit more dialling-in. I don't think that is just me finding the software an inefficient and unnecessarily restrictive interface: there really are more ways to make something sound a bit wrong. But when you get everything dead right, it can be really good.

There are plenty of other players in the tube-amp attenuator/speaker-sim market now, but in each of the products, the balance of emphasis seems to lie in a slightly different place, just as it does between the OX and the TAE. Anyone entering the market now should perhaps make a choice based on the balance of their intended live/studio usage. I don't see existing OX users, myself included, deserting a product that they love for all the things it does well, but if you are frustrated by what it doesn't do, the chances are that the TAE has it covered. You might also look at a TAE if you need a unit that can multi-task as the 'controller of everything' in your rig. That said, the more you ask the TAE to do, the more likely you are to come up against one of the things it doesn't do quite so well. It is conceivable that a future software update may be able to address some of the things that I presently perceive as limitations, but right now you'll have to work with what it does.

Adjustable reactance is not a 'magic bullet' that automatically makes everything better. There are combinations of perfectly respectable real amps and speakers that still manage to feel stiff and unresponsive, so perhaps this is not yet an exact science, but being able to tweak the impedance curve certainly does seem to increase your chances of getting a dummy load to 'feel right'. And when the amp is happy, a good IR will take care of the rest. This really is a great time to be a tech-savvy guitar player with an undimmed passion for tube amps. I have no doubt that the Boss Waza Craft Tube Amp Expander stands ready to join Universal Audio's OX as one of the biggest players in this new and expanding market. I don't think it is quite yet the sum of its parts, but some of its parts are very good indeed.


I've referenced the UA OX in this review as the nearest equivalent, and there are a number of things that do bits of the same job, but none that do it all. A Fryette Power Station 2 will get you a real tube amp for re-amping after attenuation, but I think the load is resistive rather than reactive, whilst the new Suhr Reactive Load IR is purported to be the most accurate of all speaker-like loads. And of course, one should never ignore Two Notes, who were pioneers in this field.

Audio Examples

The author has created a few audio examples to accompany this review. You can find them on the SOS website at:

You can download a ZIP file of hi–res WAV audio examples in the righthand Media sidebar or use the link below.

Download | 96 MB

Integral Cabinets

  • 4x12" STANDARD: The sound of a closed-back cabinet equipped with four standard 12-inch speakers.
  • 4x12" R-FIRE: The sound of a MESA/Boogie Recto cabinet (closed-back) equipped with four Celestion Vintage 30 (12-inch) speakers.
  • 4x12" BRIT STACK: The sound of a Marshall 1960B cabinet (closed-back) equipped with four Celestion G12T-75 (12-inch) speakers
  • 4x12" CLASSIC STACK: The sound of a Marshall 1960B cabinet (closed-back) equipped with four Celestion Greenback (12-inch) speakers.
  • 4x12" GREEN/ V30: The sound of a Friedman 412 cabinet (closed-back) equipped with two Celestion Greenback (12-inch) and two Celestion Vintage 30 (12-inch) speakers.
  • 4x12" for FOH: A closed-back cabinet equipped with four standard 12-inch speakers. A well-defined sound that's appropriate for an FOH mix.
  • 4x10" STANDARD: The sound of an open-back cabinet equipped with four standard 10-inch speakers.
  • 4x10" SUPER COMBO: The cabinet sound of a Fender Super Reverb. This is an open-back cabinet equipped with four CTS alnico (10-inch) speakers.
  • 4x10" TWEED COMBO: The cabinet sound of a Fender Bassman — an open-back cabinet equipped with four Jensen P10R (10-inch) speakers.
  • 4x10" for FOH: An open-back cabinet equipped with four standard 10-inch speakers. A well-defined sound that's appropriate for an FOH mix.
  • 2x12" STANDARD: The sound of an open-back cabinet equipped with two standard 12-inch speakers.
  • 2x12" DIAMOND AMP: The cabinet sound of a Vox AC30 — an open-back cabinet that's equipped with two Celestion Greenback (12-inch) speakers.
  • 2x12" for FOH: An open-back cabinet equipped with two standard 12-inch speakers. A well-defined sound that's appropriate for an FOH mix.
  • 1x12" STANDARD: The sound of an open-back cabinet equipped with one standard 12-inch speaker.
  • 1x12" TWEED DELUXE: The cabinet sound of a Fender Tweed Deluxe. This is an open-back cabinet equipped with one Jensen P12Q (12-inch) speaker.
  • 1x12" DELUXE COMBO: The cabinet sound of a Fender Deluxe Reverb. This is an open-back cabinet equipped with one Jensen C12K (12-inch) speaker.
  • 1x12" for FOH: An open-back cabinet equipped with one standard 12-inch speaker. A well-defined sound that's appropriate for an FOH mix.
  • 1x10" STANDARD: The sound of an open-back cabinet equipped with one standard 10-inch speaker.
  • 1x10" for FOH: An open-back cabinet equipped with one standard 10-inch speaker. A well-defined sound that's appropriate for an FOH mix.
  • 1x8" STANDARD: The sound of an open-back cabinet equipped with one standard 8-inch speaker.
  • 1x8" MINI COMBO: The cabinet sound of a Fender Champ.
  • 1x8" for FOH: An open-back cabinet equipped with one standard 8-inch speaker. A well-defined sound that's appropriate for an FOH mix.

Recommended Setting Of Reactive Load

Name Tube Amp Cabinet Resonance Z Presence Z
R-FIRE STACK MESA/Boogie Rectifier 4x12, closed LO LO
HIGH GAIN STACK Friedman BE-100 4x12 closed H.MID L.MID
BRIT STACK Marshall JCM800 4x12 closed HI HI
CLASSIC STACK Marshall 1987 4x12 closed H.MID HI
SUPER COMBO Fender Super Reverb 4x10 L.MID L.MID
TWEED COMBO Fender Bassman 4x10 H.MID H.MID
TWEED DELUXE Fender Tweed Deluxe 1x12 L.MID HI
DELUXE COMBO Fender Deluxe Reverb 1x12 L.MID H.MID
MINI COMBO Fender Champ 1x8 L.MID H.MID


  • Adjustable reactance can optimise performance with a range of amps.
  • Open system that can work with third-party IRs.
  • Comprehensive footswitching and MIDI spec.
  • Un-stepped 'attenuated' volume.
  • Integral power amp can make low-powered amps usable on stage.
  • Can add line–level effects loop to vintage amps.
  • Editor is multi-platform and connects via USB.


  • Slightly odd and limited virtual mic choice.
  • No double-miked options on integral cabs.
  • No variable level of virtual room mics.
  • Software interface is unambitious.
  • Poor software control scaling.
  • No gain-reduction metering or EQ-curve graphic in software editor.
  • Effects loop return is mono when line outs are stereo.


The Boss Waza Craft Tube Amp Expander is a potential powerhouse of a product — speaker sim, power amp, user-IR host, on-board effects, footswitch and MIDI controllable, and above all a dummy load with variable impedance curve. It can work very well when you get everything set just right, but you will want to use the editor software, and that feels a little undercooked to me. Of course, software is upgradable...


£1143 including VAT.