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Positive Grid Spark 40

Modelling Guitar Amplifier By JG Harding
Published September 2021

Positive Grid Spark 40

The affordable and stylish Spark 40 packs Positive Grid’s powerful modelling software into a convenient, portable practice amp. What’s not to like?

My own miniature Tonehenge is a lovely boutique tube amp, plus a custom 2x6 cab and an oversized 1x12 cab with a Celestion Alnico Cream. It would pass the cork‑sniffing test of all but the most lofty gear snobs but, sometimes, even a top‑degree tone‑mason can’t be bothered to deal with tubes and cabs. The amp is unreasonably loud, it gets hot, and fooling with attenuators to bring the output of the cabs down to a sensible volume for practice defeats the point of their lush tone somewhat. Plus if you want to move any of it… well, let’s just say I skipped the gym during lockdown. So small practice amps still have a place in my life and after some incredibly positive experiences with Positive Grid’s Bias Amp modelling plug‑in, I figured it couldn’t hurt to try their take on the idea.

Launched around a year ago, the Spark 40 combines a power amp with DSP amp and effects modelling, along with some interesting practice‑oriented features. It had something of a rocky start to life, because the crowdfunding campaign was so popular that it took quite a while for everyone to receive their units. That’s all out of the way though, and the Spark is now available to buy in shops. I recently did just that.

Conceptually, the Spark is pretty simple: you plug in your instrument, as with any amp, and a DSP modelling engine gives access to most guitar and bass tones you can dream up. The resulting sound is pumped out by a 40W solid‑state amplifier driving two four‑inch full‑range speakers in a stereo configuration.

You can access presets using the hardware controls but can do much more tweaking using the Bluetooth smartphone app. With the app, you can also pull tones from a vast online library, and the Spark can even listen to your playing and create a basic backing track for you, or help you to learn a well‑known song using its Smart Jam feature (see box). There’s also a USB connection, which allows you to record the emulated amp sound directly into your DAW as audio, without the influence of the Spark’s own cab‑and‑speaker combination. No driver is required for Mac/iOS, but there’s an ASIO driver for suitably low‑latency performance on Windows. This feature is a great bonus, since the modelling is good enough that you could easily use this amp to make records, and they’d more than likely sound better than if you attempted to mic a cab at home!

Getting Hands On

The Spark looks like a real amp, and Positive Grid have done a great job of hiding just how high‑tech it is inside; knowing guitarists generally to be a conservative bunch I’d guess this is a conscious choice. All the knobs feel solid and consistent and the build feels decent generally. There’s a carry handle and, at 5.2kg, the Spark 40 is neither so light as to feel cheap nor so heavy as to be a burden. When I bought mine, they threw in the carry case for free, and this is a snug fit — great for lugging it to the studio and back, taking it on holiday as hand baggage, or maybe transporting it to a quiet jam.

Top‑panel controls allow you to use the Spark much as you would any conventional amp, which makes it familiar to use — though you’d be mad to avoid the app completely!Top‑panel controls allow you to use the Spark much as you would any conventional amp, which makes it familiar to use — though you’d be mad to avoid the app completely!

If you really can’t be bothered with the app or any customisation of tones you’d be missing out, but you can stick to using the amp’s physical controls if you want. With the left‑most knob you can choose one of seven on‑board factory presets: one for acoustic, one for bass and five for electric guitar, with increasing levels of distortion. These presets remain fundamentally as they’re set up, but each one features a controllable mod effect, delay and reverb, and lets you dial the EQ (bass, mid and treble) and virtual pre‑ and power‑amp stages to taste. At the other side is an Output knob, which controls the volume put out by the Spark’s solid‑state amp, while a Master knob emulates the master output of the virtual tube amp, with all the attendant effects on distortion characteristics. There’s also a dual‑purpose button for delay tempo and accessing the built‑in tuner.

A line input on the rear means you can play along to a backing track without having to use the app, and you can control its volume relative to your instrument. There’s no dedicated line output, but the headphone output (which has plenty of juice on tap) could serve as one. There’s no footswitch support either, so you won’t be switching channels or effects on the fly — unless you want to modify the unit (see the ‘Spark Mods’ box). As with a regular amp, though, you can always use a favourite effects pedal for a different sound, or as a boost for extra gain.

App & Tone

You can do much more using the iOS/Android control app, and setting this up is simple, with the usual sign‑in and Bluetooth connection. It worked well on my Samsung Galaxy phone, with no hitches. I was a bit irritated, though, that it insisted on me allowing location access on my device; I can’t think of a good reason why this should be a requirement rather than an option.

We’ve covered Positive Grid’s Bias software in depth in SOS before, so I won’t spend too much time here describing all the modelling options. Suffice to say that you get access to the same great Positive Grid tone, courtesy of software that’s a cut‑down version of Bias Amp and Bias FX. As of the July 2021 update, it offers 25 guitar amps, four acoustic amps and four bass amps, plus 43 effects. The tube‑amp models are responsive and realistic: they react well to volume alterations and dynamic playing, breaking up nicely and doing a great impression of the real things they’re based on. You can find the full list of the amps they’re based on here: Some things, including the emulated cab and mic, can’t be customised, but in a practice unit like this you don’t really need to do that, and other Bias products cater for that depth of tweaking.

The app is laid out nicely. You have a preset chain with noise gate, compressor, drive, amp, modulation or EQ, delay and reverb. This order can’t be changed, and neither can you add extra stages, but a practice amp is about playing guitar, not getting lost in a forest of knobs. This approach keeps things nice and simple, prevents you being distracted by tweaking too much, and means that the dedicated knobs on the amp always match the emulations. You can save up to four profiles from the app to the user preset buttons on the hardware’s top panel: just press‑and‑hold to save, and press to recall.

Positive Grid have done a great job of hiding just how high‑tech it is inside.

OK Boomer?

Playing through the amp itself, which, after all, is the main purpose of the Spark, was slightly less satisfying than hearing the same tones via USB and a good set of speakers/headphones. For an emulation‑based amp or a Bluetooth speaker, which the Spark doubles up as, the ideal cab/speaker is a flat‑frequency‑response, full‑range device, but that’s quite a tall order for a small box with small speakers which is catering for both bass and guitar players. In order to extend the Spark’s bass response, there’s a bass reflex port in the front of the cabinet, and its tube leads into what is an entirely undamped speaker enclosure. Sadly, the thunderous extension from this combination of port and undamped chamber creates a sort of exaggerated tilt EQ, which by default leaves everything sounding very muffled compared with that great modelled sound recorded over USB.

There are a few ways you can try to counter this effect. For instance, you can try turning the amp up to the point where the treble feels louder in general. But by this point you’ve got mountains of bass too, and with heavy metal tones in particular things get really thunderous, so the treble never really feels very extended. It’s similar when using the Spark as a Bluetooth speaker: by the time you’ve turned it up loud enough to hear the treble clearly, the bass end of things is very overblown and boomy.

The Spark 40 has something of a timeless design, and fits in well on a ‘shelf of many things’.The Spark 40 has something of a timeless design, and fits in well on a ‘shelf of many things’.The Spark 40 is really designed to be used at ear height or on a desk, and I imagine some of this treble roll‑off is intentional, to reduce ear fatigue from speaker-beaming and close proximity to distorted high‑mid and high frequencies. The obvious downside of this is that if you leave the amp on a carpeted floor or any other position that’s not straight in front of you, the bass‑heaviness seems exaggerated because you hear less of the high frequencies.

For bass guitar sounds this tonality isn’t so bad: the stock configuration works fine, as it produces way more level than you’d expect, with rich lows and just enough twang for most players. But for most electric and acoustic guitar patches, you’ll probably find yourself wanting to get heavy handed with the EQ. Before the July update, I found myself frustrated by how little effect the bass and treble knobs had when I tried to rebalance things this way, since these knobs control the modelled EQ circuit of the virtual amp, not the final output of the Spark’s solid‑state amplifier. Even dialling out all of the bass on some patches couldn’t prevent them sounding way too dark and boomy. Similarly, turning the treble knob up all the way doesn’t increase the treble enough for some models.

Really, the Spark is crying out for an EQ at the end of the signal chain, to give you control over the sound emanating from its own speakers. There’s still no global EQ immediately before the Spark’s power amp, so you can’t do anything to the Bluetooth playback, but the July update added more EQ options in the modelled effects, meaning you now have enough scope to get things sounding good over the built‑in speakers. Using the new EQs means you can’t use modulation effects, though, and if you record an EQ’d patch over USB the EQ will be baked into the recording. So while it’s a big improvement, it’s still not ideal.

Since the tonal balance was my only major issue with the Spark, I decided to try making some very simple mods to the cabinet, and managed fairly easily to reduce the boom to an amount I could live with. You can read about this in the ‘Spark Mods’ box.

As if that weren’t enough, the Spark 40 can also be used as a recording interface, streaming the modelled audio straight into your computer.


Positive Grid produce my favourite amp‑sim software, Bias Amp. It’s incredibly realistic and I’ve had no qualms at all about using it alongside real‑world recordings in record releases. The Spark squeezes some of this magic into a tiny, tastefully designed unit. There’s enough variety for practice, the quality of the modelling is superb and, for such a small amp, it’s capable of going very loud. It really does ‘fill the room’, and the reverb has a sort of voodoo surround‑sound effect if the amp is directly in front of you at head height. It’s quite a dramatic effect, certainly not a subtle one; I often left the reverb setting very low or off unless going for an epic post‑rock soundscape.

At the time of writing, the cost‑option carry case is being offered free with the Spark.At the time of writing, the cost‑option carry case is being offered free with the Spark.As if that weren’t enough, the Spark 40 can also be used as a recording interface, streaming the modelled audio straight into your computer. It can access a vast library of tones inspired by popular songs, and can even compose a drum and bass backing track for you to practice along to. All this is done with style and finesse: it’s easy to control, produces great results and if you’re burdened with too many pedals and amps, it will help you fall in love with just playing and experimenting again.

The one ‘con’ is a bit of a shame: while the USB feed sounds great, the sound from the built‑in speakers is very tilted towards the bass. The new EQ options improve matters but you can’t EQ Bluetooth playback and can’t use modulation at the same time, and if you record these settings over USB your recordings will be too bright. The Spark is already great value as a versatile practice amp, but the mods I decided to make have made it so much more useful: I love everything else about the Spark, and in mildly modded form, it’s now getting plenty of use.

You’ll find some audio examples at the start of this article. Alternatively, download the ZIP file and audition the hi-res WAVs in your own DAW.

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Bass‑friendly Features

The Spark 40 is intended for bass players as well as guitarists, and its four bass‑amp models are based on the Gallien‑Krueger 800RB, Sunn 300T, Eden WTP600 and Aguilar Tone Hammer 500. While the sound recorded over USB is great, physics isn’t your friend when trying to get big tones from a small box like this; it’s never going to sound like a 2x15 cab. But the Spark’s low‑end extension means it works surprisingly well in this role. It’s worth noting that the Smart Jam feature is aimed at guitarists, and isn’t designed for bassists: if you play in a bass line in, it will come back to you with drums and another bass line!

Smart Jam

For guitarists (it doesn’t work with bass) the Smart Jam feature is fun, and lets you select a basic genre to jam along and practice to, which can then be customised to taste. It also includes a mode that will listen to your playing and then create a backing track automatically. This works pretty well if you just solo into it: it will create a chord sequence in key for you. Attempting more complex songs seems to confuse it though. The most useful part of Smart Jam for me is the ability to search in the app for any song, and have the app bring up the chords to play along to as it plays back from YouTube. Not every song has been analysed, but a lot have, and 90 percent of those that I searched for brought up functioning chords to play along with.

Spark Mods

There’s a very active Spark modding community online, and people have devised various tweaks and improvements to it, including ways to address the bass‑heavy response. The Spark Amp Mods Facebook group already has over 11,000 members at the time of writing, and people have experimented with different speakers, added a line out and created a downloadable custom programme to build a Bluetooth‑compatible switcher, which controls the four channels and pedals (

The grille is held on with Velcro and thus easy to remove, which makes stuffing the port and damping the cabinet a quick‑win modification.The grille is held on with Velcro and thus easy to remove, which makes stuffing the port and damping the cabinet a quick‑win modification.One aspect of the Spark’s design is really handy if you want to tame the bass: as the front grille is held on with velcro, you can pull the grille off to access the bass port and speakers. Simply stuffing the bass port (you need only stuff a piece of sealed‑cell foam or even a sock in there) removes plenty of the ‘fake octave’ it generates, and instantly makes things sound a bit less boomy. Partially filling the cabinet with wadding will damp the resonances even further. To do this, I used some polyester packaging that came with a food delivery, but you can buy wadding on Amazon and elsewhere. All of this is all easily reversible, too.

If you’re feeling braver, you could experiment with a capacitor soldered between the positive terminal of each speaker and the appropriate cable. This will act as a simple high‑pass filter, thus turning down the bass; the filter’s turnover frequency depends on the value of the capacitor you choose.

Speaker swapping is another possibility, since the Spark’s four‑inch drivers are a standard shape. Some modders, for example, have tried installing various four‑inch dual‑concentric car‑audio drivers, which include a dedicated tweeter, in order to extend the treble response. If you go down this road, it’s worth remembering that using less sensitive speakers than the stock ones will result in a lower overall volume coming from the amp.


It’s a competitive market out there for modelling practice amps, with plenty to choose from! Yamaha’s THR30iiW premium desktop amp includes plenty of models and excellent tones, as well as optional battery power and wireless play if you buy a transmitter, but it costs around twice the price, while the Yamaha THR10ii is a smaller 20W version minus the wireless feature. Blackstar’s 20W ID:Core Beam is another worthy contender with a very similar feature set and low price. The Boss Katana Air is a 30W amp, including a Bluetooth transmitter for your guitar. Boss’s Tone Studio app offers 50 effects with COSM modelling and many amp models.


  • Compact and portable, with stylish low‑key aesthetics.
  • A great range of tones and effects courtesy of Positive Grid’s excellent modelling engine.
  • Access to a vast online tone library.
  • Smart Jam works surprisingly well for guitar practice.
  • Guitar, bass and even acoustic settings work well.
  • Active modding community.
  • Bluetooth smartphone control.
  • Great price!


  • Excessive bass output makes the Spark 40 very muffled and boomy without either radically EQ’ing many presets, or even modifying the hardware.
  • No dedicated analogue line output, speaker out or footswitch compatibility.


A fully featured, wonderfully designed amp that encourages practice and experimentation, the Spark 40 has tonnes of tones on tap and is extremely convenient — and you get all this for a keen price. Stumbling at the last hurdle, the in‑built speaker delivers a very muffled sound in its stock form, but EQ or very simple mods can address this.


£219 including VAT.