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Grace Design m303

Grace Design m303

Did you think all DI boxes were created equal? If so, you may need to think again...

Best known for their superb mic preamps and monitor controllers, Grace Design also make an impressive range of instrument preamps, such as the Alix, Bix and Felix, the last of which was reviewed in SOS October 2015. These provide varying degrees of EQ complexity, adjustable input impedance, effects loops and more, but all of them feature an active DI output. Although not everyone needs the sophistication of these ‘ix’ models, a great‑sounding, reliable active DI box is a common requirement for live and recording work, so it’s not surprising that Grace’s engineers have adapted their proven DI circuitry into a dedicated standalone box.

Overview

Called the m303, this single‑channel, phantom‑powered DI box is the size and shape you’d expect, measuring 81 x 44 x 134mm (WHD) and weighing just under 0.5kg. The strong case is manufactured from extruded aluminium, finished in a gorgeous anodised cobalt blue, with silver lettering that’s easily readable in low‑light conditions to identify the socketry and switches. The base is covered with a rubber pad, which grips very well, helping the m303 stay where it’s placed. A central square is cut out of that rubber pad to reveal a printed label carrying a unique serial number for each m303, so it won’t get damaged or worn off. The whole product is built in America and comes with a generous five‑year warranty.

The review unit arrived in a recyclable cardboard box with a ‘Korrvu’ suspension that acts as a very effective shock‑absorber. There was no printed user manual and, to be fair, it doesn’t really need one; there’s nothing unusual in the m303’s use or operation. Nevertheless, a detailed 13‑page PDF manual is available, and this describes the switches, connections, and how to use the unit for novices. It also includes an unusually comprehensive set of genuinely impressive technical specifications.

Grace Design m303High‑quality components are used throughout, with gold contacts in the switches, and comprehensive RF interference and ESD protection. All I/O connectors and switches are mounted on recessed aluminium end plates, where they’re well protected by the outer case, so I have no reason to doubt the m303 will survive decades of stage use (and abuse).

At the left (input) end of the box is a pair of quarter‑inch sockets wired in parallel as unbalanced input and thru connections. To accommodate high‑level line sources, a recessed toggle switch engages an input pad. A green LED indicates when the unit is powered, while a red LED illuminates roughly 6dB below clipping.

In normal use (without the pad), the input impedance is specified as 1MΩ, which is pretty much the standard for active DI boxes and will keep all guitars and basses with passive pickups happy. Engaging the input pad reduces the input impedance, of course, in this case to a still pretty high 115kΩ. While this would affect the tone of electric guitars significantly, the m303 has such good headroom that the pad would never be needed for such sources — it will only be required when working with very hot line‑level equipment (eg. DJ decks) and possibly some electronic music sources such as modular synths. For all of those, 115kΩ is more than high enough.

I mentioned headroom, and in all active DI boxes the maximum signal level that can be handled directly (without the pad engaged) is determined by the power rails feeding the active input circuitry. To that end, the m303’s audio circuitry runs on symmetrical ±5.5V power rails, accommodating a peak‑to‑peak signal level of 11V (equivalent to +14dBu). This is more than most active DI boxes — the Radial J48 clips at +10dBu, for example — so it has a very generous headroom margin, and is extremely unlikely to be overloaded in normal use.

Unusually, the pad switch attenuates the input signal by 13dB. It might seem an odd choice, but it means the maximum clean input level is a whopping +27dBu — more than enough for the loudest line‑level equipment, even if running with its meters fully pegged (most pro equipment clips at around +24dBu). It’s not enough to handle directly the output from a beefy power amp or backline loudspeaker, though, and the user manual specifically warns you not to try that!

At the right‑hand (output) end, there’s a male XLR for the balanced (microphone‑level) output, and another recessed toggle switch to isolate the output ground from the input ground (to break any ground loop between the source and destination equipment).

Like many top‑quality active DI boxes, the m303 can only be powered by standard 48V phantom power, with no provision for internal batteries. However, it consumes a miserly 4mA of current from the phantom supply, which is less than many mics and lower than most active DI boxes, so it won’t stress your mixer channels, interfaces, or mic preamps.

Phantom Power & Galvanic Isolation

Although using phantom power to energise a DI box is a very convenient and reliable option — avoiding the issues associated with failing, exhausted, or missing batteries — it poses a significant engineering challenge: as the output XLR ground connection is a necessary part of the phantom power supply circuit, the usual ground‑lift solution of simply disconnecting pin 1 of the output XLR isn’t viable (it would stop the DI box from working). Manufacturers have evolved various workarounds to this problem, but Grace Design’s sophisticated and elegant approach ensures total galvanic isolation (meaning no direct electrical connection at all) between the input and output connections, when required — which is a good thing!

All passive DI boxes (and most active ones) use an audio transformer to provide galvanic isolation between the input and output grounds — the audio signal is passed between the primary and secondary windings inside the transformer via a varying magnetic field. The electrical isolation this provides is not only helpful in preventing ground‑loop noise problems, it also provides an important level of electrical safety. For example, should a serious electrical mains fault occur on the PA system the transformer prevents the guitarist from receiving an unwelcome electric shock. So, the m303 uses a very high‑quality, fully‑shielded, custom‑wound Lundahl LL1975 audio transformer (with a high‑nickel core content to minimise LF distortion), which provides the required electrical isolation between input and output.

A photo of the circuit board with the Lundahl audio transformer at the top and the internal power transformer at the bottom. The audio circuitry is the op‑amp to the left of the Lundahl transformer. The chip between the sockets drives the level LED. All the rest is power‑related!A photo of the circuit board with the Lundahl audio transformer at the top and the internal power transformer at the bottom. The audio circuitry is the op‑amp to the left of the Lundahl transformer. The chip between the sockets drives the level LED. All the rest is power‑related!

However, this still doesn’t solve the phantom‑power ground connection problem, because the audio electronics have a ground reference via the internal power supply. With phantom power, there will still be a ground path connection, so the internal power supply must be isolated in some way too. Like several other manufacturers, Grace achieve that by using the external phantom power only to drive an internal AC oscillator, in this case running at around 68kHz. The output voltage from that oscillator feeds the primary winding of a miniature high‑frequency transformer, whose electrically isolated secondary winding passes that AC voltage into a bridge rectifier and smoothing components, to generate an isolated DC supply providing ±5.5V to power the input audio buffer circuitry. In this ingenious way, both the output XLR and phantom‑power circuitry grounds are completely isolated electrically (via the two transformers) from the input circuitry ground. The ground‑lift switch can couple them together, or leave them isolated, as required. The whole scheme is very neat and works brilliantly.

Most of the circuitry inside the m303 is involved in this internal powering system, and the audio circuitry components really only comprise the sockets, Lundahl transformer, input pad, and a dual FET op‑amp, that serves as the high‑impedance input buffer. For this, Grace chose a Texas Instruments OP1652, which has impressively low noise and distortion specs, and the ability to handle signal all the way up to the power‑rail voltage (a choice that maximises headroom).

An illustration of the block diagram concept, highlighting the separation of input and output sections via transformers.An illustration of the block diagram concept, highlighting the separation of input and output sections via transformers.

Technical Performance

With the pad disengaged, which will be the normal way of working for most, the signal level at the balanced XLR output is around 6dB lower than the unbalanced input level, though the exact attenuation depends to some degree on the destination load impedance. Thus, a typical ‑20dBu instrument level going into the DI box will produce a fairly hot mic level of around ‑26dBu at the output. With the pad engaged, the total attenuation through the unit rises to a little more than 19dB. In both cases, the maximum balanced output level is around +8dBu (dependent slightly on the destination impedance, again), from a source impedance of 330Ω.

This DI box is perfectly transparent at all practical signal levels.

Hooking the m303 up to my Audio Precision analyser, I found the Total Harmonic Distortion (THD) at 1kHz measured 0.0005% (for an input level of ‑20dBu), rising to 0.007% with an input level of +12dBu (just before clipping). So this DI box is perfectly transparent at all practical signal levels. Intentionally overdriving the input required a level of +15dBu and didn’t sound nice — there is no graceful saturation before clipping!

An Audio Precision plot of input level versus THD+N.An Audio Precision plot of input level versus THD+N.

All transformers introduce greater distortion at low frequencies, but with the same input level at 50Hz, the THD figure reached only around 0.007% (and at 25Hz about 0.03%). So, again, that’s completely unnoticeable and inaudible. Intermodulation tests were similarly impressive, with the worst case being 0.001%.

Measuring the frequency response across a range of input levels, with and without the pad, always delivered consistently ruler‑flat traces between 22Hz and 40kHz (‑1dB limits), or 15Hz to 70kHz (‑3dB limits). There’s a fixed 12dB/octave high‑pass filter which rolls off below 20Hz, and the high corner frequency is affected slightly by the source impedance.

The m303’s signal‑to‑noise ratio measured over ‑112dBu (flat) or ‑117dBu A‑weighted (20Hz to 20kHz). While not quite as good as the published specifications, that’s still very good indeed, and an FFT spectrum of the output showed that all signal, power and distortion harmonics were comfortably below ‑120dBu.

Another Audio Precision plot, showing the ruler‑flat frequency response across a range of signal levels.Another Audio Precision plot, showing the ruler‑flat frequency response across a range of signal levels.

Best In Class?

The m303 is a very solidly built, skilfully engineered and, most importantly, great‑sounding active DI box. There’s nothing confusing about its use or settings. Plug the output to a desk or interface mic channel, set with low gain. Turn on the phantom power, and check the green power LED. If there’s ground‑loop noise, activate the ground isolation switch, and if the source is really loud and the peak light flashes, activate the pad. It’s that simple and straightforward.

I’ve long been a fan of Grace Design’s mic preamps and monitor controllers, because they sound so transparent and are easy to use. The m303 never disappoints in these departments either. It’s extremely quiet, even with low‑level sources, the headroom margin is epic, and together these afford an enormous dynamic range capability. The audio path is amazingly transparent and natural‑sounding, too, with a crystal clear and airy top end nicely balanced by a powerful, deep and perfectly clean low end.

I’ve owned and used a lot of DI boxes over the years, and I can genuinely say that the m303 is the best I’ve heard.

I’ve owned and used a lot of DI boxes over the years, and I can genuinely say that the m303 is the best I’ve heard. A few other designs have matched its headroom and transparency, but the big deal for me is that the m303 ticks almost all the boxes, where everything else seems to miss at least one. Others don’t also offered the total galvanic isolation on offer here. Others can’t match the 1MΩ input impedance, or can’t provide ground lifting when phantom powered... The only box left unticked by the m303 is its ability to cope with direct amplifier outputs. That’s a technique I use rarely and would in any case prefer to use a passive box for, so that’s an irrelevance.

Naturally, a fully isolated active DI box from Grace Design is never going to be inexpensive enough to give away with breakfast cereal. But the m303 is priced comparably to similar high‑quality DI box offerings from Radial and Little Labs, for example, and it’s undoubtedly an investment that will last a lifetime and never disappoint sonically.

Pros

  • Phantom-powered.
  • Massive headroom.
  • Completely transparent, sonically.
  • Lundahl audio transformer.
  • Total galvanic isolation between input and output.
  • Easy to use and extremely robust.
  • Five‑year Warranty.

Cons

  • Some might want a direct speaker connection mode.

Summary

The m303 phantom‑powered active DI box is an impressive design in every respect, and sets a quality and practicality benchmark to which others must aspire.

Information

£295 including VAT.

ASAP Europe +44(0)20 8672 6618.

sales@asapeurope.com

www.asapeurope.com

gracedesign.com

$299

Grace Design +1 303 823 8100.

dwood@gracedesign.com

gracedesign.com

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