Microphone Preamplifier & Compressor

Published November 2024

Both of these devices are part of ART’s new ‘multivoice’ series, which aims to provide a wide range of tonal options.

Over the years, ART (Applied Research and Technology) Pro Audio have been one of the more prolific manufacturers of valve‑based equipment for home and project studios. The arrival of their new ‘multivoice’ products, the Solo MPA (a hybrid microphone and instrument preamplifier that offers solid‑state and valve amplification) and Solo VLA (which offers a choice of optical compressor with a valve stage or a FET compressor), takes ART’s lifetime total of valve‑equipped products to a very commendable 24! They’ve always offered decent bang for buck, and I hope there’ll be many more to come. Below, I’ll cover the functionality of each of these two new products in turn, before explaining how they performed for me during the few weeks I had them in for review.

MPA Preamp

Arguably, it’s quite hard for anything to make the front panel of a mono mic/instrument preamp in a black 1U enclosure intriguing! However, the legends above and around the Solo MPA’s control knobs somehow manage that trick very well. Balanced, rear‑panel XLR and TRS jack inputs accept mic and line‑level signals, while an unbalanced front‑panel TS jack provides the high‑impedance (>800kΩ) instrument input. The first thing to catch my eye was the gain control: scaled from 0 to 40 dB, this sets the gain of the separate FET‑based preamplifier stages and it’s an unusual gain range on the face of it. But it can be augmented with two other stages: first, there’s a +20dB gain switch; and, second, you can apply up to 15dB of gain at the master output. So you have a generous total available gain of up to +75dB. This unusual arrangement appears to me to be designed to give the Solo MPA the ability to accept either mic‑ or line‑level signals.

As with the Solo VLA, the Solo MPA’s I/O are paralleled on XLRs and quarter‑inch jacks, and can be set to operate at +4dBu or ‑10dBV.

The Solo MPA’s wide‑ranging impedance control can coax different characters from some mics.Next to the gain knob sits a variable input impedance control for the microphone input’s FET preamp. This ranges from 600Ω to a whopping 80kΩ — that’s by far the highest input impedance I’ve seen on a general‑purpose mic preamp. As Hugh Robjohns explained back in SOS September 2023 (https://sosm.ag/qa-mic-preamp-impedance), lowering the input impedance of a mic preamp will significantly reduce a microphone’s output signal. This requires higher preamp gain to obtain a usable output level, which in turn leads to an increase in noise, and maybe also in distortion. The tonal effects of lower input impedances can include: high‑frequency roll off; changes in the mic’s response to fast transients; a reduction in dynamic range; and an increased prominence in the low to low‑mid frequencies. All of which can lead to a subjectively ‘thicker’ overall sound. Increasing the input impedance, on the other hand, will not only lead to an increase in the level of the mic’s output signal, but will also tend to improve the mic’s dynamic range and linearise its response at low, low‑mid and high frequencies.

Whether or not these effects are desirable — or even audible — will depend both on the individual microphone concerned, and on the source being recorded. Dynamic (including ribbon) mics and older transformer‑equipped capacitor mics are usually more responsive to changes in preamp input impedance, because their output impedance varies with frequency. The balanced mics that we use these days typically have a low output impedance, so that they can drive very long cable lengths with no loss of signal level or quality (unlike high‑impedance electric guitar pickups, for which a long cable run can have a deleterious, though sometimes desirable, effect on the sound). This makes them less sensitive to these changes.

Next comes a conventional low‑cut (aka high‑pass) 6dB/octave filter, with corner frequencies from 10‑200 Hz. This is followed by a tilt EQ, whose shelving response acts either side of a pivot frequency (typically in the region of 900Hz), so that when the treble is boosted, the bass is cut and vice versa. In contrast to the older tilt controls, such as the ±3dB one in the Quad 34 preamp I owned back in the day, the Solo MPA’s tilt EQ has a fairly radical ±12dB gain range.

The Voice control that follows is the source of the unit’s ‘multivoice’ moniker, and sets the balance between the Solo MPA’s solid‑state and valve preamplifier stages. The idea is that, by varying the blend of these two stages, you can ‘voice’ the output to suit your preference — be that for the pristine cleanliness of solid‑state, or the warmer, more characterful tonality of the Solo MPA’s single Class‑A 12AX7 valve stage.

A line of three illuminated (when active) buttons bridge the gap between the Voice control and the illuminated analogue VU meter. The first is a +20dB gain switch, and that’s followed by buttons for +48V phantom power and ‘phase’ (polarity) reverse. After the VU meter, there’s the aforementioned master output level control, ranging from full attenuation to +15dB. The Solo MPA’s maximum output level is a whopping +27dBu, with maximum input levels of +19dBu (XLR) and +17dBu (instrument input).

VLA Compressor

The Solo VLA could be thought of as the Solo MPA’s more conventional cousin — it’s pretty much a standard, soft‑knee, mono compressor. Having said that, though, this is again a ‘multivoice’ device: it has two parallel, switchable gain cells so as to give you a choice between the speed, punch and transient control typical of a FET compressor, and the slower attack and release of an optical compressor, the latter with added colour courtesy of a 12AT7‑based Class‑A valve stage.

The Solo VLA’s two different compressors (one an optical type with valve stage, the other based around a FET) share the same front‑panel control set and rear‑panel I/O.

The Solo VLA’s rotary controls comprise threshold (‑30dB to +20dB), ratio (1.25:1 to 20:1), attack (0.25ms to 50ms), release (150ms to 3.0s), blend (dry to 100 percent wet) and master output level (full attenuation to +15dB) knobs. Its three selection switches (optical or FET, compression in/out, and the meter display of gain reduction or output level) illuminate yellow when active, with the meter source selector switch turning red to indicate when gain reduction is being displayed.

Flying Solo

In overall sonic terms, the Solo MPA preamp performed very well with a selection of dynamic, passive and active ribbon mics, and both large‑ and small‑diaphragm capacitor microphones, reflecting its specified ruler‑flat 15Hz to 50kHz frequency response, and dynamic range of over 110dB. I’m well used to microphone preamps with 70dB (or more) of gain being preceded by a ‑20dB pad to cope with line‑level inputs, but the Solo MPA is the first that I’ve used with a lower initial gain followed by a ‘reverse pad’ of 20dB of gain and a final boost of 15dB on the way out — it’s unusual to include the output gain in a preamp’s specified gain figure, but this didn’t cause any problems and there was always plenty on tap.

The input impedance control was very interesting to experiment with. It doesn’t reach below 600Ω, so I couldn’t access exceptionally low impedances, but I could definitely hear changes in the sound of some mics as I slowly raised or lowered the impedance. Lowering the impedance tended to make a mic sound fuller and heavier in the lower frequencies and duller in the highs, which I sometimes found a useful option. The low‑pass and tilt controls then proved very useful in optimising this change for a given source, and I found myself spending more time than I’d initially thought that I would in this area! With all my mics, increasing the input impedance to 15kΩ evened up their overall frequency responses to the point that I had to rethink my approach to each mic. I couldn’t discern any additional changes occurring above 15kΩ, which means that half of the impedance control’s travel was wasted on me, but your own microphones and perceptions may well lead to a different conclusion.

Blending these two preamp voices together offers smooth and often beguiling characters to work with.

Once I had the input impedance and tilt EQ dialled in for a given mic and source, I started experimenting with the solid‑state/valve blended voicings. As you’d expect, the solid‑state stage sounded clearer and cleaner compared with the warmer, weightier and (to my ears) more interesting tonality of the Class‑A valve stage, and blending these two preamp voices together offers smooth and often beguiling characters to work with. Since this palette of ‘blends’ interacts with your chosen impedance, low‑pass and tilt settings, a need for experimentation arises that can easily lead to several happy hours passing by (perhaps something to do outside of a busy tracking session, then!).

Setting up the Solo VLA compressor was a much less complex task. Plug it in, plug in a source (a single cable from the MPA to the VLA — I used a TRS jack patch cable; it did the job) and hook its outputs up to the desired destination and you’re good to go. Other than giving you the choice between FET and optical compression, the Solo VLA can be thought of as a stereotypical soft‑knee compressor. The full suite of front‑panel controls plus the wet/dry blend control that gives you instant access to parallel compression makes it a really easy‑to‑operate, flexible compressor.

For a wide range of sources, its FET‑based compression delivered just the clarity, transparency, speed, precision and effective transient control I’d expect of a FET gain cell, underscoring the versatility and controllability of this type of compression. By way of contrast, switching over to the optical gain cell, with its Class‑A valve output stage, without changing any of the control settings, delivered a more languorous, vintage style of compression, combined with a degree of valve coloration and weight that I found very attractive. As with the Solo MPA, there’s plenty to experiment with in the Solo VLA. Indeed, my only criticism really is the absence of any side‑chain high‑pass filtering, which I’d have found helpful.

But Is It ART?

There’s a lot to like in these two units, both of which offer excellent value for money for the performances on offer. Despite the unusual internal gain‑structure arrangement, the Solo MPA is a very impressive microphone preamplifier for close‑miking applications. The variable impedance, low‑pass filter, tilt control and ability to blend between solid‑state and valve stages gives you a ton of voicing options. Up until this review, I’d rarely used a mic preamp with more than a 3kΩ input impedance, and I don’t own a tilt EQ: and on the back of this experience, those are two situations that I need to change!

It’s perhaps overshadowed somewhat in the voicing department by its companion preamps, but don’t let that put you off the Solo VLA: it’s a very good compressor, and one that I’d be happy to own. Its FET compression delivers exactly what you’d expect it to and its optical compressor with added valve‑derived colour does likewise. Having the two compression styles in one box is really useful, in terms of flexibility of use and experimentation, and also in terms of value for money.

If you’re in the market for a versatile mono microphone preamp and/or a mono compressor, or perhaps want a couple of options to add colour and versatility to your collection, you really should consider the Solo MPA and Solo VLA. There’s a lot to be gained from having them to hand.

Pros

Excellent audio performance from both devices.
Fully variable preamp input impedance up to 80kΩ!
Preamp features low‑pass filter, tilt EQ and blendable solid‑state and valve stages.
Solo VLA offers both FET and optical compression.
Optical compressor includes a valve stage.

Cons

Compressor lacks a side‑chain high‑pass filter.

Summary

ART’s valve‑equipped Solo MPA mic preamp and Solo VLA FET/optical compressor offer great performance, value for money and a range of voicing options that you won’t find anywhere else.