The Solo 110 and Solo 610 are a pair of high-quality, no-frills preamps - one solid-state, the other valve.
There can be very few people interested in recording sound that have not come across the name of Bill Putnam, Senior. He was a legend, not only as one of the great recording engineers of the '50s, '60s and '70s, but also as a designer of studios and recording equipment. In 1999, Bill Putnam Jr. and his brother James re-founded their late father's Universal Audio company and today the company focuses on two main lines of work: authentic hardware reproductions and innovative software emulations of classic analogue equipment.
The subject of this review is a pair of very neat, single-channel preamps based reasonably closely on existing UA technology. The first is a single-channel version of the Precision 110 series Class-A solid-state preamps found in the 4110 and 8110 units, while the second is yet another incarnation of the classic Putnam 610 valve preamp. The two units form the Solo series, and are called the Solo 110 and Solo 610, respectively.
The unique aspect of these preamps is really their form factor — they are compact, portable, single-channel units, with a refined feature set offering just the essentials. Both units share exactly the same rugged steel chassis and have identical features and facilities, but the internal electronics are obviously completely different. The chassis measures 146 x 127 x 355 mm (WxHxD), with a strap handle on the top, and the top and sides extend beyond the chassis frame to protect the switches on the front and back panels. The sides have stylish cooling slots cut in the shape of the UA logo. The solid-state Solo 110 weighs 2.4kg while the valve Solo 610 is heavier at 4kg, thanks to the additional transformers the latter model contains.
The rear-panel facilities for both units comprise a pair of XLRs for a mic input and balanced output, plus the ubiquitous IEC mains inlet with a separate mains on/off rocker switch. In keeping with its vintage styling, the 610 features a pair of metal toggle switches associated with the output XLR, which provide a ground lift and adjust the output level between mic and line settings. The Solo 110 has identical facilities but uses illuminated blue push buttons.
The Solo 610's IEC inlet includes a fuse-holder-cum-voltage-selector catering for 115 or 230V operation, while the 110 model has a switched-mode power supply that can accommodate any mains voltage from 90 to 250V. Both units consume a maximum of 18W of power.
Moving around to the front panel, both units have identical facilities, but the 610 controls them with metal toggle switches and large, vintage-style knobs, while the 110 uses the same illuminated blue push buttons and modern knobs as are found on the 4110 and 8110 units.
In both cases, the left-hand knob controls the input gain, while the right-hand knob sets the output level, and both are scaled arbitrarily from 0 to 10. The first of the five switches selects between the rear-panel mic input and front-panel DI input, while the second configures the input for high or low impedance. Next is a phantom power switch, followed by a 100Hz high-pass filter switch and, finally, an output polarity reverse switch. A blue LED shows when the power is switched on, and a tri-colour LED gives a crude indication of signal level. The LED shows green when the signal is at a nominal signal level, turns amber as the headroom limit is approached and the amount of distortion increases, and turns red when the amplifier clips.
The front-panel DI input is joined by a parallel-wired 'thru' output (both are on quarter-inch jacks) allowing the preamp to take a 'sniff' of a guitar signal en route to the guitar amp, for example. The DI input is about 20dB less sensitive than the mic input, which is pretty much ideal.
The switchable input impedance affects both mic and DI inputs. When the DI input is used, the options are 47kΩ or 2.2MΩ, while the mic input can see 500Ω or 2kΩ. The high-impedance mode could be considered the default condition, being ideally suited for 90 percent of circumstances, but the lower value provides an alternative flavour that may prove useful. In particular, dynamic mics often sound more interesting when loaded with 500Ω or so.
As might be expected, the all-valve Solo 610 has a lower maximum gain than the solid-state Solo 110, the figures being 60dB and 77dB respectively. These figures are both with the low-impedance mode selected — the gain drops a few dB in high-impedance mode — and the maximum gain is roughly 20dB less when the DI input is selected, as already mentioned. Similarly, the audio bandwidth of the two versions differs, with the valve model spanning a respectable 20Hz to 20kHz (±1dB) and the solid-state version comfortably beating that with 10Hz to 60kHz (±0.2dB).
The maximum input levels for the Solo 610 are +4dBu for the DI input and -12dBu for the mic input (at 1 percent distortion). The output level is dependent to a degree on the loading because of the output transformer, but it can produce +18dBu into a 100KΩ load, falling to +14dBu into 600Ω. The solid-state Solo 110 can accept up to +3dBu on the mic input and produce +19dBu into a 600Ω load, rising to a massive +31dBu into 100KΩ.
At the expensive end of the market, the single-channel preamp format is not a particularly popular one — at least not without EQ and dynamics thrown in. I can really only think of two direct alternatives: the SPL Gain Station 1 and the Avalon M5. The Avalon is an impressive all-discrete Class-A solid-state preamp, while the SPL is a hybrid, incorporating a switchable tube stage to add that special thermionic character when required. Both are excellent preamps in their own right, with the M5 leaning more towards the neutral and transparent end of the range, and the Gain Station being a little more colourful when required, but still capable of very faithful reproduction when appropriate.
The Solo 610 is a pretty faithful recreation of the original 610 preamp design, also found in the Universal Audio LA610 and 2-610. The Solo 610 uses the same dual-triode 12AX7 and 6072 valves — the former to handle the input, and the latter to drive the output — and the output transformer is also the same as that used in the 2-610. I believe the input transformer is slightly different, though, to suit the revised input circuitry. WIMA capacitors are used throughout alongside conventional components set out neatly on a relatively spacious circuit board, roughly half of which is given over to the linear power supply with a generously sized mains transformer. There are only two ICs in the whole box — one is a voltage regulator in the power supply and the other drives the signal level LED. The audio signal only passes through the two valves and two transformers — the way Putnam intended!
Unlike its peers, the Solo 610 has a single, continuous Gain control instead of the rotary 5dB stepped switch of the original and its subsequent recreations, and there is no line input facility or 15dB pad to accommodate high-level sources.
The characteristic aspect of the 610 circuit design is that to increase the input gain, the amount of negative feedback around the input 12AX7 valve is reduced. This inevitably results in an increase in harmonic distortion, which affords the user the ability to juggle the Gain and Level controls to optimise both signal level and sound quality. Crank the input gain up for a slightly crunchier sound, back it off for a cleaner sound, and adjust the Level control to provide the appropriate peak levels at the output. It's simple, elegant and surprisingly versatile.
By comparison, the internal layout of the Solo 110 seems ultra-modern and complex. Most of the passive components are surface-mounted with just a few conventional WIMA capacitors thrown in where it matters. The power supply is a compact switched-mode module, with additional regulation and filtering on the main board — the whole lot taking up about a quarter of the internal real estate.
The same input transformer is used as the Solo 610, but there is no output transformer. The audio path is via an all-discrete Class-A, DC-coupled design using matched FETs as the active devices, and a lot of them! The Precision 110 series is famed for its superb dynamics and clarity, while still retaining something of the warmth that characterises the vintage Putnam designs. Again, the few ICs in the box are power regulators and the signal level LED driver.
The Precision 4110 and 8110 preamps feature a switch labelled 'Shape' which is absent from the Solo 110. On the Precision models, this switch alters the transformer and input circuitry loading in three stages to provide a range of harmonic colourations. It's absence from the Solo 110 is basically a cost-cutting move, and the unit is effectively permanently set to the 'Vintage' position. Of the three modes, this is the most flexible and best-sounding in my opinion, providing a wide range of sonic control from the mildest of mid-range presence to a fairly obvious rich character, just by adjusting the input gain — just like the 610.
While both Solo models are able to provide a range of sonic colours, the two units also have a fairly distinct 'signature' character of their own. The Solo 610 has a slightly rounder, fatter, richer character, and it sounds slightly more closed at the top end compared to the 110. This is not a criticism, though. Rather, it is an essential part of the preamp's vintage personality. The 110 sounds more modern and open — a little more detailed and crisp, and with a more up-front dynamic, yet without the clinical harshness that many would associate with generic solid-state preamps. The gain and headroom provided by the 110 is quite substantial — making it virtually bomb-proof in that regard — although I had no problems with the 610 in terms of running out of gain or headroom, even when using a modern ribbon mic.
Given that the majority of the recording work I do involves mostly classical music, I generally favour accurate, transparent and characterless preamps, and given the choice I would probably opt for the Solo 110 as the closest to that ideal for me. It is quite capable of usefully neutral results, but it also has the flexibility to introduce some subtle richness and body to the sound when driven harder — something that worked well when DI-ing some 'clean' electric guitar parts.
The Solo 610 is noticeably darker and richer in character from the off, and wouldn't be my first choice for most classical recording, but is ideal for recording vocals and percussion parts, and sounded great as a DI for guitars and basses. There's plenty of scope for dialling in loads of character here, and it's almost impossible to make a nasty sound! There is a subtly forward quality to the sound, helping it to cut through the mix nicely.
The Solo units aren't cheap as single-channel mic preamps go, but they are very solidly built using quality components. Consequently, I would expect them to be providing sterling service in 20 years without any problems at all — and that's saying something in these days of cheap-and-cheerful disposable equipment.
They offer simple facilities, yet are able to condition and tailor the signals to meet most if not all technical and sonic requirements. The convenience of a reasonably compact and portable mic preamp should not be underestimated, and the Solo units will appeal to a wide range of users and applications, spanning location recording engineers, to commercial studios, to stage vocalists with deep pockets. If the format suits your needs, and the budget allows, these are both worthwhile additions to your shortlist.