[Review] Clef Soloist 50 amplifier

Clef Soloist 50 Class A/B amplifier

'A' heavyweight contender?

Product name: Clef Soloist 50 amplifier
Manufacturer: Clef Audio - Thailand
Cost: 1,195 pounds (in the UK). (Currency conversion)

Reviewer: Nick Whetstone - TNT UK
Reviewed: September, 2017

In an age of increasing miniaturization, our assessment of hi-fi equipment may have changed somewhat from those days where we made a preliminary judgment of something based on how much it weighed. None the less, it is still reassuring to purchase something like an amplifier, and find that it has substantial weight. Thus, the Clef Soloist 50 immediately made a huge impression with me! It's heavy, built like a tank, and despite that, looks gorgeous!

The Soloist 50 comes in a stout cardboard box, and is well protected by just two, but substantial polystyrene inserts. Also in the box is the remote control, and a booklet of (comprehensive) instructions. The remote control takes its cue from the amplifier, and, made of metal, weighs enough to make it useful to practise some bicep exercises while you are listening to the music. The review sample was loaned by Winters Audio.

The Soloist 50 is designated as a class A/AB amplifier, meaning that it will operate in class A mode, with an output of 10 watts per channels, or in class AB mode, with an output of 50 watts per channel. I hope that I don't have to tell you that audiophiles lean toward class A amplifiers for their smoother, more natural output. This is what Clef Audio say:

Class-A Design Circuit-

This is the most linear of the amplifier classes, meaning the output signal is a truer representation of what was imputed. The output device (transistor) conducts electricity for the entire cycle of input signal. In other words, they reproduce the entire waveform in its entirety. These amps run hot, as the transistors in the power amp are on and running at full power all the time.

Because of these factors, Class A amplifiers are very inefficient for every watt of output power, they usually waste some watts as heat. Because of the watts of heat energy released they run very hot, needing lots of ventilation. But the upside is that these amps are the most enjoyed of all amplifiers. These amps dig out musical detail, since the transistor reproduces the entire audio waveform without ever cutting off. As a result the sound is cleaner and more linear; that is, it contains much lower levels of distortion. They are the most accurate of all amps available.

The Soloist 50 operates in class A for the first 10 watts (into 8 ohm speakers). For most listening, no more power will be required. It would only be at 'party levels' that this amplifier would then go into class AB mode to provide the necessary extra power.

Sitting on the rack, the Soloist looks both beautiful (to my eyes) and imposing. Those rounded corners do a lot to give it a less 'industrial' look, and the large, clear, illuminated display in blue adds another touch of class. The display dims when you are not making adjustments to the volume control or input selector, so is not too imposing. On the rear of the amp are the usual inputs, ie an IEC mains socket, RCA (phono) input sockets, and one pair of balanced inputs, and the loudspeaker terminals, all of which look like quality items. There is also a reset switch. Rather than left and right channel connectors being grouped in pairs, as is more normal, the Soloist 50 connectors are placed on each side of the rear panel, as is befitting of a truly dual-mono design.

[Clef Soloist 50 rear panel.]
A well laid out rear panel with left and right channel connectors kept separate.

This dual-mono approach no doubt accounts for some of the hefty weight of the Soloist 50. Two transformers take up much of the internal space together with the two large heat-sinks for the output transistors. Nearly all of the amplifier's working parts reside on a PCB to the front of the case. Why dual-mono? Again, I'll let Clef Audio explain:

Dual Mono Design-

Dual mono design topology. This gives the amplifier better isolation between channels. Soundstage imaging is improved, offering true sound-stage width and depth. Instrument placement and layering are more accurate.

With seemingly no expense spared to make the Soloist 50 an excellent amplifier, the volume control is a stepped-attenuator design,built using SMD resistors, with relays used for the switching. At lower volume levels, you can hear the relays switching as you adjust the volume level, but unless you do all your listening late at night, and continually adjust the volume level, this is not an issue. And very sensibly, the Soloist 50 uses a speaker protection circuit to protect both the amplifier and speakers in the event of a fault.

Here is the full specification:

Type: Solid State
Configuration: Dual-Mono
Power Supply Transformer: 2 x 200VA
Power Supply Capacitance: 120,000µF
Class of operation: A/AB (10W Class-A)
Single Ended Inputs (RCA): 3 pairs
Balanced Inputs (XLR): 1 pair
Input Sensitivity: 160mV RMS
Input Impedance: 44 Kohms
Maximum Input Level: 3.6 Vrms Balanced/UnBalanced
Output Device Types: Bipolars / 6 per channel
Output Binding Posts: Gold-plated
Output Power @ 8 ohms: 50 Watts per channel
Output Power @ 4 ohms: 100 Watts per channel
Volume Steps: 64Steps / 1.0dB
Gain: 42dB
Signal-to-noise Ratio: 100dB @ full power
Frequency Response: 20Hz - 56kHz (+0/-3dB)
Crosstalk @ 1kHz: -100dB
THD (20Hz - 20kHz @ 1 watt): < 0.01%
THD (20Hz - 20kHz @ 50 watts): < 0.04%
Power Consumption @ idle/Max: 200 / 300Watts
AC Power Requirements: 220-230V / 50Hz
Shipping weight: 18 kgs
Dimensions (w x h x d): 432 x 112 x 400 mm.

So, having connected a source, and speakers to the Soloist 50, I sat down to listen. I had already listened to my own class AB amplifier that I was using as a reference. My immediate impressions of the Soloist 50 were twofold. First off, it sounded like it looks, solid, weighty, and well controlled. Secondly, the transparency that I enjoy from my class AB amp was slightly lacking with the Soloist 50. This is a fairly common finding with brand new amplification, so after a short audition, I connected the Soloist 50 to a CD player on the repeat function, and left it to play continuously for 24 hours. That had the desired effect, and I could then begin my auditioning more seriously.

Did I mention that bottom end? You simply can't help noticing it. It's not just that it goes low, it is so tightly controlled, and tuneful too. Talvin Singh's compilation album 'Back to Mine', contains a number of tracks with bass that will rattle a loudspeaker if that bass is not controlled by the amplifier. I need not have had any worries with the Soloist 50!

That tightness and control is evident right through the frequency range, providing a level of realism that is most impressive. Everything stops and starts almost instantly, so percussion for instance, is crisp, just like it is when you listen to it live. That realism applied particularly with voices. Singers sounded more real, more 'there'.

I also noticed that the Soloist 50 sounded good at lower listening levels, Some amps sound good at higher levels, but lose their magic when listening quietly at night. That weight in the bass that the Soloist 50 is so good at, is still there as the volume level is reduced. And when the volume was turned up (while the neighbours were out), that tight control over everything was still evident. The remote control works as expected, the volume change is both smooth and responsive.

The sound-stage produced by the Soloist 50 is both large and deep, and the imaging is also very good. In fact, when I played some Q-sounds off the Roger Waters album 'Amused to Death', it did the best rendering of them of anything that I have yet listened to. Those little sounds that have been designed to come well away from the sound stage were both substantial, more tangible, and placed further out from the speakers.

P.R.a.T. was also good with the Soloist 50. I constantly found my foot tapping away while I listened to Paul Simon's 'Gracelands'album. There was an effortless way in which the Soloist 50 produced the goods, even at high volume levels, with the music coming from an inky black background. All the tiniest sounds were evident.

Listening to Tchaikovsky's violin concerto, I noted the large scale presentation. This was the case with other classical recordings that I put through the Soloist 50.

The remote control worked perfectly, and is unlikely to get broken given that it is built like the amplifier, ie like a tank! In use, the amplifier never got very hot., the substantial heat-sinks obviously doing a great job in an amplifier designed in a country much warmer than the UK.

The Soloist 50 is one of those products that is a bit embarrassing for a reviewer in that it is almost impossible to find criticism with. Although a good amplifier shouldn't have a sound signature, anybody who has listened to more than a few will know that they do. Our ears, and the rest of our systems, will always prefer one particular amplifier over another. Having said that,it is hard to imagine anybody finding the Soloist 50 in any way objectionable. It does its job so well, and is such a joy to listen to that it should put a smile on anybody's face, and therefore be high on a wish list of amplification in its price-range.

Let's be honest, hi-fi production isn't the first thing that comes to mind when we think of Thailand. But Clef Audio have chosen to enter a competitive (and some may say, shrinking) market. If the Soloist 50 is indicative of their product range, then they should do well. The Soloist 50 is a well-built, well-specified, great sounding amplifier that also looks good. It is a clear step up from mid-range, amplifiers. Just watch your back if you are lifting it!

[Clef Soloist 50 top of case and vents]
The substantial heat-sinks run from the front to the rear of the case on either side.