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This review first appeared in the July 2012 issue of hi-end hifi magazine High Fidelity of Poland. You can also read it in its original Polish version here. We publish its English translation in a mutual syndication arrangement with publisher Wojciech Pacula. As is customary for our own articles, the writer's signature at review's end shows an e-mail address should you have questions or wish to send feedback. All images contained in this review are the property of High Fidelity or Soulution. - Ed

Reviewer: Wojciech Pacula
CD player: Ancient Audio Lektor Air V-edition
Phono preamplifier: RCM Audio Sensor Prelude IC
Cartridges: Miyajima Laboratory Shilabe & Kansui
Preamplifier: Ayon Audio Polaris III Signature with Regenerator power supply
Power amplifier: Soulution 710
Integrated amplifier/headphone amplifier: Leben CS300 XS Custom
Loudspeakers: Harbeth M40.1 Domestic + Acoustic Revive custom speaker stand
Headphones: Sennheiser HD800, AKG K701, Beyerdynamic DT-990 Pro 600Ω vintage, HifiMan HE6
Interconnects: CD/preamp Acrolink Mexcel 7N-DA6300, preamp/power amp Acrolink 8N-A2080III Evo
Speaker cable: Tara Labs Omega Onyx
Power cables (all equipment): Acrolink Mexcel 7N-PC9300
Power strip: Acoustic Revive RTP-4eu Ultimate
Stand: Base IV custom under all components
Resonance control: Finite Elemente Ceraball under CD player, Audio Revive RAF-48 platform under CD player and preamplifier, Pro Audio Bono PAB SE platform under Leben CS300 XS
Review component retail in Poland: 110.000zł

The Soulution 720 preamp completes the virtual chart-topping Swiss audio system I had the pleasure to test at home over the last two years. It included the 745 SACD player, the 710 power amplifier and now the 720 reviewed today. Although the manufacturer does not give its products catchy names, it puts into them a lot of knowledge. Soulution is very much engineering based, appealing to the best traditions of measurements, electronic and mechanical engineering. What sets it apart from similar hifi manufacturers is considerable capital behind it as part of a larger company which manufacturing industrial engines - Spemot AG.

As in their other products, the 720 preamplifier implements many proprietary solutions based on earlier company guidelines and design philosophy. The most important determinants are minimum noise, negligible distortion, wide frequency response and an ultra-short signal path. The 720 boasts a S/N ratio of 130dB (yes!), THD distortion below 0.0006% and a frequency response s from DC to 1MHz. To achieve that, Soulution’s engineers pursued the minimal number of parts in the main circuit. Although the interior is incredibly crowded, the actual signal path is very simple. To keep the number of components low, the designers settled for an unbalanced circuit topology (“as few components in the signal path as possible suggested an unbalanced circuit design.” The full description can be read here).

The 720 is a dual-mono design with separate PCBs for left and right channels. The inputs are switched by precise hermetically sealed best-of-kind parts – reed relays. To minimize cross talk, the input selector disconnects both hot and ground paths. The XLR signal is converted to single-ended and each input has a three-step gain level adjustment – 3/6/9dB. There are no signal path capacitors. The input signal is continuously monitored for DC voltage to avoid damage to your amplifier and/or speakers. Thus a coupling capacitor is switched in and indicated as such on display whenever a DC voltage greater than 15mV presents itself. As soon as that DC component decreases, the monitoring circuit automatically switches the capacitor out after 15 seconds. For safety reasons the capacitor is also switched in whilst powering up the device and during input switching.

Volume control is by means of a stepped resistor matrix consisting of high-precision metal resistors switched again by reed relays. Since this scheme may result in switching noise (bear in mind the absence of coupling caps or DC offset compensation in the signal path), the 720 is equipped with a secondary PGA programmable gain amplifier volume control which is only active during the actual volume change process. As soon as the new desired level value has been fixed, the unit simply reverts to the precision resistors. The end result is similar to using a potentiometer or digital volume control whose volume adjusts without clicking noises. The downside is a slight inertia. The display will show the  new volume level before we hear it through the speakers.

The output stage employs ultra-wideband transistors—2MHz -3dB—linearized by an analogue computing network. I reviewed the 720 with a phono stage. This model is also available as the 721without phono. Unfortunately time constraints preventing me from assessing the phono capabilities. For context Soulution’s 710 stereo power amplifier has been part of my reference system for almost two years.

Sound. A selection of recordings used during auditions: Audio Accessory - T-TOC Records High Quality Data Master Comparison, TDVD-0002, DVD-R (2011), rips 16/44.1, 24/96, 24/192 FLAC; Paganini for two, Gil Shaham, Göran Söllscher, Deutsche Grammophon/JVC, 480 246-5, XRCD24 (1993/2009); Stereo Sound Reference Record. Jazz&Vocal, Stereo Sound, SSRR4, SACD/CD (2010); Stereo Sound Reference Record. Nobu’s Popular Selection, Stereo Sound, SSRR5, SACD/CD (2010); André Previn, After Hours, Telarc/Lasting Impression Music, LIM UHD 051, CD (1989/2011); Assemblage 23, Bruise, Accession Records, A 128, Limited Edition, 2 x CD (2012); Beverly Kenney, Beverly Kenney sings for Johnny Smith, Roost Records/EMI Music Japan, TOCJ-9731, CD (1956/2012); Clan of Xymox, Subsequent Pleasures, Metropolis, Met 204, CD (1983/2001)...

... Depeche Mode, Personal Jesus 2011, Sire/Reprise 21328-2, MS CD (2011); Dominic Miller & Neil Stancey, New Dawn, Naim, naimcd066, CD (2002); Dominic Miller, Fourth Wall, Q-rious Music, QRM 108-2, CD (2006); e.s.t. Esbjörn Svenson Trio, 301, ACT Music + Vision, ACT 9029-2, CD (2012); Händel, La Maga Abbandonata, Simone Kermes, Maite Baumont, Il Complesso Barocco, dyr. Alan Curtis, Deutsche Harmonia Mundi/Sony Music Entertainment, CD 88697846212, CD (2003/2011); McCoy Tyner, Nights of Ballads & Blues, Impulse!, IMP 12212, 20-bit Super Mapping, CD (1963/1997); Me Myself And I, Do Not Cover, Creative Music, 005, CD (2012); Pat Metheny Group, Offramp, ECM, ECM1216, CD (1982); Sara K., Don’t I Know You From Somewhere?, Stockfisch, SFR 357.6055.2, CD (2008); The Beatles, Rubber Soul, Parlophone/Apple/Toshiba-EMI, TOCP-51116, CD (1965/1998).