Integrated Amplifiers

The single-ended valve amplifier is still something of a niche product. Low power outputs and often equally low damping factors mean that very careful system matching, plus sensitive loudspeakers, are a prerequisite if you are to hear such designs give of their very best. However, the UK's very own guru of all things thermionic, David Shaw of Icon Audio, has decided to address these issues with his £2299 single-ended Stereo ST30SE, an integrated amp having, shall we say, a tad more welly, thanks to it being equipped with KT150 output valves.

The KT150 is a relative newcomer to the valve scene, though Icon Audio was an early adopter, upgrading its Stereo 60 MkIIIm, MB30SE, MB90 MkIIm [HFN Mar '14] and MB150 amplifiers with this very tube way back in 2013. A development of the KT88 and KT120 pentodes, it offers higher power, and given that single-ended amplifiers are often built around lower-powered valves such as EL34s or 300Bs, it is something of a surprise that few other designers seem to have thought of using the beefier KT150. The result, Icon Audio claims, is a more 'real-world' power output.

Pots 'N' Panels
David Shaw explains that the idea came to him following enquiries from customers for the company to add a further single-ended amplifier to its catalogue. Pondering the idea, he felt that the 8-10W offered by the average 300B triode tube design just wasn't sufficient to do justice to the kind of music he listened to through the loudspeakers he prefers. So he set about designing an amp up to the job.

The result – the compact and stylish Stereo ST30SE – boasts a claimed power output of 28W per channel in Ultralinear mode, and 18W per channel in Triode]. According to the user manual the KT150s themselves are fed by 6SN7 driver valves, though our review sample came fitted with CV181 equivalents bearing David Shaw's name, no less.

Yet while the Stereo ST30SE may appear a straightforward design, albeit one enjoying a healthy dollop of extra power, it's certainly not one stripped of features. There are three line-level inputs, designated CD, Aux and Tuner, plus a tape input with matching record output. The unit is a fixed-bias design and the settings for each output valve can be tweaked via the central front panel meter, with a switch to select which valve is to be checked, and two small trimpots on the top panel. This meter shows signal level when not used for bias purposes but I found it to be rather insensitive in this role, barely making it above the '20' mark most of the time.

Sensitivity Switch
As mentioned, the Stereo ST30SE can be operated in Ultralinear or lower-powered Triode mode. The user manual suggests that most listeners may well prefer the Triode mode, but cautions that the amp may run out of power on this setting while reminding users to resist the urge to change mode on the fly. Specifically, the amplifier should always be set to standby first in order to avoid stressing the output transformers.

If that isn't enough to play with, then more fun is to be found at the rear of the unit, in the form of a three-position sensitivity switch. Select 'H' (High) and the result is that sensitivity is increased with lower feedback applied. Choose 'L' (Low) and the amp uses higher feedback and thus offers lower gain. The 'L' position is recommended for most modern line-level sources where the full range of Icon Audio's volume control is more likely to come into play. Finally, there is a central '0' position where no feedback is applied – this is designated for test purposes only.

Internal construction of the unit is excellent and it is blessed with a fine selection of high-quality components. These include an ALPS volume pot, Rubicon and Nichicon electrolytic capacitors, and SCR audiophile polypropylene audio capacitors. The loudspeaker and input terminals are all gold-plated and the valve bases are high-quality ceramic items.