Product Review -
Hart Audio Kits: Phono Preamplifier K1450 - April, 1995
By Richard Black
CD might have replaced LP almost entirely as a medium for new issues, but that doesn't mean that LPs are no longer worth playing. Indeed, there are still plenty of turntables available, ditto cartridges and arms, and the last few years have seen the emergence of a new hi-fi item: the stand-alone phono preamplifier. Intended either to supplement line-level-only preamps and integrated amps, or to upgrade phono stages already fitted inside amplifiers, this type of device is becoming quite popular.
Also currently increasing market share in leaps and bounds is kit hi-fi, fueled perhaps by the interest in "tweaking" that has been a feature of the last decade or so. Thus, it is no surprise that a few firms have brought out kit phono amplifiers. One such is Hart Audio Kits, a UK-based supplier of kits with over 30 years' experience, boasting in its catalogue several designs by John Linsley Hood, one of the most respected figures in UK audio design.
Hart's K1450 is a fully discrete transistor-based phono amplifier suitable for use with moving magnet and moving coil type cartridges, the latter with or without step-up transformer. Hart is keen to point out that the design is based on good solid electronics engineering first and foremost, although naturally high quality components (fibreglass PCBs, metal film resistors, low loss capacitors, etc.) are used throughout. Built on a single board (excluding the power supply, which is a separate, external unit), the amplifier in its smart black aluminum case measures 9 by 6 by 2.5 inches. A switch selects moving coil or moving magnet gain, while two pairs of sockets allow connection AC or DC coupled. The circuit uses the "shunt feedback" principle of RIAA equalization to give low distortion and accurate frequency response, with simple but elegant amplifying stages similar in concept to those used inside integrated circuit op amps. In common with some other manufacturers, Hart has chosen to use power transistors at the input stage to give low noise with very low source impedances, such as moving coil cartridges. This slightly compromises moving magnet noise performance, but some compromise in this area is almost unavoidable in any design.
Hart's instructions for assembling the kit are excellent, and the components are all supplied, even down to a small reel of solder. Component positions are marked on the boards, and all connectors are also boardmount to avoid errors. A multimeter comes in handy to check resistor values before installing them on the board (the paint bands on modern resistors are so thin that they can easily be misread), and purchasers with little soldering experience are strongly advised to practice, but aside from that, it is nearly impossible to go wrong. Anyone at all used to kit assembly should get the whole thing together in an evening.
The standard kit does not include a power supply; Hart can supply one as an optional extra, or the user could build one from parts or other available kits; all that's needed is a pair of +/- 15V rails at about 30mA. For review, a laboratory power supply of similar performance to Hart's design was used.
For our tests, the completed amplifier was used with a Highphonic MC-A3 cartridge (very low output moving coil) and a Shure VST-V (moving magnet), both mounted in a Pink Triangle/SME turntable, together with ATC loudspeakers and EAR-based, semi- active amplification. The K1450 quickly established itself as a very revealing amplifier with a detailed yet unforced sound. It coped effortlessly with a wide range of musical material, from string quartet to opera to rock, without losing control or becoming strident. Of particular note was its bass quality which was extended and tight, qualities not always achieved by phono amps. This allowed listeners to hear exactly what was going on in orchestral passages with lots of deep string bass plus percussion, for instance.
In the treble, the sound was possibly slightly less "open" than in some esoteric preamplifiers, although such openness can be a deceptive trait, sometimes merely a fortuitous side-effect of slight distortion. There was no question of distortion from this amp, which remained clean and uncolored throughout the heaviest LP cuts. However, I had a feeling that just a touch more detail could be wrung from the top end of well-cut LPs. Bearing in mind the price of the K1450, this is a very mild criticism. The only other problem was very slight radio pickup (very common at the test site in an inner city), which was completely cured with a couple of ferrite sleeves.
The two cartridges used were chosen partly because they sound quite similar given good amplification, and it was interesting that the Highphonic fared rather better in terms of both bass control and treble clarity. The Shure seemed distinctly more shut-in in the upper reaches of human voice, for instance. Both cartridges preferred the DC- coupled input; obviously the small DC offset current present (under 5 microamps) was insufficient to cause any serious problems, and this connection gave a small advantage in low-frequency detail.
A quick check in the lab showed that there is nothing untoward in the K1450's measured performance. Distortion is barely measurable, frequency response is dead flat to above audibility (rolled off in the low bass to give -3dB at 13Hz, a sensible figure for warp rejection), and noise is particularly low with moving coil cartridges. Of course, one of the attractions of kit hi-fi is that with the circuit diagram and layout to hand, the intrepid builder can adjust factors like low-frequency roll-off to taste, and, if necessary, varying circuit gain to cope with extremely low output cartridges.
If you have never tried kit hi-fi before and have an LP collection you care about, this kit is an excellent place to start. It is easy to build and capable of high performance, and it looks very smart too! Even if you are an experienced home constructor, it is a satisfying project and one quite amenable to after-build tweaking if you wish (obviously Hart will be unable to support such tweaking, although they do offer help to those following the instructions). It is available from Hart Audio Kits, Penylan Mill, Oswestry, Shropshire, SY10 9AF, UK, phone (+44) 1691 652894, fax 662864, price 112 UK pounds ("audiophile component" version 133 pounds).
© Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
Return to Table of Contents for this Issue.