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HIFIREVIEW Amcron PSL-2 preamplifier & PS-400 stereo power amplifier Amcron is a name which is usually associated with professional sound reinforcement rather than domestic hifi equipment so we were very interested to take a look at these two pieces of gear. The PSL-2 is a stereo control unit with everything for the keen audiophile while the PS-400 is a power amplifier built like a tank but which gives a very fine performance. By LEO SIMPSON With such a strong background in the professional sound reinforcement field you can bet on Amcron to produce domestic hifi equipment which is just about bullet proof. The amplifier in particular is built like a battleship. For its part, the stereo control unit offers lots of user facilities and will have plenty of appeal to those who like to dub material to cassettes for subsequent listening on a W alkman or in a car. The Crown PSL-2 is housed in a 34 SILICON CHIP standard one-unit high rack mounting case which means that it measures 483mm wide and 44.5mm high. It is also 333mm deep, including the knobs and rear projections, and it weighs 5kg. Finish of the case is matt black enamel while the knobs and pushbuttons are in natural matt aluminium. Since the front panel is only 44.5mm high, it looks quite busy with 7 knobs, 6 pushbuttons, 7 LED indicators and the headphone jack socket. Tape enthusiasts will be interested in the PSL-2 as it offers full dubbing facilities, from two tape decks. Purists, on the other hand, will favourably note that the bass and treble control stages can be deactivated. A tiny pushbutton between the treble and bass controls is used to "activate" these controls and when you do so an adjacent LED lights. An interesting feature is the use of two red LEDs to indicate overload of the control unit's output stages in each channel. Associated with these is a pair of green LEDs which act as "signal presence indicators". These are particularly handy if you often change your listening set-up swapping speakers and other gear. The LEDs tell you that signal is present even though a bad connection on your part may prevent any sound from coming from the speakers. Another LED tells you whether there is rumble on the record, giving you the option of switching in the low frequency filter. This is a 'fair dinkum' filter too, giving a steep rolloff for frequencies below 33Hz at the rate of lBdB/octave. This is desirable if you have infinite baffle (ie, sealed box) loudspeakers. If these are fed large signals below their low frequency cut-off they can be badly overloaded, particularly if the bass and loudness controls are being used. We liked the mode control, a feature which is often not included on today's stereo equipment, particularly stereo receivers. The control gives stereo and mono modes as well as a "reverse" position - handy if your speakers have been inadvertently swapped or, more rarely, if the channels on the record require reversing. The mute control also provides a mute position . . .:. . handy if you don't want to alter control settings while you answer the phone. Interior details Naturally, we had a look inside the control unit (curiosity always gets the better of us, you see) and found that all the circuitry is housed on one vast printed circuit board which takes up virtually all the chassis space. Interestingly, the printed circuit board is double sided (ie, copper pattern on both sides of the board) with the pattern on the component side acting as a ground plane or shield for the multiple input tracks. We also noted the individual earth tracks in between the input tracks - these have the effect of reducing crosstalk between channels. Close tolerance capacitors and resistors have been used in the preamp equalisation circuits. This is necessary if the RIAA curve is to be closely adhered to. A further refinement is the provision of separate preset pots for precise adjustment of the phono The PS-400 power amplifier is built like a battleship and really delivers the goods. The power transformer would have to be rated for at least lkW while the filter capacitors are computer grade 13,500µF units. preamp output levels - this is a nice touch. There are quite a few unusual features to the internal circuitry of the Amcron PSL-2 control unit, apart from those we have already mentioned such as the overload, rumble and signal presence indicators. The RIAA preamplifier stages are most unusual and consist of a pair of transistors, one NPN and one PNP, driving an op amp. This is quite different from the common arrangement of a low noise differential pair of transistors driving an op amp. The balance control is unusual too. It precedes the high level signal circuitry instead of being in the more usual position in the output stages. The volume control is a 4-gang unit and operates in the feedback loops of two op amps in each channel. All of this goes towards producing a very quiet control unit, at any gain setting. Another very worthwhile feature is the inclusion of signal muting circuitry. A relay provides muting of both channels at both switch-on and switch-off to avoid any thumps from the loudspeakers. This is most important for a control unit used to drive a powerful amplifier. At the rear of the control unit are quite a few pairs of RCA sockets for the various input and output connections, including a ''processor loop". This is handy if you want to use a graphic equaliser or some other signal processor device. We are not keen on the provision of 2-pin AC sockets. It is too easy to get an electric shock when plugging into or removing the small plugs from these outlets. We'd prefer to see these sockets blanked off. PS-400 Now let's have a look at the PS-400 power amplifier. If the PSL-2 control unit is impressive For the hifi enthusiast, the PSL-2 stereo control unit features comprehensive facilities. Its specifications are excellent and it delivers impressive performance when teamed with the PS-400 power amplifier. AUGUST 1989 35 The power amplifier boards mount directly on the big multi-finned heatsinks. Each heatsink carries 10 power transistors to give a rated power output of 165 watts per channel into B!J. power mono operation. Oddly enough, there are no RCA sockets for the amplifier inputs. Instead, there are the barrier terminals already mentioned and a pair of 6.5mm jack sockets. This reflects the amplifier's " professional" heritage but means that you need a pair of adaptor leads to connect it to the control unit. The top and bottom panels of the amplifier are made of perforated steel mesh and this same mesh covers the 10 power transistors on each of the large heatsinks. Removing the screws for the top mesh panel reveals a massive transformer that would have to be rated for at least 1 kilowatt output. Remember that this is an amplifier which is rated for 165 watts per channel so a transformer of this size will just be loafing along. The filter capacitors are big boomers computer grade 13,500µF units rated at 70VDC. Everything about this amplifier has the flavour of . heavy duty engineering. It is rugged and built to take the rigours of professional use. Having said that though, it is designed to give a very fine audio performance. In fact, it is one of the quietest audio amplifiers we have ever measured. Power ratings This is the rear view of the brute. As well as operating in stereo mode, the two channels can also be bridged for high-power mono operation. because of the amount of careful thought that has obviously gone into its design and manufacture, the PS-400 is impressive because of its sheer bulk. It is a brute of an amplifier - large and very heavy, and not at all easy to lift because of its lack of any handles and its sharp edges on the front panel, multifinned heatsinks and so on. There's just no getting away from the fact that it's big and heavy. It is in a 4-unit high rack mounting case, measuring 180mm high, 482mm wide and 298mm deep. Weight is 36 SILICON CHIP 24.9kg (55 pounds). On the front panel there is a pair of level controls, two green LED "signal presence" indicators and two red LED overload indicators. At the rear of the amplifier are the large hea tsinks already mentioned, two pairs of large binding post terminals and two sets of barrier terminals. One set is for input connections while the other, normally shrouded under a steel plate, is for loudspeaker connections. These allow both channels of the amplifier to be bridged for high Rated power output for stereo operation is 165 watts per channel into 80 loads over a bandwidth of 1Hz to 20kHz at a rated harmonic distortion of .05 % . Into 40 loads, the rated power output is 265 watts over the same bandwidth and for the same rated distortion. In bridged mono mono mode, the rated power is 330 watts into 160 loads while into 80 loads the power rises to 530 watts. Both figures apply for a signal bandwidth of 1Hz to 20kHz and a rated distortion of .05%. Incidentally, you may wonder about the bandwidth figure of 1Hz to 20kHz. The 1Hz figure is not a mistake. In fact, for a small signal, the amplifier's bandwidth is quoted from DC [no Hertz at all) to lO0kHz ± ldB. How many amplifiers do you see that are rated down to DC? That is why the matching stereo control unit really needs a rumble the PSL-2 control unit and without going into chapter and verse, we can report that it performs as it should, right on spec. Power figures The circuitry of the PSL-2 stereo control unit is all contained on one large double-sided PC board. The input tracks are separated by earth tracks to reduce crosstalk. filter. Otherwise, with this amplifier, really low frequency signals can be delivered to the speakers at quite high power levels. As a safety feature, the Amcron PS-400 has "Low Frequency Interrupt" . This interrupts the output drive signal if a DC voltage of more than ± 10V is present or if there is a low frequency output of more than 10V RMS at 2Hz or below. Which brings us to the question: why do the makers rate the amplifier for a full power response down to 1Hz? The answer is that the Low Frequency Interrupt feature monitors the output every four seconds. In between each sampling period, the amplifier can deliver its full power. However, the amplifier will deliver its full rated power continuously at any frequency above 10Hz. Silence is golden As we noted above, the PS-400 is one of the quietest around and is particularly quiet for an amplifier intended for professional use. The quoted figure is -112dB unweighted for a noise bandwidth of 20Hz to 20kHz, with respect to 165 watts into 80. Well, we don't have the space to quote the manufacturer's specs for the PSL-2 and PS-400 in detail they are the most complete we have seen for any amplifier and preamplifier. So let us detail the measured performance instead, starting with the preamp unit. The key specs are the frequency response, adherence to the RIAA equalisation curve, total harmonic distortion and signal to noise ratios for phono and line level inputs (eg; CD, tuner, tape). Frequency response at the rated output level of 2.5V for the line level inputs was - 0.25dB down at 20Hz and - 0.5dB at lO0kHz - the proverbial "ruler flat". The phono equalisation was within ± 0.25dB from 20Hz to 20kHz which is pretty good. And harmonic distortion for the line level and phono inputs was very low, typically less than .0015% at the rated output level of 2.5V RMS. Signal to noise ratio for the high level inputs is quoted at - 97dB unweighted. We measured it at - 104dB unweighted (20Hz to 20kHz BW) and - 106dB Aweighted. For the phono inputs, the spec is - 87dB unweighted below lOmV at lkHz and - 93dB with Aweighting. We measured it at - 87.5dB unweighted and 91.2dB A-weighted - pretty close, eh what? In fact, these figures are about as good as any we have seen for signal-to-noise ratios. We did quite a few other tests on The results for the power amplifier could actually make pretty boring reading and are just a matter of checking off the specs as you go down them: power in stereo, for 4-ohm and 8-ohm loads, bridged mono operation etc - yep, yep, yep. Typical total harmonic distortion at 265W into 40 at lkHz is .0015% . We also confirmed the signal to noise ratios. The amplifier is even quieter than the spec figure. We measured it at better than - 115dB unweighted with respect to 165W into 80 and better than - 122dB Aweighted under the same condition. That is a very quiet amplifier. Enough of figures; it all translates to excellent quality sound. We hooked the Amcron PSL-2 and PS-400 up together with a CD player and our standard reference speakers for our listening tests. That is one awesome power combo. Whether it is the really extended bass response, the vastly underrated power transformer, the overall hifi performance or whatever, the Amcron combination seems to deliver a lot more punch than a set-up rated at 165 watts really should. It really does have grunt. And it performs to perfection on the pianissimo as well as the fortissimo. It is definitely one of the quietest, if not the quietest, amplifier set-up we have listened to. With that wrap-up, you have to concede that the Amcron is well worth considering if you are in the market for a new stereo system. Buy an Amcron and you won't have to worry if your loudspeakers are efficient or not. Prices are quite reasonable. The Amcron PSL-2 has a recommended retail price of $1299.00 while the PS-400 power amplifier retails for $1999.00. For further information on Amcron equipment, contact Bose Australia Inc, 11 Muriel Avenue, Rydalmere, NSW 2116. Phone (02) 684 1255. i1t:1 AUGUST 1989 37