Powertran PSI 4002 Amp
George Chkiantz assembles his very own Powertran PSI 4002 amp. Now it's your turn.
The International Powerslave amplifier kit incorporates several novel features at a most reasonable price and comes in two versions.
The one I built, the 4002 'Studio' version, requires 0.775 V for full output and is graced with two output level meters which are calibrated to tell you what output power you would be delivering into a 4 ohm resistive load, other front panel controls being separate mains on/off switches for each channel, input level controls and input connectors. Output connectors (two paralleled mono jacks per channel) various fuses and a neat but brittle Bulgin mains socket are to be found on the back panel.
On the 4001 the level meters are replaced by an engraving of a lady of dubious virtue dragging the remnants of her chains (presumably fleeing from a rampant slew rate). This version also has the inputs on the front panel, mains switches, and in addition to the level controls a switch labelled 'overdrive' which increases the input sensitivity by 10 (optionally 20) dB. The input buffer which provides this facility also gives this version a higher input impedance. Both versions receive their inputs via mono jacks from an unbalanced line.
As I am neither enamoured of the sound of current bipolar amplifiers, nor enthused by the technical or commercial motives for their shortcomings, but I found that the features of this design, such as the use of long-tailed pairs in the input stages which allow greater linearity and input range with reduced distortion, fast output stages - 4 x 250 watt 2 MHz devices are used per channel - and low overall feedback (app 14 dB), bring the advantages of up-to-date thought at a price you can afford.
Having now built and used the amplifier for an extended period in various locations, I can recommend it for any studio or PA application. In fact it would not be out of place in any enthusiast's home, although the output capability might be excessive for this application. I would like to see a lower-power version offered for this market. As will be seen, construction presented only minor problems which, as they are of a mechanical rather than a technical nature, make this a satisfying and worthwhile project for anyone with a modicum of patience who is reasonably competent with a soldering iron.
The kit arrives in a large, well-padded box which opens to reveal the metalwork to which heavy components like the mains transformers are temporarily mounted. The rest of the kit is packed in some ten or so packs, which divide up the components into the various sections they are associated with. This is an excellent idea as it makes checking and building much easier and less liable to cause confusion, although I still fail to understand why the same value resistors, which must have been sorted originally, are carefully mixed up before they get to you; no doubt it's something to do with the fact that the entropy of the universe is increasing!
Kit manuals seem to vary from the types in which every detail of construction is elaborated to such a degree that no variation is possible (making assembly a tedious task), to those in which a creased schematic diagram and a poorly photostated list of parts is all that stands between intrepid constructors and their fate. Considerable thought has obviously been given to this offering however, as is evidenced by the most explicit rundown on soldering and the excellent details and hints on how to get a really professional-looking printed circuit board made; for example cleaning the excess resin from the board once it has been completed, something that only too many manufacturers skimp, but which also makes checking for dry joints, solder runs and the like much easier. The instructions otherwise are sufficiently detailed that even the relatively inexperienced should have no difficulty, while scope is left for individuality by those who are competent. I would, however, have liked to see the check list spread over more space so that components found could be ticked off without obliterating all the neighbouring values and, as the wiring runs have to be worked out from these, clearer photographs and more diagrams.
On checking the kit against the components list I found that every part was there, even including those necessary to set up and check the completed amplifier, but my euphoria was extinguished when I discovered that two 3-way terminal blocks had neither been supplied nor were on the check list. The only other shortfalls were that the bolts supplied (to hold on the rubber feet) were not quite long enough and that none were supplied for the fuse holders. Powertran will I am sure have corrected this by now; at all events the missing parts were promptly supplied.
To assemble the metalwork you will need a large Pozidriv screwdriver (for M4 self-tapping screws) in first class condition, to quote the manual, as these 'require a lot of pressure the first time they are fitted'. A stout vice and a helping hand from the Incredible Hulk would not go amiss either! It is essential to do this task before mounting any components as it will be far more difficult later on. Otherwise mechanical assembly was straightforward. The amplifier boards are mounted on L-shaped brackets which form part of the heat sinking for the output transistors and these in turn are bolted to the generously proportioned heat sinks which form the major part of the back of the amplifier. The holes which mount these L brackets are in two rows of four, the bottom row being so close to the angle of the bracket that once the amplifier is complete they are obscured by the output transistors and the cooling plates attached to the driver stages. If these had been re-drilled higher up so that they were completely clear of the output transistors when these are mounted, and the cooling plates on the driver stages were not mounted until these boards are in position, both the initial installation of these boards and their subsequent removal in the event of any failure would be much easier. This would also avoid covering everything with silicon grease (what dissolves it?). Then, sticking the temperature correcting diodes could be readily done with the board mounted. I obeyed the manual in every detail and fitting the 16 screws took me well over an hour, while the output transistors only narrowly escaped damage from the edge of my screwdriver.
Electrically the amplifiers proper are on one large board (each); the power regulators are on separate boards while the power transformers, rectifiers and smoothing capacitors, fuses etc are chassis-mounted and hard wired.
The unit is in effect two totally independent mono amplifiers in the same box so there are two of everything. Depending on the model there is also a small board for the buffer amplifier (4001) or the metering circuit (4002), this last being mounted on the back of its respective meter. All these PCBs are of high quality glass fibre and only lack the luxury of being silk screened. However, the layout is very easy to follow and all the electrolytics and diodes face the same way making construction and checking easy.
Protection of the loudspeakers (in the event of catastrophic failure of the unit) is provided by in-line fuses, which I hope will prove adequate although I didn't test it myself. Connections to the boards are via pins and, in the case of the regulator boards, two tags on the power transistors. All components are of high quality and clearly coded, and the hole spacing in the PCBs is such that no contortions of leads are necessary to fit any component to the board. The amount of power being handled dictates a construction using a lot of thick wire for interconnections; as many of the connecting pins - notably the centre earth pin on the regulator board, the tags on the 50V regulator outputs and the connections on the main switch — are awkward to reach when in situ, it would be easier to prewire these with the requisite length of wire wherever possible before mounting them. This would avoid the risk of the insulation running back along the wire while soldering, or a tear of solder causing a potentially disastrous short circuit, making it easier to avoid dry joints, and helping to save innocent components from death by singeing.
The manual gives a stage-by-stage check when the kit is almost completed, with a timely reminder not to attempt to discharge large value smoothing capacitors with a screwdriver (they recharge anyway if you do). You will probably lose a screwdriver and gain a healthy respect for the amount of charge that can be contained in 0.01F. The setup of bias and offset voltages are easier to perform if you solder the lamp-resistor combination (supplied) to a dead 20 mm fuse (not supplied) to avoid wrecking your beautiful soldering to the fuse holders. I found that thermal equilibrium was not reached for nearly two hours initially, but subsequent tests indicated that there was no need to change the adjustment.
During this procedure the only problem experienced occurred. This was that while one amplifier checked out perfectly, the other would not reach a stable operating point. Hooking a scope on to the output disclosed that this side was apparently convinced that it was working for Auntie on an irregular basis. I searched everywhere for signs of a dud joint, misplaced component and other nameless horrors likely to cause the RF instability, found nothing and was beginning to give way to despair when the fault came to light. The BD419 driver transistors are both attached by a metal screw to the same cooling plate and it so happened that one of these had got so smeared with silicon grease that the screw was no longer making electrical contact between the transistor tab and the cooling plate. I cleaned up the screw, mainly because I thought it looked a bit messy, and found that I had unwittingly solved a very elusive problem. After this all went like a dream.
Right. Now it's built, what is it like? I found that the musical quality of the amplifier was astounding, proving that the design criteria (chosen for reasons that some may consider dubious) are right and that these are, in my opinion, certainly the shape of things to come. It was immediately apparent that the spec was being met with great ease although I do not possess test equipment - other than ears - accurate enough to check the claims. The amplifier was tested in a variety of conditions ranging from a professional recording studio where Tannoy loudspeakers - not the most easy units to drive successfully — were the load, to what remains of a dedicated Hi-Fi fanatic's home, where the speakers used were made by Mission Electronics. People present during these tests were heard to mutter words rather like 'Wow!', 'What a clear, warm sound' and 'How much did you say it cost...?', 'I wonder if I could persuade my bank manager to...', and other comments in the same vein.
In the studio the amp was given a thorough pacing and I found that it produced a sound which was exceptionally clean and easy to listen to. It was equally at home at high and low levels, and was accurate as a monitor amp with regard to perspective in a way that has so far been the prerogative of only the best of valve (sorry, vacuum state) amplifiers. The response in the extreme top end of the spectrum was a new experience altogether; I have not yet heard anything to compare with it, regardless of price or pedigree. This showed up the excessive top equalisation (and attendant distortion) on many master tapes which had been unwittingly added by engineers who, I suspect, simply couldn't hear what they were doing. As I was one of them I can speak with certainty on the matter! One could have mistrusted the amplifier except that the natural quality of the sound was such as to inspire an enduring confidence, and of course, the studio was the ideal place to check that this was indeed so. Material listened to covered a range from direct cut discs and master tapes - and of course the occasional live mic! — right down to cassettes and FM radio.
The variety of sounds used were calculated to test the amplifier as severely as possible, and the listeners were being as critical as possible, yet apart from one minor shortcoming mentioned below, I cannot report any faults. Pop music sounded earthy, natural and full of guts, although tending to suffer from the equalisation problem mentioned above. Classical music was exciting; the clarity of the various instrumental lines was obtained without sacrificing the total blend, whereas choral music — notably some treble voice choirs which are notoriously difficult to reproduce at all — were an aural experience not easily forgotten. A recording of King's College Choir showed that the depth perspective of the amplifiers (handling such things as low-level reverb after a burst of sound) were beyond reproach.
Finally, to really test the intermodulation distortion, we tried a record of Benjamin Britten's Antiphon, where a very deep organ continuo bass note, with soft but insistent strings and pulses of treble choirs in close harmony must form the most powerful total distortion test known to man. This was reproduced with absolute clarity, which astounded us because the recording had sounded so bad previously that we'd always assumed it to have been at fault. Subsonic frequencies (from a recording of whales at normal speed) showed that the amplifier wasn't particularly bothered by this test of its stability and only served to show that the speakers we were using had rather more range than had previously been suspected. Subsequently the person with whom I had conducted these tests phoned me up to say that he had gone through some of them again with his amplifier and that he now considered me responsible for his next overdraft — praise indeed! All this confirms my opinion that this type of circuit (with low overall feedback) could become the standard quite shortly. Perhaps Powertran have a latter-day Williamson on their hands.
Now to the matter of the one little shortcoming mentioned above. There was, in the test amplifier, a certain amount of spikey buzz at very low level, but enough to be annoying. This obviously emanated from the power supply, and I was rather surprised to notice that no 'spike catching' capacitors were used across the rectifier diodes, as this effective deterrent is in quite common use even in cheap domestic equipment.
Other points are that all the internal wiring — and there is quite a bit of it - is self-supporting: no ties or other provisions for securing this are provided. In an amp intended for PA use, where the handling it receives will probably be far from gentle, this cannot be considered satisfactory. Also, while I can just about accept the use of mono jacks as connectors on the PA version, this would scarcely be considered adequate in any serious studio. I feel very strongly that the 4002 should be issued with a different back panel cut to receive XLR speaker and mains connectors, possibly with the addition of heavy wiring posts for those who eschew connectors at such points. The back plate is too thick for normal chassis punches to cope with (so you can't easily fit XLRs yourself) and if this were done I think the amplifier would gain considerable credence in a field in which it undoubtedly deserves to be. Obviously, as it is provided in kit form, individual requirements can be easily included. And then, what about balanced XLR input connections? This is also almost universal studio practice and should be catered for.
In conclusion, I am very impressed by the performance of the Powertran 4002 which I built (I assume that the same results would have been obtained from the 4001) and I think that it is excellent value for money. You don't 'have to believe' in this monitor amp; you can afford to try it out and I am sure you will find it exceptional. The kit is easy to build — and would be even easier if the points raised (which are after all only minor irritations) are looked into - and I am sure that this amp will soon become quite a common sight in all those places where a good sound is important.
George Chkiantz is a freelance sound recording engineer and producer with several famous albums to his credit. One of his major interests is the development of amplifiers for studio monitoring applications, and the problems of eliminating subtle forms of distortion in audio systems.
Review by George Chkiantz
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