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Yamaha PC1002 and PC2002M Amplifier


Yamaha PC1002.


Aside from Fostex, (see review in December HSR), Yamaha is the sole Japanese manufacturer with a stake in the UK professional power amp market. At first sight the PC1002 has average output power capabilities - a nominal 100 watts per side into 8 ohms, or 150 watts into 4 ohms. But then there's also power bridging, brought into operation by a switch on the rear panel, to give 300 watts into 8 ohms or 200 watts into 16 ohms, in mono only, of course. Because these amplifiers are principally aimed at the USA market, where the strict rules of the FTC (Federal Trade Commission) now govern the description of amplifiers' output capabilities, the power figures given are conservative by our standards, and considerably higher output power is to be expected in normal (musical) applications.

PC1002



The 1002 boasts a recessed 'alternate' switch - you depress once to switch on, and depress a second time to switch off. Of course, for studio applications, this arrangement serves no vital purpose, but it does symbolise good, detailed design, for the value of these mechanical security features will be clear if you've ever had to squeeze past a stack of power amplifiers on a cramped stage, holding your breath lest you wrench out a vital lead or knock against the vulnerable power switches found on many other amps.

There are 5 indicator LEDs, one being for 'power on', of course. The others announce (i) excessive heatsink temperatures (ii) operation of the protection circuitry and (iii, iv) clipping on either channel. Signal level metering with a clip (or 'error') LED may seem rudimentary, but it does answer the quintessential question, "Is the amplifier being driven within its limits?", and for the back-end of the monitoring system, this is all that truly matters. The input attenuators are of the genuine stepped genre, and with 0.2dB matching between channels, so you won't have to reset the channel balance at the lower settings, as with lesser pots. The knobs travel over 22 clicks, from 'off', then in handy 1dB steps, from -20dB up to zero level. And for those who have to guard against interfering fingers, the knobs, once set, can be covered with push-on rubber caps, which come with the amp - a thoughtful inclusion.

Moving aft, recessed (again) and nicely stiff slide switches give a choice of balanced or unbalanced inputs, but with one snag: the XLRs are only accessible (ie. connected) in the balanced mode, and the jacks solely in the unbalanced position. The latter is obvious, but many XLR users will have unbalanced feeds at this point, which at first sight means rewiring with jack plugs. However, don't forget that it's perfectly acceptable to drive an unbalanced source into a balanced input: using 2 core screened cable wired pin-to-pin, simply disconnect and isolate the screen from pin 1 of the source end. Then link pins 1 and 3, again at the source end of the lead, so that the unbalanced ground goes 'up' the cold (pin 3) input at the amplifier end. With this minor rewire, you can then link the PC1002's XLR inputs to a normal unbalanced source, with the slide switch in the balanced position of course.

Another switch isolates pin 1 on the XLRs to avoid possible ground loops via cable screens and chassis, although in a properly balanced system, or with the unbalanced to balanced arrangement described above, this shouldn't happen. In this particular amplifier, it appears to be necessary because the unisolated, metal jack sockets are making a direct link between the chassis and signal ground, ie. pin 1 on the XLRs. This will very likely result in a loud buzz when we come to to connect the chassis to the mains earth, or mount the unit in a rack with other equipment - more on this shortly. As for evaluating the effects of lifting pin 1, do this initially with the amp switched to -20dB, just in case something very loud happens, otherwise simply set the switch for minimum hum.

The XLRs are precision replicas of Cannon's AXR series, and the jacks too, despite their cheap appearance, are of high quality, making a tight, positive fit with a selection of jack leads.

PC1002 rear panel sockets.


Bridge-Mode Operation


Looking now at the outputs, a third slide switch function selects stereo or bridge-mode operation. The latter is different to conventional mono switching, where one input only feeds to both channels. Instead, the channel A input signal is fed in antiphase to each amplifier channel, and the speaker is connected across each output's hot (+) terminal, the ground (-) terminals being left unused. With each channel working from a single input signal in antiphase (a push-pull motion), the voltage swing is doubled, which in theory means the output power is quadrupled. However, this assumes adequate current ratings and heatsink capacity. In other words, to make use of the extra power, the cost of the components in the amplifier would have to be raised. So in common with other manufacturers, Yamaha have settled for limiting the extra power to a (nevertheless useful) two-fold increase. This is achieved by setting 8 ohms as the minimum nominal speaker impedance in the bridge-mode; the amp will protect itself at lower impedances, whilst extra power is available into high impedance speakers. In fact, very few amplifiers are capable of driving over 100 watts into a 15 ohm speaker without this power bridging trick.

The rear panel design, which is otherwise well appointed with useful facilities, is spoilt by the output terminals, which accept bared wires only - no XLRs and no bunch pin plugs permitted. Again, this clearly relates to the American market; the voltage across any high power amplifier's outputs is unlikely to kill, but it can certainly shock you - especially when a 16kHz tone propagates across sweaty flesh - and with it being relatively easy to sue companies for negligence in the USA, decently insulated terminals are a must.

For a good, reliable connection with screw terminals, wires must be tightly clamped to provide a gas-tight seal, but this isn't readily attained on the PC1002, as the terminals are both uncomfortable and too close together for a firm grip. The over-proximity arises because Yamaha have placed the broader-than-usual terminals at the customary 19mm (¾") spacing. This standard allows tandem bunch-pin plugs to be used, but has no relevance to screw terminals; there would be no harm and much good in adopting a wider (say 1") pitch.

A neat finishing touch on the rear panel comprises sturdy pillars on each side; these protect rear panel components if the amp isn't rack-mounted, and also make the amp stable if stood on its back.

Circuitry


The interior is inevitably complex, with error detection (which, incidentally, when triggered, opens relay contacts in series with the outputs), balanced input processing, and a DC servo (in place of conventional AC coupling using capacitors) all of which call for additional circuitry and subsidiary power supplies. Thus there's a total of four major PCBs. Of these, two concern auxiliary functions (as opposed to the actual power amplification), and it's good to see that latching IDCs (Insulation Displacement Connectors) are used here, to make servicing quick and simple. At the same time, cigarette smoke, aerosols, indeed, a whole range of exotic airborne chemicals exclusive to recording studios will build up a non-conducting film on the best connectors. This is no cause for immediate alarm, providing the contact's initial gastight seal isn't disturbed. However, you must clean connectors of this type, if they're ever disconnected, after deposits have had time to build up.

One switchable function lurks under the cover - a 220/240V switch held in position with a locking plate. You shouldn't need to adjust this often - it's really just a luxury, and presumably exists so that the quoted output power will be available on continental 220V supplies, eg. in France. In any event, the amp seemed happy with a 245V input whilst switched to 220V, and although we obviously don't recommend this, it's reassuring to know that there's little chance of anything drastic happening if the switch is misused. In fact, the only kamikazee aspect here is that the switch is adjacent to bare mains terminals, so please switch off before adjusting.

Results


As noted in the introduction, the power capability into a test load proved to be in excess of specification, with due care being taken to see that measurements were made against a steady 240V mains input. For instance, in the bridge mode, power is slightly over 400 watts into 8 ohms, whereas the specification indicates 300 watts. This difference won't be especially audible as extra volume (SPL) but it adds a useful safety margin in respect of clipping.

As noted in previous HSR reviews, input sensitivity (ie. the input voltage just prior to overload) depends upon the load impedance. The vast majority of power amplifiers, the PC1002 included, have unstabilised power supplies, and when loaded heavily, the supply rails droop, leading to premature clipping and/or apparently higher 'sensitivity'. In other words, less input signal is required to clip, as clipping is happening at a lower voltage, when driving into say a 4 ohm load contrasted against 8 ohms. So, for the PC1002, the nominal - and very sensible - 0dBu sensitivity (775mV) specification is correct with an 8 ohm load on a continuous test signal, whilst on a 15 ohm load, and in the bridge-mode, sensitivity 'falls' to + 1dBu, or 870mV, which reflects the light loading conditions. None of these variations should present any problem, for although they make a mockery of accurately calibrated LED metering on the console, the PC1002 clip LEDs (unlike many) were found to accurately indicate the onset of waveform clipping, regardless of load impedance, so you can rely on them absolutely. Frequency response is slightly down (-½dB) at 20kHz, and -3dB at 55kHz (although the spec quotes -1dB at 50kHz). This is good practice for PA equipment, but for studio monitoring, a slightly higher (say 80kHz) -3dB roll-off would be preferable.

PC1002 Conclusions


As a 150W into 4 ohm stereo amp, amongst dozens of competing models, the PC1002's power is expensive, at around £1.70 per watt (contrast 50p to £1/watt for many high power amps), and even in the bridge mode, given an effective 400 watts into 16 ohms, the price remains above £1 per watt.

Along the same tack, volumetric 'efficiency' is low - the PC1002 is undoubtedly built to impress sound engineers in Texas, and the average British sound engineer isn't so easily fooled by overlarge enclosures (not after Marshall amps at any rate!), and though rack space isn't yet a crucial factor in many studios, you should think carefully about how easily space is swallowed up by equipment. Size and weight are also a major consideration if you aim to use your studio gear for gigging. The PC1002 employs obsolete transformer technology, making it far heavier than it need be, but this aside, the amp is eminently suitable for road use. Indeed, Yamaha is the sole Japanese manufacturer with any credibility in PA business, and they've also demonstrated a commitment over several years to serious users. Thus the PC1002 and the backup that goes with it won't be as ephemeral as certain brands. Anyone remember Aurex?

Continuing on a positive note, the quality of assembly meets impeccable Japanese standards, although the capacitor screws weren't tightened properly! There are also many tasty design features, and a few flaws - such as the earthing. The review sample came with a 2 core mains cable, but the amp isn't double insulated, so the amp must be fitted with a 3 core cable to be legal, decent and safe. This involves connecting the chassis to the mains earth, of course, with a ground loop buzz as the immediate consequence of this well intentioned move; at least if you're using the jack inputs.

So, pending factory modification by Yamaha, you should ensure that your dealer (i) fits a 3 core cable, (ii) connects the chassis to mains earth, and (iii) isolates the jack sockets and lifts all other signal earth connections from the chassis.

Yamaha PC2002M.


The PC2002M



The PC2002M is the higher powered sister of the 1002, the 'M' suffix indicating optional power meters. The principal difference over the 1002 is one of power and rated load impedance - the 2002 is made specifically to drive high power into 8 and 16 ohm loads, and isn't intended for lower impedance speakers, say below 6 ohms. In the stereo mode, a 16 ohm speaker can be driven at 480 watts. The specification also quotes a nominal 700 watts into 8 ohms, and the amp was quite happy doing precisely this over a long period, and despite a correction slip in the Yamaha manual which implies that the bridge-mode is suited only to speakers of 16 ohms and above!

Other specifications are substantially identical, as too are the facilities (and earthing anomalies), but there are physical differences. First, the 2002 is larger, taking up 4U of rack space (7"), or slightly more if you retain the rubber feet. Yet despite the size, the weight isn't excessive, for the 2002 is fitted with a toroidal transformer which more than halves the mass of steel and copper that a laminated transformer (à la PC1002) would entail. And, although the interior is tightly packed, access for servicing is excellent. For instance, the side panels, consisting of heatsinking and the main amplifier boards, can be unscrewed and then folded outwards, whilst subsidiary PCBs can be unplugged.

PC2002M rear panel connectors.

The 2002M comes with accurate peak-reading meters, and this is the version supplied for the review. The dB scaling has its zero level (0dB) pitched at 100 watts into 8 ohms, which means that 240 watts comes out at around a convenient +4dB, but to be comprehensible, you still have to tie these levels in with dBu. With the attenuators set to -4dB, a +4dBu output from the console will drive the amplifier at a nominal 240 watts, so the dB scale on the meters will then relate directly to input level in dBu.

Initially, these power meters will provide lots of fun for those of us who've had to forsake Japanese Hi-Fi gimmicks for the sake of affording essential musical and pro-audio gear, and they'll also provide an excellent demonstration of how power relates to loudness and speaker efficiency. On efficient (say 105dB/1W/1m) monitors, it's easy to calculate that 15mW (0.015 watts) will provide enough SPL to drown out conversation, but this fact is all the more convincing when displayed on Yamaha's meters, which exhibit true peak-reading ballistics, unlike gimmicky Hi-Fi power meters.

However, once you've seen a few programme dynamics, these meters frankly aren't a lot of use, although they are undeniably attractive: power meters, however accurate their ballistics, cannot compensate for variations in the overload point and power output - for instance, you're stuck with mentally rescaling them in the bridge mode, where 700 watts into 16 ohms occurs somewhere around the 240 watt mark, and here there's no way of being sure that the amp is driven within its limits without underdriving to leave a wide safety margin. Thankfully, Yamaha have retained the vital clip LEDs which are really all you should need.

Summary



On the topic of value, and in the stereo mode, the 2002 is a better buy than the 1002, at around £1.50 per watt of power, and when driven in the bridgemode into 8 ohms, the price-per-watt drops to a very reasonable £1. Sadly, you will then need to buy two amps simply for a basic stereo set-up.

Summing up, it's ironic that Japanese amplifiers should be expensive compared to the British response, but the high price is a reflection of an altogether higher level of amplifier - at least insofar as component quality, design, and facilities are concerned.

Retail price of the PC2002M is £759 (inc VAT) and £516 (inc VAT) for the PC1002.

Further details from Yamaha, (Contact Details).



Previous Article in this issue

Using Microphones

Next article in this issue

DOD R830A Stereo Graphic Equaliser


Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.

 

Home & Studio Recording - Feb 1984

Donated & scanned by: Mike Gorman

Gear in this article:

Amplifier > Yamaha > PC1002

Amplifier > Yamaha > PC2002M

Review by Ben Duncan

Previous article in this issue:

> Using Microphones

Next article in this issue:

> DOD R830A Stereo Graphic Equ...


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