This P Series amplifier from Yamaha is advertised as a cost-effective variant of the road-worthy PC series. The P2150 occupies 3U of rack space, in exchange for providing 100W stereo into 8Ω, or 150W into 4Ω or in bridge mode, 300W into 8Ω (mono).

The Panel Functions

Grabbing hold of the forward rack handles is painless: no sharp edges and they're wide enough to make carrying this amp for any distance tolerably comfortable. Light weight contributes to this. The mains switch is mounted in a shrouded enclosure at the rear of the box, (presumably to satisfy bizarre US/Scandinavian safety regulations), and is push rod operated. This means even if someone jams the button hard, nothing gets broken, the push rod just bends.

Small but bright, a red LED confirms power on. The centre stage is taken up by the fan's grille, behind which there's a reticulated foam filter. If your environment is dusty, or more likely, filled with smoke particles, this will need to be removed and washed in detergent about once a year. You'll also need an Allen key to remove the grille.

The input attenuators are standard rotary pots, but detented. Nonetheless, sensible 1 dB steps are available all the way down to -20dB. The final ⅓rd of the scale then encompasses -20 down to -60dB, where the increments broaden out to 10dB. This finesse is excellent for a 'cost effective' amplifier.

Illuminations are threefold. To begin with, there are Signal and Clip LEDs above each channel's attenuator. Signal is green and is simply meant to confirm that a signal is coming out of the amplifier. When troubleshooting, and with practice, the LED's flash rate provides a clue as to the size of signal. For example, at high output levels, the indicator LED will be on most of the time. The 2150's signal LED isn't perfect in this respect, in that it needs 2 volts, equivalent to about 1W to trigger. But with sensitive horn-loaded monitors, this corresponds to a sound level of 104dB SPL+, meaning that they'll not even flicker at more civilised monitoring levels. In fact the LED could do with being sensitive to levels of around 200mV. This is important in so far as the Signal LED is driven from the output, meaning that a short circuit will extinguish it. So an un-lit green LED could mean one of a dozen things: which is scarcely helpful.

Other than being red and visible at all angles, the Clip LED is conventional, a simple peak indicator. Third, there's an LED marked Protection. This lights if the series output relay opens. In turn, this can happen for four different reasons. It always occurs for about 6 seconds after switch-on, or if there's a DC fault, namely more than +/- 2 volts at the outputs, or if the heatsink's temperature exceeds 85°, or finally if the load impedance is (or dips) below 2Ω. Overall, this arrangement is beneficial, in that it gives us daily confirmations that the protection relay is operative, with the protect LED coming on briefly each time the amp is powered up.

Facilities

The power bridging switch is confusingly marked Stereo-Mono. This is ambiguous, of course; Stereo means channel 1 and 2 inputs are independent, and exit from channel output 1 & 2 respectively. But Mono isn't exclusive to bridging, although it means just that for the 2150. Mono can also mean that the amp is tandem-linked, or daisy-chained, so both channels operate as independent entities, but are fed from a common input signal.

The output terminals are the same, awkward binding posts, as on the PC2002 (See HSR Feb 84). Just to remind you, these don't accept plugs, but are limited to skimpy wires (up to 1 mm²), pins or spades. Their insulation is also well over the top, making them nearly impossible to grip and tighten satisfactorily by hand. Needless to say, the Kilovolt-rated insulation performs no useful function at the power levels involved, and is invariably defeated in real life by the bare part of spades or the wires themselves. Also, there are no XLR outputs. All this is courtesy of infantile US safety legislation.

Inputs are via XLRs, and balanced, or via a parallelled pair of jack sockets, which can be fed balanced using an 'A' gauge (stereo) plug, or unbalanced, with a standard 2 pole mono jack plug. The spare socket can be used to daisy-chain, out to another amplifier, or across to channel 2.

"In the mid-band, and when driven at high levels, the 2150 wasn't as dynamic and vocals were a shade nasal, but also rougher..."

Below the main fuseholder there's a slide switch for mains voltage trim: 220 volts for Europe, 240 volts for the UK and Australia. The ready accessibility of the slide switch is a good move for touring equipment, but if left (or switched) accidentally to the 220v position when operating on 240v mains, it's possible that long term reliability would be prejudiced. On the plus side, the switch is red, so its position stands out well against the black panel.

Although it's a healthy 3m in length, the mains lead is 2-core only, and not easily replaced. This is bad, because in practice, many professional users will want to fit a 3 core cable, consistent with earth-grounding the rack frame, for safety. Last on the agenda, Yamaha-style plastic standoffs, Cord reel in Nipponese-English, allows the amp to be safely stood on its back, and double as something to wrap the mains cable around. They wouldn't however, withstand much in the way of rough treatment, though the back panel is just as likely to be torn asunder if the amp is dropped.

Interior Design

At first sight, the PC2150 innards represent a fine example of Japanese production: it's neat, and every card is both visible and readily accessible. But we shouldn't lose sight of the 2150's 3U rack height: relative to the present 'State-of-the-Art', namely shoe-horning 4 or more kW into the same rack space.

Despite this, there are many good signs: all the PCBs are connected by latching molex-style plugs, via ribbons, and the extensive legend means components and connector sockets are readily and positively identified. The 2150's cooling is unique; the fan blows air first onto the transformer, where it's deflected sideways, at the output/driver cards. Subsequent exhaust is upwards, up a pair of chimneys, which double as the heatsinks. By making compulsory use of natural convection currents, the 2150 can be adequately cooled, with just lightweight metal. Again, the 2150 is different to others, in that its fins comprise a series of lightweight, folded alloy leaves, not unlike a vehicle radiator; and a relatively quiet, low speed, low power fan, incorporating what looks like a Dansette motor...

The Bi-polar power transistors ('devices') are plastic encapsulated, but they're modern types, much improved and more likely to be as rugged, and long lasting, as the traditional metal can variety. What's more, in being 'socketed' they're easy to replace, should they expire: just unscrew and plug in a fresh device. Even thermal grease is dispensed with, thanks to silicone-rubber insulating washers, which make a good thermal contact without pastes. There are just two devices 'per side', or 8 overall. So this is no PA or stage amp, but then it's not advertised as such. Rather, its contents are clearly intended to be attuned to gentler studio or domestic environs. If so, we needn't worry that the 2150's space saving exercise has turned out to be half-hearted, in so far as a the transformer is of the ordinary, hefty laminated species. But it is well shielded.

"...a mild HF emphasis and 'glossy sheen' was evident, between 7 and 14kHz."

Bad points are just three. First, the chassis is bonded to 0v (signal ground), thereby positively guaranteeing ground loop hums in earthed racks. None of this would be necessary if Yamaha accepted with good grace that the grounding techniques used in the UK are a good deal more advanced than those in the US (whose pattern the Japanese have chosen to emulate). Adding a 3-core mains lead, and reconfiguring a few signal grounds off the deck is hardly a big deal, and Yamaha would score some valued marks from pro users, right at the outset. End of outburst!

Next, on the same tack, the mains lead is terminated on the least accessible of the circuit cards, making it damned awkward to replace.

Thirdly, if the attenuator knobs get knocked off then the metal spindles (which reach across to the pots at the rear of the box), will very likely come adrift, and short out one or more of the bare power fuses on the PCB below. Bang! Bang! In fact, it would be good to see plastic insulating covers over these fuses in future production.

Testing

At The Chapel 24-track studio, owner Bram and Matthew praised the combined XLR/Jack inputs. On the other hand, the output terminals initially came loose, when I gave a pair of speaker wires, terminated with chunky spades, a good, firm tightening. In studio time, this means spending 20 minutes shifting the amplifier from the rack, unscrewing the lid, and fully tightening the guilty nuts. The dutiful studio engineer or repairman will also add a dab of nail varnish to the nut, so it never happens again. Needless to say when we took the lid off the 2150 terminals were found to be without the vital star washers or nylon lock-nuts, or varnish, which are surely doubly necessary when the other end is a big lump of plastic, very likely to be sorely twisted by an over enthusiastic pair of pliers.

Using the Audio Precision test set, we found that our measurements of the 2150's curves and figures were good, and closely reflected in Yamaha's own published specification (given below), within ±10%. So rather than bother you with all the curves, here's an opportunity to focus on just three of the interesting ones.

Kicking off with Figure 1, it shows how distortion varies with frequency for the two channels, when driven at half the rated power. That's -6dB below clip. This is unconventional, but it accords with Yamaha's own specification. Although the Left and Right channels differ, they're both below 0.01 % THD. (Total Harmonic Distortion is the sum of all harmonics generated by the equipment, 2nd up to the nth). This is a healthy sign at this drive level. Meanwhile, the inter-channel differences are very likely due to asymmetries in the circuit layout. If the curves differed this much at much higher THD levels, this would 'fog out' the stereo image's depth.

In Figure 2, the output on the LHS is in dBu. +30dBu is 24 volts, spelling (V²/R) = (24V²/8Ω) = 72W into 8Ω. Here though, the output voltage is being tested without a load, against frequency. Looking now at the frequency response, it's fallen to a shade less than -0.2dB at 20Hz. This low end figure is good news for strongly asymmetric bass sounds emanating from the likes of kick drums and synths, whereas the top end figure is just satisfactory, -0.25dB at 20kHz. Above 20kHz, the roll-off looks steep, but it's really unusually shallow, at just -3dB/octave. So there's just (+20dBu - +30dBu) = -10dB of attenuation at 200kHz, compared with 20kHz.

Conclusions

My own listening test involved a co-observer dragged into the nearfield, close up to pair of Celestion's SL6, driven by the 2150, with a DVT50s as our new reference power amplifier. The 2150's had a thinner bass, which would be judged adversely, because the SL6s are themselves short on real bass, below 300Hz. Of course, the 2150's lightness may be seen as a relative measure, implying that we'd best steer clear of monitors which are this way inclined. Higher up, a mild HF emphasis and 'glossy sheen' was evident, between 7 and 14kHz. Evident on hi-hats and bongos, this is a sure sign of indifferent capacitors, probably ones with a polyester dielectric. In the mid-band, and when driven at high levels, the 2150 wasn't as dynamic and vocals were a shade nasal, but also rougher, which is a fair sign that the 2150's distortion is rising sharply as it approaches its rated power. Being the less powerful of the two amplifiers, the 2150 would overload before the DVT. Now putting 80W or 90W into perspective, it represents just 1 or 1½dB below 100W whereas music encompasses another 50 to 100dB below this. So while a premature rise in distortion matters, it's really insignificant for the vast majority of music's (uncompressed) amplitude domain, in everyday nearfield monitoring, where levels of 1W to 25W are nearer the average. Elsewhere, in the quieter passages, the 2150 is fully detailed. In other words, the 2150 stresses fidelity at low levels.

Back at The Chapel, Matthew hooked the 2150 up to Yamaha's own S20X minimonitors, and found a good match (surprise, surprise). In particular, Bram enthused loudly about the stereo depth and the clarity within the depth. In ironic description, Bram added: 'A good, toppy sort of sound'. It follows that mild, so called 'subjective' colourations in power amplifiers are real, in so far as other people do notice them independently, but of course, everyone's interpretation is a unique function of their environment, and their own kind of mental filtering. Again, much depends on your monitors, and how their own abberations combine with the amplifier's. Depending on these, the subtleties that become apparent with prolonged, medium-level listening may indeed enhance their sound. If this is true of your own speakers, then the 2150s can be strongly recommended for static, nearfield monitoring. It's not a PA amp but it does have a subtlety at lower drive levels that the vast majority of macho PA amps can't reach down to.

The Yamaha P2150 amplifier costs £429 including VAT and further information can be obtained from: Yamaha-Kemble Music UK Ltd, Mount Avenue, Bletchley, Milton Keynes, Bucks MK11JE. (0908) 640202.