I've seen mentioned in Hugh Robjohns' monitor reviews that he owns PMC AB1 monitors and is happy with them. I've got a pair of PMC AB1s, and I drive them with a Studer A68 amp (2x100W). I've been told by PMC that my power amp is not powerful enough and that the 10‑inch bass driver the AB1 employs is quite difficult to drive, so I should use a more powerful amp. I would like to know which kind of amp Hugh uses with the AB1, and whether he thinks the Hafler P4000 (2x200W into 8Ω) would be a good choice.
I've found the frequency response a little bit scooped in the lows (200Hz), and I was wondering if this could be due to the power amplifier not driving the woofer happily.
Ronnie Amighetti
Technical Editor Hugh Robjohns replies: I was very happy with my AB1s and ran them for almost five years. I've always thought they represent excellent value for money. Having said that, I have recently upgraded to the similarly sized three‑way PMC IB1. That extra mid‑range driver really takes the resolution up another notch (albeit at a greater price) and the bespoke planar bass driver and developments to the line design have also brought benefits at the bottom end.
I found with my AB1s that they really did need a powerful amp to make them give of their best — puny amps really did make themselves known through a poorly controlled bottom end. But 'powerful' doesn't just mean a rated power in Watts. Loudspeakers are complex loads — they are rarely purely resistive but are typically very reactive (all those inductors, voice coils and capacitors), which makes for a difficult load to drive. A lot of amplifiers have difficulty driving current and voltage peaks which are not perfectly in phase — but that is exactly what happens with reactive loads.
I used a Bryston 4B amplifier, which is a real powerhouse, with the AB1s (PMC recommend Bryston). It is rated at 250W per channel but I'd be very surprised if I got anywhere near using that much power. Its real strength is in its ability to drive complex loads without straining at all. Incidentally, the IB1s are a noticeably easier and more sensitive load for the Bryston 4B, despite having a far more elaborate crossover.
I haven't used the Hafler, although I have heard good things about it. The extra power should certainly help but, as I have said, it's more about the ability to control complex loads than sheer power. If possible, try to compare the Hafler with a Bryston 3B or 4B. One advantage of the Bryston is that the company warranties its products for 20 years, and you can't say that about many manufacturers!
Your problem with the low‑end frequency response is unlikely to be caused by the amp. A puny amp will affect the control of the speaker — its ability to start and stop notes (particularly bass notes) properly — and not its frequency response. A frequency‑response problem, especially at this kind of frequency (and below) is more likely to be room related — the dreaded Eigentones, or standing waves. Try moving the speakers around to find a location where they can drive the room better. Moving them away from side walls and varying their position within the length of the room will make a huge difference. It takes time but will pay dividends when you find the best place. Try positioning them so that they fire across the short axis of the room if they are currently working down the long axis (or vice versa), or even have them firing across the diagonal. There are no real hard and fast rules in acoustics — there are just too many variables.