Linn Isobariks

[RojDW48]

A friend is playing with a pair of these and has asked for advice re amps. I had a google and got the impression that they are liable to alarming dips in impedance over the frequency range. Would the output Tx of a valve amp be able to cope with that?

[wd40addict]

These are famous for being hard to drive. NAD supposedly used them to demonstrate that their 3020 was pretty bomb proof for a budget amp. More normally the natural partner would have been a Naim NAP250, lots of Naims if the speaker was used with an active crossover.

A valve amp should be fine on its 4R tap, but not sure how well controlled the bass will be.

[Radio Wrangler]

The dips go down to 2 Ohms.

The efficiency isn't wonderful so lots of power is needed to use them to their capabilities.

Valve amps tend to protect themselves against too low impedances and avoid damage. They can be damaged by too high impedances and open circuits.

These speakers really suit fairly brutal transistor ampifiers. Naims are the classic companion for them, but watch out for the Naim's intolerance (smoke!) if asked to work with trendy high capacitance speaker cables. Their designer didn't like Zobel network load isolators...

David

[wd40addict]

He didn't like series resistance on the preamp outputs either. When subwoofers became popular Naim had to sell a special cable fitted with a 100R series resistor to avoid instability with a typically long cable.

[Patrick Dixon]

I once ran Isobariks with a Border Patrol 300B amp and it worked surprisingly well. I've also used NAP135s and a LM3886 based amp.

I don't have any of them or the Briks any more.

[rontech]

I wonder if a QUAD 606 power amp would work well?. It is pretty powerful and supposed to be unconditionally stable.

[Diabolical Artificer]

A half decent PA amp might do the job, something like a Citronix PPX450 which have a very good reputaion for being bullet proof. Any Crown power amp or maybe an old Phase Linear 700. As always it depends on how much cash one has, but decent PA amps can be got cheap, one brand to look out for is TOA, a canadian amp builder, I have a TOA A906 mono amp that is superb, it has a OPT in it, a very well built OP stage with various IP plugins, I've been looking for another for years. Lastly another excellent vintage amp builder is Stage Accompany, I have an SA 900C which is two monoblocks in one amp, it's built like the proverbial brick outhouse, is pretty bulletproof when it comes to difficult loads.

Valve amp wise a McIntosh MC3500 or Audio Research M300 maybe,though they aren't cheap. If the speakers were mine I'd build a very beefy valve amp, something with 6 KT88's or similar with lots of over current protection etc, with the right OPT's it could chug along in Class A for the most part only jumping into Class AB1 when extra ooomph is needed. Just fantasising there.

Andy.

[wd40addict]

TOA are Japanese not Canadian! Founded in 1934:

https://www.toa.jp/sp/profile/histor...ail.html#01_01

[Diabolical Artificer]

Your right, could have sworn they're Canadian, oh well, they still make good amplifiers.

Andy.

[RojDW48]

Thank you gents - very helpful.

[Craig Sawyers]

The idea behind the Isobaric principle was that if you drove both woofers in phase, the air pressure between them is constant, so the front driver believes it is operating in a much larger enclosure.

Except that is not what happens. Since the two drivers are closely coupled by the air volume between them, they simply act as a single driver with twice the moving mass. So it pushes the resonance down by 1/root2; in fact not quite that because the two bass drivers occupy a larger volume of the cabinet.

There is a quirky arrangement of drivers. Two KEF B139's in the "Isobaric" arrangement. A forward firing midrange and tweeter, and an upward firing midrange and tweeter. And because of that a rather involved crossover, which is what will be the root cause of the difficult impedance.

Craig