I've sold most along the way, but some of the real classics are of such quality I've kept them in my store.
Around 2008 I bought 2 pairs of the Acoustic Research AR-LST from a little old lady's son after she become too old to manage them. I don't know why she needed 2 pairs, but they were in perfect condition and I paid $4000 for them, which was quite a lot for speakers made in 1975.
I knew of the formidable reputation the Laboratory Standard Transducers (LST) had and loaded all 160 kgs into my car.
The LST's are an unusual speaker with multi-drivers. One 12" bass driver in a sealed cabinet, 4 of one and a half inch midrange dome drivers and 4 of three quarter of an inch dome tweeters.
The cabinet has two offset side panels which contain the midrange and 2 of the tweeter units.
Kind of like a Bose 901 cabinet, but much bigger and meaner!
With the 2 pairs stacked I have a total of 36 drivers playing. I've always believed that small speakers are a compromise and for realistic sound a speaker has to have the physicality to move lots of air, hence my love of big stats, horns and big moving coil speakers.
The crossovers in the LST are quite interesting, 12dB slopes at 575 and 5000Hz. Nice big air coils and quality caps , I only needed to replace the midrange electrolytic cap for a poly cap.
The speaker had an input transformer with many taps to alter the tonal frequency, to adjust for room response and taste. Unfortunately this required a 2500uF electrolytic cap to counter sub bass resonance and I can't say I was too happy about having this in the signal path.
As I intended to actively tri-amp I had no need for level adjustment and rebuilt the crossovers on the back panel of the cabinet, removing the transformer and hardwiring all the crossover components .
The printed circuit boards also went in the bin.
I also put an extra set of speaker connectors for each set of drivers on back panel so that I could either use the passive crossovers or run the drivers directly from the seperate amps I intended to use for bass, mid and treble, utilizing an active crossover, either analogue or digital.
With this set up I could choose whatever types of amps I wanted to use with either the bass, mid and treble sets of drivers.
The AR-LST's came along at a defining moment in amplifier design- the new high power solid state designs had just replaced the tube designs of the 50's/60's and the LST was a difficult load- it needed the high power and current abilities of the solid state amps, but these new designs typically used a lot of negative feedback and were not the nicest sounding amps. Amps like the Crown DC300 were commonly used and the results were not in the LST's favour.
Modern day solid state amps fare much better, but part of this restoration was to enable me to use my tube power amps with this speaker.
Dividing the crossover into 3 separate sections- bass, mid and treble makes the amplifiers load much easier and I was positive my tube amps, which use positive current feedback would drive the LST's very well.
The only question was would they also drive two sets of speakers, in parallel and with the passive crossovers presenting a heavy load.
Fortunately it was not a problem, and the amps have no problem at all in driving the LST's cleanly at high levels.
I'm using a 100watt KT88 ultra-linear amp on bass, an 80 watt KT88 triode amp on mids and a 60 watt 7027 pentode amp on treble, all with positive current current.
I've been using current feedback on all my amps for many years now and it is interesting to see positive feedback now becoming used by quite a few well respected tube designers- its a win/win design that enables tube amps to work exceedingly well with just about any difficult speaker.
I've been a fan of digital crossovers for around 5 years, owning DEQX and DBX active digital crossovers, but I must admit I'm enjoying the passive crossover/ tri-amping approach used on the LST's.
My big reference electrostatic system demands solid state amplification and I somewhat miss the use of an all tube system, but with the LST's use of 6 tube power amps and a tube preamp , I am in tube heaven!
Incidentally I also went away from using a digital crossover in my big reference system, and built the speaker crossover function into the tube preamp, which works very well.
I dont think there is anything wrong with digital crossovers, but getting tubes or passive components to do the job if possible makes it simple, and simple is often better.
Although it is obvious that 6 monoblock power amps is not exactly simplistic!
I'm very happy with the AR-LST sound, it has the big dynamic openness I like, and is completely natural and un-restrained sounding. It is the type of sound you can listen to indefinitely and is simply very enjoyable.
Used in this fashion the LST's almost sound like a cross of moving coil, electrostatic and horn speakers- they have something of the good points of all 3, even a touch of ribbon sound to them.
This is the beauty of active amplification- the finest of adjustments can be made to the levels of each crossover function and the system can be precisely fine tuned to suit the owners room and taste.
