AR-LST-2

[Guest Rob LST-2]

Hi there. I've been forwarded to this forum from Audiokarma as I have recently been introduced to some LST-2's in my attic. I have yet to get them down but hope to have some pictures very soon. I have been told that they are fairly valuable and very much sought after speakers but my knowledge of them is very limited, hence my presence on here.

I am certainly going to be restoring them back to their former glory and I hope all of you can help me out and point me in the right direction. I have been told that these speakers need a rather hefty amp to drive them which means I'll need to invest in another one, any suggestions and general comments to help me gain some knowledge of these great speakers would be gratefully received. Cheers. Rob

[Steve F]

A little historical perspective on the LST-2, from a post of mine last year:

"#1. "RE: Different between LST and LST 2?"

In response to Reply # 0

The original AR-LST ("Laboratory Standard Transducer") was introduced in late 1971 as AR’s attempt to offer a quantifiable, repeatable, accurate acoustic reproducing device, suitable for use in recording studios and other applications where known, verifiable accuracy was required.

Since ferro fluid-cooled tweeters did not yet exist in 1971, AR used multiple (4 ea.) 3/4" tweeters and 1 1/2" midranges to achieve the high levels of power-handling needed for its intended application. AR also had a design goal of achieving a virtually flat acoustic power response over a 180-degree solid angle (hemisphere) when the speaker was mounted very close to the wall behind it. They considered the radiation patterns of the MF and HF radiators and then calculated the angles of the cabinet side panels to minimize any interference/cancellation between those drivers, while being able to achieve the wide MF/HF dispersion needed.

The LST was perhaps the most breathtakingly accurate speaker ever devised up until that time in the history of consumer electronics, and is still more than remarkable even by today’s standards. The reviews in Stereo Review, Audio, and High Fidelity were amazing. High Fidelity summed it up when they said of the LST’s name that the speaker "could lay claim to such an auspicious title with complete authority."

The LST used a 6-position autotransformer spectral balance control that kept the midrange level unchanged, while raising or lowering the tweeter/woofer balance in 1 dB steps relative to the midrange. Position 1 had a slightly elevated tweeter level with a slightly lower woofer level; 2 was nominally flat; 3 thru 6 progressively raised the woofer by 1 dB and lowered the tweeter section by 1 dB. According to AR, positions 5 and 6 approximated the balance of the AR-3a with its controls set to ‘normal.’

The speaker was an unexpected commercial success at its (very high for 1971) price of $600 ea. As with most companies, AR tried to capitalize on the original LST’s success by coming out with a second model.

The LST-2, introduced in March 1974, used a 10 " woofer instead of the LST’s 12," and 3 ea. of the mids and tweeters instead of 4 ea. (Interestingly, the LST-2 used a midrange and tweeter in the forward-facing front panel, while the LST had all four of its midranges on the angled side panels with just two tweeters and the 12" woofer on the front panel. The forward-facing mid of the LST-2 gave that speaker a very slightly more "forward" character than the LST.)

The other major difference was that the LST-2 had a 3-position autotransformer instead of a 6-position control. The LST-2 had a retail price of $400 ea.

I used the LST-2 for almost 10 years as my main speakers and found them to be quite excellent—neutral, room-filling, and very easy to listen to."

The LST-2 was not a particularly successful speaker, from a commercial/sales standpoint. But like the AR-5 was to the 3a, some people consider the LST-2 to hold a similar position relative to the LST: while conceding that last 1/3 octave of bass to its more costly brother, the 10" system's overall spectral balance is preferred in some quarters to that of the 12" system's.

In any event -- and by any standard -- the LST-2 was a fine loudspeaker.

Steve F.

[nfmisso]

>.......I have been told that these speakers need

>a rather hefty amp to drive them which means I'll need to

>invest in another one, any suggestions.....

Hi Rob;

On a budget, an Adcom 555, 555 II or 5500 are good amplifiers to use for your LST-2 and most other classic AR speakers. There are many other amplifiers out there that will work well, some better than an Adcom amplifier, most not as well.

AR speakers need an amplifier that will handle very low impedances at low frequencies - and in the case of the speakers without ferrofluid cooled tweeters; amplifiers with lots of headroom so that clipping never becomes an issue.

[Guest Rob LST-2]

Thanks for the info. I've had a look at some adcom amps but availability in the UK is abit crap to be honest. I absolutely adore anything in the McIntosh range... not so much the price tho. Also... I am very new to the world of vintage hifi/electronics therefore not so upto date with the lingo. I kinda know about impedance but "clipping" is a new one on me.... What does it mean? I am very willing to learn and am seriously thinking about going back to college so I can get the best out of my new hobby. I want these LST's to sound and look awesome!!

[onplane]

>Thanks for the info. I've had a look at some adcom amps but

>availability in the UK is abit crap to be honest. I

>absolutely adore anything in the McIntosh range... not so much

>the price tho. Also... I am very new to the world of vintage

>hifi/electronics therefore not so upto date with the lingo. I

>kinda know about impedance but "clipping" is a new

>one on me.... What does it mean? I am very willing to learn

>and am seriously thinking about going back to college so I can

>get the best out of my new hobby. I want these LST's to sound

>and look awesome!!

Rob, clipping occurs when an amp runs out of "headroom" and the audio signal is "clipped". Problem with clipping is creates all kind of high frequency harmonics (also know as distortion) which will frequently overload the rather fragile tweeters.

It seems counter-intuitive, but more speakers are destroyed by under powered amps than over powered amps.

As for going back to college, gaining formal training is always good. The people on this forum, however, can help you get the most out of your LST’s as these folks have been experimenting with AR speakers for many, many years. Further, there is no formal institute that can duplicate the years of knowledge and experience on these very special speakers that exists here.

Next, never feel embarrassed to ask questions. Everyone is very helpful and everyone remembers that at one time they also were new to this hobby.

In short, the only “stupid question” is the question NOT asked.

Regards,

Jerry

[KlausDK]

Hi Rob

I use an NAD 208 PA (2x225 watt RMS) to drive my LST's and the NAD does so with excellence. NAD 208 is about 10+ years of age in model, and should be available at reasonable prices and NAD is probably easier to come by in UK than the Adcom models.

BR Klaus

[onplane]

Rob,

One other way to solve the power issue is to bi-amp. That is, instead of buying a huge 200+ wpc amp, you purchase two smaller amps ie (100 wpc).

It sounds crazy, but two smaller amps splitting the load of your LST's actually achieves far more head room than a single 200 wpc amp.

Here is an interesting article:

http://sound.westhost.com/bi-amp.htm

Regards,

Jerry

[KlausDK]

Hi Jerry

I Agree that bi-amping is preferable in many situationa, I use it my self, but there is a problem bi-amping speakers like LST, LST-2 and 10Pi because of the autotransformer. Unlike the more conventional cross-overs used in the other AR speakers, You would have to completely redesign the cross-over and by-pass the transformer.

BR Klaus

[onplane]

>Hi Jerry

>

>I Agree that bi-amping is preferable in many situationa, I use

>it my self, but there is a problem bi-amping speakers like

>LST, LST-2 and 10Pi because of the autotransformer. Unlike the

>more conventional cross-overs used in the other AR speakers,

>You would have to completely redesign the cross-over and

>by-pass the transformer.

>

>BR Klaus

Hi, Klaus!

I agree the autotranformer makes it more interesting to bi-amp, but NOT impossible.

First off, in positions 4, 5 and 6 the autotransformer has ZERO impact on the woofer. So the obvious simple solution is to remove the woofer, coil and cap from any connection to the autotransformer and power directly with a separate amp. Now, to balance the woofer with the other speakers, you may need to adjust volume on the woofer amp a tad, but that should be pretty minor.

Balance of the speakers COULD be connected just as they are today, but with a separate amp powering them. This might work just fine.

Naturally, that's NOT what I would do. I'd remove competely any connections to the autotransformer, since we can balance sound between the mid-range and woofer via controls on the individual amps.

This leaves as the only issue the balancing of sound volume between the mids and the tweeters. Here a much "neater" solution is to add a series of low value resistors to your existing 6 position switch, IF NEEDED! You might be able to balance just fine with the treble tone control.

What do you think, Klaus?

Regards,

Jerry

[KlausDK]

Hi Jerry

I am not sure that the cap and coil values would fit a setup without the autotransformer, they might, I simply do not know enough about it. If the do, I agree that you can balance the output from high and mid with high quality resistors in front of caps and coils. (Never underestimate the influence of resistors in series with speakers/drivers, they have as much influence on the sound quality as the caps).

Some would also argue, that the autotransformer influences sound in a desireable way, I have AR 10Pi and 11, which has exactly the same drivers, and the do not sound alike, the 10Pi has a more soft touch to the mid and high range.

BR Klaus

[onplane]

>Hi Jerry

>

>I am not sure that the cap and coil values would fit a setup

>without the autotransformer, they might, I simply do not know

>enough about it. If the do, I agree that you can balance the

>output from high and mid with high quality resistors in front

>of caps and coils. (Never underestimate the influence of

>resistors in series with speakers/drivers, they have as much

>influence on the sound quality as the caps).

>

>Some would also argue, that the autotransformer influences

>sound in a desireable way, I have AR 10Pi and 11, which has

>exactly the same drivers, and the do not sound alike, the 10Pi

>has a more soft touch to the mid and high range.

>

>BR Klaus

Hi, Klaus!

Well concerning the coil and cap alone with the woofer, that is exactly what you have in switch positions 4, 5, and 6. That is, the transformer is by-passed for the woofer in these positions. Further, the values are very, very close to the AR-3a's and I power that woofer, coil and cap with a single amp every day.

In the other positions (1,2 & 3) the autotransformer reduces energy sent to the woofer. Again, you can achieve the same results by reducing the volume in the woofer amp.

Klaus, there is, in my opinion, some truth to the notion that transformers impact the resulting sound. Even harmonics are added and many folks fine these pleasing. I don’t, but this is personal preference.

Nevertheless, if you keep the autotransformer powering the mids/tweeters you’ll still get the lion's share of these even harmonics.

Klaus, if you have another amp (even a modest power amp), you should try a bi-amping experiment. The hardest part will be making some new terminals for the woofer, coil and cap. Returning to original design will be easy, but I doubt that you will want to.

Regards,

Jerry

[KlausDK]

Hi Jerry

I agree that this solution would be rather easy and that it would not require changes in the crossover, other than "freeing up" the connection to the woofer. This would most likely improve the alredy very good performance and transient response of the woofer, by improved control of the speaker.

This setup would also have the advantage of not requirering an electronic cross-over between the power amps, however, there are many people (I for one) that do believe that one of the great advantages of bi-amping is, that the Power Amp can concentrate on the frequencies for the speaker(s) it delivers to.

When deciding upon the cross-over frequency in the electroninc cross-over, one should allow for some space for the speaker cross over to work with; AR has for the AR-90 recommended 1 octave above or below the speakers crossover frequency (depending on whether it is a low- or highpass filter).

BR Klaus

[onplane]

>Hi Jerry

>

>I agree that this solution would be rather easy and that it

>would not require changes in the crossover, other than

>"freeing up" the connection to the woofer. This

>would most likely improve the alredy very good performance and

>transient response of the woofer, by improved control of the

>speaker.

>

>This setup would also have the advantage of not requirering an

>electronic cross-over between the power amps, however, there

>are many people (I for one) that do believe that one of the

>great advantages of bi-amping is, that the Power Amp can

>concentrate on the frequencies for the speaker(s) it delivers

>to.

>

>BR Klaus

Hi, Klaus!

As for "improved transient response of the woofer" our good friend Tom Tyson would argue that there can be no significant difference with bi-amping. That is, the woofer response will be unchanged.

Now, Klaus, I agree with you to a certain extent. I believe that I can hear a significant improvement in transient response, but ... but it's NOT in the woofer. The improvement I hear is clearly in the mid and tweeter. I believe what's going on is that once an amp is no longer burdened with the heavy current drain of the woofer, it will response with a higher magnitude output voltage to transients. This in turn we perceive as better transient response.

Now, I need to design some experiments to test this theory.

Next, Klaus, your comment upon focusing the amps to a narrower frequency range is a good one. I believe by limiting the frequency range that an amp must handle achieves two desirable objectives:

1. reduced intermodulation distortion – whenever we can separate harmonics from fundamentals we reduce the RISK of IM

2. significant gain in headroom, particularly in the amp driving the mids/tweeters.

The way I go about narrowing the frequency band is with tone controls. That is on the woofer amp I turn the treble tone control full OFF and on the mid/tweeter amp I turn the bass control to the 8:00 o’clock position (not quite full OFF).

Regards,

Jerry