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Mixing Tweeters in an LST


Perry Muscelli

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I own two LSTs which haven’t been listened to in many years since the woofers were in need of re-foaming and I never got around to it. My examination has now uncovered two dead tweeters (two outboard drivers) on one speaker. Both speakers had been serviced for blown drivers under warranty by AR in the 1974 time frame and both have a 50/50 mix of “A.19” and “A.20” surface wired tweeters as depicted in the Restoring the AR-3a guide. I have blown one A.19 and one A.20. The fine wire leads seem to be intact so no external repair will be possible.

I have spent hours reading the pros and cons of various replacement alternatives. I am not aware of a reliable source for the original drivers and the best I can conclude is that the HiVi Q1R with a .07 mH inductor wired in parallel with each driver is currently the most recommended solution. I would like to retain the six still functioning drivers and only replace the two bad units. However, because of the impedance mismatch, I am concerned about using these new six ohm drivers mixed with the original 4 ohm drivers in the series-parallel wiring scheme that the LST uses. I have attached an image of this portion of the schematic for reference. Here are my specific questions:

1. Should I wire the two replacements so that they are parallel to each other six ohm and in series with the original 4 ohm drivers? For illustration purposes replace DR 1 and DR 2 on the schematic? My instincts are that wiring in this manner assures that there is no lower impedance path for the signal to favor.

2. Assuming the new six ohm drivers should be wired in parallel, should each have its own .07 mH inductor or should I use a single .035 mH 18AWG inductor wired in parallel with the two tweeters? Maybe I am thinking about this all wrong and I need to up the inductance on both so that I end up with .07 mH total in parallel with the two replacement tweeters.

3. Would there be a preferred location for these less disperse new drivers? Front firing above the woofer or the sides above the mid-range drivers? (I would have to make the wiring work as noted above).

4. Is there a veteran tweeter replacer who can tell me how many turns I need to unwind from a .10 mH Jantzen inductor to get to .07 mH? Alternatively, since they are wired in parallel, could I just put a .1 mH on one tweeter and a .05 on the other presenting a combined .033 uH? Or, if the single inductor is the way to go, how many turns from a .05 mH to get to .035 mH?

Most of what has been written refers to the replacement of the single AR-3a tweeter. I am concerned about the effect of mixing impedances and inductors with the LST’s unique crossover. However, it seems that the primary crossover component now influencing the tweeters is the 6 MFD capacitor wired in series.

My goal here is to blend the replacements in with the original drivers. If there are good reasons not to do that then I would welcome that feedback as well. I am planning to recap the crossovers while I am at it. Thanks in advance!

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My goal here is to blend the replacements in with the original drivers. If there are good reasons not to do that then I would welcome that feedback as well. I am planning to recap the crossovers while I am at it. Thanks in advance!

Perry,

I do not recommend combining the HiVi tweeter with the original tweeters in the LST. A better way to go would be to find a pair of used AR-11 or 10pi tweeters and place a single .10mh inductor in parallel across both of them (assuming both are wired together in series). The parallel .10mh inductor is used in the AR-11 and 10pi crossover. It won't be an exact match, but will be closer than the HiVi's. Another way to go would be to install one AR-11/10pi tweeter in each cabinet to balance the system...each with its own .10mh parallel inductor.

If you decide to try the HiVi tweeters, two series-wired HiVi's in the LST would be used with a single, parallel .05mh inductor across the pair. (Recent feedback suggests the HiVi tweeter should be used with a smaller parallel inductor of .05mh, and perhaps a reduction in series capacitance, when used in the AR-3a, AR-2ax, or AR-5). Alternatively, as mentioned above, you could try one in each cabinet, each with it's own .05mh parallel inductor.

Roy

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