Michele Posted March 29, 2018 Report Share Posted March 29, 2018 Good day, somebody can help me to understand the reason of the caps installed on the input (negative wire) of the LST crossover? Why is not installed also on the 3a? Sorry for my stupid question. Quote Link to comment Share on other sites More sharing options...
KlausDK Posted March 29, 2018 Report Share Posted March 29, 2018 It is due to the transformator in the LST cross-over, which is not a part of the 3a cross-over. Early versions of the LST did not have the large Capacitor, but they blew a couple of power amplifiers, since the speaker impedans (due to the transformator) went towards 0 with lower frequencies, thus the capacitors were installed; same goes for AR-LST2 and AR 10Pi. both with transformers in the cross-over. Later a 10 Ohm resistor was added i parallel with the capacitor. BRgds Klaus Quote Link to comment Share on other sites More sharing options...
Michele Posted March 29, 2018 Author Report Share Posted March 29, 2018 Dear Klaus good afternoon, many thanks for your reply. You mean that these caps are to keep the impedance in safe value for the ampli load. Just to better explain my first question, is due to the fact that the low frequency of my LST are very poor and make one test bypassing these caps and resistance only for the woofer, immediately change the sound and coming good. Could be possible that the caps are exhaust? and affect the sound? What do you suggest? Really many thanks for your help. Michele Quote Link to comment Share on other sites More sharing options...
frankmarsi Posted March 29, 2018 Report Share Posted March 29, 2018 Michele, you haven't mentioned anything about the condition of the woofers. Might they need a re-foaming job? I'm one of the lucky and the few here that own LST's. In terms of room placement, required handling power and such, they require some different conditions than your regular speaker box. The 'AR-LST' is not a common speaker and as such special considerations must be adhered to. If not properly observed this could add to your situation of lessened bass response. FM Quote Link to comment Share on other sites More sharing options...
RoyC Posted March 30, 2018 Report Share Posted March 30, 2018 8 hours ago, Michele said: Dear Klaus good afternoon, many thanks for your reply. You mean that these caps are to keep the impedance in safe value for the ampli load. Just to better explain my first question, is due to the fact that the low frequency of my LST are very poor and make one test bypassing these caps and resistance only for the woofer, immediately change the sound and coming good. Could be possible that the caps are exhaust? and affect the sound? What do you suggest? Really many thanks for your help. Michele Michele, Try by-passing the fuse on the back of the cabinet. On three separate occasions, I have found deteriorated slow blow fuses to cause this issue. About three years ago, upon completion of a restoration of a very nice pair of LST's, Larry Lagace (aka "Vintage_AR") and I spent a number of hours trying to figure out why there was virtually no bass response from one of the speakers. After replacing the woofer (twice) and carefully testing the crossover (the caps were new) we were still stumped. I finally by-passed the fuse simply because there was literally nothing else left to evaluate, and there it was! The difference was not subtle! Since then we have run across 2 other LST's with the same issue. It is advisable to replace those old slow blow fuses as part of any LST work and/or evaluation. Btw, Most all of the crossover series capacitors of old AR speakers are on the negative side of the crossover circuit. I believe it was originally done to accommodate the 3 input terminal/jumper arrangement. Let us know if by-passing the fuse solves the bass issue. Roy Quote Link to comment Share on other sites More sharing options...
Michele Posted April 3, 2018 Author Report Share Posted April 3, 2018 Dear Roy, ..... unbelievable.... I bypassed the fuse and the sound is coming back !! Really many thanks for your support.. and what do you think about to leave in place the bypass jumper or change type of fuse? Quote Link to comment Share on other sites More sharing options...
RoyC Posted April 3, 2018 Report Share Posted April 3, 2018 Hi Michele, You're welcome...I'm glad it turned out to be the fuse issue, and not something more complicated. Sure, you can use them with the fuse by-passed, if you are careful about very high volume levels. Roy Quote Link to comment Share on other sites More sharing options...
Michele Posted April 4, 2018 Author Report Share Posted April 4, 2018 Ok Roy, thanks again... I will keep you update regarding my restoring job. keep in touch michele Quote Link to comment Share on other sites More sharing options...
KlausDK Posted April 4, 2018 Report Share Posted April 4, 2018 Very interesting fact that the fuses can cause degradation of the sound. However, bearing in mind how unobtainable/expensive the tweeter are, I would never play mine w/o fuses installed. BRgds Klaus Quote Link to comment Share on other sites More sharing options...
RoyC Posted April 5, 2018 Report Share Posted April 5, 2018 7 hours ago, KlausDK said: Very interesting fact that the fuses can cause degradation of the sound. How ever, bearing in mind how unobtainable/expensive the tweeter are, I would nev er plays mine w/o fuses installer. BRgds Klaus Hi Klaus, To be clear it is not fuses in general causing the problem, only old degraded slow blow fuses. You would know it if you have one causing the issue described above. The problem is obvious...there is literally no bass response. The fuse is no longer usable. We now replace all original LST slow blow fuses as part of any repair or restoration. Roy Quote Link to comment Share on other sites More sharing options...
KlausDK Posted April 5, 2018 Report Share Posted April 5, 2018 Thanks Roy, I never had a problem with mine, I replaced the fuses with new, when I got mine 10+ years ago. I use original Bussmann Fusetron FNM2 fuses, hard to get by here in Europe. BTW I apologize for errors in the text in the above post (now corrected, but visible in Roy's quotation , looks like I was drunk when I wrote that (and I was not!!! ); Must have been the autocorrect function on my phone. BRgds Klaus Quote Link to comment Share on other sites More sharing options...
lakecat Posted April 5, 2018 Report Share Posted April 5, 2018 I am curious about your tweeters in your pic. Quote Link to comment Share on other sites More sharing options...
Michele Posted April 6, 2018 Author Report Share Posted April 6, 2018 Good day, the tweeters are with new dome due to damaged ones from previous owner... Quote Link to comment Share on other sites More sharing options...
Michele Posted May 14, 2018 Author Report Share Posted May 14, 2018 Good day to every body... I'm again asking your good experience regarding the proper connection of the tweeters on the frontal terminal... Most probably are not properly connected, positive and negative, ( during past restoring ) and up to now I don't know how to check... Have you an idea how the positive and negative wires must be connected on the front terminals? ( parallel and series of the speakers ). Really many thanks for your kind help. Michele Quote Link to comment Share on other sites More sharing options...
tysontom Posted May 15, 2018 Report Share Posted May 15, 2018 On 4/4/2018 at 11:31 PM, RoyC said: Hi Klaus, To be clear it is not fuses in general causing the problem, only old degraded slow blow fuses. You would know it if you have one causing the issue described above. The problem is obvious...there is literally no bass response. The fuse is no longer usable. We now replace all original LST slow blow fuses as part of any repair or restoration. Roy Michele, Klaus and Roy, When the fuse is blown in an AR-LST, rather than have a "totally silent" speaker as you might expect, there remains a high-impedance path through the autotransformer causing a 30 dB drop in system output, but the speaker is not completely shut off. The older Buss Fusetron FNM fuses do sometimes deteriorate over time and can open partially or completely, causing this sensation. --Tom Quote Link to comment Share on other sites More sharing options...
Michele Posted June 8, 2020 Author Report Share Posted June 8, 2020 Hello to everybody, i think that the 2500 uF capacitor in the cross-over start to be tired, due to some strange noise in all audio spectrum with medium power. Do you suffered the same in all your experience? In case this capacitor is exhausted, where i can find? The value is not standard ( not polarized value ). Many thanks for yo comments/help. regards, Michele Quote Link to comment Share on other sites More sharing options...
Aadams Posted June 8, 2020 Report Share Posted June 8, 2020 3 hours ago, Michele said: due to some strange noise in all audio spectrum with medium power. From what you describe it seems like the problem should be somewhere before the cap. Is it in one channel or both? If one channel only, have you tried to move the problem to the other channel? Adams Quote Link to comment Share on other sites More sharing options...
Michele Posted June 8, 2020 Author Report Share Posted June 8, 2020 Hi, good evening, happen on both speakers, medium volume... I discovered this problem with cd sound test low frequency track. Sound good up to abt 20 watts after that start this noise. Left and right speakers not at the same time. Same test with AR 3a and ok, sound good. actually the speakers are connected to McIntosh 2205. Quote Link to comment Share on other sites More sharing options...
ar_pro Posted June 8, 2020 Report Share Posted June 8, 2020 Two things: Does the MC2205 Power Guard indicator lamp ever come on? To which impedance tap do you have the LSTs connected? They are nominally 4 ohm speakers, but might be happier connected to the 2 Ohm tap. Quote Link to comment Share on other sites More sharing options...
Aadams Posted June 8, 2020 Report Share Posted June 8, 2020 In your original post you said "all audio spectrum" but apparently you meant low frequencies. It seems improbable that both 2500uf caps would malfunction at the same point in time. The problem could be degraded fuses, perhaps dirt in the spectral balance controls, both of which are more likely. Quote Link to comment Share on other sites More sharing options...
Michele Posted June 10, 2020 Author Report Share Posted June 10, 2020 I recognize this problem using sound test cd with the low frequency track.... in the beginning there was some vibration coming out from the cross-over back panel due to gasket worn out. i replaced in both cabinets and tested again with the same low frequency track with good result. i tried to increase little bit the volume looking the woofers moving good without any noise, but after few extra dB start to have some scratch noise coming from all speakers, woofer, mids and tweeters.. no any power guard alarm from ampli and the power showed by wu-meter was 20 watts... not both speakers left and right in the same time, first left and with one extra dB also the right. Please note that the fuses has been already changed, and spectral balance control cleaned. With this scenario a start to think about the exhausted capacitor. I measured the impedance of my LST and shoved on my Fluke instrument 10,4 Ohms: looking into cross over layout there is one 10 Ohms resistor connected in parallel of capacitor. So with this impedance, where is better to connect on my McIntosh output? 2, 4, or 8 Ohms? Really many thanks for your comments, help. Quote Link to comment Share on other sites More sharing options...
RoyC Posted June 10, 2020 Report Share Posted June 10, 2020 2 hours ago, Michele said: I recognize this problem using sound test cd with the low frequency track.... in the beginning there was some vibration coming out from the cross-over back panel due to gasket worn out. i replaced in both cabinets and tested again with the same low frequency track with good result. i tried to increase little bit the volume looking the woofers moving good without any noise, but after few extra dB start to have some scratch noise coming from all speakers, woofer, mids and tweeters.. no any power guard alarm from ampli and the power showed by wu-meter was 20 watts... not both speakers left and right in the same time, first left and with one extra dB also the right. Please note that the fuses has been already changed, and spectral balance control cleaned. With this scenario a start to think about the exhausted capacitor. I measured the impedance of my LST and shoved on my Fluke instrument 10,4 Ohms: looking into cross over layout there is one 10 Ohms resistor connected in parallel of capacitor. So with this impedance, where is better to connect on my McIntosh output? 2, 4, or 8 Ohms? Really many thanks for your comments, help. Continue investigating your electronics. The large capacitor is not the problem. Roy Quote Link to comment Share on other sites More sharing options...
ar_pro Posted June 10, 2020 Report Share Posted June 10, 2020 From the MC2205 manual: If a load impedance is used that is lower than the output impedance tap, then reduced power and possible distortion will result. The LST is nominally a 4 ohm system, but the manual advises that its actual impedance will depend upon the position of the autotransformer switch. Here's the McIntosh connection recommendations for different loads. Quote Link to comment Share on other sites More sharing options...
Michele Posted June 12, 2020 Author Report Share Posted June 12, 2020 On 6/11/2020 at 3:57 AM, ar_pro said: From the MC2205 manual: If a load impedance is used that is lower than the output impedance tap, then reduced power and possible distortion will result. The LST is nominally a 4 ohm system, but the manual advises that its actual impedance will depend upon the position of the autotransformer switch. Here's the McIntosh connection recommendations for different loads. Quote Link to comment Share on other sites More sharing options...
Michele Posted June 12, 2020 Author Report Share Posted June 12, 2020 Many thanks. I connected to 8 ohms ampli tap, lst impedence 10,4 ohms Grazie Quote Link to comment Share on other sites More sharing options...
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