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#1
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
A previous poster stated that the Heathkit AR1500 and AR3a are a poor
combination. This is true if you listen at any level other than quiet. A couple of years after introduction, the AR1500's output amplifiers were updated and the AR1500A was born. The AR1500A has specific circuits that limit clipping and other distortion when it is driving low imperance speakers at high levels. It is possible to modify the AR1500's output amplifiers to include this improvement; you'd have to go to a Heathkit users groups to obtain that information. Driving AR3a's hard with an unmodified AR1500 produces a lot of "clipped sine waves" that will blow the mid-range driver. If you don't want to burnout the mid-range, but want to turn the volume up, add a 1 ohm resistor in series with each mid-range. This will change the mid-range balance, but that's better than burning out the driver. The AR3a was sold in kit form as the Heathkit AS103a. Let me know if you have any questions. |
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#2
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
wrote in message ...
A previous poster stated that the Heathkit AR1500 and AR3a are a poor combination. This is true if you listen at any level other than quiet. Rolf, I have an AR-3a and an AR1500 and they seem to work together fairly well. I don't know about "quiet" as that is a relative term. My den is quite small and my normal listening level is less than 1 watt, but occasionally I'll listen at levels higher. A couple of years after introduction, the AR1500's output amplifiers were updated and the AR1500A was born. The AR1500A has specific circuits that limit clipping and other distortion when it is driving low imperance speakers at high levels. It is possible to modify the AR1500's output amplifiers to include this improvement; you'd have to go to a Heathkit users groups to obtain that information. Rolf, any chance you have the schematic for the power amp on the "a" version. If I could see the "a", I could document the differences. I would think you'd really have to to be driving the speakers hard to get anywhere near clipping. My guess is the heavy current drain forces the rail voltages way down, and this allows even modest signals to clip. Only real solution is to "beef up" the power supply. Driving AR3a's hard with an unmodified AR1500 produces a lot of "clipped sine waves" that will blow the mid-range driver. If you don't want to burnout the mid-range, but want to turn the volume up, add a 1 ohm resistor in series with each mid-range. This will change the mid-range balance, but that's better than burning out the driver. Well, Rolf, I've done just the opposite. I've taken all the pots out of the AR-3a circuit and this significantly INCREASES the sensitivity of the mid-range and tweeter. (Frankly, I'm a little surprised that the mid is most susceptible to clipping damage. People on the AR forum worry far more about the tweeters.) In any event, "clipping" is no longer an issue for me. I'm now bi-amping my AR-3a's. My AR1500 drives only the woofers. I have a nice little 30 watt Kenwood powering the mid-range and tweeter. The Kenwood has rail voltages of 40 volts! With the pots out AND the fundamentals greatly reduced, it's hard to get peak voltages of 3 volts going to the mid-range and tweeter. I mean, at 3 volts I can barely stand the volume. Now, there is no question that at the same time that I have 3 volt peaks in the Kenwood, the peak voltages in the AR1500 are much, much higher. I can't see any clipping, but even if there were some, the mid and tweeter are totally isolated from that amp. The AR3a was sold in kit form as the Heathkit AS103a. Let me know if you have any questions. Rolf, if you have an AR1500 and a set of AR-3a's, I'd strongly recommend you try a bi-amping experiment. Not only will you protect your mids and tweeters, but there is a significant tightening of the bass that results. Regards, Jerry |
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#3
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
Jerry wrote:
wrote in message ... A previous poster stated that the Heathkit AR1500 and AR3a are a poor combination. This is true if you listen at any level other than quiet. Rolf, I have an AR-3a and an AR1500 and they seem to work together fairly well. I don't know about "quiet" as that is a relative term. My den is quite small and my normal listening level is less than 1 watt, but occasionally I'll listen at levels higher. A couple of years after introduction, the AR1500's output amplifiers were updated and the AR1500A was born. The AR1500A has specific circuits that limit clipping and other distortion when it is driving low imperance speakers at high levels. It is possible to modify the AR1500's output amplifiers to include this improvement; you'd have to go to a Heathkit users groups to obtain that information. Rolf, any chance you have the schematic for the power amp on the "a" version. If I could see the "a", I could document the differences. I would think you'd really have to to be driving the speakers hard to get anywhere near clipping. My guess is the heavy current drain forces the rail voltages way down, and this allows even modest signals to clip. Only real solution is to "beef up" the power supply. Driving AR3a's hard with an unmodified AR1500 produces a lot of "clipped sine waves" that will blow the mid-range driver. If you don't want to burnout the mid-range, but want to turn the volume up, add a 1 ohm resistor in series with each mid-range. This will change the mid-range balance, but that's better than burning out the driver. Well, Rolf, I've done just the opposite. I've taken all the pots out of the AR-3a circuit and this significantly INCREASES the sensitivity of the mid-range and tweeter. (Frankly, I'm a little surprised that the mid is most susceptible to clipping damage. People on the AR forum worry far more about the tweeters.) In any event, "clipping" is no longer an issue for me. I'm now bi-amping my AR-3a's. My AR1500 drives only the woofers. I have a nice little 30 watt Kenwood powering the mid-range and tweeter. The Kenwood has rail voltages of 40 volts! With the pots out AND the fundamentals greatly reduced, it's hard to get peak voltages of 3 volts going to the mid-range and tweeter. I mean, at 3 volts I can barely stand the volume. Now, there is no question that at the same time that I have 3 volt peaks in the Kenwood, the peak voltages in the AR1500 are much, much higher. I can't see any clipping, but even if there were some, the mid and tweeter are totally isolated from that amp. The AR3a was sold in kit form as the Heathkit AS103a. Let me know if you have any questions. Rolf, if you have an AR1500 and a set of AR-3a's, I'd strongly recommend you try a bi-amping experiment. Not only will you protect your mids and tweeters, but there is a significant tightening of the bass that results. Regards, Jerry October 5, 2005 update from Rolf: Both versions of the AR1500's output amplifier have a current limiting circuit to protect the amplifier's power transistors (Motorola MJ802's) from burning out. For the original AR1500, the circuit limited the current too soon when driving low impedance speaker like the AR3a. The AR1500A added a modification (resistors and capacitors only) that limited the response time of the current limiters. In effect, the new current limit circuit let short high-current transients through (which don't heat up the MJ802's enough to cause trouble) and only limited the current for non-transients. With the original AR1500 power amplifier, the too fast-acting current limiting circuit introduced "harmonics" not in the original signal; at high sound levels these harmonics have enough energy to burn-out the mid-range drivers. I have both versions of the AR1500, a pair of AR3a's (the kit version Heathkit sold as AS103) and a pair of AS101's (the kit version of the Altec Lansing Valencia). (The AR1500 and AR3a's were my dad's.) I modified the power amplifier of the original AR1500 many years ago, and unforunately no longer have the instructions. The only problem I've had with the AR1500A was the need to replace the front panel bulbs more frequently than I want to. I added silicone grease into the bulbs' sockets, and the bulbs now last much longer (due to better heat transfer away from the bulbs). I also spray painted the inside of the AR1500A's cabinet flat black, this reduces the internal heat of the receiver. |
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#4
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
Jerry wrote:
My den is quite small and my normal listening level is less than 1 watt, but occasionally I'll listen at levels higher. Jerry: There are a lot of misconception to do with "volume", and especially as it relates to "watts". At one watt average, with moderately dynamic source, you will need at least a 100-watt amp to get clean peaks absolutely without clipping. But you already know this. Put another way, a speaker with an 86dB @ 1-watt SPL, a 30-watt amp simply will not cut it, even bi-amped at anything above a very moderate volume. Agreed, mostly one listens at a very moderate volume. But not always. Components of the vintage of your Heath more-or-less send straight DC into the speaker when driven to clipping. And depending on the source (where, as it usually happens most of the signal is at the mid-range), that could be a lot of the time and pretty rapidly fatal to your speakers. Again, put another way, it is (Very typical of vintage SS amps) amplifiers of low power that burn speakers, not high power. Almost any well-made speaker can handle normal signal at very high power, well beyond their nominal ratings, for brief periods. Even the very best conventional PM speakers will fry in short order when fed DC. I think this is what the OP was trying to convey. Those of us who dabble in tube equipment tend to ignore this simple truth as output transformers will not pass DC, so clipping is much softer and relatively harmless (to the speaker, that is). Also why those of us who have high-powered amps (tube or SS) also tend to ignore this as there is enough headroom in any case to minimize consequences from clipping. Bi-Amping, even as you have applied it will create some advantages if very carefully managed. But the real-world difference between a 30 watt amp and two 30-watt amps split is limited in this *particular* application given that at any given moment about 75% of your signal will be at/within the midrange, and if you include the tweeter, that goes to about 90%, at least as it applies to the need for headroom. I would very strongly suggest that you make it your mission in Audio to beg, borrow or steal a well-made high-powered amplifier (200W/RMS/CH @ 4 ohms or better) and re-evaluate your position. Enjoy the results in any case. Peter Wieck Wyncote, PA |
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#5
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
Peter Wieck wrote:
Some misconceptions: There are a lot of misconception to do with "volume", and especially as it relates to "watts". At one watt average, with moderately dynamic source, you will need at least a 100-watt amp to get clean peaks absolutely without clipping. A peak-to-average ratio of 20 dB: moderate?? Components of the vintage of your Heath more-or-less send straight DC into the speaker when driven to clipping. Some pathological cases do, the vast majority don't. Again, put another way, it is (Very typical of vintage SS amps) amplifiers of low power that burn speakers, not high power. This has been pretty thoroughly debunked elsewhere, check out Rane's application notes on apolifier clipping and tweeter life. Those of us who dabble in tube equipment tend to ignore this simple truth as output transformers will not pass DC, so clipping is much softer and relatively harmless That tubes clip more softwly has almost nothing whatsoever to do with the fact that the output transformers do not pass DC. The clipping is software for two reasons: 1. The upper end gain transfer characteristics of the tube do not change as abruptly when limiting is reached, 2. The conbination of the tube's HF bandwidth, the transformer's HF bandwidth and the overall circuit's HF bandwidth can suppress some of the higher harmonics resulting from clipping. |
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#6
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
"Peter Wieck" wrote in message
... Jerry wrote: My den is quite small and my normal listening level is less than 1 watt, but occasionally I'll listen at levels higher. Jerry: There are a lot of misconception to do with "volume", and especially as it relates to "watts". At one watt average, with moderately dynamic source, you will need at least a 100-watt amp to get clean peaks absolutely without clipping. But you already know this. Put another way, a speaker with an 86dB @ 1-watt SPL, a 30-watt amp simply will not cut it, even bi-amped at anything above a very moderate volume. Agreed, mostly one listens at a very moderate volume. But not always. Peter, please remember that the 30 watt amp is driving strictly the mid and tweeter. Further, this amp sees greatly reduced fundamentals, so the voltage swings are NOTHING like what a single amp would experience. When playing music at levels I can just barely stand (somewhere around 10 -20 watts), PEAK output voltages in this amp are LESS than 7 volts. Now, one reason for that low output voltage is that I removed those blasted pots from the AR xover circuit. This netted a huge gain in sensitivity on just those drivers. So I must compensate by reducing volume or the mid/tweeter will overpower the woofer. To give you a feel ... the average voltage swings in the woofer amp are 5 times higher! Based upon all of the above, there is just no way that peak output voltage could ever get to the 40 volt rails in the mid/tweeter amp. Further, rail drop will be minimal given the low current drain of the mid and tweeter. Peter, please remember that I don't have the fundamentals going through that 30 watt amp and this significantly changes the game and the math. Components of the vintage of your Heath more-or-less send straight DC into the speaker when driven to clipping. And depending on the source (where, as it usually happens most of the signal is at the mid-range), that could be a lot of the time and pretty rapidly fatal to your speakers. Even if this is the case, (and I doubt that because I measured the filter caps and they are just fine), the mid and tweeter will never see the impact of the clipped waves and resulting harmonics in the woofer amp. Further, neither will the woofer as it's internal xover will reject those harmonics. Peter don't forget that the Heath amp sees reduced harmonics as well. Now this reduction is NOT as great as the reduction of fundamentals in the 30 watt amp, but still there is a significantly less harmonics "riding on fundamentals" than what a single amp would see. Again, put another way, it is (Very typical of vintage SS amps) amplifiers of low power that burn speakers, not high power. Almost any well-made speaker can handle normal signal at very high power, well beyond their nominal ratings, for brief periods. Even the very best conventional PM speakers will fry in short order when fed DC. I think this is what the OP was trying to convey. Peter, if we drove this 30 watt amp to the 40 volts rails, clipping would be the least of our problems. Both the mid and the tweeter would be smoking long, long before. With the fundamentals removed, the impact of stacking 40 volt rails on top of 40 volt rails in the Heath is equivalent to a 750 watt amp. Now this is from the vantage point of the mids/tweeters. (The woofer still sees the same 100 watt amp with but even it "goes further" without the harmonics.) Those of us who dabble in tube equipment tend to ignore this simple truth as output transformers will not pass DC, so clipping is much softer and relatively harmless (to the speaker, that is). Also why those of us who have high-powered amps (tube or SS) also tend to ignore this as there is enough headroom in any case to minimize consequences from clipping. Peter, you can ignore it, but the truth is that I have far, far more headroom than you with your "brute" ss amp. You must pass fundamentals with harmonics on top through that amp. Further, you still have the pots "sucking up power" in your AR-3a's and this also contributes to a reduction in headroom. Bi-Amping, even as you have applied it will create some advantages if very carefully managed. But the real-world difference between a 30 watt amp and two 30-watt amps split is limited in this *particular* application given that at any given moment about 75% of your signal will be at/within the midrange, and if you include the tweeter, that goes to about 90%, at least as it applies to the need for headroom. I would very strongly suggest that you make it your mission in Audio to beg, borrow or steal a well-made high-powered amplifier (200W/RMS/CH @ 4 ohms or better) and re-evaluate your position. Peter, are you saying that 75% of the signal STRENGTH is going to the mids/tweeters? This is totally incorrect and I have a dual channel scope to prove it. Less than 25% of the voltage swings are going to the mid/tweeter amp. Then if we integrated the signals to compute power, once again the real power is in the woofer amp ... BY A LOT! It's not even close. I have the equivalent of a 750 watt amp (@ 4 ohms - 375 watts @ 8 ohms). Why in heavens name would I want a mere 200 watt amp. That would be going backwards. Regards, Jerry |
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#7
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
Jerry wrote:
I have the equivalent of a 750 watt amp (@ 4 ohms - 375 watts @ 8 ohms). Why in heavens name would I want a mere 200 watt amp. That would be going backwards. Jerry: Pulling the pots from the AR3(a) is about the first thing that should be done, they were barely functional when-new. So we are 100% agreed there. But per any of the several web-sites dedicated to these speakers (Layne, Vintage, et.al.) this gives only about a 3dB gain (assuming otherwise good pots) over the pots in the max (theoretically straight-wire) position. And, as to the "equivalent" of a 750 watt amp, _ALL_ I am suggesting is that unless you have tried the brute-force amp and rejected it as being unsatisfactory as compared to your present set-up, you REALLY SHOULD try it. The results may well be revealing. At the very least they will endorse your present position. Yeah, a good part of the stuff I listen to regularly does have a 20dB P/A, the room is fairly large, and I do like to run at a moderately high volume... trumpets like that. And even the solo human voice with an orchestral background likes that. Peter Wieck Wyncote, PA |
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#8
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
Peter Wieck wrote on 10/7/2006:
Jerry wrote: I have the equivalent of a 750 watt amp (@ 4 ohms - 375 watts @ 8 ohms). Why in heavens name would I want a mere 200 watt amp. That would be going backwards. Jerry: Pulling the pots from the AR3(a) is about the first thing that should be done, they were barely functional when-new. So we are 100% agreed there. But per any of the several web-sites dedicated to these speakers (Layne, Vintage, et.al.) this gives only about a 3dB gain (assuming otherwise good pots) over the pots in the max (theoretically straight-wire) position. Hmmm, 3db seems low, but could be right. I should have, but didn't measure before and after. In any event, a 3db reduction in power is equivalant to cutting the power to the mids/tweeters in HALF. That is, I now send half as much power to the mids/tweeters and get the same SPL. This equates to a terriffic gain in headroom and issures clipping in the mid/tweeter amp will never happen. Peter, you never told me you removed the pots from your AR-3a's! You can't do this, Peter, and mainain the proper sound balance UNLESS you compensate with fixed resistance in the xover. So, Peter, what mod did you perform upon your 3a's? And, as to the "equivalent" of a 750 watt amp, _ALL_ I am suggesting is that unless you have tried the brute-force amp and rejected it as being unsatisfactory as compared to your present set-up, you REALLY SHOULD try it. The results may well be revealing. At the very least they will endorse your present position. Alas, I can't do this anymore. With the pots out, there is no way to power my AR with a single amp and maintain any semblance of balance between the energy sent to the woofer and that sent to the mids and tweeters. And I'm NOT putting those pots back in the circuit. I itched for three days the last time I opened those boxes. Peter did you ever look at your audio signal on a scope to see the height of the transients? I look fairly often and I've never seen peak voltages as high as you describe. Regards, Jerry |
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#9
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
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#10
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
John Stone wrote:
I thought all your bi-amping adventures were done with the purpose of not altering the original design of your AR3a's. John, the various mod sites noted emphasize the need for a fixed resistor in place of the pot, simply bypassing the adjustable function. Yes, *maybe* get them out of the circuit, no, do not alter the overall function of the circuit. http://layneaudio.hypermart.net/AR3aXorig.gif shows the OEM crossover arrangement. One may either bypass the wiper directly to the 0-ohms position, or remove the pot inserting a 16-ohm resistor and tying the correct leads at the 0-ohm position. (above link courtesy Layne Audio) Classic Speaker Pages: http://www.classicspeakerpages.net/a...ar-3a/ar3a.htm All sorts of information and suggestions. Peter Wieck Wyncote, PA |
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#11
Posted to rec.audio.high-end
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AR3a/AS103a speakers and the Heathkit AR1500 receiver
John Stone wrote on 10/7/2006:
I thought all your bi-amping adventures were done with the purpose of not altering the original design of your AR3a's. Well, now that you're in there changing things around, I'm wondering if you did anything to compensate for the removal of those pots from the circuit. They place a 16 ohm load across the mid and tweeter crossover circuits, and that load forms part of the filter network. Did you put a fixed 16 ohm resistor in the circuit in their place? If not, you've certainly altered the crossover responses and consequently, the speaker response. (I can't wait to see whether you're going to agree with this). I'm also trying to figure out how you got such a huge gain in sensitivity by removing the pots. Turned all the way up, the mid and tweeter are directly connected to the crossover outputs, so the pot is out of the circuit other than presenting a parallel 16 ohm load. Unless those were the world's most defective pots-and you would have easily been able to tell from the intermittent operation-then I don't see where you're picking up all the sensitivity from. No, John! No pots and no fixed resistors. Pots are removed from the circuit. The real purpose of the pots was to balance the energy going to the more sensitive drivers (mid and tweeters) in relationship to the energy going to the woofers. In short, they were "padding" to bring all the drivers in balance, plus they allowed some minor tweaking. Now most folks have long set the tweeter pot to max as is recommended everywhere. Further, removing the tweeter pot was even suggested by Chuck McShane. See: http://www.classicspeakerpages.net/ar/ar-9/3and9.txt So, John, this leaves just the mid pot and it has a dramatic impact on the balance of music ... no question about it. Any adjustments to this pot dramatically impact the balance of high vs low frequencies. I don't think anyone would argue this as a simple test of moving the pot form end to end results in dramatically different sound. Your question is how can I remove this pot and still maintain the proper balance. Well, for me, John, the pot is redundant. I still have total control over the balance of energy going to the woofers vs the energy going to the mids/tweeters ... only instead of pots, I use volume controls. Yes, the volume controls on the amps that independently power each half of the speaker. I think you'll agree, John, that over time it's far, far easier to maintain low power volume controls than it is to maintain the high power pots in the speakers. As for the improvement in sensitivity, all I can say it's significant and very noticable. That 16 ohms still draws current that produces no sound. If we assume that the impedance of the driver over it's frequency range averages 4 ohms. Then if the pot is set to max increase, approx 25% of the current going through the driver is also flowing through the pot and ... producing zero sound. On top of this, I never had the mid pot set to max. That would be way too "bright" and in my room would NOT be balanced. So when I removed the pots the padding that was going on, is no longer happening. So to balance the speakers, I send significantly less power to the mids and tweeters and my guess is the actual current flowing through them is very similar to what it was when the pots were in the circuits. John, when I removed the pots, I never claimed any change in sound with the minor exception that my left speaker no longer loses mid-range completely any more. I'm also amazed at how much work you've gone through to deviate from the original design intent of the AR3a's. If it was me, I would put them back to stock and sell them. They still bring decent money. Then I would invest in some real DIY loudspeaker building using the much more robust and better performing loudspeaker components that are available today. You could then tweak to your heart's desire without the worry of blowing fragile drivers that are no longer available. You seem like the type that loves to experiment. Why limit yourself to such old technology that will never reach today's performance levels no matter what you do? John, I don't believe that I have changed the intent of the AR3a's. I think they sound terrific and I believe that any amp will drive them better with the complex xover network split. Further these drivers are somewhat unusual. AR made both the mid and tweeter because they could NOT find drivers of comparable quality. The woofer they purchased, but they specified all parameters so that it matched the closed cabinet. Regards, Jerry |
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