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#1
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Best way to connect multiple Speakers?
Here is my question:
I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas |
#2
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Kevin Killebrew wrote:
Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch..... Kevin Killebrew Austin, Texas This subject is treated in the RDH4 book, chapter 21, bottom of page 885: proper loading of two sets of two speakers, all of the same impedance, using either pair or both at once. ---- Chapter 21 is only 569 kb so it's not much to download to take a look, if you don't have a copy of RDH4 around. |
#3
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Kevin,
I was wondering the same thing as you just yesterday when I picked up two speakers in someone's garbage. I live nearby in Round Rock Texas. I also have a Sherwood S8000 that I restored, and then got carried away with modifications on. The wires were really bad with insulation falling off everywhere, and the power transformer wires were all bare! You might laugh, and then sigh, when you see the pictures at the following link: http://pages.prodigy.net/jcandela/Sherwood_S8000/ I hope your transformers are in better shape than mine. I also had to rebuild the audio output transformers in a similar way Here is where the modifications began: Proto boarding with 500 volts can be interesting! Always use a variac, ac ammeter, use one hand, and stand on a piece of dry plywood!: http://pages.prodigy.net/jcandela/Sh...rotoboard1.JPG Close up of the proto board: http://pages.prodigy.net/jcandela/Sh...rotoboard2.JPG Final circuitry with description of changes: http://pages.prodigy.net/jcandela/Sh.../regulator.htm As for your question, I personally would run on the 4 ohm tap. That way you will have maximum power available when running both sets of speakers. Also if you load the amplifier with 4 ohms when on the 8 ohm tap, the reflected plate to plate impedance will be reduced, and if you crank the volume when in this condition, expect higher peak currents drawn by the 7868 plate and screen, as well as higher distortion. The poor "stock" power supply regulation of the Sherwood will really show it's ugly head too and this will muddy the bass, and cause the FM multiplex to switch in and out of lock in response to the audio bass notes. Let me know how you proceed, and how it works out.. Regards, Jim Candela "Kevin Killebrew" wrote in message ... Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas |
#4
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Kevin Killebrew wrote:
Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load Running 4 ohms on the 8 ohm tap will not hurt the amp. Tube amps don't mind lower impedance loads, what they don't like is no load. The amount of undistorted audio power will be a bit lower with the mismatch, but other than that it will work fine. |
#5
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Kevin Killebrew wrote:
Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas Whatever you do, don't wire the speakers in series. For a two speaker hookup, that removes the amplifier damping from both speakers since they would be then be sourced through each other, an intolerable situation. Better a lower Z match than higher since PP pentode amps benefit from that. The 3rd harmonic is reduced. Most speaker loads are far too high anyway at their resonance(s). A parallel hookup tends to smooth that out provided the speakers are dissimilar. Good Luck, John Stewart |
#6
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John Stewart wrote: Kevin Killebrew wrote: Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas Whatever you do, don't wire the speakers in series. For a two speaker hookup, that removes the amplifier damping from both speakers since they would be then be sourced through each other, an intolerable situation. The DF issue is not always important You can have 10 speakers in series and if each has the same enclosure, and each driver is the same, the response will be identical to a single driver driven with the same low Rout, and this proves the amp damping doesn't reduce. In fact it increases. If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10. If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50. People who build line array speakers using a dozen or more drives know what I am saying to be true, or else they would be compelled to connect all speakers in parallel, giving input impedances of very low value. However, where you have two different brands of speakers, different boxes and drivers, series speakers will produce big variations in response from each because the impedances of each are very dissimilar at different F. So its for this reason you wouldn't want to series speakers. Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel. Pentode amps are a bit queer. They have a load optimum where thd is lowest and each side of that thd tends to rise and thd is very high when RL is very high. But the amp NFB also becomes most effective when RL is high, when the tube gain is high, and also the PP pentode amp works in class A when RL is high, when it isn't producing power in class AB where RL is low. This is an idiotic statement, but speaker Z varies for different F, and either its working in AB or it isn't, but one has to contemplate that the amplifier is sending a voltage x current signal to a constantly changing load value. Since its almost impossible to think of 3 things at once, I leave you to contemplate the tube's operation knowing that to win a game of chess against a naked woman and keep an erection and do your tax return all simultaneously is downright impossible. Better a lower Z match than higher since PP pentode amps benefit from that. The 3rd harmonic is reduced. Most speaker loads are far too high anyway at their resonance(s). A parallel hookup tends to smooth that out provided the speakers are dissimilar. Using two brands of speakers in parallel with different box resonances at LF will indeed tend to produce a more even impedance. But also its likely that at some F the impedance will be lower then 4 ohms. Some makers have deliberately different enclosure volumes and Fb for a pair of bass drivers in the one speaker cabinet. It makes the resonant Z peaks overlap, and a much flatter Z is realised. Its all in RDH4, with a graph. Patrick Turner. Good Luck, John Stewart |
#7
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On Mon, 25 Apr 2005 15:28:52 GMT, "Kevin Killebrew"
wrote: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Modern multiple-room home installations often use stepped autoformer level controls, wall-mounted in each room. Two advantages: the volume is adjustable locally, and the unused speakers don't load the amplifier, proportionally. Disadvantages: pentode output stages might well resist being so treated, and decent-but-not-Nirvana level controls like the "Niles" brand run about $70US. And whatever you decide to do, remember to (be able to) turn off the outside speakers, especially if your neighbors are Texans. Good fortune, Chris Hornbeck "This has been an account for those who don't keep them" J-LG, _Tout Va Bien_ 1972 |
#8
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Patrick Turner wrote:
John Stewart wrote: Kevin Killebrew wrote: Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas Whatever you do, don't wire the speakers in series.For a two speaker hookup, that removes the amplifier damping from both speakers since they would be then be sourced through each other, an intolerable situation. The DF issue is not always important We can see that as some folks still choose to use pentode output amps with no voltage NFB. For example, it may work well for some musicians in order to get the sound they want. You can have 10 speakers in series and if each has the same enclosure, and each driver is the same, the response will be identical to a single driver driven with the same low Rout, and this proves the amp damping doesn't reduce. Within the limits of the similarities of the enclosures & drivers, true enough. As well as the minor differences between enclosures & drivers, in actual practice there will be a spread of response since the boxes will all have to be in physically different space. As a group they do not constitute a point source. So each speaker it it's box will have a unique response curve, unlike any of the others. Your statement doesn't prove anything. It simply makes an assumption & a poor one at that. In fact it increases. If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10. If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50. In your example the DF for the group is 50. However, for each speaker the source impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of the other nine speakers in the group. That is a condition I would prefer to avoid. An even more important consideration is, how will we deliver power to the load? In your example the load reflected to the output tubes (or transistors) is now 10X nominal. So power available is drastically reduced. As well, if a pentode output, distortion goes way up due to a gross mismatch. People who build line array speakers using a dozen or more drives know what I am saying to be true, or else they would be compelled to connect all speakers in parallel, giving input impedances of very low value. Not at all. Systems are easily built using series/parallel connexions. A set of four 5 ohm speakers is still 5 ohms when series paralleled. The same is true for 16 speakers & these kinds of things have been built by their fans for many years. I recall well a friend building something called the 'Sweet 16' more than 45 years ago. It used a set of sixteen 6X9 speakers in a single enclosure. It sounded pretty good too. In a recent magazine article I see a project using four speakers per enclosure, again connected series/parallel in order to maintain a useable impedance level. However, where you have two different brands of speakers, different boxes and drivers, series speakers will produce big variations in response from each because the impedances of each are very dissimilar at different F. That is what I recommended against in the first place. Why do we need to hear that again from you? I certainly don't need your confirmation. So its for this reason you wouldn't want to series speakers. Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel. Pentode amps are a bit queer. They have a load optimum where thd is lowest and each side of that thd tends to rise and thd is very high when RL is very high. True enough. But how does that make a pentode a 'bit queer'? It is simply a property of pentodes, which can be exploited. That is why I recommended using the 8 ohm tap. By paralleling the load resulting in a mismatch on the low side, the 3rd harmonic is reduced. If we were to go the other way as you have recommended, 3rd harmonic instead goes way up. The choice IMO is obvious to anyone who carefully considers how a pentode works. But the amp NFB also becomes most effective when RL is high, when the tube gain is high, and also the PP pentode amp works in class A when RL is high, when it isn't producing power in class AB where RL is low. When the Rl is higher (or lower) than optimum in a pentode amp, distortion increases. Than more FB is required to get the distortion down to a reasonable level. In a PP amp the even order (2nd, 4th, 6th...Etc) harmonics are reduced by the circuit. All the other noise & distortion products are still in there. In all the ordinary amp topologies, voltage NFB is applied around the entire amp to reduce the D. However, by doing that there is a danger of manufacturing some intermodulation products that were not originally present. NFB of this kind does not fix the D in the output stage. It simply tries to correct what is already there while at the same time it is causing other problems. Better no miss match in the first place. This is an idiotic statement, but speaker Z varies for different F, What is idiotic about that? It is a set of complex impedances, a property of the speaker in it's enclosure & dependent on the space it is in, as well. and either its working in AB or it isn't, but one has to contemplate that the amplifier is sending a voltage x current signal to a constantly changing load value. Since its almost impossible to think of 3 things at once, I leave you to contemplate the tube's operation knowing that to win a game of chess against a naked woman and keep an erection and do your tax return all simultaneously is downright impossible. What has any of that got to do with the original query? Or is it simply another 'Fog of Bull****' to cover something else you are unsure of? Better a lower Z match than higher since PP pentode amps benefit from that. The 3rd harmonic is reduced. Most speaker loads are far too high anyway at their resonance(s). A parallel hookup tends to smooth that out provided the speakers are dissimilar. Using two brands of speakers in parallel with different box resonances at LF will indeed tend to produce a more even impedance. But also its likely that at some F the impedance will be lower then 4 ohms. Some makers have deliberately different enclosure volumes and Fb for a pair of bass drivers in the one speaker cabinet. It makes the resonant Z peaks overlap, and a much flatter Z is realised. And easy to simulate for anyone with simulation software. Its all in RDH4, with a graph. And many other references, as well. If one thinks about what is happening here, this whole exercise (the debate) is ridiculous. Kevin K asked a simple question & several responded with straight forward & too the point answers. Kevin told us the speakers are in different locations ( And probably different speakers). As one of my points, I simply recommended against a series connexion for the speakers. But our Patrick T knows better & proceeds to go into another of his many long winded explanations, much of which was completely unrelated to the query. Worse still, parts of his response are in error. It seems Patrick T is for some unknown reason is driven to respond to every post that shows up on the NG. He is far from the expert he pretends to be. Others should be aware of that. Be sure to get a second opinion! John Stewart Patrick Turner. Good Luck, John Stewart |
#9
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"John Stewart" wrote
... In your example the DF for the group is 50. However, for each speaker the source impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of the other nine speakers in the group. That is a condition I would prefer to avoid. ... Seems to me this is the crucial point, John. Could be seen as another example of current sources in series. cheers, Ian |
#10
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John Stewart wrote: Patrick Turner wrote: John Stewart wrote: Kevin Killebrew wrote: Here is my question: I've got a tubed receiver (Sherwood S8000) that is going to be connected to two sets of speakers via an external switch. One set is outside on the patio, the other inside at the bar. Most of the time only one set of speakers will be playing, but occasionally I forsee both sets being played simultaneously. Both sets of speakers are 8 ohms so when I occasionally run both pairs I will have four ohms (assuming they are paralleled) per channel. Which receiver output should I run to the switch box? Four or eight ohms? Is it best to run an occasional four ohms on the eight ohm tap? or run mostly eight ohms on the four ohm tap having a four ohm load only when both sets of speakers are playing? I suspect the eight ohm tap is the right choice because it will be correct most of the time, but I wanted some RAT opinions of the effect of running mismatched impedances, i.e., four ohm load on an eight ohm OT tap. I want to avoid an overly complex switch arrangement. Kevin Killebrew Austin, Texas Whatever you do, don't wire the speakers in series.For a two speaker hookup, that removes the amplifier damping from both speakers since they would be then be sourced through each other, an intolerable situation. The DF issue is not always important We can see that as some folks still choose to use pentode output amps with no voltage NFB. For example, it may work well for some musicians in order to get the sound they want. Many old radios used a 6V6 with no NFB. Above a very low level they sounded attrocious. Better than no radio. And sure, with muso amps, DF doesn't matter much but many guitar amps do have some NFB. You can have 10 speakers in series and if each has the same enclosure, and each driver is the same, the response will be identical to a single driver driven with the same low Rout, and this proves the amp damping doesn't reduce. Within the limits of the similarities of the enclosures & drivers, true enough. I didn't mean it any other way. As well as the minor differences between enclosures & drivers, in actual practice there will be a spread of response since the boxes will all have to be in physically different space. Nope, you can have a line away of speakers and they all share the same box. The dozen+ drivers used act like one big long driver, with a dozen times the box volume. You could even replace all the cones with a single long lightweight diaphram glues to the voice coils, and curved in one direction, like a pipe cut down the centre, and with a suspension rubber along the edges and at the ends. That would make all the drivers improve their team work. As a group they do not constitute a point source. That doesn't matter to the builders of large line arrays. So each speaker it it's box will have a unique response curve, unlike any of the others. Your statement doesn't prove anything. It simply makes an assumption & a poor one at that. You are not proving anything either, except you are giving a lesson in side tracking. Muliple 2 way bookshelf speakers can be hooked up in series without changing the near field response of each or how they operate. They just have to be the same brand, model number, and fairly well matched. Its routine to make speakers closely matched. Such matching gives the best imaging where a stereo pair is used. But we are talking about DF and whether or not is is decreased, ie, DF number is lower, just because we series connect speakers. Clearly I have made my point that if one connects a dozen of the *same* bookshelf speakers in series, the DF is actually improved if the same amp with the same Ro is used. In fact it increases. If you have Ro = 0.5 ohms and a 5 ohm speaker, DF = 10. If you have 10 series speakers of 5 ohms for 50 ohms, same Ro, DF = 50. In your example the DF for the group is 50. However, for each speaker the source impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of the other nine speakers in the group. That is a condition I would prefer to avoid. So would I, so line array builders connect groups of say 4 speakers in series, so 4 give 32 ohms, then they might have 4 groups of 4 in parallel, pulling the Z to 8 ohms, and the same as one single driver. DF then is the same as one driver. An even more important consideration is, how will we deliver power to the load? In your example the load reflected to the output tubes (or transistors) is now 10X nominal. So power available is drastically reduced. As well, if a pentode output, distortion goes way up due to a gross mismatch. See above. When multiple drivers are seriesed and paralleled for the correct Z wanted for a good amp match there are no problems. If somebody wants to ties multiple speakers together more tightly rather than hooking them up in series and expecting them all to behave with the same parameters then I suggest one use a tapped auto transformer with say 4 equal windings in series and one could have 4 x 8 ohm speakers across each winding for 2 ohms, then with 4 windings loaded by 2 ohms each you have 8 ohms input for 16 drivers, and they are all locked together magnetically. People who build line array speakers using a dozen or more drives know what I am saying to be true, or else they would be compelled to connect all speakers in parallel, giving input impedances of very low value. Not at all. Systems are easily built using series/parallel connexions. Ah, but as I said you have to series speakers in line arrays. A set of four 5 ohm speakers is still 5 ohms when series paralleled. The same is true for 16 speakers & these kinds of things have been built by their fans for many years. I recall well a friend building something called the 'Sweet 16' more than 45 years ago. It used a set of sixteen 6X9 speakers in a single enclosure. It sounded pretty good too. In a recent magazine article I see a project using four speakers per enclosure, again connected series/parallel in order to maintain a useable impedance level. So, what's wrong with seriesing speakers? However, where you have two different brands of speakers, different boxes and drivers, series speakers will produce big variations in response from each because the impedances of each are very dissimilar at different F. That is what I recommended against in the first place. Why do we need to hear that again from you? I certainly don't need your confirmation. So its for this reason you wouldn't want to series speakers. Set the amps for 4 ohms where you want to have a pair of 8 ohms in parallel. Pentode amps are a bit queer. They have a load optimum where thd is lowest and each side of that thd tends to rise and thd is very high when RL is very high. True enough. But how does that make a pentode a 'bit queer'? It is simply a property of pentodes, which can be exploited. That is why I recommended using the 8 ohm tap. By paralleling the load resulting in a mismatch on the low side, the 3rd harmonic is reduced. If we were to go the other way as you have recommended, 3rd harmonic instead goes way up. The choice IMO is obvious to anyone who carefully considers how a pentode works. But if you palce two 8 ohms speakers in parallel across a 8 ohm outlet from a pentode amp, RLa-a is halved, and all the measurements i HAVE EVER CONDUCTED indicate thd rises heaps, and at high sustained levels the tubes will glow red hot and expire. The gain of pentodes is about proportional to their loads, so if load is halved, so is the output tube gain and so is the amount od applied NFB so the thd rises because of this fact and the fact that the lower load increases thd anyway. I am suggesting ppl try to keep the amp working with its recommended correct RLa-a, ie, with a pair of parallel 8 ohm speakers = 4 ohms, and connected to a 4 ohm outlet if there is one. But if ppl connect 8 ohms to the 4 ohm outlet, the pentode AB amp will make marginally less maximum power, and its output tube gain will near double, and the slight increase in thd due to a higher than ideal load match will be offset by a near doubling of the applied NFB. Applied NFB depends on open loop gain Without any load, and with NFB, pentodes don't have an alarming increase in thd. But the amp NFB also becomes most effective when RL is high, when the tube gain is high, and also the PP pentode amp works in class A when RL is high, when it isn't producing power in class AB where RL is low. When the Rl is higher (or lower) than optimum in a pentode amp, distortion increases. Than more FB is required to get the distortion down to a reasonable level. In a PP amp the even order (2nd, 4th, 6th...Etc) harmonics are reduced by the circuit. All the other noise & distortion products are still in there. But as I said, the NFB is increased hugely with the worst case situation, no load. A properly designed pentode amp with 20 Db of NFB only has 20 dB of FB when RL = the rated load, say 8 ohms. output tube gain is perhaps 25. But with no load at all, tube gain leaps to near µ for the tubes at mid frequencies, so gain = 120 for EL34. So the applied NFB with no load leaps about 5 times, or 15 dB to 35 dB. So the thd is controlled. In all the ordinary amp topologies, voltage NFB is applied around the entire amp to reduce the D. However, by doing that there is a danger of manufacturing some intermodulation products that were not originally present. IMD is more likely to be made in amps with low NFB due to NFB application. Where NFB application is high, there is less IMD, period. NFB of this kind does not fix the D in the output stage. Yes it does, where the tubes are not driven into clipping, and have enough gain and are not driven into grid current. It simply tries to correct what is already there while at the same time it is causing other problems. Better no miss match in the first place. Sure, but speakers present loads which are always going to provide a mismatch at some F. This is an idiotic statement, but speaker Z varies for different F, What is idiotic about that? It is a set of complex impedances, a property of the speaker in it's enclosure & dependent on the space it is in, as well. and either its working in AB or it isn't, but one has to contemplate that the amplifier is sending a voltage x current signal to a constantly changing load value. Since its almost impossible to think of 3 things at once, I leave you to contemplate the tube's operation knowing that to win a game of chess against a naked woman and keep an erection and do your tax return all simultaneously is downright impossible. What has any of that got to do with the original query? Or is it simply another 'Fog of Bull****' to cover something else you are unsure of? Speakers present a dynamically varying load with the signal whose frequency content varies dynamically along with amplitude. Should one contemplate what is happening at their amp's output and within the amp, one should remember that the load is not a resistance of one value. Because all speakers supply a degree of mismatch, their power ceiling is somewhat restricted below the ideal resistance value that gives the ideal power. Series speakers are OK if well considered, but not if they differ widely in specs. Nearly all pentode power amps are set up to cope with twice the average rated load value quite well, but certainly not with half the recommended load value. Better a lower Z match than higher since PP pentode amps benefit from that. The 3rd harmonic is reduced. Most speaker loads are far too high anyway at their resonance(s). A parallel hookup tends to smooth that out provided the speakers are dissimilar. Using two brands of speakers in parallel with different box resonances at LF will indeed tend to produce a more even impedance. But also its likely that at some F the impedance will be lower then 4 ohms. Some makers have deliberately different enclosure volumes and Fb for a pair of bass drivers in the one speaker cabinet. It makes the resonant Z peaks overlap, and a much flatter Z is realised. And easy to simulate for anyone with simulation software. Its all in RDH4, with a graph. And many other references, as well. If one thinks about what is happening here, this whole exercise (the debate) is ridiculous. Kevin K asked a simple question & several responded with straight forward & too the point answers. Kevin told us the speakers are in different locations ( And probably different speakers). As one of my points, I simply recommended against a series connexion for the speakers. Let us grant Kevin that he has adequate intelligence to draw from what we have said what he needs to know. The conclusive simple recommendation is series speakers are OK if they are exactly the same. This won't hurt the amp. If the speakers are different, paralleling them is all you should do. But the combined nominal load may be too low for the amp and cause heat stress, unless you can move the speakers from the 8 ohm connection point to the 4 ohm connection point if there is one. Other matters raised about series/parallel speakers are just salient matters raised, and should not distract from the simple answer. But our Patrick T knows better & proceeds to go into another of his many long winded explanations, much of which was completely unrelated to the query. Worse still, parts of his response are in error. It seems Patrick T is for some unknown reason is driven to respond to every post that shows up on the NG. He is far from the expert he pretends to be. Others should be aware of that. Be sure to get a second opinion! But I don't recommend paralleling speakers just to get the 3rd harmonic down, which won't occur as you claim, and I won't recommend loading an amp adversly with a lower impedance load that it was designed for. Feel free to get as many opinions as you want. I am not worried by 100 different opinions. And at least I take no notice of your consisten complaints about my long posts. I reserve the right to talk around and through a subject in detail, and if you don't like to have to read what people post here, then you are not interested in details, and the nitty gritty. This is a discussion group, and nobody has the right to tell me how much I should say. If its awkward for you to post an answer then so be it. And BTW, I don't reply to every post, that's another thing you got wrong. Patrick Turner. John Stewart Patrick Turner. Good Luck, John Stewart |
#11
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Ian Iveson wrote:
"John Stewart" wrote ... In your example the DF for the group is 50. However, for each speaker the source impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of the other nine speakers in the group. That is a condition I would prefer to avoid. ... Seems to me this is the crucial point, John. Could be seen as another example of current sources in series. cheers, Ian Nature (And Physics) abhor certain conditions. One of them is current sources in series. By way of 'The Principle of Duality' another would be voltage sources in parallel. The definition of each should tell us something. A perfect current source would deliver the same current to the load no matter what the load was from a short circuit & on up. Similarly, a voltage source would deliver a constant voltage to the load from open circuit & on up to ever increasing currents. Fortunately for us, most of the circuits we have to deal with include some positive resistance in parallel with a current source & a positive resistance in series with a voltage source. That helps us quite a lot to establish an operating point. Without those resistors the circuit Q point becomes indeterminate. So a simple current source while not perfect would be able to source a reasonably constant current over a wide range of loads. The max load is simply limited by the max voltage available. If a negative resistance were to be introduced into the circuit than we have the potential for an oscillator. So, I would sure agree with your point Ian, in particular at the resonance F of the speakers. Z tends to get quite high at that (those if a bass reflex) point. Even at mid-frequencies far from the speaker resonance, nine speakers would add up to 45 ohms. The source for each speaker if all were simply series connected as Patrick T remarked in the first paragraph of his original post of April 27th would than be 45.2 ohms. For each individual speaker, particularly at resonance the remaining speakers form a rather good current source. If Patrick didn't mean them to be all wired in series he should have made that clear in his post. The second paragraph of his original post has the speakers all in parallel. What next? I see in Patrick's later response to my critic of his post that he has suddenly got the clue to series/parallel the speaker array. I wonder where he got that idea! As always with anything, including technical advice, Caveat Emptor. More to go on this yet. And, Cheers John Stewart PS- I guess I will have to post the graph showing individual (2,3,4..) harmonics for pentodes. It is not it in RDH4, but it does show up elsewhere. That way one can see why a steeper load line is a better condition for pentodes, again opposite Patrick's opinion. Best example without having to look it up would be the steeper loadline applied to PP amps. That helps to reduce the 3rd harmonic & one of the reasons we do it. Depending on speaker resonance & program material, the tube is always well loaded, both above & below the knee. Below the knee at speaker resonance & above the knee at mid-frequencies. JLS |
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John Stewart wrote: Ian Iveson wrote: "John Stewart" wrote ... In your example the DF for the group is 50. However, for each speaker the source impedance would be the amp Ro of 0.5 ohms plus the entire series complex impedance's of the other nine speakers in the group. That is a condition I would prefer to avoid. ... Seems to me this is the crucial point, John. Could be seen as another example of current sources in series. cheers, Ian Nature (And Physics) abhor certain conditions. One of them is current sources in series. By way of 'The Principle of Duality' another would be voltage sources in parallel. The definition of each should tell us something. A perfect current source would deliver the same current to the load no matter what the load was from a short circuit & on up. Similarly, a voltage source would deliver a constant voltage to the load from open circuit & on up to ever increasing currents. Fortunately for us, most of the circuits we have to deal with include some positive resistance in parallel with a current source & a positive resistance in series with a voltage source. That helps us quite a lot to establish an operating point. Without those resistors the circuit Q point becomes indeterminate. So a simple current source while not perfect would be able to source a reasonably constant current over a wide range of loads. The max load is simply limited by the max voltage available. If a negative resistance were to be introduced into the circuit than we have the potential for an oscillator. So, I would sure agree with your point Ian, in particular at the resonance F of the speakers. Z tends to get quite high at that (those if a bass reflex) point. Even at mid-frequencies far from the speaker resonance, nine speakers would add up to 45 ohms. The source for each speaker if all were simply series connected as Patrick T remarked in the first paragraph of his original post of April 27th would than be 45.2 ohms. For each individual speaker, particularly at resonance the remaining speakers form a rather good current source. If Patrick didn't mean them to be all wired in series he should have made that clear in his post. The second paragraph of his original post has the speakers all in parallel. What next? I see in Patrick's later response to my critic of his post that he has suddenly got the clue to series/parallel the speaker array. I wonder where he got that idea! You have quoted me out of context and you alleges that I got the idea of series and parallel combos of speakers from you. Not so; known about all this for years before ever hearing what you had to say. But where you have say 4 speakers in series and each is 45 ohms at Fo, then then one could say each one sees a drive impedance of the Ro of the amp in series with 135 ohms, and the DF is indeed very crook. But it isn't, its very good. Where the impedance of all the series speakers is the same, there is an equal division of power in each one, and at all F, and the response from each will be the same for the lot. There is no manifestation of a speaker seeming to be driven by an impedance of 4 times its own, just because you have series speakers. This was the point I have been trying to make. I have series speakers in a Dapollito arrangement, and there is no DF problem. As always with anything, including technical advice, Caveat Emptor. More to go on this yet. And, Cheers John Stewart PS- I guess I will have to post the graph showing individual (2,3,4..) harmonics for pentodes. It is not it in RDH4, but it does show up elsewhere. That way one can see why a steeper load line is a better condition for pentodes, again opposite Patrick's opinion. One could have a very low RL, akin to a short circuit, say 500 ohms for an EL34. The distortion will be an unhappy picture. IN fact the distortion with low loads becomes dominated purely by the gm variation at varying Ia, and boy, do most pentodes vary! Triodes have varying gm with Ia. Triodes are virtual pentodes with a shirtload of internal NFB, so that with no Ia change there is maximum NFB applied electrostatically, hence they are very linear with no current change, as when loaded with a ccs. Pentodes on the other hand are attrociously non linear when no NFB is applied internally because the screen stops it. Gain when loaded by a CCS is extremenly high, about 130 for EL34, and maybe 3,000 for a 6AU6. Fortunately, the huge gain gives us an ability to apply NFBV in external loops. But still you end up with the same spectra of thd products plus a few extras from imd, although at low levels. Still not as clean as triodes. But with either a too low load, or a too high load, pentodes and beam tetrodes are disgusting. Best example without having to look it up would be the steeper loadline applied to PP amps. That helps to reduce the 3rd harmonic & one of the reasons we do it. Next time you measure a class AB pentode amp try reducing the RL to 1/4 of the load needed for maximum pure class A. Then you find the amp works in class AB, and the 3H is far greater at all power levels. Beam tetrodes fare better than pentodes, if that makes you feel any better. 6550 can be configured to make under 1% thd at 40 watts in pure beam class A. Reducing RL isn't going to reduce the thd. Depending on speaker resonance & program material, the tube is always well loaded, both above & below the knee. Below the knee at speaker resonance & above the knee at mid-frequencies. JLS But music has multiple frequencies, During bass notes where the speaker Z = 40 ohms, the pentodes are loaded with a class A load, large v swing, low current swing. But *simultaneously*, at some other F, say 300Hz, the tubes are loaded by a Z at say a crossover F between bass and mids where Z = 3 ohms. What sort of load line appears then? Speaker load lines are dynamically variable, so a an elipse coloured grey for its whole enclosed area is a better idea of a speaker load. Resistances offer the only black line load. And in class AB amps the load line changes from 1/2 RLa-a to 1/4 RLa-a during each cycle which involves transition from A to AB. So with a fixed R load value at the output, the load seen by each of a pair in PP is a bent line, or a curved one. The changes in pentode gain between A and AB causes a lot of odd order thd. PP triodes exhibit the gentlest transition from A to AB, so they have the least odd order generation due to crossover, offset by the way gm increases with Ia, so as load drops with class AB, the gm increases, and so does gain...... Patrick Turner. |
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