Home |
Search |
Today's Posts |
#1
|
|||
|
|||
Biwiring tube amp with 0-8-16 ohms OPT
Hi,
what do you think: The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz |
#2
|
|||
|
|||
Behold, Heinz signalled from keyed 4-1000A filament:
Hi, what do you think: The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz Use a crossover on the *input* of the amps and your set, eh? :-) -- Gregg *It's probably useful, even if it can't be SPICE'd* http://geek.scorpiorising.ca |
#3
|
|||
|
|||
Biwiring is the misguided practice of using two sets of wires from one
pair of terminals to another. Biamping is using one amplifier for the treble and another for the bass, with a crossover before the amplifiers. What he's doing might be made to work, but not real well, with a custom transformer and driver impedances or Zobel networks. Basically not worth fooling with. |
#4
|
|||
|
|||
"Heinz" Hi, what do you think: The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz ** Connecting between the 8 and 16 ohm output taps effectively gives you a 1.4 ohms tap. ............ Phil |
#5
|
|||
|
|||
Do not understand why 1.4ohms. If the transformer is winded
symmetrical, the 0-8 and the 8-16 windings must be the same. Heinz "Phil Allison" wrote in message ... "Heinz" Hi, what do you think: The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz ** Connecting between the 8 and 16 ohm output taps effectively gives you a 1.4 ohms tap. ........... Phil |
#6
|
|||
|
|||
"Heinz" Do not understand why 1.4ohms. ** Tricks everyone. If the transformer is winded symmetrical, the 0-8 and the 8-16 windings must be the same. ** Not at all, the 0-4 and 4-16 windings have the same number of turns while the 8 ohm will have 1.41% times the 4 ohm one. Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8 ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance goes up with the SQUARE of the turns number. In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so the impedance is 0.29 squared times 16 = 1.35 ohms. ............. Phil |
#7
|
|||
|
|||
Phil Allison wrote: "Heinz" Do not understand why 1.4ohms. ** Tricks everyone. If the transformer is winded symmetrical, the 0-8 and the 8-16 windings must be the same. ** Not at all, the 0-4 and 4-16 windings have the same number of turns while the 8 ohm will have 1.41% times the 4 ohm one. Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8 ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance goes up with the SQUARE of the turns number. In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so the impedance is 0.29 squared times 16 = 1.35 ohms. ............ Phil Phil is correct on this tricky subject of transformer turns, although I wasn't tricked one bit. People could be almost forgiven for dreaming that the turns between the common end of a winding and the 8 ohm tap were equal to the 8 ohm tap and the 16 ohm end point of the winding and that the CT of the winding was the 8 ohm tap. But the 29 turns which Phil nominates is a match for 1.35 ohms, but who would ever make use of these turns? The wire dia use for the 29 turns would be the same as for the rest of the tapped winding, so if 1.35 ohms were used, the winding resistance would losses would be quite high, and the leakage inductance also high, and somewhat limited lossy HF transfer would occur. However, with a 0-8-16 ohm winding on an OPT, it would be possible to use each of the two outlet points for different Z bi-wired speakers. There could be two 8 ohms bass-midrange in D'apolito series configured speakers connected to the 16 ohm outlet, and the single 8 ohm tweeter may be tried on the 8 ohm tap, rather than have a resistive series divider to reduce the tweeter SPL should that need doing, as it so often is. The amplifier plate circuit would then remain matched to the correct nominal anode load. The only bother is that the NFB take off for the amp is usually on the whole of the 16 ohm winding, and corrections for errors from the tweeter HF content are not directly fed back, which may not be an all important issue, but it sure isn't ideal engineering either. On the plus side, consider the case where an SET amp without loop FB is concerned. A typical 300B might be loaded with an OPT of 5k to 16 ohms, which is a 312 :1 impedance ratio, so the Ra of 800 ohms is transformed to 2.56 ohms as measured as Ro at the secondary, plus winding resistances of say 0.8 ohms, so Ro = 3.36 ohms, which gives a damping factor of 4.7 for a 16 ohms speaker, and higher at bass F where typical speakers have a higher Z. If the tap at 8 ohms is used for a tweeter, then the Ro at this tap is 1/2 that at the 16 ohm point, because the 8 ohm tap is at 0.71 times total sec turns, and 0.71 x 0.71 = 0.5. So the Ro of the amp from the 8 ohm tap for the the tweeter is around 1.7 ohms, giving the same reasonable damping factor of over 4 for the tweeter if it is 8 ohms. There is the down side of building speakers needing two values of input voltages; they won't be usable with any other single input amp. And slight variations between amps with various output configs may make incompatibility worse, however any amp with 0-4-8 would have the correct relationship of voltages, as 0-8-16 has. 8 + 16 ohm speakers could be used on any amp with 0-4-8 tappings a tube amp with no harm done, and the expense of maximum power delivery, but with the bonus of less thd and less losses, and a better damping factor. Many speaker have series resistances added in series with them to attenuate the SPL of the speakers concerned and its usually the midranges and tweeters, because a man needs to hear as much bass as possible, and one way some makers enhance bass is by cutting midrange and treble. The down side of this practice is that the mids and treble are driven with far higher series impedance than that measured at the output of the amp. Should one want lower voltages to be fed to mids and treble to get a more balanced eq and flatter response, they could do worse than use a tapped toroidal transformer, using a 500 VA rated core, with a cross sectional core area of 50mm x 30mm and allowing for a voltage of 30vrms at 0.3 Tesla at 50 Hz, one could have about 300 turns of 1.5mm dia wire for a total whole winding, and with taps all along it at 10% intervals of the turns. if the hole dia was 50mm, then approx 98 turns per layer could be neatly wound onto the core, so 3 layers with insulation between would be possible, and doable by the keen diyer. This auto transformer can thus be used for a wide variety of impedance matching, and cope with powers of up to 100 watts into 16 ohms, perhaps more into 8 ohms, at slightly higher winding resistance losses. THD would be negligible, and bw quite wide, since all the turns are located in 3 layers on top of each other. It might seem stange to have yet another transformer in the signal path but nobody complains about the step up trannies in SL speakers. If the amp is connected to one end of the auto tranny winding, then as each tap down from the top of winding is tried, the Ro becomes lower and lower, which is the opposite effect of resistances in series with attenuation for speakers. The other side effect is that the amp sees a higher value load if a speaker is connected at a tap lower down the auto tranny. This may be a good thing if there is plenty of voltage swing headroom, but not if a high power is required to the increased load effectively harnessed to the tubes. All is not completely simple, which is usual for anything to do with electronics, and there are tricks galore for the uninitiated novice who has no experience. Patrick Turner. |
#8
|
|||
|
|||
"Bob Hedberg" So I take it that if I were to utilize a split primary step-up transformer between the two 8-16 windings, I could sum those two portions of the signal and drive a center-channel speaker? ** No. ........... Phil |
#9
|
|||
|
|||
So I take it that if I were to utilize a split primary step-up
transformer between the two 8-16 windings, I could sum those two portions of the signal and drive a center-channel speaker? bob Patrick Turner wrote: Phil Allison wrote: "Heinz" Do not understand why 1.4ohms. ** Tricks everyone. If the transformer is winded symmetrical, the 0-8 and the 8-16 windings must be the same. ** Not at all, the 0-4 and 4-16 windings have the same number of turns while the 8 ohm will have 1.41% times the 4 ohm one. Imagine the 16 ohm winding has 100 turns, then the 4 ohm has 50 and the 8 ohm has 71. A 2 ohm winding would have 35 turns. Remember - impedance goes up with the SQUARE of the turns number. In the above example there are 29 turns from 8 -16 (ie 100 minus 71) so the impedance is 0.29 squared times 16 = 1.35 ohms. ............ Phil Phil is correct on this tricky subject of transformer turns, although I wasn't tricked one bit. People could be almost forgiven for dreaming that the turns between the common end of a winding and the 8 ohm tap were equal to the 8 ohm tap and the 16 ohm end point of the winding and that the CT of the winding was the 8 ohm tap. But the 29 turns which Phil nominates is a match for 1.35 ohms, but who would ever make use of these turns? The wire dia use for the 29 turns would be the same as for the rest of the tapped winding, so if 1.35 ohms were used, the winding resistance would losses would be quite high, and the leakage inductance also high, and somewhat limited lossy HF transfer would occur. However, with a 0-8-16 ohm winding on an OPT, it would be possible to use each of the two outlet points for different Z bi-wired speakers. There could be two 8 ohms bass-midrange in D'apolito series configured speakers connected to the 16 ohm outlet, and the single 8 ohm tweeter may be tried on the 8 ohm tap, rather than have a resistive series divider to reduce the tweeter SPL should that need doing, as it so often is. The amplifier plate circuit would then remain matched to the correct nominal anode load. The only bother is that the NFB take off for the amp is usually on the whole of the 16 ohm winding, and corrections for errors from the tweeter HF content are not directly fed back, which may not be an all important issue, but it sure isn't ideal engineering either. On the plus side, consider the case where an SET amp without loop FB is concerned. A typical 300B might be loaded with an OPT of 5k to 16 ohms, which is a 312 :1 impedance ratio, so the Ra of 800 ohms is transformed to 2.56 ohms as measured as Ro at the secondary, plus winding resistances of say 0.8 ohms, so Ro = 3.36 ohms, which gives a damping factor of 4.7 for a 16 ohms speaker, and higher at bass F where typical speakers have a higher Z. If the tap at 8 ohms is used for a tweeter, then the Ro at this tap is 1/2 that at the 16 ohm point, because the 8 ohm tap is at 0.71 times total sec turns, and 0.71 x 0.71 = 0.5. So the Ro of the amp from the 8 ohm tap for the the tweeter is around 1.7 ohms, giving the same reasonable damping factor of over 4 for the tweeter if it is 8 ohms. There is the down side of building speakers needing two values of input voltages; they won't be usable with any other single input amp. And slight variations between amps with various output configs may make incompatibility worse, however any amp with 0-4-8 would have the correct relationship of voltages, as 0-8-16 has. 8 + 16 ohm speakers could be used on any amp with 0-4-8 tappings a tube amp with no harm done, and the expense of maximum power delivery, but with the bonus of less thd and less losses, and a better damping factor. Many speaker have series resistances added in series with them to attenuate the SPL of the speakers concerned and its usually the midranges and tweeters, because a man needs to hear as much bass as possible, and one way some makers enhance bass is by cutting midrange and treble. The down side of this practice is that the mids and treble are driven with far higher series impedance than that measured at the output of the amp. Should one want lower voltages to be fed to mids and treble to get a more balanced eq and flatter response, they could do worse than use a tapped toroidal transformer, using a 500 VA rated core, with a cross sectional core area of 50mm x 30mm and allowing for a voltage of 30vrms at 0.3 Tesla at 50 Hz, one could have about 300 turns of 1.5mm dia wire for a total whole winding, and with taps all along it at 10% intervals of the turns. if the hole dia was 50mm, then approx 98 turns per layer could be neatly wound onto the core, so 3 layers with insulation between would be possible, and doable by the keen diyer. This auto transformer can thus be used for a wide variety of impedance matching, and cope with powers of up to 100 watts into 16 ohms, perhaps more into 8 ohms, at slightly higher winding resistance losses. THD would be negligible, and bw quite wide, since all the turns are located in 3 layers on top of each other. It might seem stange to have yet another transformer in the signal path but nobody complains about the step up trannies in SL speakers. If the amp is connected to one end of the auto tranny winding, then as each tap down from the top of winding is tried, the Ro becomes lower and lower, which is the opposite effect of resistances in series with attenuation for speakers. The other side effect is that the amp sees a higher value load if a speaker is connected at a tap lower down the auto tranny. This may be a good thing if there is plenty of voltage swing headroom, but not if a high power is required to the increased load effectively harnessed to the tubes. All is not completely simple, which is usual for anything to do with electronics, and there are tricks galore for the uninitiated novice who has no experience. Patrick Turner. Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#10
|
|||
|
|||
Bob Hedberg wrote: So I take it that if I were to utilize a split primary step-up transformer between the two 8-16 windings, I could sum those two portions of the signal and drive a center-channel speaker? bob I am not sure what you mean. I'll take a complete wild guess, and agree with Phil, to be on the safe side. That way I can't be blamed for any smoke in your lounge. Patrick Turner. |
#11
|
|||
|
|||
Kind of like using a primary to split secondary transformer as a phase
splitter for a push-pull amp, but in the opposite direction. Seems like it would work to me, as long as the primary windings were not in a phase-cancelling orientation. Bob Hedberg Patrick Turner wrote: Bob Hedberg wrote: So I take it that if I were to utilize a split primary step-up transformer between the two 8-16 windings, I could sum those two portions of the signal and drive a center-channel speaker? bob I am not sure what you mean. I'll take a complete wild guess, and agree with Phil, to be on the safe side. That way I can't be blamed for any smoke in your lounge. Patrick Turner. Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#12
|
|||
|
|||
"Bob Hedberg Kind of like using a primary to split secondary transformer as a phase splitter for a push-pull amp, but in the opposite direction. Seems like it would work to me, as long as the primary windings were not in a phase-cancelling orientation. ** The two primary windings of your additional (centre mono) transformer couple magnetically to *EACHOTHER* !! When used the way you suggested the two channels of the stereo amp would have their outputs effectively wired in parallel - very bad news. ............... Phil |
#13
|
|||
|
|||
An independant winding across the unused taps (8 to 16 ohm) for each
channel?. "Phil Allison" wrote: "Bob Hedberg Kind of like using a primary to split secondary transformer as a phase splitter for a push-pull amp, but in the opposite direction. Seems like it would work to me, as long as the primary windings were not in a phase-cancelling orientation. ** The two primary windings of your additional (centre mono) transformer couple magnetically to *EACHOTHER* !! When used the way you suggested the two channels of the stereo amp would have their outputs effectively wired in parallel - very bad news. .............. Phil Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#14
|
|||
|
|||
"Bob Hedberg" An independent winding across the unused taps (8 to 16 ohm) for each channel?. ** Please read carefully: " The two primary windings of your additional (centre mono) transformer couple magnetically to ****EACHOTHER*** !! When used the way you suggested the two channels of the stereo amp would have their outputs effectively wired in parallel - very bad news. " Two windings on the same core are never "independent" - that will act just like a primary and secondary. ............ Phil |
#15
|
|||
|
|||
Yes, I suppose channel seperation would take a bit of a hit. How about a couple of small transformers, each with their primary winding across the 8-16 ohm tap, but their secondaries combined? Bob H. "Phil Allison" wrote: "Bob Hedberg" An independent winding across the unused taps (8 to 16 ohm) for each channel?. ** Please read carefully: " The two primary windings of your additional (centre mono) transformer couple magnetically to ****EACHOTHER*** !! When used the way you suggested the two channels of the stereo amp would have their outputs effectively wired in parallel - very bad news. " Two windings on the same core are never "independent" - that will act just like a primary and secondary. ........... Phil Bob H. Just grab that plate in one hand, the chassis in the other, and FEEL the power of tube audio!!! (not literally, of course, just kidding. DON'T DO THAT!) |
#16
|
|||
|
|||
"Bob Hedberg" Yes, I suppose channel seperation would take a bit of a hit. ** Much, much worse that that - one channel sees the other as a near dead short on its output. Only when a mono signal is fed to the amp could it work OK. How about a couple of small transformers, each with their primary winding across the 8-16 ohm tap, but their secondaries combined? ** With the two secondaries wired in parallel the exact same problem exists. If the secondaries are wired in series there is no cross coupling - UNTIL you connect the speaker. Then it is just the same again. ............... Phil |
#17
|
|||
|
|||
Heinz wrote:
Hi, what do you think: The MiniMax power tube amp has an OPT with 0-4-8-16ohms taps. Can go something wrong if use the 0-8 tap for bass/midrange and the 8-16 ohms tap for the tweeter? Must be a perfect biwiring! Thanks, Heinz I'd say this is a safe experiment. I'm assuming that you'll be using the appropriate crossover components on the speakers. You're gonna get more voltage outta the woofer though as the 8-16 Ohm winding has far fewer turns than the 0-8 winding. What may be more useful, if you have un-matched drivers, is to use the 0-8 for the woofer and 0-16 for the tweeter. This is gonna serve up more voltage on the tweeter which may in turn have to be padded. FDJ |
Reply |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Tube amp 16 ohms impedance speaker has 4 ohms | Vacuum Tubes | |||
Arizona Cowpie goes to Tube School | Vacuum Tubes | |||
When did home theater take over? | Audio Opinions | |||
One for the Tube Grabbers | Vacuum Tubes | |||
For Sale: Tube Driver Blue TDB475 | Car Audio |