Looking for good 20 watt amp design to drive Jordan TL's? [message #11218] |
Mon, 10 October 2005 23:32 |
Norris Wilson
Messages: 361 Registered: May 2009
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Grand Master |
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Hi, I am trying to help a friend track down a good 20 watt SET design that will firmly run a pair of Jordan JX92S TL speakers. He use to own a pair of Welborne Apollo MKII's and would consider going back that direction. But we feel that at 20 watts, the VV52's were short lived, not a good investment at $500 a pair. Is there a better alternative short of using an 845 at 900 Volts plus, or a $500 a pair of VV52's to obtain the desired 20 watts? If a P-P circuit would get you to that musical 20 watts with plenty of drive, that could be an alternative to the big tube SET's. Any input or suggestions would be greatly appreciated. Thanks Norris Wilson
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Damping factor - SE vs. PP [message #11220 is a reply to message #11219] |
Tue, 11 October 2005 06:45 |
Damir
Messages: 1005 Registered: May 2009
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Illuminati (2nd Degree) |
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Just to add - PP amp doesn`t have a larger damping factor (or lower output resistance) then SE amp per se, but from the fact that most PP amps have a global negative feedback loop and SE amp mostly do not. If we have a typical 300B SE amp with say Ra=3k, we can expect DF~3 without neg. feedback, or DF=Ra/(rp+Rw). We can express DF on the secondary side, like ratio DF=Rsp/Rout, where Rout is rp+Rw (anode resistance and windings resistances "reffered" to the secondary). Then, we can have 300B PP amp, class A, say with Raa=6k. Primary reflected impedance is doubled, but we now have two output tubes, and DF=Raa/(2rp+Rw). We can see that in both examples (SE & PP) we have about the same DF. In fact, if we use typical UL PP output stage coupled with somewhat lower Raa (AB1 amp), then we can expect maybe DF~1 without feedback. Simplified, and there`s a more about PP/SE things...
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Re: Looking for good 20 watt amp design to drive Jordan TL's? [message #11221 is a reply to message #11219] |
Tue, 11 October 2005 10:02 |
Norris Wilson
Messages: 361 Registered: May 2009
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Grand Master |
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Hi Wayne, How is life going in the world of Pi? I want to thank you for the kind offer of hooking up your Audio Note 2 SET to my friends Jordan speakers. My friend Mike, who lives in Seattle and I have discussed the Audio Note 2 SET as an option. But, he feels that the 12 watts is not enough to drive the Jordan transmission line speakers. He feels since the Jordan's bottom frequency response goes down to 45 Hz and is 88db efficienct at 8 ohms, that it will not have enough authority since he likes to listen to classical music. Also, since he lives in Seattle, it would be diffacult to hook up the amplifier with out a dangerous jouney across our land at the hands of the delivery wrecking crew. I for one, am not convinced that 12 watts would not be enough power to drive his speakers well. Obviously, a 5" full range driver is not the best choice to play classical through in the first place, possibly at lower listening levels though. I think that is why he feels 20 watts should be at the lower end of the amplifiers output capability. His thinking is some what stuck in the direction of a triode based SET amplifier to obtain the last degee of musical purity. I again, am open to any suggestions that would get us there, triode, pentode, SET, or push pull, I'm open. I have been trying to get my friend to build the 7591A version of Poindexter's Musical Machine with some Sansui 1000A output transformers, but he is not ready to do so at this time. Thanks again and I hope someone can chime in with other DIY amplifier designs and suggestions. Norris
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Re: Damping factor - SE vs. PP [message #11223 is a reply to message #11220] |
Wed, 12 October 2005 13:47 |
Steve
Messages: 83 Registered: May 2009
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Viscount |
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I have to disagree a little. (assume no global feedback). Viewed near the operating point with minimal signal swing, there isn't much difference. But as the signal swings more and more, the DF varies more and more in an SET than a PP amp as the Ra is changing more in an SET triode while the PP triode combo is more constant. Theoretically, as one triode's Ra is rising, the other is lowering, maintaining an approx constant. This Ra swing occurs in all triodes, with the Ra varying from several hundred ohms to thousands of ohms and eventually (theoretically) infinite ohms at triode cutoff.
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Re: Damping factor - SE vs. PP [message #11224 is a reply to message #11223] |
Wed, 12 October 2005 15:39 |
Damir
Messages: 1005 Registered: May 2009
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Illuminati (2nd Degree) |
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Well, I simplified very complex tube(s)/transformer/speaker case. Although it is true that internal anode impedance of our output triode (say 300B) is not a constant resistor, and that our Zout is not constant, especially on the frequency extremes vs. mid frequencies, and that those changes are probably little larger in SE then PP case - we can`t say that rp (and Zout) of SE amp vary wildly like in your example. Measuring the Zout of SE amps showed relatively constant value throughout the frequency and power output magnitudes. The change in rp is not large, and for most practical purposes we can model our triode like voltage source (generator) with its (constant) internal resistance rp in series. Definition of DF like I explained it is correct, it is a ratio of primary (reflected) resistance and tube internal resistance, or ratio of speaker resistance and rp reffered at the secondary - rp divided with OPT impedance ratio. PP amp (class A) has two tubes in series. Then we can add complexity in our model (Rw, Lp, Lsp, Cw, Zsp...)...
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Re: Damping factor - SE vs. PP [message #11226 is a reply to message #11224] |
Wed, 12 October 2005 16:48 |
Steve
Messages: 83 Registered: May 2009
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Viscount |
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>>"we can`t say that rp (and Zout) of SE amp vary wildly like in your example." It might be partly symantics Damir, so I will say the output Z and subsequent Damping Factor changes "somewhat". Even with a relatively horizontal loadline, the plate current should be held as high as possible, or keep the power down so the signal doesn't swing near the tube's cutoff. Specs from 300b data sheet (since that is the tube you mention.) At 60ma idle current at 100 volts or above, tube Ra is approx 600 ohms. At the 30ma point of the loadline, the Ra is approx 1000 ohms. The DF is no longer 3, but 1.8 at the 30ma point. (This means a peak to peak current swing of 30ma to 90ma. One can figure the output power from there.) At the 20ma point, at 100 volts or above, results in an Ra of approx 1200 ohms, double the 600 ohm Ra at 60ma. This means that a DF of 3 now becomes a DF of 1.5 at 20ma. (This means the current swings between 20ma and 100ma, with 60ma as quiescent.) If the plate current is allowed to drop to 10ma during a portion of the cycle, the Ra is approx 1450 ohms. DF now becomes 1.25. (peak to peak swing of 10ma to 110ma.) At 5ma, the Ra increases to approx 2000 ohms, or nearly 3 times that at 60ma. Df becomes approx 1. (peak to peak swing of 5ma to 115ma.) As the tube approaches cutoff, the Ra rises towards infinity till the tube isn't actually conducting. PP has virtually no drop off of DF, except because of variations in individual tube characteristics. PP therefore does not offer such a compromise in Damping factor as SE operation does.
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