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| Re: Damping factor - SE vs. PP [message #11232 is a reply to message #11228] |
Thu, 13 October 2005 16:22   |
Steve
Messages: 83
Registered: May 2009
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Viscount |
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Hi Damir, I am not talking about frequency changes varying Ra. Ra varies, from the characteristics chart, vs current at any frequency. This varies more over the half cycle from idle current on down VS from idle current on up, where Ra tends to be much more constant. So what you might be measuring is the Average DF or output Z. But since Ra changes over a portion of the cycle, the output Z also changes over that portion of the cycle. When the higher output Z occurs, there is less control of the woofer/midrange/tweeter, not only to dampen, but also for initiation of a signal. Take care.
Steve
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| Re: Damping factor - SE vs. PP [message #11233 is a reply to message #11232] |
Fri, 14 October 2005 01:11 |
Damir
Messages: 1005
Registered: May 2009
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Illuminati (2nd Degree) |
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We are talking about dinamic anode resistance. It is increment change of alternating anode voltage "through" increment change of (inphase) anode current. Static anode characteristics are not ideal, and because of curvature, we have "added" second, third and other higher order "terms" to the simple ia=ua/rp. Contribution of third and other odd-order "terms" to the fundamental expression changes the simple static rp=ua/ia.
My point is that with typical use of SE amp (small power, max. power only on short peaks), we have rel. small ua, rel. small contribution of odd-order distortion, and we can say that rp=ua/ia, or dinamic rp is about static rp, found graphically at the operating point.
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| Re: Damping factor - SE vs. PP [message #11234 is a reply to message #11233] |
Fri, 14 October 2005 09:55 |
Steve
Messages: 83
Registered: May 2009
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Viscount |
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Right, we are talking dynamic Ra, and keeping the power as low as possible is crucial/fundamental for keeping Ra relatively constant, although never as good as PP. (I think that is one reason I have been hearing suggestions of using Even higher efficiency speakers.) rp= u/gm, both u and gm constantly change during a cycle. Both changing independently, not in sync, in value and causes rp to rise. This can be graphed in a quadrant. The Ra curve (at different plate voltages) rises gradually at first over the first 5 volts peak or 10 volts p-p; then rises rapidly. Even then, a 10 volt peak, 20v p-p, drive signal can easily cause Ra to change by 20% or more over a cycle. The slope of the loadline is very important. A more vertical loadline dramatically increases Ra's change and effect. A more horizontal loadline helps minimize, or even eliminates Ra changing. But a horizontal loadline causes the output power to lower, rather dramatically. This applies to any triode run SET mode. Just something to think about when designing an SET amp. RL is important.
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| Re: Looking for good 20 watt amp design to drive Jordan TL's? [message #11238 is a reply to message #11218] |
Fri, 14 October 2005 14:46 |
PakProtector
Messages: 935
Registered: May 2009
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Illuminati (2nd Degree) |
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20W of PP is fairly easy, even for a Class A design. KT88's and consevative operation can get there with excellent sound. There are of course other valves than the KT88, but it is an easy example. Take the Hammond 5k/100W 1650 output TX. A pair of good KT88's, a PS to deliver 350V at 250 mA DC. I suggest a choke input filter, valve rectification and film caps( oil filled AC motor run ), and borrow the Merlin amp front end( http://audioroundtable.com/GroupBuild/Projects/ ). As soon as I get off my backside and forward Wayne a copy of the hybrid mods to the front end, you'll see another option useable for Pete's amp as well. It is a MOSFET/triode cascode and does a brilliant job with the E-Linear circuit. I am a bit biased to suggest PP, but it does well for me. If you must have SE, there is plenty to borrow between the two topologies.
cheers,
Douglas
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