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In Reply to: RE: Choke coupled is going to be inefficient posted by dave slagle on April 21, 2015 at 10:06:43
Unless the chokes are pretty big, the cutoff is likely to be higher.
They have to dissipate the energy from the voltage drop across them which means that energy is not available to the speaker. The more energy developed by the output section that is dissipated by the output section, the less efficient.
Normally chokes are nice to use in a power supply because the DC voltage drop is less than that of a power resistor. So they are good at knocking out power supply noise without loosing a lot of power supply voltage. But this property does not work to your advantage in designing an output section. You want some voltage drop across such a device if you are going to make a voltage swing that is translated into power.
Additionally, the choke is not offering any impedance conversion, so that has to be done by the power tubes themselves.
As an OTL manufacturer, I'm very used to what that last statement means. I'm pretty sure that if I built an output section using two triode sections, I could make more power with my Circlotron circuit than one could do with the same power tubes and a set of chokes!
If one used an output transformer, the coupling to the speaker load would be much more efficient as the transformer would allow for impedance conversion. So any way you look at it, a choke coupled output section will be inefficient.
Follow Ups:
They have to dissipate the energy from the voltage drop across them which means that energy is not available to the speaker.
so does the primary of a transformer.
Additionally, the choke is not offering any impedance conversion, so that has to be done by the power tubes themselves.
tap the choke for your output and you have impedance transformation.
dave
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Inductors by nature do not dissipate AC power so I fail to see where the inefficiency comes into play.
dave
It will be dissipating variable DC power, not AC. The output of course is AC.
How is variable DC not AC?
Granted there will be some copper losses from the energy stored and released by the inducance but I'd hardly call that inefficient and the same exact thing happens in the primary of an output transformer. Obviously the ideal solution is to do away with the transformer altogether.... oh now I get it.
dave
An autoformer is definitely do-able but I wonder if the 'secondary' DCR tap can be made small enough to dispense with the coupling cap , maybe even using thicker wire . Obviously there would be a small amount of offset . What do you think ?
Al
Unless I've misunderstood your question... there's a pretty darn good reason to you cannot do what your thinking about...
if you have an autoformer in the output stage of a power amp (i.e., a power tube is driving it) which is hooked up to a speaker load at some tapped point... you would want to use a blocking cap to keep the dc off of the voice coil of your driver, heh?
Or have I gotten your application all wrong?
MSL
Builder of MagneQuest & Peerless transformers since 1989
I did it with the 6C33 for a NY noize. As I recall I had about a quarter of an amp of current and the secondary section had a 0.1 ohm DCR for a 25mv DC offset across the load. I really expected major driver offset and issues from the DC but didn't notice any with my lowthers.
Unfortunately the amp sounded like 6c33's so I moved up conventionally coupled GM-70's for the next year.
dave
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