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Old 5th June 2004, 01:19 PM   #1
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Lightbulb Tubes in Class D amplifiers

I don't know if someone had this ideea before or if will really work, but is nice to think about it.
I'd like to make some input first. Everyone knows that class D amplifiers are best energy efficient, and have a pretty good response in frequency too and they don't heat much.
On the other give a "nice" saturated, compressed "round" sound.

!!! I wonder what if we combine class D circuit (4ex. LM4651 ) as first stage with power tubes as finals?

Practically we combine the oldest technology with the newest. It will be really nice to have someone who would want to try it! We may have all a big surprise...



ps:interesting compromise, isn't it?
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Old 5th June 2004, 08:34 PM   #2
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Hi,

Quote:
!!! I wonder what if we combine class D circuit (4ex. LM4651 ) as first stage with power tubes as finals?
Sorry but I fail to see what you'll win by running the input stages in class D?
I mean, those hardly consumer any power except for the valve heaters...

The idea of combining valves at the input and a class D output stage seems more attractive economically speaking.

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Old 5th June 2004, 09:08 PM   #3
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Tubes as switching devices and an output transformer?

That would be a challenge
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Old 5th June 2004, 09:16 PM   #4
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Start that class D forum NOW !

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Old 5th June 2004, 09:17 PM   #5
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Default fdegrove

Maybe u're right, but I stil don't find any power class D amplifiers, and all of them (I know few) are pretty low in power handeling.
I forgot to mention that class D amplifiers compress sound a little bit, just almost like the tubes and so they have this common feature. I would ask u to tell me if there are tubes capable of switching?

Thanks, we should think about this; I'll just wait for a professional opinion.
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Old 5th June 2004, 09:20 PM   #6
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Hi,

Why would anyone want to start a Class D forum????

CLASS D

Cheers,
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Old 5th June 2004, 10:17 PM   #7
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Quote:
Originally posted by Jax
Tubes as switching devices and an output transformer?

That would be a challenge

For over 30 years, TV's used this for deflection and high voltage generation. Power? - 20 - 60w. This was generally in resonant mode though.
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Old 5th June 2004, 10:36 PM   #8
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Default dhaen

So u say it can be possible to couple the tubes and the class D circuit?
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Old 6th June 2004, 12:02 AM   #9
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Quote:
Tubes as switching devices and an output transformer?
I think it could be done without transformers quite easily using same tubes that are used in an OTL amp but the benefits would not be the same as in a SS circuit.

Also I fail to see the benefits of building class D amplifiers from a sonic perspective, class D main benefits are small size and high efficiency, there are no benefits from a sonic perspective but in many cases instead sound is suffering due to improper implementation.

Of course there are people arguing that class D amplifiers sound so much better than anything else but that is the same exaggurated sales talk that was used to sell the first transistor amplifiers, "tubes are obsolete! buy our new modern transistor amplifier" We all know what happened, when most of the transistor hype had calmed down some people who where not afraid to say what they really thought said "Hey, my old tube amp sound better than this modern stuff that shoul have been so more advanced" and people started to be interested in tube amps again.

I predict that the same thing will happen with class D, in many applications they will be dominant as non-switching SS technology is today but in those cases where the ultimate in sound production is important there will be hard battle between tubes, non-switching SS and class D.

I also predict that there always be people that fall for all the hype and sales talk for any new tecdhnology and fool themselves into believing that everything new and modern is superior, that is OK , it would be very difficult to introduce new technology without these kind of people driving the market.

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Old 6th June 2004, 12:45 AM   #10
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Bah. Tubes are only useful for quality class A or AB sound and the occasional kW class C radio using them purdy 3-500Zs or such. But since you want a design challenge...

The later sweep tubes certainly are good at switching, that being their designed purpose after all. With a saturation voltage as low as 30V while passing 500mA one gets the same performance as a transistor whose Vsat might be around 1.5V and 10A. Ran from +300V, the tube gets a max. of 300-30 = 270V across a load, drawing .5A, putting 135W into the load and only 15W in the plate. Said transistor, from +15V, would get 15-1.5 = 13.5V at 10A = 135W into the load and 1.5*10 = 15W collector dissipation. Unfortunately the heater dissipation is equal to plate dissipation here, ruining total efficiency, and let's not even touch peak screen current.

But you run into the problem of coupling the high voltage signal to the speakers. 1/2A isn't any good to an 8 ohm speaker. You can't just transform it because the supposed advantage of class D PWM is small size -- the carrier will easily transmit through a HF transformer but the audio will be lost as operating current because the transformer ignores anything so slow. To do it this way you'd have to use a regular OPT for impedance matching and do the carrier filtering before it. Although you could count it as part of the speaker's weight, that's not fair.

Tim
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