tech99 said:
The push pull amplifier shown will have less even order distortion than my suggestion for a single ended Class A beam tetrode. However, I would, be very surprised if this is audible or objectionable, and the additional complication and cost is very great.
All 3 of those points are true.
That amplifier had wonderful sound.
I have long believed that the highly controversial "Tube Sound" arises not so much from "distortion" attributed to tube amps
but from the difference in speaker cone response when driven by a current controlled source versus a voltage controlled source.
SInce amps are usually lab tested with a resistor load that difference gets missed because V = IR and R is constant.
But for a speaker, V = IZ and Z includes inductance, mechanical inertia, spring constants of speaker cone and air in the enclosure.
So the current through and voltage across a real speaker in a real enclosure in a real room will have different waveshapes.
A pentode tube being basically a current source will deliver to a
real speaker load a different voltage signal than to a
pure resistive load. That would look like distortion on an o'scope . But they're usually tested with a resistor load so it doesn't show up in reviews.
I am a "Transient Intermodulation Distortion" denier . It's the complex Z.
Here is an article from a respectable engineering publication that goes further into the subject:
http://www.edn.com/design/consumer/...periority-of-current-drive-over-voltage-drive
There cannot be found any scientifically valid reasons that justify the adoption of voltage as the control quantity - it is only due to the historical legacy originated almost a century ago, most likely by cheapness and simplicity; the quality and physical soundness of operation have not been considerations in this choice. Engineers are also more accustomed to identifying electrical signals as voltages rather than currents.
At least the hi-fi community should be interested and able to better see through this discrepancy. But they too have taken the state of affairs as a given, being largely conditioned to the wishful thinking that tightly held voltage somehow "controls cone motion," even up to middle and high frequencies. Such a notion doesn't have real scientific grounds, and it can be clearly shown by basic analysis and modeling that any damping effects that voltage drive can have on driver operation are strictly limited to the bass resonance region.
And another from a hi-fi writer who did quite a bit of actual testing ,
http://www.firstwatt.com/pdf/art_cs_amps.pdf
Conclusion
So here you have it – a re-examination of some things that have been known for a long time
but were here tested with currently available drivers. It’s easy to dismiss the older ways.
Somewhere in the 1960s, the speaker/amplifier interface took a left turn and headed off to
high-power voltage source amplifiers and speakers designed to lean on them for
performance. A small group of iconoclastic cranks has stayed interested in these fossils and
in the end, we have to recognize full-range high-efficiency speakers as offering elegance,
charm and a different sort of quality.
Current sources and amplifiers with low damping offer interesting possibilities for
improvement with these drivers but they require considerable work to get the enclosures,
electrical networks and acoustic environment just right. Remember, you don’t have to own a
current-source amplifier to put this information to work. Placing R0 in series with the output of
a powerful voltage-source amplifier instead of in parallel with a current source will give similar
results. If R0 is 47 ohms or higher, you are going to want a big amp, say 300 watts, but in any
case, be certain to use a high power resistor of 50 watts or more. That said, it’s all just
entertainment and I hope it inspires some of you to have some fun.
Bon Appetite.
He's a "true believer" in current source amps, see his site
(CAVEAT - he appears to sell them and this is NOT an endorsement,
i don't own one, just i find the concept interesting)
