"Distortionless" Tube Oscillator -Cutting Edge Technology - diyAudio
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Old 30th August 2014, 04:04 AM   #1
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Default "Distortionless" Tube Oscillator -Cutting Edge Technology

While browsing back issues of Audio Engineering I found this article on Page 13 of the August 1952 issue: >

http://www.americanradiohistory.com/...o-1952-Aug.pdf

It speaks of a "Distortionless" Audio Oscillator.
I prefer to say it's has an unmeasurable level of distortion. Graphs show its level of harmonic distortion as being as low as -110-120 db down. Not a big deal to achieve with today's technology.

The purpose of this Oscillator according to the last paragraph on page 43 was - to drive a Tube Amp, which was being developed at the time, which was producing distortion levels in the order of 0.01% or better at 60 watts at 20 cycles;- far better than required for good listening. So much for the modern day philosophy of using single ended Class-A Directly Heated Triodes for ultra-high end sound ! And how much distortion do they produce in an amp ? - just a little more than 0.01 %. Try 10 %.

So my questions are - If technology with tubes was this good way back in 1952 - an maybe seen as being at the cutting edge of audio tube technology - ( which most of today's transistor amps on the market don't even produce these ultra-low levels, as they use IC chips and built to a price - not the general domestic units I see in the average Electrical Store ) - then why haven't these high quality tube performers become more popular over time ? I fail to see cost as the only reason.
Why can't we apply the same techniques in these circuits now to get these ultra-low distortion figure's with today's tube amplifiers; considering the average tube produces upwards of 5-10% distortion depending on operational topology ?

This is not a criticism of whats currently available, so much as I believe operational performance can still be greatly improved - without the use of semiconductors - which in my purest opinion is cheating.
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Old 30th August 2014, 08:37 AM   #2
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From the schematic of the oscillator it looks like the basic idea is to create an amplifier with a very high open loop gain (about 90dB) and use lots of NFB to reduce the distortion (produced predominantly in the output stage). This is just like modern op amp topologies.

The problem with high open loop gains in tubes has always been low frequency phase shift lading to instability at low closed loop gains. The solution is direct coupling between stages and in the NFB. In op amps this is easy to achieve but in tubes, with the vast difference in input and output dc conditions, it is much more difficult. The posted circuit has four direct coupled tube stages the last of which is the output stage producing 10% distortion reduced by as much as 90dB by the NFB. It also looks like some additional compensation has been included to mitigate Miller effect which, over several stages, can lead to HF instability.

Overall, an interesting concept. Presumably the same principles are applied to the power amplifier they refer to.

Cheers

ian
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