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In Reply to: RE: There are a number of misconceptions here posted by MarkJohns on November 16, 2015 at 18:53:02
Those do like like hysteresis curves to me, but they don't apply the way you think they do to output transformers. I used to think they did; IOW I made the same mistake.
Julius Futterman built the amplifier with the low distortion specs I mentioned. It was an OTL. The OTLs I make go to 1Hz-300KHz with full power.
Power triodes tend to drift a lot less than pentodes; sounds like you don't have much experience with them. We can set the bias on our amps and 6 months later they likely will not need adjustment. They are surprisingly stable. It all has to do with design- some designers are better at getting the tubes to behave than others. It does not surprise me at all that you could hear the difference between tubes and transistors- most people can. But FWIW here's something interesting: the more feedback you can put into a tube amp the more it will sound like solid state. Most solid state amps run a lot of feedback! Interestingly though, the solid state amps that don't run feedback are some of the stand-out amps that even tube lovers like...
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In 1983 I attended a Seminar and demonstration at the WQXR auditorium during the AES convention in NYC presented by New York Audio Labs, Harvey Rosenberg's company. Their engineers had taken a lot of death bed testimony from Futterman and his widow gave them all of his technical papers. They used the same principles but provided a modern solid state power supply. They also built a rig for adjusting the bias. It used to take Futterman an entire day to get this right. They could do it in five minutes.
Among the many shortcomings of vacuum tubes is that thermionic emission of the cathode varies with the square of the temperature. That means a small change in temperature results in a much larger change in the rate electrons boiled off. Negative feedback properly used mitigates this problem. Improperly used it creates more distortion than it is intended to solve. In fact used very wrong and you've built an oscillator. The rigorous study of negative feedback and servo systems which operate on exactly the same principle entails understanding mind blowing mathematical equations. This is why tyros invariably get it wrong and should stay away from it. Like any powerful tool it is dangerous. You can build a house with a hammer or smash your thumb.
If I had to buy a tube amplifier on sound alone, it would be an OTL type. But I still prefer solid state for practical reasons.
see the link.
I'm not against feedback, but sometimes you are better off without it given some of the compromise that develops.
One problem that is well known (see Norman Crowhurst) is that feedback enhances higher ordered harmonic production that might not have existed open loop. Since the ear uses the higher ordered harmonics as loudness cues, the feedback can and does cause bifurcation of the amplifier output; IOW increased loudness cues due to the increased higher ordered harmonics, even though they might be in trace amounts.
This phenomena is old enough that Crowhurst was writing about it in the 1950s. Its not gone away- the ability of feedback (in sufficient quantity) to cause an amplifier to behave as a voltage source has been considered a desirable trait for taming speaker resonance in box speakers. So this issue is still around over half a century on.
Less quantify , how much is too much feedback ....
Go Rossi ......
Here's the problem: feedback is often inappropriately applied.
Here's a link that describes the problem quite well.
Its probably best if you read through all the parts.
Ralph , you missed my question, how much feedback is too much for Ralph , 1db, 10db, 20db, 30db ..? Would 10db be perfectly acceptable , no need to get into the academic stuff like stability and OL bandwidth , just the amount you would be comfortable with...
Go Rossi ......
All designs are different and what works for one may not do so for another.
I like a gentle overload characteristic and there is almost no way to do that if you are running loop feedback- the overload onset is rather sudden.
In addition I like lots of detail and tonal neutrality, which means not only no 2nd harmonic but also no artificial brightness. Both are colorations IMO. To accomplish 'no brightness', using feedback is tricky indeed (as it will generate higher ordered harmonics where they may not otherwise have existed at all) so I tend to avoid it.
It does not matter to me if this limits the speakers that I can use. If a speaker will not sound right unless the amplifier employs feedback; well, put another way its always going to sound bright due to higher ordered harmonic generation if feedback is used. Since that is an annoying artifact and a coloration, IMO/IME if the speaker needs the amp to have feedback the result is that it might sound like a great hifi but it will never sound like real music. IOW I like the speakers that have the ability to sound real- and that can only occur if the amp is low distortion and lacking feedback. Its a narrow path for sure, but it works.
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