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Old 16th November 2012, 11:51 AM   #31
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The distorsion figures of a SS similar to a pentode is more of a problem if you run them zero feedback.

Not THD but the curve.

I now use Vfets for ouput that has a more triode like curve.
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Old 16th November 2012, 11:59 AM   #32
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by DF96 View Post
Local degeneration raises output impedance, the opposite of what you want in a trivial amplifier.
So use local shunt feedback -same as is built into every triode. In any case, why should a high output impedance matter if the distortion is ultimately lower than the valve version?

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The problem you always have with BJTs (and to a smaller extent with FETs) is that they are essentially small-signal devices. A bare BJT will give you 1% THD with 1mV peak input voltage.
So attenuate the input signal.

This trivial valve amp is still likely to be much more expensive to build, and less efficient than the transistor version. In any case, people on this forum don't seem too concerned with circuit complexity- they are quite happy to add elaborate CCS loads and shunt regulators to their valve circuits. I just haven't figured out why they bother, if they are truly chasing fidelity. The same money and effort could have been spent on transistor design that would have been commensurately even better. Why claim to be chasing better and better fidelity, always trying to lower the distortion by another decimal point, and then choose to use valves that everyone knows won't hold a candle to a similarly complex transistor design?

I maintain that the reason we use valves is because they sound good despite their obvious technical inferiority. If you choose to use valves, its because you want an effects box. Fidelity is not required for the best listening experience, and therefore we are perfectly justified in deliberately choosing a 'coloured' input valve.

Last edited by Merlinb; 16th November 2012 at 12:02 PM.
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Old 16th November 2012, 12:58 PM   #33
DF96 is offline DF96  England
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Quote:
Originally Posted by MerlinB
In any case, why should a high output impedance matter if the distortion is ultimately lower than the valve version?
Because nonlinear distortion is not the only problem. You also need a fairly flat frequency response, which for most speakers means a lowish output impedance.

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So attenuate the input signal.
Have you ever heard of boring details like signal-noise ratio?

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I just haven't figured out why they bother, if they are truly chasing fidelity.
That is true, you haven't figured it out.

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Why claim to be chasing better and better fidelity, always trying to lower the distortion by another decimal point, and then choose to use valves that everyone knows won't hold a candle to a similarly complex transistor design?
It is mainly the SS people who strain for another zero after the decimal point, not us.

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I maintain that the reason we use valves is because they sound good despite their obvious technical inferiority. If you choose to use valves, its because you want an effects box.
No, not me. If you look at some of my posts you will see that I often criticise that view, especially when people ask for 'tube sound'.
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Old 16th November 2012, 02:27 PM   #34
SY is offline SY  United States
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Quote:
Originally Posted by Merlinb View Post
But if you ignore distortion, what other factors are left to measure fidelity?
Quite a few (eg., overload margin and recovery, signal to noise, frequency response...). But once you get well beyond the performance needed to be audibly transparent, the measurement is not very important, it's a matter of choosing what you'd rather "drive." I choose tubes (well, hybrids, really- I get a lot of grief for the electronic miscegenation that characterizes my designs).
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Old 16th November 2012, 03:02 PM   #35
rongon is offline rongon  United States
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I may be wrong, misguided, or just plain ignorant, but I thought tubes are best at voltage amplification (can swing lots of volts), while transistors are better at current drive (can swing lots of milliamps). Something like a triode followed by an emitter follower makes a nice low distortion amplifier without any loop feedback. Can even do a good job driving a giant triode and an output transformer.

But I may be wrong, misguided or just plain ignorant.
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Old 16th November 2012, 03:12 PM   #36
rmb is offline rmb  United States
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Rongon: I have enjoyed tube audio for years. One advantage of a single ended tube versus a single ended SS BJT is no "pinch off" voltage..which may be on the order of 0.2 volt for germanium and 0.6 volt for silicon. The "pinch off" may be avoided depending on where the device operating point is set. You have Miller effect regardless of whether a tube (especially a triode) or a BJT or FET is used.

As far as "standards" go, look at the EIA standard for broadcasting...the
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Old 16th November 2012, 03:19 PM   #37
rmb is offline rmb  United States
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Rongon: XP glitched and posted my reply prematurely..
As far as standards go, check out the EIA standard for broadcasting...the frequency range is 30 to 15,000 Hz. The Williamson and other "ultralinear" amplifier designs had much wider frequency range and the primary limiting factor was the output transformer. I never liked single ended power amps; impedance coupling was necessary to keep the DC plate current out of the output transformer primary. DC in the output transformer primary may create core saturation with adverse effects on frequency response and dynamic range....

As far as preamps go, the op amp NE5534 is a good one....this one is widely used in commercial equipment.
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Old 16th November 2012, 03:37 PM   #38
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by SY View Post
Quite a few (eg., overload margin and recovery, signal to noise, frequency response...).
By overload margin you mean dynamic range?

Signal to noise: again nothing special about valves there. SS can do as well or better.

Frequency response: this is just a form of 'linear' distortion. Again, nothing to recommend valves over transistors here...
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Old 16th November 2012, 03:41 PM   #39
Merlinb is offline Merlinb  United Kingdom
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I maintain that the reason we use valves is because they sound good despite their obvious technical inferiority. If you choose to use valves, its because you want an effects box.
No, not me. If you look at some of my posts you will see that I often criticise that view, especially when people ask for 'tube sound'.
I know you (and practically everyone on here) claim not to want an effects box, but I have yet to hear a rational argument why you would deliberately handicap yourself by choosing to use valves rather than transistors. (And then often use transistor CCSs to try and scrub away all the pesky nonlinearities of valves!)

Last edited by Merlinb; 16th November 2012 at 03:44 PM.
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Old 16th November 2012, 04:12 PM   #40
SY is offline SY  United States
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If the imperfections are below audibility, how is that "handicapping"?
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