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Old 4th December 2010, 01:41 PM   #11
Bigun is offline Bigun  Canada
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I'm not so sure that this speaker is typical, the peak that is being talked about isn't necessarily in the bass (at fs) - I think there's a thread on the klipsch forum mentioning a peak at 2.5kHz.

Negative feedback has many consequences, it changes clipping behaviour for the worse and gives the speaker (back emf) and it's cables (rf) an opportunity to send their own signals into the amplifier. It also changes the harmonic distortion profile of the amplifier so you will not end up with a classical SET amplifier.

My advice is to build your amplifier with good quality parts and without negative feedback and try it out with your speakers. It's DIY afterall, so you can experiment with changes to the design if you don't like the results.
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Old 4th December 2010, 02:55 PM   #12
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Quote:
Negative feedback has many consequences, it changes clipping behaviour for the worse and gives the speaker (back emf) and it's cables (rf) an opportunity to send their own signals into the amplifier. It also changes the harmonic distortion profile of the amplifier so you will not end up with a classical SET amplifier.
Concerning the clipping behaviour:
In HIFI-amplifier there should not happen any clipping during listening with nominal power levels. If such happens the amplifier is too small and the listening result is far from HIFI.
In any case, without negative feedback the amplifier is more prone to clipping - especially at bass resonant frequency of the speaker (see below).

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and it's cables (rf) an opportunity to send their own signals into the amplifier.
Please explain me in detail what you mean.


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It also changes the harmonic distortion profile of the amplifier so you will not end up with a classical SET amplifier
Yes it changes. It will reduce ALL distortion content of the reproduced signal and improve the linearity of the amplifier.

Once again, if a HIFI-amplifier is concerned, there should be as little distortion added by the amplifier as possible (the quitar amplifiers are whole other story).


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My advice is to build your amplifier with good quality parts and without negative feedback and try it out with your speakers. It's DIY afterall, so you can experiment with changes to the design if you don't like the results.
I will give an example what the final result is with and without negative feedback.
At first, the below figure is a typical impedance curve of a loudspeaker:




Click the image to open in full size.
As is quite generally known, loudspeakers are designed to be driven from an (almost) constant voltage generator. Then the effect of impedance variations of the speaker is minimized.

Below is a figure when the mentioned speker is driven from three diffrent amplifiers having output impedances of 100 ohms, 10 ohms and 1 ohm.

Click the image to open in full size.

Please quess what output power/-voltage curve is from 100 ohms representing a pentode output stage without negative feedback and which one is an UL-stage with "right" amount of negative feedback.

Conclusion: A good classical SET amplifier can be built without negative feedback. But with proper amount of NFB it will be even better.
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Old 4th December 2010, 03:16 PM   #13
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Originally Posted by antiquekid3 View Post

If the higher impedance really isn't a problem, then why do people say not to run a tube amp into no load? What will happen?


Kyle
With no load on the outputs, ie no speakers connected, there very little current flowing, so the various high voltages can go even higher, possibly popping capacitors or worse, frying the output transformers by arcing over from one winding to another.

70 ohms load is well within reasonable operation compared to completely unloaded speaker outputs as far as the voltages go....
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Old 4th December 2010, 11:40 PM   #14
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So it sounds like trying out the Klipsch certainly won't hurt my amplifier. It might be more inefficient at certain frequencies, and it might even sound bad, but it can't hurt to try. Since this is certainly a DIY project, it might take on several versions before I get it the way I like.

The whole part (okay, not the whole part...but a lot of it) part of me building this amp is to learn even more about tube circuits. I think I have a pretty good understanding of what goes on, and I even calculated my own loadline for this project, which I think is a good start. Once I get the basic amp constructed, I think it's only right if I learn how feedback works and is calculated. It will be a good learning opportunity.

Kyle
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Old 6th December 2010, 03:35 PM   #15
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Originally Posted by artosalo View Post
Without negative feedback the amplifier will obviously be overdriven at this frequency and produce severe harmonics.
Not buying. I've seen a lot of zero feedback tube amps in the last 30 years and there is nothing here so far to indicate anything like this.
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Old 6th December 2010, 03:56 PM   #16
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Originally Posted by artosalo View Post
In any case, without negative feedback the amplifier is more prone to clipping - especially at bass resonant frequency of the speaker (see below).

As is quite generally known, loudspeakers are designed to be driven from an (almost) constant voltage generator. Then the effect of impedance variations of the speaker is minimized.

Conclusion: A good classical SET amplifier can be built without negative feedback. But with proper amount of NFB it will be even better.
Clipping has nothing to do with negative feedback, although negative feedback can cause the onset of clipping to be more severe. Clipping is created by distortion as the output devices saturate, which has little to do with feedback.

Speakers are not always "voltage driven". This model is used by a lot of speakers that are designed to be used with transistors. But the voltage model falls apart if you really want the speaker to be tube friendly. Why? Tubes cannot double power as impedance is cut in half. You can add feedback to approach 'voltage source' behavior, but it is done at a price.

The price? Negative feedback results in the distortion of odd ordered harmonics, the 5th, 7th and 9th. Our ears use these harmonics to determine the volume or loudness of a sound so we are quite sensitive to them. If they are distorted the amp might sound bright or might otherwise be fatiguing. In fact this is a coloration- one that our ears are more sensitive to than minor frequency response variation.

So as a result, there is an alternative speaker drive model (it is also much older as a concept) in which the amplifier is more of a 'power source' than a 'voltage source'. And it turns out that speaker designers whether consciously or not have used this model (based on their intention of making the speaker tube-friendly). If you are aware of the design rules, it is possible to get flat frequency response despite the higher output impedance.

This allows for a zero-feedback SET to have fairly flat frequency response on a speaker like the one mentioned. As soon as it is assumed that *all speakers* follow the 'voltage model', you suddenly have an equipment matching conversation that arises, one seems to have no clear boundaries, and is one that many audiophiles have flushed at lot of money into! When you understand that the 'voltage model' is not the only game in town, its easy to see why there are equipment mismatches. Anytime equipment from one camp is used with that from the other, tonal aberrations result.

more:

Competing paradigms in amplifier and speaker design
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Old 6th December 2010, 04:38 PM   #17
20to20 is offline 20to20  United States
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The price? Negative feedback results in the distortion of odd ordered harmonics, the 5th, 7th and 9th.

Our ears use these harmonics to determine the volume or loudness of a sound so we are quite sensitive to them.

If they are distorted the amp might sound bright or might otherwise be fatiguing.
What is the 5th, 7th, 9th harmonic frequency of 1Khz?

Does this mean that if our amps have no harmonic distortion, we won't be able to hear to "Stairway to Heaven?"
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Old 6th December 2010, 04:43 PM   #18
TheGimp is offline TheGimp  United States
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I have built two SE U.L. tetrode amps (6P1P) that deliver about 3.5W into 8 ohms and tested them with Klipsch Heresy speakers. They sound great and have no problems with driving the speakers (1976 or 77 production). The speakers have never been opened and are all original.

I remember measuring my normal listening levels while in college with these speakers driven by a Sony STR5600SD, and the average power level was only 32mW with peaks reaching 1W. The listening levels were loud enough to enjoy (Dixie Dregs Sky Dive IIRC) but low enough not to require raising ones voice to carry on a conversation.

Last edited by TheGimp; 6th December 2010 at 04:51 PM.
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Old 6th December 2010, 04:56 PM   #19
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Originally Posted by 20to20 View Post
What is the 5th, 7th, 9th harmonic frequency of 1Khz?

Does this mean that if our amps have no harmonic distortion, we won't be able to hear to "Stairway to Heaven?"
5KHz, 7KHz, 9KHz.
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Old 6th December 2010, 05:14 PM   #20
20to20 is offline 20to20  United States
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Originally Posted by atmasphere View Post


5KHz, 7KHz, 9KHz.
So if we produce a 1Khz 0db sine, what would the harmonic content be with an amp that has .5% HD?

What would the levels of those harmonic frequencies be?

Would a speaker produce a different set of harmonics of a recorded tuning fork compared to a live tuning fork, played through an amp with .5% HD?
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