SY said:
Most horn loaded speakers seem to be around 8 ohms with some being 16 ohms.
But if your output impedance is as high as your nominal load impedance, you're taking off what, just 3dB?
And if your output impedance is much higher than your nominal load impedance, you're probably going to be worrying about things other than efficiency.
se
Bernhard said:
When you consider the characteristics of devices, there is no
such thing as no feedback, and so the comment by Bernhard
is most appropriate.
/pass/: I don't know the definition of art or porn, but I
recognize it when I see it.
Nelson Pass said:
Yes. And I don't care to split hairs with regard to the intrinsic feedback of the individual devices. That's something the designer really has no control over and it's rather miniscule to begin with. So might as well make that a given and ignore it with regard to claims of feedback.
But I'd like to think we can agree that one can't rightly call an amp using followers a "zero feedback" amp without some sort of qualification such as "zero global feedback," etc.
se
By the way, after Charles cited the BUF-600 datasheet with regard to their being "open loop" buffers and using "no feedback" I EMailed Texas Instruments and asked them to clarify what was meant by this.
I EMailed them Thursday but didn't receive a reply until this evening so I thought I'd pass the information on.
By "open loop" and "no feedback" they mean no externally applied feedback.
To wit:
Thank you for contacting Texas Instruments. Regarding to your query about BUF600, this device does not require any external feed back. The design does not consist of differential amplifier at input stage. Rather than internally complementary emitter followers are there.
The respondant's name was Sanjeev (whose title is Analog and Mixed Signal Product Specialist) which perhaps explains some of the grammar. 🙂
I suspect Maxim uses the same terminology for the same reasons for their MAX-4200.
se
I EMailed them Thursday but didn't receive a reply until this evening so I thought I'd pass the information on.
By "open loop" and "no feedback" they mean no externally applied feedback.
To wit:
Thank you for contacting Texas Instruments. Regarding to your query about BUF600, this device does not require any external feed back. The design does not consist of differential amplifier at input stage. Rather than internally complementary emitter followers are there.
The respondant's name was Sanjeev (whose title is Analog and Mixed Signal Product Specialist) which perhaps explains some of the grammar. 🙂
I suspect Maxim uses the same terminology for the same reasons for their MAX-4200.
se
There are two types of feedback. Degenerative and regenerative.
I think I understand degenerative feedback
the term regenerative feedback is new to me
could Fred, or anyone, give a brief definition, perhaps with an example
thanks
mike
I think I understand degenerative feedback
the term regenerative feedback is new to me
could Fred, or anyone, give a brief definition, perhaps with an example
thanks
mike
Re: There are two types of feedback. Degenerative and regenerative.
Degenerative feedback is negative feedback, meaning the signal fed back is ideally 180 degrees out of phase with the input signal. Regenerative feedback is positive feedback, meaning the signal fed back is ideally in phase with the input signal.
Example? An oscillator. Another example? A squealing microphone in a PA system. A more entertaining example? Jimi Hendrix playing the Star Spangled Banner. 🙂
se
mikelm said:
Degenerative feedback is negative feedback, meaning the signal fed back is ideally 180 degrees out of phase with the input signal. Regenerative feedback is positive feedback, meaning the signal fed back is ideally in phase with the input signal.
Example? An oscillator. Another example? A squealing microphone in a PA system. A more entertaining example? Jimi Hendrix playing the Star Spangled Banner. 🙂
se
Charles's amps
At Saturday I was hearing at the first time on exhibition " High End Prague 2004 " amps Ayre and boxes Avalon. First impression was very well - there plays mostly chamber jazz and and I had feeling, that musicians are " inside " of the room . For this " type " of music and similar gentre, such as folk and every similar " weak " recordings, will be probably this amps plays good. On mostly of exhibitions you can listen similar music and salesman knows very well why they do it. When was this aparatus " forced " to play any " nonweak " music, such as symphonic orchestra or choirs, results was not so positive, 'cos in higher levels ( not on maximum levels ) start " mischmasch " - distortion of amps with " low feedback " is still too high. I am sorry guys, many of yours probably will not agree, but it's true.
At Saturday I was hearing at the first time on exhibition " High End Prague 2004 " amps Ayre and boxes Avalon. First impression was very well - there plays mostly chamber jazz and and I had feeling, that musicians are " inside " of the room . For this " type " of music and similar gentre, such as folk and every similar " weak " recordings, will be probably this amps plays good. On mostly of exhibitions you can listen similar music and salesman knows very well why they do it. When was this aparatus " forced " to play any " nonweak " music, such as symphonic orchestra or choirs, results was not so positive, 'cos in higher levels ( not on maximum levels ) start " mischmasch " - distortion of amps with " low feedback " is still too high. I am sorry guys, many of yours probably will not agree, but it's true.
Claude,
Responding to your thesis:
Responding to your thesis:
No, Re is adding series resistance such that changes in gm/beta lead to reduced changes in Ic. Can be explained by Ohm's Law.
Thank you.
That's becaue there isn't any.
Are you trying to show you know more about transistors than I do?The EF circuit's tendency to neutralize beta AND hie (input resistance) variations is due to the presence of Re, forcing a constant Ic, which results in stable gm. The collector current Ic is hFE times base current Ib...................... from Vin to account for the base-emitter forward drop. As long as hFE is reasonably large, hFE/(hFE + 1) approaches one. This is why EF circuits are stable with regard to varying beta. The transconductance gm, is only as stable as Ic, which is only as stable as (hFE + 1)*Re being greater than hIE.
Irrelevant. The discussion is about whether the EF topology implies NF, not about parasitic stability effects.
I know.
>When was this aparatus " forced " to play any " nonweak " music, such as symphonic orchestra or choirs, results was not so positive, 'cos in higher levels ( not on maximum levels ) start " mischmasch " - distortion of amps with " low feedback " is still too high. I am sorry guys, many of yours probably will not agree, but it's true.
I'm completely agreed with this statement. There will be a lot of intermodulation products.
I'm completely agreed with this statement. There will be a lot of intermodulation products.
Re: Charles's amps
Hello -
I have no reason to doubt your observations. Based on these observations, I can think of at least four different possibilities that would explain them:
1) All zero-feedback amplifiers will always have audible levels of distortion at high listening levels.
2) A zero-feedback amplifier is capable of inaudible levels of distortion at high listening levels, but in this specific instance it did not.
3) The unspecified Avalon loudspeaker has audible levels of distortion at high listening levels.
4) The amplifiers were clipping when played back at high levels with those speakers in that room.
It seems to me that it would be difficult to draw a conclusion from this particular listening experience.
Best regards,
Charles Hansen
Upupa Epops said:
Hello -
I have no reason to doubt your observations. Based on these observations, I can think of at least four different possibilities that would explain them:
1) All zero-feedback amplifiers will always have audible levels of distortion at high listening levels.
2) A zero-feedback amplifier is capable of inaudible levels of distortion at high listening levels, but in this specific instance it did not.
3) The unspecified Avalon loudspeaker has audible levels of distortion at high listening levels.
4) The amplifiers were clipping when played back at high levels with those speakers in that room.
It seems to me that it would be difficult to draw a conclusion from this particular listening experience.
Best regards,
Charles Hansen
Re: There are two types of feedback. Degenerative and regenerative.
Hello Mike,
Once again, Steve Eddy was slightly off the mark. The examples he gave are almost always referred to as "positive feedback" (which is why you haven't heard the term "regenerative" before). The term "regenerative" feedback was normally used to refer to circuits that employ positive feedback, but at such a level that the circuit does not oscillate.
This technique was invented by Major Armstrong in the early days of radio because the tubes of that time had very low mu (amplification factor).
Here is an explanation taken from:
http://www.faqs.org/faqs/antiques/radio+phono/faq/part4/
"One other common early design was the "regenerative" set. In these sets, an RF amplifier was designed as an oscillator, but provided with a control that could be adjusted so that the stage wouldn't go into oscillation. The positive feedback in the stage provided substantially more gain than a simple tuned circuit would provide. Misadjustment of the feedback control would make the stage oscillate, producing squeals in the output, and quite powerful RFI (radio frequency interference) as well."
Please note that this technique was normally used for the RF stages of radios and not the audio stages. (There wasn't any IF stage until Armstrong later invented the "superheterodyne" circuit that for all practical purposes replaced the "regenerative" circuit.)
I have only seen positive feedback applied to a non-oscillating audio circuit once. This was in a Jadis power amplifier. By applying positive feedback, the gain of the voltage gain circuit was increased so that the overall negative feedback (that was also used) would also be increased. I presume that it was a less expensive approach than using an additional gain stage.
Hope this helps,
Charles Hansen
mikelm said:
Hello Mike,
Once again, Steve Eddy was slightly off the mark. The examples he gave are almost always referred to as "positive feedback" (which is why you haven't heard the term "regenerative" before). The term "regenerative" feedback was normally used to refer to circuits that employ positive feedback, but at such a level that the circuit does not oscillate.
This technique was invented by Major Armstrong in the early days of radio because the tubes of that time had very low mu (amplification factor).
Here is an explanation taken from:
http://www.faqs.org/faqs/antiques/radio+phono/faq/part4/
"One other common early design was the "regenerative" set. In these sets, an RF amplifier was designed as an oscillator, but provided with a control that could be adjusted so that the stage wouldn't go into oscillation. The positive feedback in the stage provided substantially more gain than a simple tuned circuit would provide. Misadjustment of the feedback control would make the stage oscillate, producing squeals in the output, and quite powerful RFI (radio frequency interference) as well."
Please note that this technique was normally used for the RF stages of radios and not the audio stages. (There wasn't any IF stage until Armstrong later invented the "superheterodyne" circuit that for all practical purposes replaced the "regenerative" circuit.)
I have only seen positive feedback applied to a non-oscillating audio circuit once. This was in a Jadis power amplifier. By applying positive feedback, the gain of the voltage gain circuit was increased so that the overall negative feedback (that was also used) would also be increased. I presume that it was a less expensive approach than using an additional gain stage.
Hope this helps,
Charles Hansen
dimitri said:
Hello Dimitri,
I'm curious as to why you would reach that conclusion.
(By the way the THD of that particular amplifier at 1 dB below clipping is 0.1%. While it is possible that Upupa Epops was able to hear 0.1% of amplifier distortion at high playback levels, I would think that it was more likely that he was hearing the amplifier clip. That is why #2 and #4 are not the same thing at all. #2 was referring to a hypothetical zero-feedback amplifier that would exhibit audibly high levels of distortion even before clipping.)
Best regards,
Charles Hansen
Dynaco PAS series of preamps used it in the phono stage. Is bootstrapping a sort of positive feedback? If so, we've got a whole lot more examples.
And there are lots of amps which have used it inadvertently, but that's a different story.
Hello SY -
You're absolutely right about the Dynaco preamps. I used to have one of those in high school, but at that time I didn't know enough to recognize that 47k resistor going from cathode-to-cathode as a positive feedback path.
I'm hesistant to go down the road of "whether boostrapping is positive feedback". I don't want to start another "discussion" like the current one! 😀
Charles Hansen
You're absolutely right about the Dynaco preamps. I used to have one of those in high school, but at that time I didn't know enough to recognize that 47k resistor going from cathode-to-cathode as a positive feedback path.
I'm hesistant to go down the road of "whether boostrapping is positive feedback". I don't want to start another "discussion" like the current one! 😀
Charles Hansen
Charles Hansen said:
Hm, now when you say it... but yes, I guess we'd better save
that discussion for a rainy day. 🙂
It's raining here at the moment.
Since there is no sociological, emotional, or marketing baggage associated with bootstrapping ("The Syzygy PA-600, a ZERO bootstrapping amp!"), I think it's safe to discuss without worrying about extraneous stuff.
My own first exposure to bootstrapping was in a Mac power amp. It used a unity-coupled output stage and bootstrapped the driver to get anough swing and gain to drive the outputs. There was also a similar circuit used in the Luxman 50 watt monoblocks (3045?). Since there's an increase in open-loop gain as well as swing, isn't it really regenerative feedback?
Since there is no sociological, emotional, or marketing baggage associated with bootstrapping ("The Syzygy PA-600, a ZERO bootstrapping amp!"), I think it's safe to discuss without worrying about extraneous stuff.
My own first exposure to bootstrapping was in a Mac power amp. It used a unity-coupled output stage and bootstrapped the driver to get anough swing and gain to drive the outputs. There was also a similar circuit used in the Luxman 50 watt monoblocks (3045?). Since there's an increase in open-loop gain as well as swing, isn't it really regenerative feedback?
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