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Old 17th September 2012, 08:22 AM   #7181
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There was an amp sometime ago that had an optional third wire for feedback from the speaker + terminal . No idea what and how good .

I saw a valve amp guy define damping factor in a simple way . He said that if he had a damping factor of 4 then the average speaker had a 3 dB exaggeration at the resonate point . As he also said probably unnoticeable over the majority of the frequency range and perhaps nice at resonance to some ? I suspect to be able to go from 4 to 100 would be fun . If I build a bass guitar amp I think it will be used as my tests with that seemed positive . It wasn't boom so much as the ability to open the sound . I could well believe an under-damped amp and Acrostic suspension speakers might be fantastic . AR grew up with Dynaco's . Perhaps that is why they were so well liked ?

I take the idea of a series resistor assisting an amp as possible . A watershed to prevent back EMF entering the global feedback loop , if so could we use much more feedback ? This technique is common in power supplies to prevent hum loops . I think Martin Colloms is about to write about this soon . He will use tests more than ears . To have a very low impedance still seems ideal . If the additional resistance helps it might do even better with a brick-wall low impedance to assist ? It was said that 0R22 helped most amps drive a Quad ESL better .

I wonder also if a negative impedance output and a series resistor has anything to offer ? Using a speaker feedback system ( 3rd wire ) . Lets say 0R47 ? Try to resolve to 0 ohms with the benefit of making the load more restive in terms of VI . I would speculate that negative impedance needs that resistance so as not to over compensate . Compression horns mentioned as the only speakers to benefit . I didn't much like the RCA valve amp that had it . However that was the amp I think ? It was worth a fortune I am told .

I would speculate that Motional feedback only needs a 50 Hz pass band ? Perhaps less if a notch filter ? I think I saw an LED and silver foil reflector used as a way to get the feedback on the centre dome . Some use a feedback tap on the voice coil . If that was used with the Goodmans inductive tweeter idea something very cheap and nice could happen . The Goodmans was like a cheaper Tannoy with no conventional crossover . It was not a wizzer cone either . It looked like a dust cover . If the experience with the Servo Sound is universal the bass cone might need an error signal to work well ? A nice paper cone seems idea .

Last edited by nigel pearson; 17th September 2012 at 08:29 AM.
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Old 17th September 2012, 10:37 AM   #7182
DF96 is offline DF96  England
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'Back EMF' and impedance are two different ways of talking about the same thing. In most cases impedance is the better method to use. For some strange reason people seem not to know this, or act as if they don't know it.
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Old 17th September 2012, 10:41 AM   #7183
AndrewT is offline AndrewT  Scotland
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Quote:
'Back EMF' and impedance are...................ways of talking about the same thing.
Are they?
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Old 17th September 2012, 11:00 AM   #7184
DF96 is offline DF96  England
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Back EMF is the voltage coming back from something as a consequence of the current signal you have put into it. Another name for this is impedance. There may be energy storage mechanisms (such as cone resonances) but these can be modelled by including LC tuned circuits in the impedance model.

If you drive something from a pure voltage source then you don't see any back EMF (by definition) but instead you see the current draw varying.

There may be an assumption in this that the equivalent circuit is not time varying, but that will be generally true except in pathological cases.

The simplest example of back EMF is a single pure inductor. The back EMF both defines and is perfectly modelled by the inductance.
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Old 17th September 2012, 11:56 AM   #7185
dvv is offline dvv  Serbia
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Nige, 3 or 4 parallel/series power devices, especiall when run "hot" (i.e. with a larger bias current), should all by themselves produce a damping factor of 6 or more open loop. Add moderate NFB and you are home and dry in that respect.

True, the DF at 20 kHz will be lower than the DF at say 100 Hz, but then remember that like 90% of the energy used is done below 1 kHz, the energy requirements at 20 kHz are really much lower.
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Old 17th September 2012, 12:05 PM   #7186
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There is an argument that the dead band of a class B is an issue . Someone called it free fall . I don't see it myself as the transition is fast . I see greater feedback with the lowest possible impedance as good . I suspect converting the load to a more resistive one would be adventurous . If the swings and roundabouts allow more to be gained than lost that is ?

If the free fall is possible then MOS FET's might be better . Class B as in Quad 405 is a sharper switch off . Anyone every put MOSFET's in a 405 ? I think it might have advantages ?
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Old 17th September 2012, 12:31 PM   #7187
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Originally Posted by dvv View Post
Nige, 3 or 4 parallel/series power devices, especiall when run "hot" (i.e. with a larger bias current), should all by themselves produce a damping factor of 6 or more open loop. Add moderate NFB and you are home and dry in that respect.

True, the DF at 20 kHz will be lower than the DF at say 100 Hz, but then remember that like 90% of the energy used is done below 1 kHz, the energy requirements at 20 kHz are really much lower.

DVV . Good to have some figures . If you get time maybe you could increase your global feedback and add 0R22 ( 12 dB perhaps ) . I have a hunch you will like it . I seem to remember you use a series resistor already ( no choke ) . Years ago a damping factor of 16 was said to be the point where the punch of a speaker is already optimum ( no idea if true ) .

Most high quality op amps sound great at unity gain ( if it is in their design to do it ) . Why not power amps ? I know why not , however is there a sweet spot where feedback is greater and sound better . My hunch is it needs a more restive load to enjoy more feedback .

I read about ricochet effect in studios years ago . To quote ESP on this .

On the other hand, the transformer serves as a 'buffer' between loudspeaker and amp proper. This can be advantageous with practical loudspeakers in avoiding the 'ricochet' effect of transistorised amplifiers faced with back-EMF from loudspeakers resulting from unwanted diaphragm motion. This output from the loudspeaker can, in some circumstances, penetrate back to interfere with a signal passing out toward the speaker; a consequence of the inherently low impedance of transistorised amplifying circuits. Valves are inherently high impedance devices, and the effects of back-EMF from the speaker are therefore far less likely to achieve significant penetration at sufficient level to degrade performance. This must not be confused with damping factor per se which is generally rather better in transistorised amplifiers than in valved models.

Valve Amplifiers

Sometimes ricochet effect was due to the room I think ? The first time I read about it was in that context . The time delay not the same as back EMF . I am not sure if it is a genuine problem .
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Old 17th September 2012, 01:43 PM   #7188
gpapag is offline gpapag  Greece
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Quote:
Originally Posted by nigel pearson View Post

Quote:
It was said that 0R22 helped most amps drive a Quad ESL better
Nigel, Quad ESL is electrostatic. No back EMF there.
Instead, it is a capacitive load for the output stages. They don’t like the elliptical shaped load “line”(current lead voltage) which they face with electrostatics, thus the "isolating" (makes the ellipse a bit thinner) effect of the series 0R22 is beneficial.

Quote:
I take the idea of a series resistor assisting an amp as possible . A watershed to prevent back EMF entering the global feedback loop
It may only reduce the EMF back signal, unfortunatelly reducing too the amp output signal feeding the speakers).

Ref. techniques for separating the back EMF speaker signal from the amp output signal, there were 2-3 articles (tube circuits) on one of the early “Audio Anthology Volumes”. I don’t remember which one. On the same volume there should be an article for a tube amp with variable output impedance.



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Old 17th September 2012, 02:04 PM   #7189
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Thanks for that, as you say back EMF not an issue , nor damping factor I presume ? .

EMI never got back over Callas . The name Peter Andre was given when I phoned , he seemed a bit young . Also he was Australian , if so he had no obvious accent .

Strange we all know George Martin , we don't know the equally important classical man . I should point out it was the archive department ( Gary ) . Surprised they didn't know . I write some risky things here . I try very hard to open conversations . I get paid nothing and do it out of the need to know . Others are paid and seem to know nothing ! I often do know nothing , I am very willing to learn .

I suspect back EMF is not the issue . I have always doubted it amounted to much power ?

Last edited by nigel pearson; 17th September 2012 at 02:15 PM.
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Old 17th September 2012, 06:39 PM   #7190
dvv is offline dvv  Serbia
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Quote:
Originally Posted by nigel pearson View Post
DVV . Good to have some figures . If you get time maybe you could increase your global feedback and add 0R22 ( 12 dB perhaps ) . I have a hunch you will like it . I seem to remember you use a series resistor already ( no choke ) . Years ago a damping factor of 16 was said to be the point where the punch of a speaker is already optimum ( no idea if true ) .

Most high quality op amps sound great at unity gain ( if it is in their design to do it ) . Why not power amps ? I know why not , however is there a sweet spot where feedback is greater and sound better . My hunch is it needs a more restive load to enjoy more feedback .

I read about ricochet effect in studios years ago . To quote ESP on this .

On the other hand, the transformer serves as a 'buffer' between loudspeaker and amp proper. This can be advantageous with practical loudspeakers in avoiding the 'ricochet' effect of transistorised amplifiers faced with back-EMF from loudspeakers resulting from unwanted diaphragm motion. This output from the loudspeaker can, in some circumstances, penetrate back to interfere with a signal passing out toward the speaker; a consequence of the inherently low impedance of transistorised amplifying circuits. Valves are inherently high impedance devices, and the effects of back-EMF from the speaker are therefore far less likely to achieve significant penetration at sufficient level to degrade performance. This must not be confused with damping factor per se which is generally rather better in transistorised amplifiers than in valved models.

Valve Amplifiers

Sometimes ricochet effect was due to the room I think ? The first time I read about it was in that context . The time delay not the same as back EMF . I am not sure if it is a genuine problem .
Aaaaaaaarrrrggghjhhhhh!

Nige, I don't WANT to increase my global feedback over the 26 dB limit. Not that it's too hard to do, I simply have to decrease local stage degeneration and increase overall NFB, but in my FIRM view, it's easier to get the amp to sound good when you do not use more than 26 dB of global NFB.

As for parallelled power resistors in series with the amp output, I saw that for the first time in a project by John Lindsley Hood, in the early 80ies, a project which used IGBT transistors from Toshiba. He claimed that three 0.22 Ohm resistors in parallel helped "significantly" regarding what the loudspeaker may choose to send back to the amp. I am a bit baffled, as 3 time 0.22 in parallel equals just 0.07 Ohms, and to me, that is low, low, low.

The other take on this is to use two diodes from the amp output to the plus and minus power supply line. US designers hardly ever use it, European designers use it almost by default - as do I. Assuming you agree with Harman Kardon that nothing ever sent back will exceed 5 Amps worst impulse case, you can use humble 1N4004/5/6/7 diodes as they do, but I prefer to use 1N5604, I like to overdo it whenever convenient.

Lastly, here I am, trying hard as hell to eliminate all capacitors and inductors from the direct signal path, and then I should stick in resistors I don't really need slap bang in the middle of my signal path? I'll do that when you Brits start serving cold beer.
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Last edited by dvv; 17th September 2012 at 06:44 PM.
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