# Amplifier myths

Discussion in 'AV Receivers' started by Yogi, Mar 19, 2004.

1. ### Yogi Screenwriter

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2. ### Kevin C Brown Producer

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The voltage/current relationship for a speaker is AC and not DC. So when he talks about "ohm's law," that isn't 100% correct, is it? He's forgetting about phase. Ohm's law works for "resistance", i.e., DC, but is different for "impedance" which is what's up with a speaker, and is AC. Right?

3. ### Shiu Second Unit

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Ohm's law does apply to both a.c. and d.c. V=IZ for a.c. V=IR for d.c., Z is the impedance. Phase angle between the voltage and current phasors only comes in the picture in power calculations. (Power=VI X cosine of phase angle between V&I). For d.c. the phase angle is zero, cosine 0 degree=1 so Power=VI, or I squareXR, or V square divide by R.

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I have only skimmed through it, but I was disturbed at the way Ohms Law was thrown around as an explanation for everything. It's like using Newton Laws to explain motion near the speed of light.

Resistance is a form of impedance. Impedance is anything that impedes current flow (truism, I know, but you would be surprised). The DC component of impedance is called resistance (The AC component is reactance).

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H

5. ### Yogi Screenwriter

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Dont you think that Ohms' law for electricity is a tad bit more general than Newtons laws for motion. Where does Ohms' law fail?

6. ### Shiu Second Unit

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Mathematically, impedance Z, can be expressed as a complex number, e.g. Z=R+or-jwL+or-j/wC, where w is proportional to the frequency (a.c.), L is the inductance, C is the capacitance. The j(imaginary term of the oomplex bumber) components are the "reactance", R is the "resistance". All 3 components react to both d.c. and a.c., just differently. From this formula you can see that jwl, the inductive reactance increases as frequency increases while the capacitive reactance increases as frequency "decreases", to infinite under steady state d.c.(frequency=0) condtion. Resistance remains constant regardless of the frequency, except there is a "skin effect" at high frequencies that affects its value, though not significantly. I thought it would be confusing to refer to resistance as the d.c. component because it does react to a.c. basically the same way (except at very high frequencies), as it does to d.c.

7. ### Chu Gai Lead Actor

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Well I think he's made some assumptions about ss amps and what they can or can't do while contrasting it with what I assume are actual measurements off of one of his amps own amps. One of the things the site doesn't tell you is that his amps will most certainly have a euphonic character to them.
Yogi, did you check out that last amp of his. Talk about a ton of tubes!
Newton's law is pretty general too.

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I stand corrected. I should have said resistance is the frequency independent component of impedance.

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H

9. ### Yogi Screenwriter

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Yes I figured. I have never heard of an OTL amp but now my interest is perked. For the moment I am very satisfied with my transformer coupled amps. I am not completely convinced with all that he says. I think one could make arguments in favor of any of the amp technologies if one was clever enough. I'll believe one when I hear one. I always trust my ears. I just thought that this was an interesting perspective on amps. Never thought of it this way. Hence the posting.

Also Ohms law is much more general than Newtons laws, IMHO. Newtons laws are based on the faulty assumption of absolute time(& coordinates) and have been very easily disproved by people (forget their names) carrying atomic clocks on airplanes all over the globe and measuring the loss in time thus proving that time does move slower as you move at higher speeds and away from gravity. You dont have to go at near the speeds of light to disprove Newton all you need is a clock thats accurate enough. Thats why I say Ohms laws are more general. But thats besides the point of this thread. So why dont we let it rest.

10. ### Michael R Price Screenwriter

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Shiu knows what he's talking about. Ohm's law holds but the speaker has to be treated as a complex impedance function. So it is possible that weird things can happen in the amplifier especially if low amounts of feedback are used, or if there's an output transformer or something.

11. ### Robert Cowan Supporting Actor

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speakers have very complex impedence characteristics. most digital meters cant even lock on for an accurate reading while music is playing. they jump around quite a bit. bass frequencies can drop really low really quick, and the opposite applies for high frequencies.

12. ### Chu Gai Lead Actor

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This means what?

13. ### James W. Johnson Screenwriter

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14. ### Robert Cowan Supporting Actor

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hah. yeah, i meant impedences can drop low with bass frequencies, and the opposite for high frequencies. you knew what i meant, thanks for taking it out of context...

15. ### Yogi Screenwriter

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I hope we dont have to resort to beer in this thread

16. ### Chu Gai Lead Actor

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Actually, I wasn't sure what was meant and it seemed like too much of a generalization but no beer is required. Consider the impedance curves of three speakers: Dynaudio Conference, Paradigm Studio 40, and Axiom M40's.

The largest impedances occur in the bass region whereas in the upper frequencies, the impedance gently rises providing, at least to me, a fairly benign scenario for an amplifier.
Now contrast that with the impedance curves of a Quad and an ML.

Once again, a fairly high impedance in the lower frequencies but a drop to some pretty damned low impedances once we get up there in frequency requiring a judicious choice in amplifiers.

17. ### Shiu Second Unit

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I think manufacturers try to design their speakers to minimize impedance fluctuation within a wide frequency range. Even if the impedance is not as low as people think at low frequencies, it could still be demanding for the amplifier because of the higher current required to move the much larger mass of the bass drivers.

18. ### Chu Gai Lead Actor

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to a point perhaps but it's hard to get away from impedance rises at resonance.

19. ### Shiu Second Unit

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I suspect you are right, the impedance of the speaker includes all 3 elements, i.e. resistance, inductance & capacitance. These things bound to resonate at some points. The impedance of the speaker wires, and possibly many other factors could also affect the impedance. This is a complicated thing, period. It takes much more than Ohm's law to analyze this apparently simple device (speaker). Unfortunately I have not come across any E.E. text books, or any other source including google search that deal deep into this topic. May be next time I visit a library I'll see what I can find. Until then, every time I see people debating whether voltage or current is more important I can laugh but cannot tell them why there is no point debating.

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