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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Re: MOSFETs work fine too... posted by Robert Karl Stonjek on December 14, 2001 at 01:37:54:
Understood. Half the disagreements on newsgroups and message boards start when one or both parties do not have enough time to outline their case clearly enough.Yes. And becuase one or both parties do not have enough time to actually read what's actually being said by the other party.
Reiterating:- the debate began on the subject of feedback and whether or not it is really necessary.
It did? Granted, my memory's not what it used to be, but I could have sworn that it began on the subject of a loudspeaker's impedance dropping during transients. Oh well. Let's move on.
I pointed out that MOSFET amps need feedback to maintain a constant output with frequency when the load changes with frequency.
Yes. And as I beleive I pointed out, such behavior is the practical consequence of an amplifier's non-zero output impedance. And as I also believe I pointed out, an amplifier's output imepedance can be lowered verus what it would be otherwise by the application of feedback.
Someone pointed out that there are several varieties of MOSFETs and not all perform in the manner I outlined. I specified Hitachi MOSFETs in later discussions.
Any device with a non-zero output impedance will behave in that manner to one degree or another. And since every device that I'm aware of has a non-zero output impedance, it really boils down to a matter of degree and what degree the designer feels is acceptable.
You said that feedback was not necessary for MOSFETs either but later said that it is necessary to correct for "inherent limitations" of some designs, and also maintained that there are other (non-feedback) methods of achieving the same thing.
First, I didn't say it wasn't necessary. Whether it's necessary is ultimately a decision for the designer to make. What I did was point out that, in the context of controlling the motion of a loudspeaker cone, particularly at its fundamental resonant point, the amplifier's output impedance doesn't have quite the substantial effect that many people believe. That even a thousand fold difference in output impedances results in a very marginal difference in the amplifier's control over the loudspeaker.
Second, I didn't say that feedback was necessary to correct for inherent limitations. I simply said that that's what the feedback is doing in this particular context. If a designer feels it's necessary to apply feedback in order to reduce the amplifier's output impedance, then obviously the amplifier's inherent output impedance (i.e. without feedback) is a limitation. I made no mention of its being inherently necessary as again that's a call for the designer to make. Other designers might be willing to live with the inherent limitation.
Thirdly, I believe I said that you can design an amplifier with low output impedance without having to resort to feedback, but I didn't intend this to apply to your specific topology/parts compliment.
I also mentioned that MOSFET amps can correct for the errant voltage output of a driver, such as might be generated by an overshooting speaker.
And I mentioned that it does this by way of its output impedance. Doesn't matter if it's a MOSFET amp, a BJT amp, a tube amp, or what have you. With feedback or without feedback. Any "errant voltage" applied across the amplifier's output will see the amplifier's output impedance and behave accordingly.
I think now might be a good time to address this whole "overshooting speaker" thing as I think you may be looking at this issue from some erroneous assumptions.
For example your assumption that any time the speaker is moving in one direction, then before it can move in the opposite direction, its momentum must first be overcome, which you say cannot occur instantaneously.
While it's true that any mass which is moving (in this context in response to an applied force) has momentum seeing as momentum is mass times velocity, not every mass which is moving in response to an applied force GAINS momentum. And it's only momentum GAINED which would need to be overcome before you could change its direction.
Take a hockey puck for example. Set it down on a smooth sheet of ice and with your finger apply a little force in one direction for a certain distance and then stop applying the force. The hockey puck will continue to move a bit beyond the point at which you stopped applying the force. That distance represents the momentum that was GAINED by the hockey puck. And if you were to apply a force in the opposite direction at the same moment you stopped applying the initial force, you would first have to overcome this gained momentum before you could get it moving in the opposite direction.
Now do the same thing only this time with the hockey puck sitting on a sheet of rough sandpaper. This time the hockey puck doesn't move past the point at which you stopped applying the force. That's because the much higher frictional losses of the sandpaper versus the ice prevented the hockey puck from gaining momentum.
Loudspeakers of course have losses as well, in all three domains; electrical, mechanical and acoustic. With respect to raw drivers, the relationships between losses and reactances are described in the various Q parameters, QES, QMS and QTS. What they describe is the damping effect the losses have on the reactive elements.
QES describes the damping of the electrical portion of the driver, QMS of the mechanical portion, and QTS is the product/sum of the two others, i.e. (QES x QMS) / (QES + QMS).
Anyway, the point here is that these figures (well, QTS really) are useful in determining a driver's propensity to overshoot. A QTS of 0.707 will have a maximally flat response with no overshoot. Above this is considered "underdamped" and you will begin to get overshoot as the driver's internal losses are overcome by the reactive elements. Below this is considered "overdamped" and internal losses dominate.
QTS for typical drivers ranges from around 0.2 to 0.6.
If it's not obvious yet, another way of putting this is that for any QTS at or below 0.0707, the driver's internal losses are such that they prevent the cone from GAINING momentum. And if the cone doesn't gain any momentum, then when your transient comes along that wants to move the cone in the opposite direction, then as far as the transient is concerned, it sees nothing different than if the cone were otherwise at rest. It's like the hockey puck on the sandpaper.
Of course raw drivers are typically used in enclosures which in combination with the driver's QTS will result in a different system Q (QTC, which will always be at least as high or higher than the driver's QTS). And depending on the designer, one may decide on a QTC greater than 0.707.
Anyway, this is just a side note to address any notion that simply because the cone is moving it must have gained momentum.
And now back to our regularly scheduled program...
I did perform an additional experiment. I build a subwoofer that uses a compound woofer - basically the compound woofer is two woofers glued together face to face.
So I have some of these lying around. In the experiment I powered one of the woofers. The other was passively driven, simulating an overshoot condition.
The input voltage was 6VRMS at 20Hz. The voltage generated by the passively driven woofer was 2.3V with no load.
I then connected various resistors to act as loads/damping and checked the voltage output of the passively driven woofer:-
No Load.2.3V
10r.....1.2V
1r......220mV
0.033R...12mV
Amp.......9mV (passive woofer connected to power amp; amp input shorted)
Short.....9mVNote that the amp gave the same result as the short. The error bars are two big to say more than that. Accuracy of around ±2mV and 1.9mV measured noise.
Ok. Basically this is a good illustration of the effects of an amplifier's output impedance. As I said previously, the reason your amplifiers don't have a constant voltage into a changing load is because of the amplifier's output impedance.
Let's say your amplifier, which isn't using any feedback at this time, has an inherent output impedance of 1 ohm. And into a 16 ohm load you're drawing 1 amp of current. Ohm's Law says that you've got 16 volts across the load.
And since the amplifier's output impedance is in series with the laod, all of the current flowing through the load is also flowing through the amplifier's output impedance. And Ohm's Law says that you must be dropping 1 volt across the amplifier's output impedance.
So basically you've got an amplifier that's ouputting 17 volts (since the total voltage is the sum of all the voltage drops) but because of its output impedance, it's only delivering 16 volts to the load.
Keeping all else equal, drop the load impedance down to 4 ohms. 17 volts into 5 ohms (4 ohms for the load, 1 ohm for the amplifier's output impedance) gives you 3.4 amps. Now you've got 13.6 volts across the load and 3.4 volts dropped across the amplifier's output impedance.
The effect of this is a 2.4 volt (1.4 dB) difference between the voltages applied across a 16 ohm and 4 ohm load.
Now compare the difference if the amplifier's output impedance is reduced two orders of magnitude, from 1 ohm to 10 milliohms. In the case of the 16 ohm load, you've got 16.99 ohms across the load and 0.012 ohms dropped across the amplifier's output impedance. In the case of the 4 ohm load, these numbers become 16.96 volts and 0.042 ohms respectively. So you've gone from a 2.4 volt (1.4 dB) difference to a 0.03 volt (0.015 dB) difference.
When you apply feedback, what you're doing is reducing voltage gain, which also has the effect of lowering output impedance. How much you lower output impedance (and reduce gain) depends on how much feedback you apply with respect to the amplifier's open loop voltage gain.
And this also goes toward your claim that the amplifier is "pushing harder" when you apply feedback. Since the feedback lowers voltage gain, the amplifier's output voltage will be lower for a given input voltage. So even though you've reduced the amplifier's output impedance, you've reduced its voltage gain by the same amount. Which means that the amplifier is pushing just as hard into the same load with feedback as it was without it.
Notably, the amplifier’s feedback circuit showed a 0.5V error voltage passed to the gates of the MOSFETs. This seems to show that the amp was ‘pushing against the voltage’ produced by the speaker.
If the amplifier was indeed "pushing against the voltage" produced by the speaker, then it would be producing a voltage of the same polarity as the voltage from the speaker in which case you'd be measuring something more like the 2.3 volts you got from the non-loaded speaker. And if that were the case, your feedback would be positive rather than negative.
If I had a couple days spare to set up the test properly I might find something interesting, but is it really worth it? My original point was that feedback is required to keep some (ie Hitatchi) MOSFET amps form varying output voltage with load. You say there is another approach to achieve the same thing.
What you're really saying though is that the MOSFET amps you're familiar with have inherently high output impedances and you feel that feedback is required to lower output impedance to an acceptable level.
If you want lower output impedance without feedback, then use devices in the output stage which have an inherently lower output impedance. I don't know that you'll be able to reach the level you'd like using MOSFETs and it appears that MOSFETs are the only devices you'd consider.
Anyway, if a person was really serious about sound they would dispense with the passive crossover and drive each speaker with a separate power amplifier. I think there is something to be said for specialist subbass, bass-mid and treble amps, though that is one further step I don’t think I’ll bother taking!! :)
No argument there. One of my friend's pet peeves are all the outrageously expensive, five figure loudspeakers out there which utilize passive crossovers. I'm inclined to agree. :)
se
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Follow Ups
- Re: MOSFETs work fine too... - Steve Eddy 16:06:40 12/14/01 (1)
- Re: MOSFETs work fine too... - Robert Karl Stonjek 00:20:28 12/15/01 (0)
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