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John Curl's Blowtorch preamplifier part III
John Curl's Blowtorch preamplifier part III
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Old 7th May 2018, 02:51 PM   #3581
Markw4 is offline Markw4  United States
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Joe, Maybe it would help in thinking about some of this stuff to recall something very basic from physics: the extent to which the voltage and current in a speaker are in phase is an indication that power is being dissipated inside the speaker system. That could either by because of frictional or resistive losses, or because the sound waves it produces are launching or coupling energy outside and away from the speaker itself.

Likewise, to the extent voltage and current are out of phase, it means energy is being temporarily stored in the speaker system (and maybe its local environment) until it is released back out (perhaps only a fraction a cycle later). In addition, when a speaker has a resonance energy storage/release process can occur over a much longer time period.

Hopefully, that more or less makes sense. Of course, if I misspoke at all please do correct me. The idea is to get to some some shared understanding of the actual physics.

Once we can agree on some basic principles, we have a big picture and we know our more detailed, lower level understanding will have to be consistent with that.

Last edited by Markw4; 7th May 2018 at 02:54 PM.
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Old 7th May 2018, 02:52 PM   #3582
Markw4 is offline Markw4  United States
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Originally Posted by billshurv View Post
Mark: you are going all hall monitor on us again... :P
Hopefully not. I just don't want to make Joe feel unwelcome when he is starting to get more comfortable talking with us.
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Old 7th May 2018, 02:56 PM   #3583
scottjoplin is offline scottjoplin  Wales
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Old 7th May 2018, 03:15 PM   #3584
scott wurcer is offline scott wurcer  United States
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Originally Posted by traderbam View Post
I think he may be getting confused about voltage feedback to a transconductance amplifier and sinusoidal phase shift at the output.
No, it's more fundamental. BTW if your still interested I thought of something with reference to the error correcting output stage, the difference in re-entrant distortion.
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Old 7th May 2018, 03:22 PM   #3585
Joe Rasmussen is offline Joe Rasmussen  Australia
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Quote:
Originally Posted by Markw4 View Post
Joe, Maybe it would help in thinking about some of this stuff to recall something very basic from physics: the extent to which the voltage and current in a speaker are in phase is an indication that power is being dissipated inside the speaker system. That could either by because of frictional or resistive losses, or because the sound waves it produces are launching or coupling energy outside and away from the speaker itself.
I hope you don't think I am anti-physics or whatever. But I can tell you that there is computer modeling and actual tests that shows a direct (and powerful) relationship with the non-volatile impedance that is the DC resistance and the voltage that appears across it and the dB-SPL variation. The method of extracting that using a current sense resistor not only works, but validates it. I have literally done this on my bench. The fact that SoundEasy, which is a pure maths based program and I would think above reproach, can actually model this as well - makes me very confident (and if you replicated it, so would you). It is a bit too complex to explain on a forum like this where (as JC pointed out) there is a tendency to shut down certain non- approved topics (I tried it elsewhere on diyaudio and it got 'the treatment' there too).

So look at an impedance plot, measure the DC resistance, which is constant with frequency. It is not frequency dependent, but it is thermally dependent, but that is a different topic. Yet the overall impedance is indeed frequency dependent, but clearly not all of it. So if Re is 6 Ohm, the RMS voltage is 6V, then under ideal conditions it should draw 1 Amp. And if current drive equivalence, by sending 1 Amp, the 6V should develop across the voice coil terminals. Yet the V/I conversion does not take place at the terminals if the impedance is above 6 Ohm. Please have an open mind and ask what is really then happening. The overall impedance determines the current drawn from the amplifier (I am surely on safe grounds there), so if the impedance is 12 Ohm, then we have 6 Ohm of stable resistive impedance and another 6 Ohm of volatile back-emf impedance. The current will be 500mA, half the current - the dB-SPL of the driver will change by 6dB and then the penny drops, the volt times current across the DC resistance accounts for that 6dB exactly. You model it and that is how it works, then you do it physically on the bench, and it does the same. The physics is clear - the DC non-volatile impedance times voltage = dB-SPL - and it is not just theory, but a verifiable fact that anyone with fairly basic equipment can do.

If you don't find that interesting, then don't blame me if I feel otherwise.
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Last edited by Joe Rasmussen; 7th May 2018 at 03:30 PM.
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Old 7th May 2018, 03:31 PM   #3586
KSTR is offline KSTR  Germany
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Originally Posted by Joe Rasmussen View Post
I am now aware that others are also looking at the above. Let's not be hasty if it leads to a better understanding of some kind. At least it can't hurt.
Yep

The point I was trying to make is that neither pure current drive nor standard voltage drive (let alone pure velocity drive) might be the best for a given driver, rather its all a tradeoff and we have an free variable in the game which not everyone is aware of, and that's the drive impedance profile vs. frequency.

By shaping this profile to get overall "best" distortion (HD and notably IMD) at any given range of usable frequency and operating point (excursion and power) we might improve the fidelity of the driver significantly. What the best distortion behavior is supposed to be is then another topic open to discuss.

With a given terminal voltage (or current, for that matter) the SPL output will be the same no matter what the drive impedance is, so for comparison we'd need to EQ this voltage/current curve to whatever fixed target. But "fine-print" in the sound will be different, notably when we're about to leave the linear range of any parameter involved.
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Old 7th May 2018, 03:39 PM   #3587
Joe Rasmussen is offline Joe Rasmussen  Australia
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Originally Posted by scott wurcer View Post
Yup, he sure did. Though I wonder what was actually said.
Would you like his email? You could ask him yourself.

I can assure you, I didn't misquote him. He is Dutch, I seem to recall you not liking the British much (why not?), so that might help?
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Old 7th May 2018, 03:46 PM   #3588
traderbam is offline traderbam  Canada
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Originally Posted by billshurv View Post
Mark: you are going all hall monitor on us again... :P
Iím not sumitting to his rules.
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Old 7th May 2018, 03:54 PM   #3589
nezbleu is offline nezbleu  Canada
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John Curl's Blowtorch preamplifier part III
Quote:
Originally Posted by Joe Rasmussen View Post
So look at an impedance plot, measure the DC resistance, which is constant with frequency.
What does that mean, exactly? The "frequency" of DC resistance is 0Hz, by definition.

Quote:
Yet the overall impedance is indeed frequency dependent, but clearly not all of it.
Right, because the impedance of the driver is not resistive, it is a complex impedance that includes capactive and inductive components.

Quote:
So if Re is 6 Ohm, the RMS voltage is 6V, then under ideal conditions it should draw 1 Amp.
No, that is wrong, who told you that? That would only be true at DC (in which case you wouldn't be measuring voltage as RMS). I know you understand the difference between resistance and impedance, not sure why you are saying thiis.
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Old 7th May 2018, 03:54 PM   #3590
Markw4 is offline Markw4  United States
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Joe,

When you say things like, "...the non-volatile impedance that is the DC resistance," and, "The method of extracting that using a current sense resistor...," it makes me think we are not speaking the same language. Until we solve that problem we will always have trouble communicating. May I ask about your background in the areas of electronics and engineering? (No disrespect intended, I'm just trying to get a better picture of where you are coming from.)
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