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In Reply to: RE: Hagerman Piccolo Zero Headamp posted by Tre' on January 30, 2024 at 07:18:42
Hi Tre'
according to Lenz's Law, if a conductor moves within a magnetic field it will induce a current in the conductor and that induced current will create its own magnetic field that opposes the motion that created the current in the first place. IMHO, that will act to dynamically reduce compliance. I am not sure if, or why, reducing dynamic compliance would be advantageous and it is not clear how small the coil resistance needs to be for Lenz's Law to actually make a difference. Electromagnetism is not my speciality, I once posed this very question to one of my EM-savvy colleagues who had no answer!
In 'normal' voltage mode, very little current flows in the coils as the total resistance is usually >100R. If you took things to the extreme and had zero coil resistance and a true virtual earth transimpedance input then a very large current would flow and generate its own magnetic field that would completely oppose the cantilever movement! Probably, you don't need much resistance to get any benefit while still allowing the thing to work (?).
I had never equated this effect with amplifier damping factor before. In that case the amplifier output voltage creates a field that moves the voice coil and the speaker moves. Considering what I wrote above, in moving a current will be created in the voice coil that opposes the movement - I've never seen that commented on before (at least not explicitly). What I thought advantageous about high damping was if the speaker overshoots and is no longer following the driving signal any 'error' current created due to that extraneous movement will create a magnetic field to compensate for the overshoot. But if there is effective opposition to an overshoot why not effective opposition to the voice coil moving in the first place? Maybe, the current sourced into the voice coil by the driving amplifier swamps out any back emf but when there is overshoot that ratio of back emf to driving current is much higher (because the amplifier isn't driving at that point)?
Enough musing for now, I have some non-EM technical problems to attend to and after that enjoy some LPs without thinking about how they work :)
Follow Ups:
Take a floppy woofer without a box and move the cone by hand. Now short across the voice coil leads with a .1 ohm resistor. Assuming an 8 ohm speaker, that .1 ohms represents an amplifier with a damping factor of 80. The cone will not move nearly as freely for the reasons you have described.
When the speaker is connected to an amplifier that is playing, the cone has to move to follow the signal provided by the amplifier but any movements, as you have described, that are outside of the signal that amplifier is feeding to the speaker will be damped out due to the low output impedance of the amplifier. Speakers are also mechanically damped. In fact, some speakers do not require electrical damping but I believe all woofers do.
With a cartridge loaded into zero ohms the movement of the cantilever, other than the movement caused by the undulations in the groove that the stylus is following, will be damped out by, not only the built in mechanical damping (that gives us the compliance number given in the specs), but now, and in addition to that, we have the electrical damping, do to the extremely low load impedance, damping the movement further.
The question I have is how much will the compliance be lowered by this electrical damping on top of the mechanical damping built into the cartridge's suspension?
So much so that a heavier tonearm will be needed to keep the cartridge compliance/tonearm mass resonance at 10Hz? And how much heavier?
I am a little disturbed that Hagerman didn't say anything about this.
He has to know that the compliance is lower. He says as much in the text while talking about Dynamic Braking. "Hence, the lower the resistive loading, the more effort it takes to move the cantilever."
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It is s slippery slope to attempt to quantify the concept of "dynamic braking". A few key general thoughts:
-With your shorted woofer example, it is impossible to tell if the driver is shorted or not from gently pushing it in and out. It takes an abrupt blow to see the effects of Lenz Law. It can be thought of as a velocity sensitive effect.
-the nature of the riaa curve typically has the highest measured velocities occurring in the 4kHz-12kHz range and Holman found measured dynamic peaks in music in the 80-100cm/sec range.
-I contend that under 'normal' conditions the change in compliance (dynamic braking) is minimal but on the dynamic peaks where mistracking occurs it can clamp down and reduce the negative effects of the needle attempting to be kicked from the groove wall.
dave
.
Have Fun and Enjoy the Music
"Still Working the Problem"
Sadly, I don't understand EM well enough to be authoritative. However, it is not obvious to me that Mr. Hagerman understands it any better.
At the link see the section Current-mode design improves cartridge tracking through superior damping. The picture shows better tracking when an MC is loaded into a transimpedance input due to 'simple physics' but not simple enough to actually describe :)
I have often mused that the 'unique' sound of the transimpedance approach has little to do with the nature of the amplification or the mythos surrounding the coil as a current generator and everything to do with the aggressive load improving the dynamic tracing ability of the cartridge. It is good to see Lino say the 'quiet part' out loud.
A must read for anyone interested in this is:
Audio Fallacies Exposed
Low impedance Loads for MC Cartridges
J. Peter Moncrief
International Audio Review #5
1980
dave
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