Same old song over at Head-Fi....

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dc
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Re: Same old song over at Head-Fi....

Post by dc » Sat Aug 13, 2011 12:04 am

Beefy wrote:A voice coil is not a purely resistive load, but actually generates a voltage of its own as it moves inside the fixed magnetic field. This might be important to measurements, it might not, but could possibly account for audible differences between amps that measure similarly.
fair point. i guess for things in the 'unknown' like this would be where DBTs benefit most. but then there will always be some asshole out there who claims to have superior hearing than the test participants.

i think the constantly variable nature of a headphone would be helped by a higher slew rate of an amp, again depending on the headphone.

re: current/voltage that goes back into the amp from the headphone: doesn't that have something to do with damping factor? (out of my depth here obviously, electrical engineer types please help!)

I suppose the fact that we have such a hard time hearing, measuring or otherwise explaining problems that may theoretically exist goes to show we are well and truly into diminishing returns territory. please refer to elitist asshole comment above however.

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Re: Same old song over at Head-Fi....

Post by Beefy » Sat Aug 13, 2011 12:23 am

dc wrote:re: current/voltage that goes back into the amp from the headphone: doesn't that have something to do with damping factor? (out of my depth here obviously, electrical engineer types please help!)
Not really. Damping factor reflects how much of the amps total energy is driving the phones, versus driving it own output impedance (or a series resistance on the output). So a low damping factor means that the amp has less electrical control over the driver, and a high damping factor means it has tighter control.

Whether high damping factor is desirable or not depends on the design of the phones. Dogma would say that very light diaphragms that are not heavily damped in the cups could easily overshoot or resonate (think orthos) need a high damping factor to keep things in check. Whereas heavier diaphragms that are heavily damped (think Beyers) require a lower damping factor so that they don't sound too sharp. Some argue this is why Beyers sound better out of 120 ohm outlets.

With regards to how reactive voltage feeds back into the amp, I really don't know how it affects things, only that a purely resistive load is not real-world. But yes, I think that an amp with a faster slew rate or lots of negative feedback would react differently to this than a slower amp or one without negative feedback.
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Re: Same old song over at Head-Fi....

Post by dc » Sat Aug 13, 2011 12:56 am

thanks, you have confirmed what I think I understand about the actual measurements needed to calculate damping factor. But I was under the impression damping factor itself is useful for 'damping' or absorbing incoming current/voltage produced by the movement of the magnetic coil from dynamic headphones and more significantly speaker cones.

I will check back tomorrow to see what others have to say, but before hitting the sack decided to check my favourite source of reliable, credible information on the internets:
Damping factor @ Wikipedia wrote:But a driver with a voice coil is also a current generator, since it has a coil attached to the cone and suspension, and that coil is immersed in a magnetic field. For every motion the coil makes, it will generate a current that will be seen by any electrically attached equipment, such as an amplifier.
...
A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a "brake" on the voice coil motion
Perhaps I'm reading it wrong (to justify my possibly incorrect understanding of damping factor's usefulness), but still interesting!

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Re: Same old song over at Head-Fi....

Post by Beefy » Sat Aug 13, 2011 1:31 am

dc wrote:
Damping factor @ Wikipedia wrote:But a driver with a voice coil is also a current generator, since it has a coil attached to the cone and suspension, and that coil is immersed in a magnetic field. For every motion the coil makes, it will generate a current that will be seen by any electrically attached equipment, such as an amplifier.
...
A high damping factor (which requires low output impedance at the amplifier output) very rapidly damps unwanted cone movements induced by the mechanical resonance of the speaker, acting as the equivalent of a "brake" on the voice coil motion
Perhaps I'm reading it wrong (to justify my possibly incorrect understanding of damping factor's usefulness), but still interesting!
I think you and I are both reading it right, but we are looking at it from different perspectives. Current generated by the voice coil does need to be sourced or sunk, and a high damping factor means that the amp is reacting to this, rather than a passive resistive element. But it still doesn't necessarily take into account how the amp actually responds. Does it hold the voltage perfectly steady, or does it distort a touch while sinking/sourcing current?
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Re: Same old song over at Head-Fi....

Post by dingostolemyipod » Sat Aug 13, 2011 1:19 pm

Yeah as far as I know, you're both talking about the same things from different perspectives.
It's only around the resonant frequency of the driver where the driver will be producing a significant back-EMF which is what give rise to the impedance hump at resonance, because the amp doesn't deliver as much current around this frequency as the driver is producing it's own => higher impedance. If you've got a low output impedance to the amp, all frequencies will receive an equal voltage signal. With an ouptut impedance present, then a voltage divider is in place that will reduce the power to the headphones, however the reduction in power around resonance will be less due to the higher impedance, which boosts bass around resonance. As for whether this is a good thing or bad thing entirely depends on the headphones, and lets face it personal preference too.
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Re: Same old song over at Head-Fi....

Post by dc » Sat Aug 13, 2011 8:27 pm

right, gotcha. thanks guys.

I'm not aware of what to measure to determine the performance while the amp is 'damping' the return current. I didn't think it was as much of an issue for headphones as it is for speakers, but then again considering the scale of size/voltage/current it probably works out about the same.

Probably some combination of measuring the slew rate and damping factor together under load. It's getting complicated!

Anyway, I'm still kind of keen to build the amp if just to prove that measurements aren't everything. Since it's meant to be all through hole I should be able to manage.

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Re: Same old song over at Head-Fi....

Post by Beefy » Sat Aug 13, 2011 8:44 pm

dc wrote:Anyway, I'm still kind of keen to build the amp if just to prove that measurements aren't everything. Since it's meant to be all through hole I should be able to manage.
Go for it. I am secretly hoping for a huge shit storm when this is out in the community, and nobody likes the sound of it...... :)

There is interesting discussion on Head-Fi re: the volume control after the input stage. I thought power dissipation in the volume control would be an issue, but snarfed my calculations. AMB has recently chipped in, showing how easily the input stage will clip if given a hot source. Fun times!
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Re: Same old song over at Head-Fi....

Post by dc » Sat Aug 13, 2011 10:45 pm

well I'm hoping that at the very least, the lesson people will learn from this is to acknowledge that they are heavily subjective rather than to create the world's most technically accurate amp - at least I don't think that is his objective

basically as was said above it's the people at the polar opposite camps that seem to make the most noise whereas the least vocal people who are probably also the most normal fall somewhere in the middle

considering the parts he's using it should cost sfa so not much to lose

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Re: Same old song over at Head-Fi....

Post by dingostolemyipod » Sun Aug 14, 2011 1:39 am

Beefy wrote:There is interesting discussion on Head-Fi re: the volume control after the input stage. I thought power dissipation in the volume control would be an issue, but snarfed my calculations. AMB has recently chipped in, showing how easily the input stage will clip if given a hot source. Fun times!
Yeah, each builder will need to pretty much know the exact sources that would be used with the amp in what situations, even to the point of using uncommon gain setting like 2.5 or 1.8 etc(if they want ideal performance).
There might be a real chance of disaster with people having cliptastic amps left right and centre.
I think i still might build one - but i will wait and see how much i like it before deciding what to do with it enclosure wise and what not, since that could easily cost more than the populated board.
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Re: Same old song over at Head-Fi....

Post by dc » Sun Aug 14, 2011 8:42 am

Haha I thought there must have been a reason that hardly any other amp designs although having 2 stages didn't put the volume control after the input stage!

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