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Will someone explain to me what ohms are? - Page 2

post #16 of 50
5/2/07 at 8:35pm
Ohm is an electrical resistence ...

EDIT: I'm trying to make it simple. A resistence of an element shows it's capacity of resisting a current flow, measured in ohms. Did i make myself clear?
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post #17 of 50
5/2/07 at 8:37pm
Quote:
Originally Posted by ericj View Post
Oh geeze. One of these days we need a sticky thread on ohm's law, efficiency, sensitivity, and what that means to you.

Lets start with the basic presumption of Ohm's Law: You Get Nothing For Free.

When you create an electrical potential across a load, the impedance of that load - how much the load impedes the flow of electricity - defines how much electricity is drawn from the source.

Thus, a low-impedance load draws more current from the amplifier than a high-impedance load. This is unavoidable, force-of-nature, 'canna break the laws of physics cap'n' stuff.

If the source doesn't have a lot of current sourcing capability, this can have a number of ill effects.

Large trasients - like a bass note - can fall flat before they reach their peak if the source runs out of current before it can finish servicing the pulse.

Increased current draw on some kinds of amplifiers can slow down their slew rate, causing the amplifier to slur the sound slightly.

The upside is that you don't need many volts to make the air move around the driver.

A high impedance load presents a very easy load to the source, but because less current is being drawn across it, if we presume that all other properties of the driver are the same, it requires more volts to move the air around the driver.

Your home stereo system has volts to spare. If anything the headphone jack was an exercise in limiting the volts on the output. It's not uncommon for a home stereo type system to be able to swing about 80 volts.

Your computer's sound card can probably only swing about 5 volts. Often as little as three.

Portable devices can generally swing three or less.

So, if you plug 600 ohm DT-990's into your iPod, it's not going to make much noise. If you were going to get an amp, you'd want one that can swing plenty of volts (say, two 9v batteries) but it doesn't need much current - a simple cmoy will do just fine.

If you plug your 32-ohm grados into your ipod, they'll get plenty loud, but they may sound slurred and distorted at high volume due to the difficult load they present to the little headphone amp.

Curiously enough, the ipod shuffle will handle it better than a full size ipod, since it appears to have a switch-mode headphone amp rather than a linear amp. If you were to look at the output on an oscilliscope, the full-size ipod will start out to service a large transient nice and clear, and then just give up. The shuffle makes it all the way across, but there's little squigglies on the top of the wave 'cause the output transistors are switching on and off at frequencies much higher than you can hear.
Thanks , good read.
Someone should make a sticky
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post #18 of 50
5/2/07 at 8:48pm
Wow, I'm really surprised that with a hobby that is so closely related to the physics of electricity, that many people dont seem to have a very good understanding of it.

Not speaking to anyone in particular. But this forum could use a crash course in some basic physics of electricity and waves. Anyone up for it? It could help eliminate alot of theories around here that contradict what physics would suggest.

I think it would be good to start breaking things down to actual facts and proper research, instead of purely opinion.

Im sure this forum must have some egineers about.
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post #19 of 50
5/2/07 at 8:52pm
Thread Starter 
Quote:
Originally Posted by LawnGnome View Post
Wow, I'm really surprised that with a hobby that is so closely related to the physics of electricity, that many people dont seem to have a very good understanding of it.

Not speaking to anyone in particular. But this forum could use a crash course in some basic physics of electricity and waves. Anyone up for it? It could help eliminate alot of theories around here that contradict what physics would suggest.

I think it would be good to start breaking things down to actual facts and proper research, instead of purely opinion.

Im sure this forum must have some egineers about.
I am down I have just been sitting back and trying to learn what I can. I might not be the brightest person at there but I welcome the chance to learn more.
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post #20 of 50
5/2/07 at 8:57pm
Excellent explanation, encj. Much thanks. If I hear this 100 times, eventually I'll understand it. It's always amazing to me that the original Earmax couldn't drive Grados without clipping, only higher impedance headphones, while the Earmax Pro with a larger power supply can. Such is the requirement for current at low impedance as you explain.

At least I hope I got that right.
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post #21 of 50
5/2/07 at 9:12pm
Quote:
Originally Posted by GregBe View Post
Why is it that in general headphones that are lower in Ohms are easier to drive, whereas, speakers with higher Ohms are eaier to drive?
My take on that:

The typical impedance you find in speakers is 8 ohms. Speakers with low impedance can have 6, 4, or even less ohms.

Notice that zero ohms would be absolute no resistance, meaning, basically a short circuit. Zero ohms would be equivalent (for the amp) to not connecting the speaker wires to the speakers, but simply shorting the red wire to the black wire. The lower the impedance of the speakers, the more the amp sees the load as such a shorted circuit.

Also, the closer-to-shorted-circuit the load is, the more the power supply needs to continuously provide higher and higher amounts of sustained current to provide the same voltage. (After all, V = I*R; notice that the closer to zero R gets in that formula, the higher the I -current- needs to become to yield the same voltage V).

But providing high levels of current (and voltages) in a sustained and reliable manner is not easy, and that is (ultimately) the tough job of the power supplies. Depending on their circuitry and specs, different power supplies have different operational limits. In general, the closer to zero ohms the load gets, the harder the power supply has to work, the closer to its operational limits it gets, and the easier it is for the power supply to attempt to exceed those limits and fail (either by making the amp produce clipped sound waves, by blowing up a fuse, or by shutting off if it has a self-protection mechanism built-in.)


Now, "low impedance" headphones are never as close to a shorted circuit as low impedance speakers. A low impedance headphone is usually around 32 ohms, quite higher than even typical "normal/high" impedance speakers, which as mentioned before, have ~ 8 ohms. Also, the voltage levels required in headphones are in general lower than those required by speakers to produce the desired volume levels from the drivers. So in general, the operational parameters are much more demanding and extreme when driving speakers (in particular, when driving "low impedance" speakers) compared to when driving headphones, whether low or high impedance ones.

This is only my take and understanding. The true knowledgeable electronics gurus here can bring more clarity on this, I'm sure.
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post #22 of 50
5/2/07 at 9:32pm
Ohm is a unit for measuring impedance/resistance. An ohm is equivalent to Volt/Ampere (voltage/current). V=IR is a very common electrical equation. This equation is know as Ohm's law (after whom the unit is named) since he discovered that voltage drop and current display a linear relationship in most cases and can be generalized as such. Many people believe that the nominal impedance decides the volume and stuff, while it might have some effect, the volume is actually affected more by the sensitivity. Since P=IV and V/R=I, it follows that P=I^2(squared)*R=V^2(squared)/R. While this equation is true, a LARGE COMMON MISCONCEPTION is that this is the power required where as this should actually be read as a relationship between POWER DISSIPATED, CURRENT FLOW and VOLTAGE DROP across an element. This is the reasoning behind transferring electricity at high voltage, since by doing so, with the same power supplied, the current drops and power loss over the wire is reduced.

Amplifier circuits have what is known as the input and output stages. Input impedance is an equivalent impedance as seen from the input and likewise for the output impedance of the amplifier. These can often be found in specifications of OP-AMPs (operational amplifiers) and other amplifier circuits. It is most desirable to match the impedance of the amplifier IIRC so that most of the power will be put to good use rather than wasted. E.G. for my ASL MG HEAD, the transformer mode is more desirable to drive Grados (low impedance) while OTL is more desirable for Sennheiser HD580/600 (high impedance) though it was probably impedance matching was probably not the reason why they put the OTL feature in that amp. NONETHELESS, if I don't match the impedance, say driving the Grado on OTL, or driving the Sennheiser on transformer, I will notice a drop in the output power from my cans.
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post #23 of 50
5/2/07 at 9:39pm
Thread Starter 
Ok so its gonna take me a few read throughs to fully understand that but I got a quick question. Are you saying that its always best to match your ohms output to input I.E. My player and headphones are a 32/32 ratio both input out put the same. AM I understanding this correctly?
All this is still floating in my head and I haven't quite grasped it. So if you have say a 32 for your source and a 16 for your headphones. Is all that wasted energy alter the sound at all? Or if you match the player and the phones is it gonna be worse cause its more taxing on the player?
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post #24 of 50
5/2/07 at 10:43pm
Quote:
Originally Posted by ericj View Post

snip...

This is unavoidable, force-of-nature, 'canna break the laws of physics cap'n' stuff.


snip...
Unless you start the matter/anti-matter engine via a warp-core implosion. It's a theory, never been tested, but its our only chance!
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post #25 of 50
5/2/07 at 10:47pm
Quote:
Originally Posted by Frostychrist View Post
Ok so its gonna take me a few read throughs to fully understand that but I got a quick question. Are you saying that its always best to match your ohms output to input I.E. My player and headphones are a 32/32 ratio both input out put the same. AM I understanding this correctly?
Yes, and no.

Yes, the ideal situation is to match the impedance to maximize the power transferred to your headphones. However, if your amp is more than capable to drive your headphones even with an impedance mismatch, then this becomes a non-issue.

Quote:
Originally Posted by Frostychrist View Post
All this is still floating in my head and I haven't quite grasped it. So if you have say a 32 for your source and a 16 for your headphones. Is all that wasted energy alter the sound at all?
Again, provided your source has more than enough power to drive your headphones, this power is just "wasted". It will not affect the sound from an electrical standpoint.

Quote:
Originally Posted by Frostychrist View Post
Or if you match the player and the phones is it gonna be worse cause its more taxing on the player?
The idea of matching impedance is to minimize the losses in power. Thus, matching impedance will actually make it less taxing on your source to achieve a certain volume level.

I will note that however, impedance is a function of frequency, but since audio signals are generally low frequencies (compared to most high-speed analog and digital circuits nowadays), this fact can be safely ignored.

P.S. I'm an electrical engineering major working towards my BSEE at the University of Texas at Austin (Graduating Fall 08 or Spr 09).
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post #26 of 50
5/2/07 at 11:13pm
so does it have a direct correlation to the sq or spl?
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post #27 of 50
5/2/07 at 11:13pm
i think generally the rule of thumb is that the output impedance of the amp at the headphone jack should be lower than the impedance of the headphones
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post #28 of 50
5/2/07 at 11:20pm
my take
if it sounds good to me, its good for me
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post #29 of 50
5/2/07 at 11:24pm
Thread Starter 
Quote:
Originally Posted by d-cee View Post
i think generally the rule of thumb is that the output impedance of the amp at the headphone jack should be lower than the impedance of the headphones
so You should have a haedphone with an OHM rating higher than the player. Did I read that right?

Thanks AZN
That I understand
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post #30 of 50
5/2/07 at 11:30pm
Quote:
Originally Posted by d-cee View Post
i think generally the rule of thumb is that the output impedance of the amp at the headphone jack should be lower than the impedance of the headphones
For voice coil dynamic headphones, ideally the output impedance of the amp should be vanishingly small, low single digits of ohms.

Impedance matching between the amp and the headphones is not desired with voice coil headphone designs.

This is not true with all driver technologies. PVDF piezoelectric films have an impedance curve that goes from near infinite at DC to almost a dead short at ultrasonic frequencies. So if you happen to have a pair of Pioneer SE-300, SE-500, or SE-700 headphones, an amp with a fairly high output impedance is greatly desired.

This sort of impedance matching is probably a big part of the $800 transformer box for the TakeT H2 piezo-heils.
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