[proton007]

Quote:

In which case, it's not "power" that's "lacking". It's more like an impedance matching issue.

 
Another way to look at this is in terms of power transfer. An amp should transfer maximum power to the load when the output impedance and load impedance is considered. This means the voltage and hence power drop should occur all at the headphone (load), not the amp itself.
 
Now if the amp itself has higher impedance, I'm sure it'll add reactance (capacitive and inductive), which means it'll also filter out some frequencies of the signal even before they reach the headphone.

 

[Bill-P]

Quote:

 
You're looking at a step response. The over-damped line shows high frequency roll-off.
The resonant frequency is usually a low frequency.
 
"Bumping up" the output impedance is the opposite of what feedback is there for. Also, there is no bass roll-off with the DT990, but tight control over the driver. 1V in equals 1V out regardless of frequency with an ideal amp with a gain of 1. With a higher output impedance 1V in might result in 0.5V at 1 kHz and 0.8V at 100 Hz, resulting in a bass "boost".

 
So in that case, too low a damping factor causes a high frequency roll off -> lack of "soundstage" then.
 
But why would the over-damped line be different for low-frequency?
 
Also, I don't see how higher output impedance can cause a bass "boost".
 
For instance, considering a 1V after-gain voltage => [V gained] = 1V
 
[V out] = [V gained] * [Z load] / ([Z load] + [Z out])
 
Assuming the same frequency => same [Z load]
 
This would effectively mean increasing [Z out] causes [V out] to drop, not increase. When [V out] drops, output power also drops because:
 
[P output] = [V out]^2 / [Z load]
 
Or did I miss something there?
 
 

Another way to look at this is in terms of power transfer. An amp should transfer maximum power to the load when the output impedance and load impedance is considered. This means the voltage and hence power drop should occur all at the headphone (load), not the amp itself.
 
Now if the amp itself has higher impedance, I'm sure it'll add reactance (capacitive and inductive), which means it'll also filter out some frequencies of the signal even before they reach the headphone.

 
So basically, [V gained] from my above example comes from the amp, and [V out] is basically what remains after accounting for [Z load] and [Z out], right?

 

[xnor]

Quote:

 
Why would the over-damped line be different for low-frequency?

 
Because a low frequency has a much longer rise time. See here.
 
 
Quote:

Also, I don't see how higher output impedance can cause a bass "boost".  
For instance, considering a 1V after-gain voltage => [V gained] = 1V
 
[V out] = [V gained] * [Z load] / ([Z load] + [Z out])
 
Assuming the same frequency => same [Z load]
 
This would effectively mean increasing [Z out] causes [V out] to drop, not increase. When [V out] drops, output power also drops because:

Yes, increasing Zout will decrease Vout, but if Zload increases at the resonant frequency for a given Zout then Vout will be bigger at the resonant frequency, resulting in the bass "boost" (quotes because the Zout actually attenuates other frequencies stronger).
 
Example: 30 ohms at 1 kHz and 60 ohms at 100 Hz, with Zout = 10 ohms
 
Vout at 1 kHz = 0.75 V, Vout at 100 Hz = 0.857 V, about +1.16 dB bass.
 
In other words: the output impedance takes away the amps control over the headphone driver, typically causing the bass "boost" described above but also higher distortion as can be seen in a benchmark whitepaper.

 

[Happy Camper]

To break from the scientific explanation for a minute, the early days of the HE-6 and LCD-2 found the problems explained. It was increasing power that corrected the problems. When I first heard both on headphone amps, the soundstage and imaging was fuzzy, ill-defined, and just terrible to listen to. More power solved that problem. Not saying that that's the solution to every problem but everybody whipping out the calculations and saying more power isn't the answer were wrong. The market suppliers recognized the problem and a plethora of new high power amps for orthos flooded the market.

 

[Bill-P]

Quote:

Because a low frequency has a much longer rise time. See here.

 
Then it looks like just a horizontal scale of the same plot to me.
 
 

Yes, increasing Zout will decrease Vout, but if Zload increases at the resonant frequency for a given Zout then Vout will be bigger at the resonant frequency, resulting in the bass "boost" (quotes because the Zout actually attenuates other frequencies stronger).
 
Example: 30 ohms at 1 kHz and 60 ohms at 100 Hz, with Zout = 10 ohms
 
Vout at 1 kHz = 0.75 V, Vout at 100 Hz = 0.857 V, about +1.16 dB bass.
 
In other words: the output impedance takes away the amps control over the headphone driver, typically causing the bass "boost" described above but also higher distortion as can be seen in a benchmark whitepaper.

 
Then in which case, I think the correct way to look at it might be that it's not "bass boost" that's taking place, but rather "over-damping" of all other frequencies, right?

 

[xnor]

Quote:

To break from the scientific explanation for a minute, the early days of the HE-6 and LCD-2 found the problems explained. It was increasing power that corrected the problems. When I first heard both on headphone amps, the soundstage and imaging was fuzzy, ill-defined, and just terrible to listen to. More power solved that problem. Not saying that that's the solution to every problem but everybody whipping out the calculations and saying more power isn't the answer were wrong. The market suppliers recognized the problem and a plethora of new high power amps for orthos flooded the market.

More power compared to what? With just 77 dB/mW the HE-6 is extremely inefficient. You'll need a lot more power compared to other ~50 ohm dynamic headphones. If any headphone deserves the "moar power!" recommendation it's this one.
The LCD-2 on the other hand is more efficient. Dunno what the problem with these should be.
 
Quote:

 
Then it looks like just a horizontal scale of the same plot to me.

If you look at the sample values, a step response looks like this: 0, 0, 0, 1, 1, 1, 1, .. The rise time is as small as possible. Ideally it is infinitely small.
A low frequency on the other hand would look like this: 0, 0, 0, 0.01, 0.03, 0.04 ... and a lot of samples later we finally reach 1 aka a sine wave. There won't be overshoot or overdamping as shown in the image you posted.
 
 
Quote:

Then in which case, I think the correct way to look at it might be that it's not "bass boost" that's taking place, but rather "over-damping" of all other frequencies, right?

I call it bass "boost" due lack of control over the driver (the higher the output impedance, the less the amp cares about the actual load, the driver will resonate more and more "out of control" at its resonant frequency), or low damping factor. As you can see in the image you posted, typical headphones are already under-damped. Higher output impedance will only make this worse, i.e. even more under-damped.

 

[mikeaj]

Quote:

More power compared to what? With just 77 dB/mW the HE-6 is extremely inefficient. You'll need a lot more power compared to other ~50 ohm dynamic headphones. If any headphone deserves the "moar power!" recommendation it's this one.
The LCD-2 on the other hand is more efficient. Dunno what the problem with these should be.

 
Also, I wonder what is meant by "increasing power".  If you increase the power delivered to the headphones, that will make them sound louder.  If you switch from one amp to another that is capable of higher output power levels, you are also changing things unrelated to power.  If you switch amps but do not change the volume, then you are not increasing power (delivered).  For example, if you're delivering 10 mW to headphones with one amp (capable of 100 mW output into that headphone) and 10 mW to headphones with another amp (capable of 4 W output into that headphone), that is not increasing the power.
 
Recently it seems to be fashionable to get and to "require" amplifiers that can output levels far above that which is actually being used—with maybe the exception of listening to HE-6 at high volumes.

 

[xnor]

Yup, it actually has a few downsides to have a very powerful amp if you only need a fraction of the power like higher noise, less usable volume control range.

 

[Bill-P]

Quote:

Originally Posted by xnor /img/forum/go_quote.gif
 
If you look at the sample values, a step response looks like this: 0, 0, 0, 1, 1, 1, 1, .. The rise time is as small as possible. Ideally it is infinitely small.
A low frequency on the other hand would look like this: 0, 0, 0, 0.01, 0.03, 0.04 ... and a lot of samples later we finally reach 1 aka a sine wave. There won't be overshoot or overdamping as shown in the image you posted.

 
Yes, but since the amplitude changes slightly (overall), the step would change slightly as well, right? Or am I to assume that there is no change at all in steps?
 

I call it bass "boost" due lack of control over the driver (the higher the output impedance, the less the amp cares about the actual load, the driver will resonate more and more "out of control" at its resonant frequency), or low damping factor. As you can see in the image you posted, typical headphones are already under-damped. Higher output impedance will only make this worse, i.e. even more under-damped.

Yeah, but in the case of high impedance headphones like the DT990 250 Ohm, where the damping factor is typically greater than 8 for any output impedance lower than approx. 31 Ohm, I think over-damping would happen quite regularly, which causes rolling off.
 
And I'd think 600 Ohm headphones like the DT880 would be even worse.
 
Also the "scale-king" HD 650, which has long been said to "scale up" really well with amping seems to fall into this exception:
 

 
I think it needs to be re-stated that I'm considering this case only for high impedance headphones (as the title suggests) rather than "typical headphones".

 

[xnor]

Quote:

 
Yes, but since the amplitude changes slightly (overall), the step would change slightly as well, right? Or am I to assume that there is no change at all in steps?

The step response would, yes, but not the rising edge as shown in the image.
 
Quote:

Yeah, but in the case of high impedance headphones like the DT990 250 Ohm, where the damping factor is typically greater than 8 for any output impedance lower than approx. 31 Ohm, I think over-damping would happen quite regularly, which causes rolling off.
 
And I'd think 600 Ohm headphones like the DT880 would be even worse.
 
Also the "scale-king" HD 650, which has long been said to "scale up" really well with amping seems to fall into this exception:
 
[hd650 impedance]
 
I think it needs to be re-stated that I'm considering this case only for high impedance headphones (as the title suggests) rather than "typical headphones".

Yeah but that's not the case. If these high-impedance headphones were under-damped, you wouldn't see the overshoot and ringing in the square wave measurements. If there was a roll-off, we'd see it in the measurements. We can also calculate the bass "boost", or lack thereof.
 
All you get with a higher output impedance is a peak at the resonant frequency (and sometimes the top octave), like a bell EQ filter, and higher distortion. Some people might like this, others don't. I prefer a low output impedance and equalizing the frequency response.

 

[Bill-P]

Quote:

The step response would, yes, but not the rising edge as shown in the image.

 
No, of course not. I wasn't referring to the edge. That doesn't occur in a time frame that would allow it to be shown. But just the fact that the response is "different" may cause a small change that some people can "perceive". I don't think we should rule that out.
 

Yeah but that's not the case. If these high-impedance headphones were under-damped, you wouldn't see the overshoot and ringing in the square wave measurements. If there was a roll-off, we'd see it in the measurements. We can also calculate the bass "boost", or lack thereof.
 
All you get with a higher output impedance is a peak at the resonant frequency, like a bell EQ filter, and higher distortion. Some people might like this, others don't. I prefer a low output impedance and equalizing the frequency response.

 
I think the problem is whether the output impedance would be high enough to cause an under-damp with high impedance headphones. In which case, I'd agree that there is some peak and distortion.
 
So far, I don't see any indication that that has ever happened for any impedance less than 1/8 the impedance of the headphone. And I think that's because less than 1/8, the amp still has about 90% control over the voltage, so it can control distortion to some extent.
 
And again, we are talking about high impedance headphones or... if I have to classify them, pretty much anything over 120 Ohm.

 

[xnor]

Quote:

 
No, of course not. I wasn't referring to the edge. That doesn't occur in a time frame that would allow it to be shown. But just the fact that the response is "different" may cause a small change that some people can "perceive". I don't think we should rule that out.

It's a bass boost that can be measured, calculated and heard if it's big enough.
 
 
Quote:

I think the problem is whether the output impedance would be high enough to cause an under-damp with high impedance headphones. In which case, I'd agree that there is some peak and distortion.

Any increase in output impedance causes more under-damping. A difference between a damping factor of 100 or 300 may not be observable by the human ear, but differences are there (measurable and calculable).
 
 
Quote:

So far, I don't see any indication that that has ever happened for any impedance less than 1/8 the impedance of the headphone. And I think that's because less than 1/8, the amp still has about 90% control over the voltage, so it can control distortion to some extent.
 
And again, we are talking about high impedance headphones or... if I have to classify them, pretty much anything over 120 Ohm.

Sure, with most headphones the bass "boost" is less than about +0.5 dB if driven by <1/8 the impedance of the headphones. Distortion increase won't be big either.
 
(But I've seen amps and receivers with hundreds of ohms of output impedance, and people plug anything with a 1/4" jack in there, regardless of impedance.)

 

[Bill-P]

Quote:

It's a bass boost that can be measured, calculated and heard if it's big enough.

 
 
I'm just wondering if the "change" would also be the inverse... or a "bass roll off", in other words. Some amps with low impedance like the O2 have a bass roll-off... though I'm not sure if that's due to the nature of the opamp or as a result of the low impedance when coupled with a high impedance load.
 

Any increase in output impedance causes more under-damping. A difference between a damping factor of 100 or 300 may not be observable by the human ear, but differences are there (measurable and calculable).

 
I'm not sure I follow you. If a DP of 300 is under-damping, then what would be considered over-damping?
 
 

Sure, with most headphones the bass "boost" is less than about +0.5 dB if driven by <1/8 the impedance of the headphones. Distortion increase won't be big either.
 
(But I've seen amps and receivers with hundreds of ohms of output impedance, and people plug anything with a 1/4" jack in there, regardless of impedance.)

 
Yeah, I concur. I have seen people try to plug their headphones into vintage amps that go up to hundreds of ohms indiscriminately.
 
Weren't those amps and receivers meant to drive 2K Ohm headphones?

 

[xnor]

Quote:

Originally Posted by Bill-P /img/forum/go_quote.gif
 
I'm just wondering if the "change" would also be the inverse... or a "bass roll off", in other words. Some amps with low impedance like the O2 have a bass roll-off... though I'm not sure if that's due to the nature of the opamp or as a result of the low impedance when coupled with a high impedance load.

Are you sure about that? -0.6 dB at 5 Hz doesn't seem to be audible. There can only be an "inverse" change if the output impedance is negative, which is possible but I've never seen such a headphone amp. This should also answer your over-damping question.
 
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Weren't those amps and receivers meant to drive 2K Ohm headphones?

Yeah but even new products are released with high output impedance. Those manufacturers just don't care enough about headphone-fidelity it seems.

 

[Steve Eddy]

Quote:

 
Yeah but even new products are released with high output impedance. Those manufacturers just don't care enough about headphone-fidelity it seems.

 
It's about cost savings. Less expensive to just run a couple of resistors off the speaker outs to the headphone jack than to include a dedicated headphone circuit.
 
Having said that however, my LCD-2's sound just fine out of the headphone jack of my TEAC A-H500 which drives it through a pair of 390 ohm resistors.
 
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