Does the amplifier voltage swing limits headphone dynamics?

[JohnYang1997]

A good amplifier is good amplifier. You can add extra resistor to increase output impedance. It won't cause difference in sound except for a little bit higher distortion. For a good amplifier without Ro, distortion can be 0.0001%, after adding the 10ohm resistor the distortion across the load can be 0.0003%.

 

[Fluffy]

So maybe it's an indirect effect? If the power transfer becomes less efficient and the headphones low in volume, than logically you need to turn the volume up. And if the amplifier is not powerful enough you can ran into its limit (not sure if power or voltage…), which will result in distortion. No?

 

[solderdude]

When an amp runs into audible distortion (think > 1%) it is already clipping and is simply not suited to drive the load.

Connect the Verum1 to a 50 Ohm out amp that is designed to drive 300 Ohm headphones you are in trouble.
Connect it to a C'Moy amp which can only supply 50mA peak = 35mA and it runs out of steam with just 10mW.

Maybe what you heard was caused by other parameters or something else entirely such as a dodgy contact.
For instance a Verum 1 with a 6 Ohm impedance and a source resistance varying 0.01 Ohm (Verum 1's have been known to have dodgy contacts) could cause amplitude variances at an audible -50dB.

 

[lucian]

There is no such thing. Damping factor only guarantees the efficiency of power transfer.

Damping factor also has impact on the mechanical driver itself. Low damping factor means that the feedback from speaker can be better controlled by the amp.

Imagine a step response e.g. 1V to the speaker. The coil accelerates follows the step to e.g. 10mm movement. But at the end of the 10mm move the coil/cone still has momentum, it moves beyond the 10mm, you get overshoot and due to spring forces damped oscillation around those 10mm (e.g. betwenn 11...9mm). This additional motion also generates current inside of the coil (Lorentz/Lenz law) which feeds back to the amplifier. This introduces an voltage error at the output of the amp and it starts to counter these currents. Effectively this reduces unwanted motion of the speaker coil. The less the impedance of amp, the more can the amp *see* the feedback current and the more damping effect you have. Thats where the name comes from: it dampens unwanted motion of the speaker coil.

This might play a bigger impact at big speaker than on headphones though. The moveable mass of headphones is much smaller and you already have the damping of the surrounding air.

Actually it would be better to drive magnetic speaker directly with current and not with voltage, this would greatly reduce the effect of impedance. But voltage is defacto standard and all speacker/equipment are optimized to be driven by voltage.

 

[solderdude]

Actually it would be better to drive magnetic speaker directly with current and not with voltage, this would greatly reduce the effect of impedance. But voltage is defacto standard and all speacker/equipment are optimized to be driven by voltage.

Consider the following: Acurrent source supplies a current from a very high output resistance (with infinite voltage available)
what would be closer to a current source... a voltage source with 0 Ohm output resistance or a voltage source with say a 470 Ohm output resistance ?

 

[lucian]

Consider the following: Acurrent source supplies a current from a very high output resistance (with infinite voltage available)
what would be closer to a current source... a voltage source with 0 Ohm output resistance or a voltage source with say a 470 Ohm output resistance ?

I like how we oscillate around the original topic from the OP

You describe a constant current source. That's not what I meant.

a) Look at operational amplifier. You have voltage feedback and you have current feedback. Current feedback OP's a in general faster and therefore mostly used in video application, but also better for feedback at higher frequencies and therefore with less distortion.

b) The driving force of the coil is directly related to current. If you control the current - impedance doesn't matter. If you apply voltage - the current is directly dependent of impedance, from amp and speaker. And speaker impedance varies a lot with frequencies, which you don't want. Speaker/headphone producer work around that and optimize for voltage. But you still can see the effect on frequency response depended of impedance of the amp.

That's why they use 4..20mA current loops in industrie - impedance doesn't introduce errors.

 

[Fluffy]

Damping factor also has impact on the mechanical driver itself. Low damping factor means that the feedback from speaker can be better controlled by the amp.

Imagine a step response e.g. 1V to the speaker. The coil accelerates follows the step to e.g. 10mm movement. But at the end of the 10mm move the coil/cone still has momentum, it moves beyond the 10mm, you get overshoot and due to spring forces damped oscillation around those 10mm (e.g. betwenn 11...9mm). This additional motion also generates current inside of the coil (Lorentz/Lenz law) which feeds back to the amplifier. This introduces an voltage error at the output of the amp and it starts to counter these currents. Effectively this reduces unwanted motion of the speaker coil. The less the impedance of amp, the more can the amp *see* the feedback current and the more damping effect you have. Thats where the name comes from: it dampens unwanted motion of the speaker coil.

That's pretty much the explanation I'm familiar with regarding damping factor. If it's not true, that'll be an opportunity to learn something new. @JohnYang1997

 

[solderdude]

a) Look at operational amplifier. You have voltage feedback and you have current feedback. Current feedback OP's a in general faster and therefore mostly used in video application, but also better for feedback at higher frequencies and therefore with less distortion.

a) both designs using either current or voltage feedback will ultimately be used to create a voltage source with a low output impedance.
Video has frequencies above 1MHz, audio does not.

b) The driving force of the coil is directly related to current. If you control the current - impedance doesn't matter. If you apply voltage - the current is directly dependent of impedance, from amp and speaker. And speaker impedance varies a lot with frequencies, which you don't want. Speaker/headphone producer work around that and optimize for voltage. But you still can see the effect on frequency response depended of impedance of the amp[/QUOTE]

b) It actually is the other way around. Speakers and headphones are designed to be driven by a voltage source. Connect a headphone with a substantially varying impedance to say an amp like the Bakoon and tell me it measures and sounds the same as any voltage output amp.
The rising impedance, seen from the amplifier side, just means less current is drawn.

That's why they use 4..20mA current loops in industrie - impedance doesn't introduce errors

They use 4-20mA in current loops so one always knows there is a signal arriving (that's where the 4mA is there). One can also choose 0-20mA but in this case an inoperable source cannot be distinguished from one that has 0mA.
See the 4mA as sort-of a 'heartbeat' in this case.
It is used for practical reasons that also include impedance variances in cables and loads.

 

[JohnYang1997]

Damping factor also has impact on the mechanical driver itself. Low damping factor means that the feedback from speaker can be better controlled by the amp.

Imagine a step response e.g. 1V to the speaker. The coil accelerates follows the step to e.g. 10mm movement. But at the end of the 10mm move the coil/cone still has momentum, it moves beyond the 10mm, you get overshoot and due to spring forces damped oscillation around those 10mm (e.g. betwenn 11...9mm). This additional motion also generates current inside of the coil (Lorentz/Lenz law) which feeds back to the amplifier. This introduces an voltage error at the output of the amp and it starts to counter these currents. Effectively this reduces unwanted motion of the speaker coil. The less the impedance of amp, the more can the amp *see* the feedback current and the more damping effect you have. Thats where the name comes from: it dampens unwanted motion of the speaker coil.

This might play a bigger impact at big speaker than on headphones though. The moveable mass of headphones is much smaller and you already have the damping of the surrounding air.

Actually it would be better to drive magnetic speaker directly with current and not with voltage, this would greatly reduce the effect of impedance. But voltage is defacto standard and all speacker/equipment are optimized to be driven by voltage.

It's all measurable. Like I said. If you measure directly between the headphones terminal the distortion can go from 0.0001% to 0.0003%. Does that matter? Idk. If you think this is what you want then yes it matters. But is it really audible concern or is it a big difference. Absolutely not. If you manage to get headphones that has more than 0.1% thd when measured across the terminal then it can be really audible. Otherwise it's less than the transducer distortion itself which means won't matter. The issue with all the traditional experiences is that it's all different depending on amplifiers. There are of course some amplifiers can have large distortion from emf. But with well engineered amplifiers, adding a series resistor will not change performance much. People often only look at the surface without actually thinking through. That's exactly what's happening here. If you look deeper the solution/answer is much less arbitrary and clean and clear.

 

[solderdude]

Food for thought:

Imagine we have a 300 Ohm headphone driver, which is also a source as it acts as a 'generator' as well, with a 1V amplitude (voltage in unloaded conditions)

We now short this source with a practical 0.1 Ohm resistor. Current will flow 3.3mA
We now load this source with a practical 10 Ohm resistor. Current will flow 3.2mA
How much difference in 'damping current' will be there ?

Of course this changes when the impedance is lower. Say 30 Ohm
We now short this source with a practical 0.1 Ohm resistor. Current will flow 33mA
We now load this source with a practical 10 Ohm resistor. Current will flow 25mA
Do you see that as something that damps much more, this is a damping factor of 120 vs 3 yet the actual damping current varies just slightly.

There is the damping current story.. a nice invention from the sales department in the eighties.

What does matter here is that a varying impedance in the lows means that when an amplifier has a higher output resistance due to voltage division there will be a boost at the lower frequencies. Between 50 and 150Hz a boost there means 'muddier' sound, darker sound.
Damping current differences or voltage division effects ? What is heard/perceived ?

Consider a driver has back EMF and likes to resonate at 1 or more frequencies. It just acts as a generator (microphone) and thus generates those frequencies.
We apply a pulse that makes the headphone move and should stop but having resonance(s) doesn't want to.

Now imagine we are measuring a signal at the output of a black box (an ideal 0 Ohm amplifier + output resistance).
After the pulse the output voltage is back to zero but the driver (acting as a source) generates its specific frequencies.
We measure voltage at the black box. This is done versus the applied input signal of the black box.
Input signal is 0V. output R is close to 0 means the bulk of the generated EMF is dissipated in the driver and since driver and output R are in series, due to voltage division only a very, very tiny voltage is measured resulting in a low distortion number.
Increase the output R to 10 Ohm. Same source... due to voltage division 10 Ohm and 300 Ohm the current will show a much higher 'unwanted' voltage drop across the 10 Ohm resistance. This means the distortion number goes up as it compares unwanted signals to desired signals.

Is this increase in measured THD actually increased distortion or an artifact of voltage division and measurement method ?

 

[JohnYang1997]

A damping factor of around 1 will almost eliminate back emf completely under the audible range. If you have an output impedance of a few hundred owns and a transducer of 8ohm then yes it can be an issue. But practically 8ohm load with 10ohm source is almost worst as you can get. It does not introduce distortion over the distortion of the transducer itself.

 

[Frank Dernie]

As it's planar meaning flat impedance curve. Inefficient but still not an issue.

Mine are not inefficient at all.
They sound fine with my iPad and Macbook pro without unusual volume control settings.

 

[JohnYang1997]

Mine are not inefficient at all.
They sound fine with my iPad and Macbook pro without unusual volume control settings.

It will only be more efficient with lower output impedance.

 

[lucian]

a) both designs using either current or voltage feedback will ultimately be used to create a voltage source with a low output impedance.
Video has frequencies above 1MHz, audio does not.

b) The driving force of the coil is directly related to current. If you control the current - impedance doesn't matter. If you apply voltage - the current is directly dependent of impedance, from amp and speaker. And speaker impedance varies a lot with frequencies, which you don't want. Speaker/headphone producer work around that and optimize for voltage. But you still can see the effect on frequency response depended of impedance of the amp

b) It actually is the other way around. Speakers and headphones are designed to be driven by a voltage source. Connect a headphone with a substantially varying impedance to say an amp like the Bakoon and tell me it measures and sounds the same as any voltage output amp.
The rising impedance, seen from the amplifier side, just means less current is drawn.



They use 4-20mA in current loops so one always knows there is a signal arriving (that's where the 4mA is there). One can also choose 0-20mA but in this case an inoperable source cannot be distinguished from one that has 0mA.
See the 4mA as sort-of a 'heartbeat' in this case.
It is used for practical reasons that also include impedance variances in cables and loads.

a)
one of the best - if not THE best audio op is the LME49720NA. If you look at the datasheet you see a gain bandwidth product of 55 Mhz. You need fast feedback loop for accurate error compensation.

b)
Of course they sound different, if you drive speaker via voltage vs. driven via current. They optimized to sound good with voltage, cause that standard. However - the ultimate force that drives the coil is current. So if you supply current to the speaker you can directly control the force generating movement.
With voltage you effective current through the coil (and damping) depends on contact + wire resistance + change of resistance due to heatpup of coil + impedance ...
You don't have that with current. If you current driver delivers 10mA - the coil will see 10mA.

c)
4..20mA current loops are not inventend cause of live sign with 4mA. 4mA is actually 0 signal - and Error would be 3.6mA. But here are further variants to 0..20mA ; 4..24mA.
Also you can give a *live* signal with voltage output, e.g. analog sensor with supply of 5V typically have an output range of 0.5 ... 4.5V.

The main reason for 4..20mA is that you signal gets independent of resistance, be it the length of the wires or the resistance of contacts. (If you want to do that with voltage you would need 4 wires, which would make it more expensive.)

 

[lucian]

It's all measurable. Like I said. If you measure directly between the headphones terminal the distortion can go from 0.0001% to 0.0003%. Does that matter? Idk. If you think this is what you want then yes it matters. But is it really audible concern or is it a big difference. Absolutely not. If you manage to get headphones that has more than 0.1% thd when measured across the terminal then it can be really audible. Otherwise it's less than the transducer distortion itself which means won't matter. The issue with all the traditional experiences is that it's all different depending on amplifiers. There are of course some amplifiers can have large distortion from emf. But with well engineered amplifiers, adding a series resistor will not change performance much. People often only look at the surface without actually thinking through. That's exactly what's happening here. If you look deeper the solution/answer is much less arbitrary and clean and clear.

Yep, higher damping might increase the distortion of the amplifier. But it is also expected to reduce distortion of the speaker due to better control. So if you trade little additional distortion inside the amp against much lower distortion of the speaker - its a win for system.

The most critical part of the audio chain are the speakers. And I am not convinced that mechanical distortion of the mechanical system automatically translate into distortion measurable at the electrical terminal. These would need to be measured by microphone.

Food for thought:

Imagine we have a 300 Ohm source with a 1V amplitude (in unloaded conditions)

We now short this source with a practical 0.1 Ohm resistor. Current will flow 3.3mA
We now load this source with a practical 10 Ohm resistor. Current will flow 3.2mA
How much difference in 'damping current' will be there ?

Of course this changes when the impedance is lower. Say 30 Ohm
We now short this source with a practical 0.1 Ohm resistor. Current will flow 33mA
We now load this source with a practical 10 Ohm resistor. Current will flow 25mA
Do you see that as something that damps much more, this is a damping factor of 120 vs 3 yet the actual damping current varies just slightly.

There is the damping current story.. a nice invention from the sales department in the eighties.

What does matter here is that a varying impedance in the lows means that when an amplifier has a higher output resistance due to voltage division there will be a boost at the lower frequencies. Between 50 and 150Hz a boost there means 'muddier' sound, darker sound.
Damping current differences or voltage division effects ? What is heard/perceived ?

Consider a driver has back EMF and likes to resonate at 1 or more frequencies. It just acts as a generator (microphone) and thus generates those frequencies.
We apply a pulse that makes the headphone move and should stop but having resonance(s) doesn't want to.

Now imagine we are measuring a signal at the output of a black box (an ideal 0 Ohm amplifier + output resistance).
After the pulse the output voltage is back to zero but the driver (acting as a source) generates its specific frequencies.
We measure voltage at the black box. This is done versus the applied input signal of the black box.
Input signal is 0V. output R is close to 0 means the bulk of the generated EMF is dissipated in the driver and since driver and output R are in series, due to voltage division only a very, very tiny voltage is measured resulting in a low distortion number.
Increase the output R to 10 Ohm. Same source... due to voltage division 10 Ohm and 300 Ohm the current will show a much higher 'unwanted' voltage drop across the 10 Ohm resistance. This means the distortion number goes up as it compares unwanted signals to desired signals.

Is this increase in measured THD actually increased distortion or an artifact of voltage division and measurement method ?

I read a couple of times but don't understand your point here. Are you saying emf damping is valid or not?

Do you happen to have an stepper motor at hand? If yes, try to turn it with open wires and than with shorted wires. Its getting much more difficult to turn with shorted wires, cause the generated voltage is shorted and you get an current flow acting as brake.

This is the same effekt you will have at your speaker. Shorting terminals will counteract / damping motion of the coil in the magnetic field. This effect is less effective if you not short the terminal but include an resistor. And this resistor is corresponds to the output resistor of the amplifier. The higher the resistor - the less the damping effect.

And yes, increase of THD is actually increased distortion. Voltage division is linear and doesn't cause distortion. THD is caused by nonlinear transfer function.

 

[lucian]

A damping factor of around 1 will almost eliminate back emf completely under the audible range. If you have an output impedance of a few hundred owns and a transducer of 8ohm then yes it can be an issue. But practically 8ohm load with 10ohm source is almost worst as you can get. It does not introduce distortion over the distortion of the transducer itself.

Sorry, don't get it.

a) Where did you get the number of DF = 1 ( e.g. output imp. of 8 Ohm into 8 Ohm load) to be enough? Design rules state > 20, modern amps. designed > 1000. All for nothing ?

b) Why is than ~ 1 (10Ohm source vs. 8Ohm load) the worst ? - even worse as the output impedance of few hundred ohm vs. 8Ohms ????

c) What do you want to say with the last sentence?

 

[restorer-john]

Sorry, don't get it.

Fair enough. There's a lot of statements being peddled as absolutes in this thread...

Suffice it to say, low output impedances for driving low impedances are better in general. Control of the transducer, particularly at or near its resonant impedance peak requires low output impedance. Speakers are where it really matters, not headphones. They rarely have wild impedance peaks, the amplitudes are tiny and the back emf or currents generated are trivial compared to a loudspeaker.

Skyhigh damping factors were obtained in the 80s by putting the speaker cable into the NFB loop (Kenwood Sigma Drive). Various manufacturers have played with adjustable damping factor since the 1960s. Personally, I'm surprised more headphone "amplifier" manufacturers don't include controls for adjustment. The headphone guys would fall all over themselves to have some more chit to tweak.

Here's a 1974 amplifier with adjustable DF (3 position switch on the back panel):

And for any of you who think the switch positions would all sound the same, you're wrong.

 

[solderdude]

b)
Of course they sound different, if you drive speaker via voltage vs. driven via current. They optimized to sound good with voltage, cause that standard. However - the ultimate force that drives the coil is current. So if you supply current to the speaker you can directly control the force generating movement.
With voltage you effective current through the coil (and damping) depends on contact + wire resistance + change of resistance due to heatpup of coil + impedance ...
You don't have that with current. If you current driver delivers 10mA - the coil will see 10mA.

yes all very true except speakers and headphones are meant to be voltage driven, not current driven.
Of course with planars and quite a few dynamic headphones you can drive them with a current.
If that were so ideal wouldn't there be a bunch of headphone amplifiers around with current output and raving reviews of audio magazines ?

And as mentioned by restorer-john it is more of a problem for speakers where impedances are low compared to cables and contacts and even X-O inductors and masses of cones are high.

c)
4..20mA current loops are not inventend cause of live sign with 4mA. 4mA is actually 0 signal - and Error would be 3.6mA. But here are further variants to 0..20mA ; 4..24mA.
Also you can give a *live* signal with voltage output, e.g. analog sensor with supply of 5V typically have an output range of 0.5 ... 4.5V.

That's what I said.

The main reason for 4..20mA is that you signal gets independent of resistance, be it the length of the wires or the resistance of contacts. (If you want to do that with voltage you would need 4 wires, which would make it more expensive.)

the main reason for the 4..20mA is the '0' being an active signal instead of an absent signal (as in the 0.. 20mA standard).
The reason for using a current is to be independent of wire resistance (within limits) among other reasons.

The fact remains that speakers and headphones are designed to be driven from a voltage and the loads draws a current.
I remember the signal for recording in the old 5 pin DIN plug where (1 & 4) was a current signal (actually voltage driven with a very high source resistance) where the playback signal was voltage driven.

 

[Fluffy]

I've seen some headphone amps with variable output impedances, which would translate to adjustable damping factor. But it's fairly rare.

Anyway, I think I've got a sort of an answer to my initial question, but the rest of the thread got way too complex for me to draw a definitive conclusion on the subject of damping factors. I think I'll stick with the common wisdom of having low out impedance at the amplifier, since good ones already come with it.

 

[restorer-john]

I've seen some headphone amps with variable output impedances, which would translate to adjustable damping factor. But it's fairly rare.

I think we've seen a couple of LoZ/HiZ outputs on headphone amplifiers reviewed by Amir, but from what I read, it looked to be a simple series resistor.

The adjustable damping factor mentioned above was a little more sophisticated. It lifted the common (-/return) and inserted switched, very low value resistors (0.1R/0.033R) to 0V and at the same time changed the low end RC time constant in the NFB loop and the tap point with respect to 0V.