Question about headphone amplification in speakers amp

[Roy_L]

I've had several headphone amps over the years (Fiio, JDS labs, Schiit, Drop THX 789 etc.). I also own an old Audiolab 8000A integrated amp (the early 90's UK version with the bass & treble knobs), which is mostly just stored away.
The old Audiolab has a feature that I think is being faded out in recent years from speaker amps- a headphone jack. Every now and then, I take out the old Audiolab to be used just as a headphone amp. And I gotta say, it sounds every bit as good as any dedicated headphone amp I own with one major advantage- it has orders of magnitude more power.

With my Drop HD6XX, with certain recordings, I have to use the THX 789 at maximum volume to get an enjoyable listening experience, with some headroom still lacking (this is fixed if using XLR signal path which manages to give the few more extra DB needed). However, with the integrated amp, there seems to be an endless abundance of power, far more than the headphones (or my ears) can handle. Actually, the only reason I don't just use the Audiolab is that it takes way too much space on my table.

However, I just wondered- Is there a fundamental difference between a speakers amp with a headphone jack and a dedicated headphone amp? Are the dedicated headphone amps just a form factor sacrificing power for taking less space? Or are they designed in a way specifically benefiting headphones in some way?

 

[RickSanchez]

Typically — I’m not saying it’s the case here with your amp — vintage stereo amps will have much higher output impedance vs. modern headphone amps. Do you happen to know what the output impedance is for your Audiolab?

It might be moot for you as I believe your headphones are 300Ω, so if it’s not a big gap in output impedance between your Audiolab vs. your THX 789 you likely wouldn’t notice a difference.

 

[Soandso]

Roy_L, - Old amplifiers usually had up to 120 Ohm output impedance headphone jacks out. Which corresponded to the old industry standard of marketing 120 Ohm and up rated headphones.

Contemporary headphone amplifiers have really low headphone jack output impedance. Which corresponds to how the industry standard of dynamic driver headphones' rating has shifted to (say) 32 Ohm and less.

[For example my iPhone 6 headphone jack has 4.5 Ohm output impedance; while my more modern iPad Air has only 1.6 Ohm output impedance. Which are different from even more specialized headphone amps - like (for example) the Chord Mojo's 0.075 output impedance.]

All of which means that, generally:
* if you are using high impedance rated headphones you will enjoy them "better" (ex: low distortion) when driven out of a regular old amplifier's headphone jack ...
* vs. if you are using low impedance headphones you will enjoy them "better" because will get better low frequency transients (ex: tighter bass) when driving them out of a headphone amplifier .... (Conversely, using low impedance headphones with a 120 Ohm output impedance jack the bass tends to "boom" while mid-frequencies "bloom" more predominantly.)

 

[solderdude]

With my Drop HD6XX, with certain recordings, I have to use the THX 789 at maximum volume to get an enjoyable listening experience, with some headroom still lacking (this is fixed if using XLR signal path which manages to give the few more extra DB needed).

This may not caused by a lack of power but rather a lack of gain.
The used DAC is either not having enough output voltage, the amp is not set to high gain or the digital volume is lowered too much somewhere in the chain.

In 300 ohm the 789 can supply 6.7V which is enough to play impressively loud (120dB peaks) so possible with SE out (TRS plug)
Balanced out (XLR 4-pin) it can supply 13V (0.6W) which is more than what would ever be needed to even exceed impressively loud levels (125dB peaks !)

 

[Soandso]

Roy_L, - I propose listening to your 300 Ohm headphones is preferable through your old integrated amplifier.

Consider that dedicated headphone amps are designed for low impedance headphones. And thus headphone amps have some design emphasis on supplying current.

High impedance headphones are relatively less demanding of current. And your integrated amplifier's headphone jack out is usually designed with a resistor(s) between it's circuit and it's speaker circuitry giving attenuation - it will give voltage to the headphones demand and commonly in designs current roughly in an inverse proportion.

Your old integrated amp's high output impedance headphone jack interacts with your 300 Ohm headphone's impedance. So that, although you may increase the voltage (turn it up) the dynamics of the dual impedances' combination alters (limits/reduces) the degree to which voltage may off-set the drivers of the headphones.

Which is to say (as per above) the driver oscillations occur in more controlled manner. So that high impedance headphones, with their relatively lighter voice coil mass (thinner wire in windings) benefit from less distortion and thus sound better with the added effect of more/better bass.

Running high impedance headphones from dedicated headphone amplifiers with relatively low output impedance the tendency is to do so at higher gain in order to listen louder. And, as I understand it, in most designs switching to the high gain setting on dedicated headphone amplifiers usually interposes a resistor.

But for high impedance headphones dedicated headphone amplifier (gain) resistor does not create the high enough level of output impedance which, unlike you old integrated amplifier headphone jack does, which smooths oscillations affecting excursion related to voltage "off-set" at a driver (Note: probably the better/best headphone amps' design limit the variability of off-set, which is a way of describing potential movement when peculiarty of electric charges push driver over a bit. Balanced headphone amps' output incidentally will not provoke such off-set.) I suggest that generally summarized such high gained music in a dedicated headphone amp has enough volume "punch", yet with less desirable tone for your 300 Ohm headphones than when plugged into your old integrated amplifier.

 

[solderdude]

Roy_L, - I propose listening to your 300 Ohm headphones is preferable through your old integrated amplifier.

Consider that dedicated headphone amps are designed for low impedance headphones. And thus headphone amps have some design emphasis on supplying current.

Depends on the amp in question. Some are perfectly capable or even better equiped to drive high impedance loads.

Which is to say (as per above) the driver oscillations occur in more controlled manner.

driver ringing is damped acoustically, not electrically.

So that high impedance headphones, with their relatively lighter voice coil mass (thinner wire in windings) benefit from less distortion and thus sound better with the added effect of more/better bass.

Most (even high impedance) headphones perform optimally with low impedance sources (i.e. not jack outputs on receivers)

Running high impedance headphones from dedicated headphone amplifiers with relatively low output impedance the tendency is to do so at higher gain in order to listen louder. And, as I understand it, in most designs switching to the high gain setting on dedicated headphone amplifiers usually interposes a resistor.

It alters the gain of the amplifier. It never introduces a series resistor in the headphone out. Very few amplifiers offer a variable output resistance.
Some (older) headphones gain a little bass/lower mids when driven from higher output resistance amplifiers and may actually sound better. In some cases the bass becomes bloated (voltage division) and other headphones may not change tonal balance at all.

But for high impedance headphones dedicated headphone amplifier (gain) resistor does not create the high enough level of output impedance which, unlike you old integrated amplifier headphone jack does, which smooths oscillations affecting excursion related to voltage "off-set" at a driver (Note: probably the better/best headphone amps' design limit the variability of off-set, which is a way of describing potential movement when peculiarty of electric charges push driver over a bit. Balanced headphone amps' output incidentally will not provoke such off-set.) I suggest that generally summarized such high gained music in a dedicated headphone amp has enough volume "punch", yet with less desirable tone for your 300 Ohm headphones than when plugged into your old integrated amplifier.

I cannot make heads nor tails of it...

 

[Soandso]

Solderdude, - Then I stand corrected ... as per ASR motto "addicted to learning and fun."

 

[Soandso]

So, I used my old 600 Ohm moded headphones, an old iPhone low output impedance headphone jack, an old integrated receiver headphone jack and low level dB[C] volume matched Rod Stewart rendition of "They Can't Take That Away From Me" to satisfy my curiosity. That version has a steady bass beat and about 1:40 into the song a melodic saxophone comes in.

Anyway, after many personal sighted comparisons cuing not only from the beginning, but also starting from the introduction of the saxophone, listening using both low and high impedance outputs, my conclusion is that I liked the sound of whichever set-up was playing. However, for what would seem to be features of reduced distortion, I repeatedly interpreted that the song's bass notes "sat down" more decisively and the tones were "cleaner" from the saxophone using the integrated amplifier's headphone jack (high output impedance).

Of course I can not rule out confirmation bias on my part. And I have not used an actual dedicated headphone amplifier for comparing low to high output impedance, nor any balanced amplifiers.

 

[solderdude]

iphone can only provide 1V which is too little for 600 ohm so clipping/compression will occur and bass will not be loud enough.
Integrated amps, can provide voltages well over 10V in 600 ohm.

 

[Soandso]

solderdude, - Tonight I set up a Fiio A5 amplifier fed by the old iPhone 5 (hey, was most practical music source) for the low output impedance signal path example. A5 is capable of up to +/- 11V output and Amir reviewed this as having negligible distortion.

[I turned the FiioA5 low gain setting to maximum and volume matched in comparison with the old integrated amplifier (Kenwood KA-128) fed by the iPhone5 playing the same RodStewart song as before, but up a bit to the 47-50 dB[C rated] level. That was loud enough in terms of dB comfort in this quiet setting for me; but I wondered if the voltage output was significant enough and only did a few comparisons.]

I chose to use the A5 high gain setting and engage that signal path to volume match the old integrated amplifier signal path wherein both in turns played the same song from the iPhone 5 at the 49-54 dB[C rated] level. There was approximately <25% additional range left in the A5 gain dial, but using more of the potential gain was too loud in this situation.

Anyway, my repeated (sighted, biased?) comparisons of the song's 1st minute bass line, the piano bars 1 minute in and the saxophone melody about 1:40 into the song were basically the same as reported earlier. Namely, the old integrated amplifier high output impedance headphone jack gave comparatively "cleaner" tone and dynamics for my old 600 Ohm moded headphones - even though the trialed 49-54 dB[C] level was higher than I would have chosen to listen at when indoors relaxing at night. (I acknowledge that synergy of headphone pairings with amplifiers can be case specific.)

[Just chatting: Those same 600 Ohm personally moded headphones are my favorites for out walking around my neighborhood connected to the Fiio A5 and fed streaming music from an old iPhone 6 headphone jack. There I turn the A5 high gain up past 75% and they're not too loud, which is probably because of the background street noise.]

 

[solderdude]

What headphone is used ?
The amp has a 330ohm output resistance which is quite high.
It is to be expected to have a tonal balance change in bass and treble.
The fact that you prefer a change in tonal balance opposite 'flat' response or that the headphone in question actually improves being driven from a higher output R is possible.
The 330 ohm is not a standard (120 ohm used to be) so no headphone is ever designed/optimized to work 'correctly' on that specific R out.
R outs of headphone outs on integrated/power amps can vary between 40 and 700ohm or so.
To hear what the output resistance does you can also connect your 600ohm headphone directly to the speaker taps (requires making an adapter).

 

[Soandso]

solderdude, - I have been testing in the above reports using a vintage AKG240 "sextet". I rewired it balanced and used to run it from balanced amplifiers - but over the years all my balanced amps have lost a channel.

Actually I did run them after that from speaker taps of different integrated amplifiers with assorted resistor arrays. But over the years integrated amp have gone bad and currently down to just my old Kenwood amp being fully functional. Which prompted moving some audio equipment around and I decided to terminate all my balanced headphones' cables in TRS plugs.

I thank you for the specific Kenwood KA-128 headphone jack 330 Ohm information. I'd just assumed it was 120 Ohm.

Since (only) the headphone jack of my old Yamaha R-5 amplifier still works I'd be curious if you would be able to tell me it's Ohms. Which I thank you here in advance for considering.

Maybe my comparative report of preferences above were also colored by my headphone modifications. I use loose glass wool (~ 0.008 mm diameter spun borosilicate having no binders/oil/dye) inside the ear ups, 1 disc of thin craft felt and several small individual pieces of sorbothane stuck against the inside of the ear cup in a symmetric spaced array. I have fabricated fine leather ear pads for them for a nice seal in spite of wearing eyeglasses.