• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!

How headphone amplifier affects the sound?

PingWine

Member
Joined
Jun 6, 2020
Messages
86
Likes
34
Location
Finland
With my limited understanding I have always assumed that if headphone amplifier has output impedance at least 10 times lower than the headphones load, the frequency response will be unaffected. (Provided that the amp itself has a flat frequency response)

So what has made me question my sanity are the comments amirm makes in his headphone amp reviews. For example in the review of the Apple's USB-C dongle he said:
"Switching to high-impedance 300 ohm Sennheiser HD-650 nearly killed performance. The sound was OK but bass is week and dynamic range just not there."

I understand that the little dongle probably didn't have the voltage to drive the headphones to high desibels but I can't wrap my head around that comment about bass. How can a 0.9Ohm output impedance amp affect the sound of 300Ohm headphone?

I suppose that this has something to do how different frequencies have different impedances, but this is just a guess. Can anyone enlighten me on what is going on?



Oh and another question (that is kinda the same as the first one but worded a little different):
What is this "scaling" people keep fussing about. In many reviews I see people praising the HD650 and how well it scales with high-end amps. Like how can a 300Ohm headphone that isn't even that insensitive sound different with Topping L30 vs SPL Phonitor X

Edit: Shit I posted to wrong channel
 

dfuller

Major Contributor
Joined
Apr 26, 2020
Messages
3,335
Likes
5,050
I understand that the little dongle probably didn't have the voltage to drive the headphones to high desibels but I can't wrap my head around that comment about bass. How can a 0.9Ohm output impedance amp affect the sound of 300Ohm headphone?
In this case it's mostly about a lack of power. HD650s are high impedance cans nominally, but right around the driver's resonant frequency (110hz ballpark, IIRC) they need substantially more voltage drive. So something like the apple dongle just didn't have enough oomph to drive them loudly.

What is this "scaling" people keep fussing about. In many reviews I see people praising the HD650 and how well it scales with high-end amps. Like how can a 300Ohm headphone that isn't even that insensitive sound different with Topping L30 vs SPL Phonitor X
Part of this is subjectivist nonsense, but part of it is that the 650s don't suddenly become a super weak link with a higher end cleaner headphone amp - and to a degree, they can sorta "eat" power. You can drive them to hearing damage levels without seriously audible distortion.
 
Last edited:

maverickronin

Major Contributor
Forum Donor
Joined
Jul 19, 2018
Messages
2,527
Likes
3,308
Location
Midwest, USA
I understand that the little dongle probably didn't have the voltage to drive the headphones to high desibels but I can't wrap my head around that comment about bass. How can a 0.9Ohm output impedance amp affect the sound of 300Ohm headphone?

That's probably it just not getting anywhere near loud enough for Amir's tastes, due to the equal loudness curves de-emphasizing the bass at low volumes.

What is this "scaling" people keep fussing about. In many reviews I see people praising the HD650 and how well it scales with high-end amps. Like how can a 300Ohm headphone that isn't even that insensitive sound different with Topping L30 vs SPL Phonitor X

This is people imagining that some amps sound better in some mysterious unmeasurable ways combined with them imagining that specific headphones also interact with amps in their own unmueasreable ways.

Here's both sides of this particular audiophilia nervosa coin.

Some headphones are just always poor or average regardless of an amplifier's mystical qualities. These headphones don't scale.

Some really good headphones need an amp which is "good" in one of those mysterious unmeasureable ways to be worth listening to at all and that the headphone will sound terrible on "lesser" amps. These headphones don't scale either.

The HD800 has become the classic example of this as audiofools try everything except EQ to fix its 6KHz peak and eventually just get used to it.

Some other headphones are said to sound ok on "lesser" amps but dramatically improve when used with "better" amp. This is a headphone that scales.

For some reason the HD650 has garnered this reputation.
 

outerspace

Active Member
Joined
Aug 18, 2020
Messages
111
Likes
76
About amp output impedance you can read here. Also interesting experiment with amps (including iPhone 6s, Topping DX7 pro, SMSL SU-8, SMSL SP200) you can view and listen here and discussion about results here.
 
Last edited:

JIW

Senior Member
Joined
Nov 11, 2019
Messages
378
Likes
551
Location
Germany
HD650s are high impedance cans nominally, but right around the driver's resonant frequency (110hz ballpark, IIRC) they need substantially more voltage drive.

This incorrect. How much voltage is required for a given SPL at a given frequency is the frequency response. At resonance, the SPL of the HD650 is about the same as at 1 kHz for the same voltage.

Rather, it is current flowing through the coil producing a magnetic field, which by interacting with the static magnetic field from the driver's magnet assembly makes the driver's diaphragm move. According to Ohm's law, the voltage merely tells you how much current will flow through the coil depending on the impedance: voltage equals the product of current and impedance, i.e. current equals the ratio of voltage to impedance.

At resonance, the driver requires less stimulation, i.e. less current, to produce the same SPL. This is reflected in the impedance being about 500 ohm at resonance compared to 300 ohm at 1 kHz. Thus, the HD650 requires less current at resonance compared to 1 kHz for the same voltage and thus requires less power for the same SPL. In other words, the HD650 is more efficient at resonance compared to 1 kHz. How much depends on the exact impedance. According to Amir's measurements, efficiency is about 50% higher at resonance compared to 1 kHz.
 

RHO

Major Contributor
Joined
Nov 20, 2020
Messages
1,182
Likes
1,087
Location
Belgium
About amp output impedance you can read here. .
Regarding the "exceptions to the rule" section: Could it be that mostly older headphone designs intended for studio use fall into this category?
Could this also be that this is the reason we see a bass roll-off with "many" higher impedance headphone models?
 

Nango

Major Contributor
Joined
Aug 6, 2018
Messages
1,463
Likes
985
Location
D:\EU\GER\Rheinhessen
With my limited understanding I have always assumed that if headphone amplifier has output impedance at least 10 times lower than the headphones load, [...] ..... [...]

IF your central assumption is headphones have all the way the same load/impedance, that is indeed the first mistake. The load in dynamic headphones varies according with the frequency range and usually doubles around 100Hz the figure printed in the box before it returns to that level.
 

solderdude

Grand Contributor
Joined
Jul 21, 2018
Messages
15,891
Likes
35,912
Location
The Neitherlands
So what has made me question my sanity are the comments amirm makes in his headphone amp reviews. For example in the review of the Apple's USB-C dongle he said:
"Switching to high-impedance 300 ohm Sennheiser HD-650 nearly killed performance. The sound was OK but bass is week and dynamic range just not there."

I understand that the little dongle probably didn't have the voltage to drive the headphones to high desibels but I can't wrap my head around that comment about bass. How can a 0.9Ohm output impedance amp affect the sound of 300Ohm headphone?

I suppose that this has something to do how different frequencies have different impedances, but this is just a guess. Can anyone enlighten me on what is going on?

It simply can't get loud enough for bass to become impressive. Has to do with how the hearing works. Look for equal loudness contours.
Given that most music is recorded at 85dB average (so with peaks reaching 100dB) and you would really like to hear undistorted and good bass you need a bit more using EQ.
Such dongles and phones simply cannot reach those levels undistorted. When you play softer (you'd have to) you just do not reach satisfying bass levels.

It has nothing to do with output resistance or amps being rolled off. It's a level thing, simply as that.
 
Top Bottom