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Old 9th February 2013, 04:04 AM   #1
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Location: Hamilton, was Ottawa (Canada)
Default The No Headphone Amp!

That's right the No Headphone Amp!

It's very inexpensive because, well there's no headphone amp and it's very transparent because ... you guessed it, there's no headphone amp!

Looking at Blues' Cool Head 'amp' (The WarpSpeediER) pdf I noticed this:

WarpspeediER - The Lightning Bug Headphone attenuator.pdf

The output impedance of our Source/DAC will determine how much max current it is capable of delivering. For a given max output voltage (>/= 2 Vrms), the lower the Source/DAC Zout (</= 200 ohms) the more current is available, and is preferred for all phones -from 16 ohms through 600 ohms. A Source or DAC that touts very low Zout (<50 ohms) but smokes its output stage when accidentally shorting the output connectors is not a good design.

He makes a good point that we have all the voltage swing we need and really it's about can the output stage supply the current?

Well, how much power do headphones need?

To rock out on a 32-ohm/98dB/1mW HP we essentially will need lower than 600 ohms for Source Zout. For higher impedance HPs like 300-ohm Sennheisers (102dB/1V = 107dB/1mW!) or 600-ohm BeyerDynamics, higher Source Zouts(>/= 300 ohms) will still generate thunder in our heads. Remember that an 88dB SPL directly into our ears (a mere 0.1mW on 98dB/1mW HPs -that's 1.77mA on a 32-ohm HP or 245mV on 600-ohm HP!) is already very loud, albeit good quality loud through the Warpspeed. I hope not one of us aspires for 100dB SPLs directly into our ear canals –this number is acceptable with loudspeakers in a typical listening room volume with furniture, other sound absorbing materials, and we are 8 feet away. 90dB to 93dB peaks however are plenty loud for headphones.

Hmmmm ... I have some sort of optical volume around here ...



SKA's optivol is very basic: 28k in series and then LDR shunt. The output is off the LDR shunt. (so just a single matched pair of LDRs for stereo)
A single log pot drives a current source I think through both LDRs. It was my entry into LDRs for a nice $30!

I hacked it to make the No Headphone Amp: LDR in series then headphone element as shunt (no shunt resistor)! About as simple as you can get: a variable resistor then the headphones!

No Headamp, LDR series phones shunt.jpg
No Headamp underside.jpg
No Headamp in out connections.jpg

I have Sony MDR-EX71SL IEMs:

Driver Unit : 9mm
Frequency Response : 6Hz - 23,000Hz
Impedance : 16 ohms
Sensitivity (db) : 100dB/mW? (higher!)
Power Handling Capacity : 100mW

and AKG311 'ear buds':

Frequency Response 18Hz - 20.5kHz
Maximum Input Power 15 mW
Input Impedance 32 ohms
Sensitivity: 125 dB SPL

and old over-the-ear AKG K301s:

Sensitivity 94 dB SPL/mW
Frequency range 20 to 25,000 Hz
Rated impedance 120 ohms
THD <1%
Max. input power 200 mW

I measured the impedance with the 3 different headphones hooked up and regular listening was between 700 ohms and 1.5k.

I have the No Headphone Amp hooked up to a USB dac with 2V out and reasonable drive and ... it sounds good! On the AKG K301s (120 ohm) it sounds perfect but with the IEMs and loud volume I think I'm detecting some distortion (700 ohm)

And really, you don't have to use an LDR. You could use a pot, attenuator or even a few resistors (next test!)

No Headphone Amp really is best!


Last edited by AudioLapDance; 9th February 2013 at 04:38 AM.
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Old 9th February 2013, 04:36 AM   #2
Blues is offline Blues  United States
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Thanks for referencing my work, Jeff...very much appreciated. Welcome to direct-drive headphones listening, it's quite an ear-opening experience.

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Old 9th February 2013, 05:37 PM   #3
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Location: Hamilton, was Ottawa (Canada)
Ok, now I've gone oooooooooooooollllld school!

No Headphone Amp with a single resistor!

1 resistor!

USB Dac --> 909 ohm resistor --> headphones

usb DAC and single resistor No Headphone Amp.jpg

1 resistor! (and spaghetti connectorization)

No Headphone Amp 909 ohm.jpg

Ya, alligator clip old school: Samuel L. Jackson: "Oh, now we gonna alligata' yo ***!"

I'm using the big AKG K301s (120 ohm) so my DAC is driving 909 + ~120 = 1029
So about 1k ... so far so good!

Later I'll try the 32 and ... 16 ohm earbuds and IEMs = 925 ohm

The question is ... can your output stage drive your headphones like this? Other headphones might be more efficient or higher res (600 ohm) and your DAC / phono pre would only have to drive 2 or 3k!

Blues puts-it-in-a-nutshell at the end of the doc:

In summary, with the correct components combination of 2Vrms Sources having 200-ohm Zout or less; 16 thru 600-ohm headphones with 93dB/1mW or more are all we need to complement the WarpspeediER –which about covers most dynamic HPs, IEMs, and even some planar mags. Again high voltage electrostatics and low sensitivity planars are not recommended.

Last edited by AudioLapDance; 9th February 2013 at 05:47 PM.
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Old 9th February 2013, 06:00 PM   #4
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Interesting. The .pdf author credits nwavguy but ignores his recommendations.

NwAvGuy: Headphone & Amp Impedance
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Old 9th February 2013, 06:36 PM   #5
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I agree, if you want proper damping you should use a <4 ohm resistor (for 32 ohm headphones) in parallel with the load. Most sources won't like such a load..
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Old 9th February 2013, 06:59 PM   #6
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Now my setup is a little different than the WarpSpeedIER because it has a shunt resistor in parallel with the phones.

But lets look at my case, you have a pretty pure 909 ohm resistance into something like:


(from NwAvGuy, I'll reference later)
This is the HD280 but its typical of a 120 ohm phone: resonance around 100Hz, dip to 60 ohm and slight rise back towards 100 ohm.

I should be hearing a big peak at 80 Hz but I don't ... Hmmm....
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Old 9th February 2013, 07:19 PM   #7
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Nvm. Tired

Last edited by Neutrality; 9th February 2013 at 07:26 PM.
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Old 9th February 2013, 07:23 PM   #8
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Should be almost +8 dB at the resonant frequency. Less control over the driver also increases distortion.

Btw, the HD280 is a 64 ohm (nominal) headphone.
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Old 9th February 2013, 07:36 PM   #9
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Excellent! Exactly the sort of things running through my head!

This will provide concrete anchors:

WHY DOES OUTPUT IMPEDANCE MATTER? It matters for at least three reasons:

1) The greater the output impedance the greater the voltage drop with lower impedance loads. This drop can be large to enough to prevent driving low impedance headphones to sufficiently loud levels. A real world example is the Behringer UCA202 with a 50 ohm output impedance. It struggles with some 16 - 32 ohm headphones.

2) Headphone impedance changes with frequency. If the output impedance is much above zero this means the voltage delivered to the headphones will also change with frequency. The greater the output impedance, the greater the frequency response deviations. Different headphones will interact in different, and typically unpredictable, ways with the source. Sometimes these variations can be large and plainly audible.

3) As output impedance increases electrical damping is reduced. The bass performance of the headphones, as designed by the manufacture, may be audibly compromised if there’s insufficient damping. The bass might become more “boomy” and less controlled. The transient response becomes worse and the deep bass performance is compromised (the headphones will roll off sooner at low frequencies). A few, such as those who like a very warm “tube like” sound, might enjoy this sort of under damped bass. But it’s almost always less accurate compared to using a low impedance source.

From a great article:
NwAvGuy: Headphone & Amp Impedance
And related:
NwAvGuy: Headphone Impedance Explained

(thanks xnor, the K301 is ooooold ... for now I'll just double everything to explore the concept: peak 300 ohm drop to 120 ohm and rise to 200 ohm)

Just quick because I have to go play in the snow with my son,

1) Not a problem as stated because what I'm driving is:
909 + lowest dip (120 ohm) = ~1k
And the volume is fine--this is an 'attenuator'!

2) 120/1k = 12%, 300/1k = 30% a diff of 18% ... not too big and I don't think I'm hearing the resonance or the high freq lift

3) Damping ... hmmm this I'm not sure about and would love to hear some thoughts. My first one is that this sort of thing may matter more for loudspeaker divers than headphones. Whatever it is, it can't be too bad because I just don't have a 100 ohm output impedance but almost 1000 ohm!

I'd like to explore this apparently non-ideal setup and it's real effects and then move on to what happens when adding a lower value shunt resistor across the headphones.


PS Counter Culture, I was getting to that ... and I thought the light tone of the thread was apparent?

Last edited by AudioLapDance; 9th February 2013 at 08:04 PM.
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Old 9th February 2013, 07:39 PM   #10
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Damping still matters.
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