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An appropriate op amp for low impedance headphones (25-65 Ohm load)?

Doug Mytty
Prodigy 25 points
Other Parts Discussed in Thread: NE5532, OPA2134, LM4562, TPA6120A2, OPA2604, OPA1652, OPA1612, OPA1662-Q1, OPA1622, OPA1688, OPA627

I am putting together a solution for a pc based hi fi audio system utilizing low impedance headphones.  There are several motherboard designs that are emphasizing high impedance headphones (they seem to consider 'audiophile' as 600 Ohm), however, the Op Amps that are utilized seem inappropriate for lower impedance headphones.  The design guideline that I have been using for for impedance voltage matching is as follows: 

amp output impedance <= 1/8 of headphone impedance.

The low impedance headphones for this reference system have been chosen to allow playback with cellphones and other portable devices.  Headphones with the rated impedance include for example, the HiFiMan HE400i, Audeze Sine, and AKG 7xx (massdrop).  These headphones range from $200-500.  The iPhone 6 and other mobile devices drive these headphones well.


For the PC side of things, to minimize distortion (in frequency ranges <1 Khz and < 100 Hz), I am looking for an op amp with an appropriate output impedance (see 1/8 rule above).

The NE5532 has been described as a low impedance Op Amp.  However, when looking for a frequency dependent rating for output impedance, I do not see this graph in the data sheets.  Quoted impedance is 0.3 Ohm with 30 dB signal at 10 Khz into a 600 Ohm load.

A summary of op amps that I have looked at include:

#1 NE5532 ... 0.3 Ohm (see above) (Used by Asrock for their Purity Sound 2/3 motherboards)

#2 OPA2134 ... 10 Ohm (This is used by Gigabyte for their high end motherboards)

#3 LM4562 ... 13 Ohm (Alternative OpAmp used by Gigabyte and Asus)

#4 TPA6120A2 ... 13 Ohm (open loop specification) however a (reference?) layout diagram shows 39.2 Ohm resistors on the outputs (or is that the headphone load) (Used by Asus in their external SupremeFX Hifi DAC)

#5 OPA2604 ... 25 Ohm (open loop specification)

#6 OPA1652 ... graph shows 45 Ohm @ 10 Khz ... 120 Ohm @ 100 Hz ... 1000 Ohm @ 10 Hz

#7 OPA1612 ... graph shows 2 Ohm @ 10 Khz ... 200 Ohm @ 100 Hz ... 2000 Ohm @ 10 Hz

#8 OPA1662-Q1 ... graph shows 32 Ohm @ 10 Khz ... 320 Ohm @ 100 Hz ... 380 Ohm @ 10 Hz

For Op Amps #6-8, there seems to be very steep impedance jump below 100 Hz.  I am associating this with a steep roll off of bass as the voltage drops dramatically (ie lower 'damping factor').

Would appreciate feedback on the following:

A.  Commentary on the above op amps

B.  The relationship between frequency and output impedance (and why some opamps don't have this chart)

C.  General recommendations on an op amp that's well matched to 25-65 Ohm headphones.

Note:  I have been researching these and other 8PDIP package type OpAmps so as to allow swapping out OpAmps on the motherboards that support this (Gigabyte), but this is not a necessary package type.

  • 0
    TI__Guru 63185 points

    Hi Doug,

    First, I recommend you take a look at the OPA1622, OPA1612, and OPA1688 devices.  These three devices are commonly used in high-performance low impedance headphone applications.  The OPA1622 features many performance curves with 16 and 32 Ohm loads.

    For Question "B", older high-voltage popular audio products often were non-rail-to-rail BJT input/output stage devices which typically feature a resistive open-loop output impedance.  Therefore, it's common for older op amp datasheets to specify a single value for the open-loop output impedance because the value didn't change much over frequency (See the OPA627 datasheet for an example of this).  The shape shown in the OPA1652 datasheet is common many CMOS devices.  Many newer devices, specifically 3-stage BJT and CMOS rail-to-rail output stages often feature complex output impedances, similar to what you've found in the OPA1612 datasheet. The OPA1622 was specifically designed with an output stage structure that produces a low resistance open-loop output impedance that is very advantageous in high-fidelity, high-power headphone applications. 

    One extremely important thing to keep in mind is that most TI precision op amps specify the open-loop output impedance, not the closed-loop output impedance.  This is the case for the OPA1652, OPA1612, OPA1688, OPA1622 and other similar devices.  Some competitors (and occasionally some older TI datasheets) specify the closed-loop output impedance.  The closed-loop output impedance (Zout) is equal to the open-loop output impedance (Zo) divided by 1 plus the loop-gain (Aol*B).  Zout = Zo / (1+Aol*B).  Therefore the closed-loop output impedance presented to the load is dependent on the device open-loop output impedance, open-loop gain, and feedback factor (Beta (B)).  Therefore while the open-loop output impedance increases for the devices above at lower frequencies, keep in mind that the closed-loop output impedance presented to the load is the open-loop output impedance divided by the loop-gain.  The loop-gain is equal to the open-loop gain (Aol) for unity-gain applications and decreases proportional to the circuit noise-gain.  So, for a low-gain headphone driver application the closed-loop output impedance is divided by the very large >100dB open-loop gain of the op amp at the lower frequencies.  These effects are covered in great detail in the TI Precision Labs Videos on op amp bandwidth and stability.

    As an example I've copied the Aol and Zo curves for the OPA1652 below.  At 100Hz the Zo is equal to roughly 130Ohms and the Aol is equal to roughly 105dB or 177,828V/V.  In a unity-gain configuration, Beta, is equal to 1V/V.  Therefore, the closed-loop output impedance presented to the load at 100Hz in a unity-gain configuration is equal to:  Zout = Zo / (1+Aol*B) = 130Ohms / (1 + 177,828V/V * 1V/V) = 731uOhms (7.31E-4 Ohms).