Post Go back to editing

AD8592 for Headphone Amp

Hi All,

One of my customer is looking for low cost headphone amplifier.

Specification is >100mW into 32 ohm headphone.

Is AD8592 eligible device for his application?

Best Regards,

Ricky

  •      Hello Ricky,

         AD recommends the AD8592 for driving headphones,and it should work very well for most applications.  However, its output impedance won't allow a full 100mW RMS output per channel into 32 ohms.  With a 5 volt supply, the highest available output is 5Vp-p.  This is 2.5V peak, or 1.77 V RMS.  With P = V^2 / R, we get only 98 mW RMS into 32 ohms.  Close enough, yet the amplifier's output MOSFETs have enough resistance to significantly reduce the available output power.

         Data sheet Figure 6 shows the output voltage drop vs. load current for 5V supply operation.  I added the headphone's load line in red, calculated for two conditions:

    • With no drop (max 2.5V peak), the headphone draws 2.5 / 32 = 78 mA peak.  This defines the bottom point of the red curve.
    • With a one volt drop on both source and sink transistors, only 3V p-p (1.5V peak) is available for the headphone, at which point it draws 1.5 / 32 = 47 mA peak.  This marks the top end of the curve.

         The load line runs between these two points.  We can read the voltage drop at maximum output at the point where the red and black curves meet.  We find approx. 0.3 volts sink drop, and 0.5 volts source drop.  Subtracting this from the available 5V p-p, we have 4.2V p-p (1.48V RMS) remaining.  This provides an absolute maximum output power of 68mW RMS per channel into 32 ohms.

         Finally, the data sheet recommends 20 ohm series resistors for short-circuit protection.  If required for your application, these resistors will cut further into the available output power.

         BTW -- 100mW into headphones is likely quite deafening!

         Best regards,

         Bob

  • Sorry to stalk you Bob, but 100mW is actually a reasonable specification.  This is because people don't listen to sine waves (well maybe except in EDM), and the difference between average program peak and RMS is at least 10dB more than the same measurement for a sine wave.  Thus, a 100mW sine wave spec will yield around 10mW of clean "music" power.  You might further think that is also very loud, maybe 110dBSPL for a typical headphone, but again that's not the A-weighted SPL the listener will experience, because music will roughly follow a 1/f curve that will be around 100dBSPL-A.  That's still loud, but plenty of customers will desire that volume.

    I find that 100mW is a bare minimum for a satisfying experience with adequate headroom for program material.  This gets harder as headphone impedance increases as the voltage supply must increase and that might not be available in a typical circuit these days.  The typical "pod" device has Vpp of only 1.8V.  This is why earbuds are 16 ohm and placed directly in the ear canal.  Better quality over-the-ear headphones of 32 ohm and up need at least 5V, ideally even more.

  •      Hi Jon,

         Thank you for your clarifying comments.  If I understand correctly, a headphone amplifier based on the AD8592 would be good only for PC or consumer devices, not for any audiophile application (where at least 100 mW is really needed).  Meanwhile, I'm perfectly happy with my iPod driving 32 ohm on-ear phones at "my" kind of volume.  Being 60 years old likely has something to do with that.

         Best regards,

         Bob

  • Hi KJBob-san, jon71-san

    Thank you for both of your advice.

    I have talked to my customer based on your advice.

    Thank you again for your help,

    Ricky