Why would a product have a headphone amp AND an audio op amp for headphones?

[HzMeister]

I recently came across the Vivo X5 Max and in the spec sheet it has a Sabre ES9018K2M DAC, Sabre ES9601 headphone amp AND an OPA1612 amplifier.

Likewise, the Xiaomi Mi Note Pro uses an ESS Sabre ES9018K2M DAC, 2x OPA1612 AND 2x ADA4896 amps.

I thought headphone amps and audio op amps served the same purpose. Why would a product design require both? What's the difference between the two? Couldn't the ESS dac output directly to the OPA1612 op amp?

 

[dotneck335]

A headphone amp is designed to drive low impedances---generally 32 to 300 Ω, but sometimes as low as 8 Ω. A TPA6120, for instance will deliver 700ma into an 8 Ω load. An OPA1612 opamp will only deliver 30ma into a 2K Ω load.

 

[Monte McGuire]

A popular trick with amps that have to deliver power is to use a high quality op amp to provide feedback around a "headphone" amp that actually delivers the power, but probably has more distortion. In this way, you get the power of the larger amp with the accuracy of the op amp.

So, it can make sense to use both in one circuit.

Also, these words like "headphone amp" are really serving suggestions. It's like saying that pizza cannot be eaten for breakfast - you can do whatever the heck you want with anything, regardless of what the packaging says it's intended for. A circuit or an IC has no idea which market it was designed for, it simply does what it does, and a clever engineer can make use of it (or not).

 

[mlackey]

As stated earlier, it is common to use a single hi-quality opamp for voltage gain, which in turn feeds a "power" opamp used for current gain. The "power" opamp is usually wrapped within the first opamp's feedback loop, which can actually improve the performance of an already good power amp, making it's performance even better. Since the 1612 is a dual opamp, it is very possible that one is used for voltage gain (previously mentioned) while the second is used as a DC servo, keeping DC offset to the headphones at very low levels, potentially less than 1mV per channel. A D/C servo also eliminates the need for coupling caps in the audio path, another sonic improvement. For an example of this all this see the TI datasheet for the LME49600, Figure 28.

http://www.ti.com/lit/ds/symlink/lme49600.pdf

Substitute OPA1612 for LME49720 and ES9601 for LME49600. the TI design is a solid, reliable, stable, inexpensive design.

Edit: Walt Jung has been a proponent of "composite amps" for many years. A composite is a high-quality voltage gain stage (opamp) driving a current gain stage (opamp). Check here:
http://www.waltjung.org/PDFs/ADI_1992_Seminar_Audio.pdf

He has many other papers on composite headphone amps, but if interested you will have to dig just a little. They can be found here:
http://www.waltjung.org/PDFs/Site_Pages.pdf



Also as stated earlier, remember that all sales literature is designed to make the product sound fantastic and sway you into believing that you can't live another hour without it. Buyer beware.

 

[ammel68]

Since the 1612 is a dual opamp, it is very possible that one is used for voltage gain (previously mentioned) while the second is used as a DC servo, keeping DC offset to the headphones at very low levels, potentially less than 1mV per channel.

Edit: Walt Jung has been a proponent of "composite amps" for many years. A composite is a high-quality voltage gain stage (opamp) driving a current gain stage (opamp). Check here:
http://www.waltjung.org/PDFs/ADI_1992_Seminar_Audio.pdf

I don't think anyone would use the 1612 for a servo. JFET op-amps are used typically used for servos.
The 1612 would make for a decent sounding voltage gain stage and that's about it.

Your Jung "composite amps" link above doesn't seem to work.

 

[Salar]

Any measurements on the ES9601 headphone amp available?
Seems to be very often used in many (portable) devices
claiming to be High End - like Onkyo or Pioneer.
I have a HiFiMe SPDIF DAC, using the combo of ES9018k2m & ES9601 headphone amp for connecting to line level.

No datasheet or clear statement if the 9601 can drive hedphone and / or line level or has dedicated outputs for each purpose.