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Original question:

OPA1622: Any comments or concerns using OPA1622 to drive balanced headphones

OPA1622: Any alternatives, such as OPA1688, to drive larger headphones?

Moreno Hassem
Prodigy 90 points

Part Number: OPA1622
Other Parts Discussed in Thread: OPA1688, OPA1612

I'm currently using the OPA1622 as the output amplifier of a DAC product I'm developing. The sound is really good and the part is quite easy to work with. However, I'm feeding it with +5V and -5V and finding difficult to driver certain types of headphones, as the Hifiman HE-6:  

  • Sensitivity : 83.5dB/mW 
  • Impedance : 50 Ohms

And the AKG K-1000:

  • Sensitivity 74 dB/mW
  • Impedance 120 Ohm

However, it has no issues driving the Sennheiser HD800: 

  • Sensitivity 100.10 dB/mW SPL
  • Impedance: 430 Ohm

I understand this is mainly due to sensitivity of those devices. Do you think that by using a device that is rail-to-rail like the OPA1688 will make the situation any better? Are there any advantages of using this part over the OPA1622? 

I've seen from previous threads that the 1688 will not put as much power on phones as the 1622. Can you please clarify the capabilities considering these corner case devices?

Thanks!

  • 0
    TI__Guru* 93105 points
    Hello Hassem,

    Regarding your statement, "However, I'm feeding it with +5V and -5V and finding difficult to driver certain types of headphones, as the Hifiman HE-6." Can you provide a bit more detail what you mean by difficult to drive?

    The OPA1688 operates with lower supply current than the OPA1622, and does not have as high of output current capability as the OPA1622 either. In actuality, if you compare the OPA1622 datasheet Figures 38 and 39 Output Voltage vs Output Current graph,to the OPA1688 Figure 9. Output Voltage Swing vs Output Current graph, it is evident that the OPA1622 output swings much closer to the supply levels, than the OPA1688 for a given high current output level. Therefore, when driving the lower impedance headphones that require more current for a specified output power level the OPA1622 is more capable of providing the higher current.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0
    Guru 140200 points
    Hi Moreno,

    the AKG K-1000 is known to need a very high signal amplitude. You can see this from the very low sensitivity. You would need a supply voltage which is much higher than +/-5V.

    Kai
  • 0 in reply to Thomas Kuehl
    Prodigy 90 points

    Hello Thomas, 

    thank you for your reply. By difficult to drive I'm stating that the headphones are not perceived to be loud enough by a person utilizing the system. I was demoing the product to an owner of the headphones, so I did not have a chance of measuring the output voltage swing. 

    From the sensitivity figures, my rationale is that the two examples I posted need a higher output power from my system, in order to be loud enough so users can enjoy their music at a reasonable value. I used some industry standard DAC to compare volumes. I'm not talking deafening levels of dBSPL on the output. 

    I'm sharing schematics for my output circuit. I'm mainly using the OPA1612's as output filters for the DAC (not sure about the necessity of doing that, hence the optional 0R resistors so I can bypass that section entirely using a different assembly). 

    I'm using a voltage gain of ~2 at the exit stage on the OPA1622, which should place the output at near the limit of what my supply can do. Am I oversimplifying this? Any pointers on things I could do to improve output power using the OPA1622?maha_dacone_audio.pdf

  • 0 in reply to kai klaas69
    Prodigy 90 points
    Hello Kai,

    this is within the realm of possibility. Question that stands is: if I increase voltage gain and supply voltage, is the OPA1612 capable of sourcing current enough to drive these cans? If so, I can perform some changes to the design and incorporate different regulators.

    Thanks!
  • 0 in reply to Moreno Hassem
    Prodigy 90 points
    Answering my own comment here: by looking at the documentation, if I increase the voltage gain and supply, it should increase the output power to levels capable of providing somewhere near 100dBSPL to the K-1000.

    However, there is the noise tradeoff. The OPA1622 datasheet states I can include a switch to select the gain, which is something I might do. Do you have any preferred way of including such switch in a "solid state" manner that won't introduce too much noise on the audio line? Any specific devices I can use or implementing this with transistors will do?

    Thanks.
  • 0 in reply to Moreno Hassem
    Guru 140200 points

    Hi Moreno,

    you could provide two different outputs, individually driven by well matched amplifiers.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 93105 points
    Hi Moreno,

    I agree with Kai's assessment regarding increasing the supply voltage levels. I am pretty certain in the low sensitivity headphone cases, it is not an issue of the OPA1622 current output capability but rather the combined output voltage swing and current (power) needed to overcome the low sound pressure level (SPL) of the headphones.

    Also, I advise against trying to incorporate any kind of active switch in the OPA1622 circuit; doing so will severely degrade the very high THD+noise performance of the op amp. Although it may be a little more difficult to implement because of the OPA1622 difference amplifier connection I suggest a two gain approach for two different SPL ranges. The R17/R24 or R18/R27 pairs would be mechanically switched to a different value to affect gain. A mechanical switch shouldn't degrade the THD+ noise

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 90 points
    I will study mechanisms to implement such switching or provide two different outputs. Removing the OPA1612 circuitry will allow me a affording a new output!