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Original question:

TPA6120A2: TPA6120A2 Power

TPA6120A2: High Impedance Bose A20 headset driver

Barry Klein1
Intellectual 580 points
Part Number: TPA6120A2

I am looking at this part for a pair of audio drivers for a military Bose A20 headset, which has a mono impedance of 160 ohms and stereo impedance of 320 ohms.

First off, I'm new at this and a simple general question: would the quoted mono impedance of 160 ohms be for one earpiece or the result of both in parallel?  Similarly when they quote stereo impedance of 320 ohms what are they saying?  I know this a super common thing but I don't see it explained!  

Typically we would be using the headset in stereo L and R mode as we have individual L and R drivers. But even our spec gets a bit nuts as it is apparently spec'ing output levels given two headsets are connected in parallel to the drivers.

I found a headphone calculator at http://www.digizoid.com/power.php

I'm confused by it too...  You enter a headphone impedance and a SPL - SPL in either per V or per mW.   The A20 headset spec says 92dB SPL measured at 1mW, 1kHz, full volume on a KEMAR ear simulator.

So do I enter a number in SPL per V or mW?  Regardless the results are not that different, for SPL Fairly Loud to Very Loud, 110dB-115dB 4.5-8Vrms for 320 ohms and 3-5.6Vrms for 150 ohms.  The guy in the other thread here for this part was having issues.  What do you think, do you have any suggestions starting out that I should adhere to?  Seems I should be able to do ok with +/- 15V supplies maybe?

  • 0
    TI__Genius 14501 points

    Hello Barry,

    I will try and respond to your questions in sections below:

    1) The quoted monaural & stereo impedances are likely referring to the newest revision of the Bose A20 having a mono/stereo selector switch.  I did find the following article from Bose: https://www.bose.com/en_us/support/articles/HC2672/productCodes/a20/article.html.html.html.html   This article suggests monaural mode routes a mono (single-channel) input signal to both ears.  Based on this, it would be fair to continue your design with the 160 Ohms representing the headphone impedance in monaural mode for BOTH ears, whereas the 320 Ohms headphone impedance applies during stereo mode only.  In either mode of operation, you are operating (i.e. sending an electrical signal to) both sides of the headphones.  

    2) I would advise you to match your selection of SPL in per V or per mW to the specification present in the devices or systems for which you are going to interface.  Since the Bose A20's specify SPL in dB per/mW, I would use the values from the calculator set to dB per mW.  

    3) As you are designing for a high impedance headphone, the necessary output voltage must be high; a higher voltage is needed to provide an equivalent output power to the Bose A20 that would be seen in the TPA6120A2 datasheet for a lower impedance headphones at 16, 32, or 64 Ohms.  You are correct that you cannot successfully operate the TPA6120A2 with these design requirements on a +/- 5V supply range.  I would definitely recommend using the larger +/- 15V supplies for this Bose A20 application.  If you can achieve an output voltage/power which satisfies your design requirements, you will benefit with better distortion products due to the high impedance load.

    Please take a look at the total harmonic distortion (THD) + Noise curves in the TPA6120A2 datasheet, plotted against Output Voltage and Output Power at various load impedances.  Your available output voltage swing is specified for a 25 Ohm load in the datasheet at 12.5 to -12.2 V.

    Best,

    Alec