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OPA310: Is it suitable as a headphone amp?

Jerry Catanescu
Prodigy 20 points

Part Number: OPA310
Other Parts Discussed in Thread: OPA2990, OPA2196, , OPA1678, OPA1671, OPA322, OPA396, TINA-TI, OPA4990, OPA2310

I am considering OPAx310 for a battery-powered headphone amplifier due to its high output current and low quiescent, but I'm concerned about excessive crossover distortion when driving heavy loads (as low as 16 Ohms). The datasheet does not show THD plots for loads below 2kOhm -- probably for good reason!  What should I expect? I'd be happy with <1% THD across the audio band, but I suspect it's not achievable based on my experience with other low-power CMOS op amps (OPA2990, OPA2196)... 

  • 0
    TI__Guru 71711 points

    It would be really helpful if you show the circuit or describe circuit configuration. If you plan to use 1.8V supply in a battery powered headphone application, OPA310 may be a challenge due to its limited short-circuit current (see below) BUT on 5V supply it could work for as long as the output signal is not higher than 1Vpp (Iout=1V/16ohm =~60mA). 

    Keep in mind that the linear output voltage swing is a function of the output current (see below), thus you need to make sure to maintain enough voltage headroom. 

    Excessive crossover distortion could occur in a buffer configuration driving heavy loads but if your input is at least 0.6V below supply voltage this will eliminate it.

    OPA2990 and OPA2196 are high voltage supply op amps (4.5V to 36V) and thus poorly suitable for battery power applications-for high voltage I would recommend OPA1678.

    For a low voltage buffer configuration one may consider OPA396 but for high gain OPA1671 or OPA322 would be the way to go.

  • 0 in reply to Marek Lis
    Prodigy 20 points

    My supply would be 5.5V or even 6V (dropped down from a 9V battery), so output current is not an issue. The configuration is inverting, so I don't expect any crossover distortion from the input stage of the OPA310. It will all come from the output section.

    OPA2990 works down to 2.7V BTW. I tried both sections in parallel with a 9V supply, and while it delivers up to 150mW into 16 ohms and even into 32 ohms, I'm getting too much THD (crossover distortion, visible on a scope) at higher frequencies (above 1kHz or so). On the order of 2 or 3%. Very audible (and nasty sounding). I was hoping that OPA310 would have less distortion due to higher bandwidth. 

  • 0 in reply to Jerry Catanescu
    TI__Guru 71711 points

    With THD distortion devil is in the details. By inverting gain you mean -1?  What’s the output signal magnitude under 16ohm load and 5.5V supply?  What resistor values do you use for the inverting gain? 

  • 0 in reply to Marek Lis
    Prodigy 20 points

    Yes, gain would be -1. I did not physically build an OPA310-based circuit yet because I wasn't happy with the outcome of the OPAx990, and I I don't have any OPA310 on hand, so I figured I'd ask about it first. The circuit below gets significantly more than 1% THD at several kHz with a 16ohm load and there's nothing I can do about it short of trying a different opamp. Unfortunately Tina-TI (or at least the models for OPAx990/OPA310) do not model crossover distortion, as best as I can tell. 

     

  • 0 in reply to Jerry Catanescu
    TI__Guru 66666 points

    Hi Jerry,

    Have you looked at the dedicated headphone drivers? Such as the LM4881? Though most are limited to 5V supply.

    https://www.ti.com/product-category/audio/amplifiers/specialty/products.html#-2=contains%3Bheadphone&

  • 0 in reply to Paul Grohe
    Prodigy 20 points

    The main thing that made me consider OPA310 is the low quiescent current. It's important for my application. 

  • 0 in reply to Jerry Catanescu
    TI__Guru 71711 points

    Jerry,

    It would help to know what peak voltage you apply under a given loads. 

    Average 120mW into 16ohm load implies voltage across the load: V = sq-rt(P*R) = sq-rt(0.12*16) = 1.38Vrms or 1.95Vp

    Average 130mW into 32ohm load implies voltage across the load: V = sq-rt(P*R) = sq-rt(0.13*32) = 2.04Vrms or 2.88Vp

    Average 90mW into 80ohm load implies voltage across the load: V = sq-rt(P*R) = sq-rt(0.12*16) = 2.68Vrms or 3.78Vp

    Driving 16 ohm with 1.95V peak voltage would imply 60mA output current in each of the two op amp and minimum Vo of 2.132V - see below. 

    This would mean that sourcing 60mA load the minimum OPA4990 supply voltage would need to be 6.132V (2.13V2+4V) - see below.

    For this reason, I believe the distortion you saw was caused NOT by crossover distortion but by clipping of the output signal.

    In case of OPA2310, assuming load connected to ground, the minimum supply voltage while sourcing/sinking 60mA would be +/-2.332V (2.132V+0.2) - see below.

    OPA2310.TSC

  • 0 in reply to Marek Lis
    Prodigy 20 points

    My OPA2990 testing was done at a 9V supply. You might have missed this in my drawing:

    There was definitely no clipping of the top/bottom of the waveform - I kept the levels below clipping. In any case, the crossover distortion happens in the middle section of the waveform (around zero crossing) so it's actually worse at lower output power (it's a bigger percentage of the amplitude).

  • 0 in reply to Jerry Catanescu
    TI__Guru 71711 points

    In the above plot was this using 9V single supply or +/-9V dual supply and what was the Rload and frequency?  Also, was Rload connected to ground (negative supply) or to mid-supply (4.5V)?  This does look like output crossover issue thus I think the load was connected to mid-supply.  On the paper OPA2310 should perform better BUT we never tested it under 60mA load so can't be sure whether it suffers from crossover distortion - only test would show.

  • 0 in reply to Marek Lis
    Prodigy 20 points

    "we never tested it under 60mA load so can't be sure whether it suffers from crossover distortion - only test would show."

    I'm just surprised that a part that's specifically advertised as high-output curent (150mA) doesn't have THD plots at high-output current in its datasheet. At this point I think we can close this thread, I'll have to find the answer myself on the bench.

  • 0 in reply to Jerry Catanescu
    TI__Guru 71711 points

    There are standard THD loads (10k, 2k, 600ohm) we test for but 60mA load is not one of them. 
    However, we do test for short-circuit and AOL under 50mA so we know the part is able to sink/source at least 75 mA at 5.5V supply (typical Iout is 150 mA while min is 75 mA). 

  • 0 in reply to Marek Lis
    Prodigy 20 points

    For the benefit of whoever might stumble upon this thread in the future, I did finally measure the THD of an OPA310 under heavy load (16 ohm and 32 ohm). I achieved the lowest THD in a unity-gain, non-inverting configuration. The inverting configuration pretty much doubled the THD, so I would avoid it for this kind of job. Of course, any above-unity gain would also degrade THD due to the reduced negative feedback available.

    To answer my initial question, I find OPA310 very suitable for driving headphones if a typical THD on the order of 0.1% to 0.5% is acceptable. With a 6VDC single supply, capacitor-coupled output (470uF) and load tied to real 0V, it puts out as much as 150mW into 16ohms and 110mW into 32, which is quite impressive and better than many dedicated headphone amp chips. In the image below, the higher-THD traces are at the lowest tested output power (0.1mW) and the lowest THD is at max power before clipping. THD was measured with a 22kHz bandwidth, because we can't hear distortion products outside the audio band.

         

  • 0 in reply to Jerry Catanescu
    TI__Guru 71711 points

    Thank you for the info - a couple of comments here:

    An inverting configuration avoids the distortion caused by the change in the input common-mode voltage BUT places the circuit in the noise gain of 2 - this would explain why you see its doubling of THD vis-a-vis buffer configuration.   

    This is not to say that you may not do so but only to bring to your attention that even though 6V single supply is below the absolute maximum rated voltage of 7V, nevertheless it is above the recommended maximum of 5.5V.  Thus, if used outside the Recommended Operating Conditions but within the Absolute Maximum Ratings, this may affect device reliability, functionality, performance, and/or shorten the device lifetime.