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Old 30th December 2013, 12:49 PM   #1
maxw is offline maxw  United Kingdom
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Default DAC driving headphones directly from I/V opamp?

I have a DAC that has a maximum current output of 2.5mA and then an OPA2132 for I/V conversion. Can I drive headphones directly from the opamp or would I need an extra buffer?
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Old 30th December 2013, 01:12 PM   #2
maxw is offline maxw  United Kingdom
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What I mean is, would the 2.5mA be exceeded or will the opamp do all the work?
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Old 30th December 2013, 02:18 PM   #3
jcx is offline jcx  United States
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the op amp output can drive added load - up to ~20 mA (I use ~ 1/2 the short circuit current as a practical max linear output)
you also have to consider the V required - opa2134 can't swing much current at Vout within 2 V from the rail

a single op amp I/V used as output doesn't give much choice for the required low pass filtering
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Old 30th December 2013, 10:46 PM   #4
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Why not use TPA152 ? A little chip will give you all that you need with minimum headache. But before you have to put one more OP Amp to go from diff. signal into single polarity.
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Old 31st December 2013, 01:29 AM   #5
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Not sure low-pass filtering is required when only driving headphones? Certainly if driving an amp, an LPF will minimize IMD but headphones are generally going to be better than amps for IMD.

@maxw You don't say what impedance your headphones are? I've done this but I didn't use OPA2132, rather I used AD8016 which has a lot more current output. More akin to jcx's TPA6120.
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Old 31st December 2013, 10:42 AM   #6
maxw is offline maxw  United Kingdom
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Thanks for the replies.

I'm not sure I need any filtering. This is not a standard DAC, this is a multiplying R2R DAC being used as an attenuator:
Click the image to open in full size.

My headphones are 32Ω.

Basically, I don't care too much about the opamp, I can change it later but I'm just curious if I can damage the DAC? Is there any way that using the opamp to drive a 32Ω load would cause that DAC to output more than 2.5mA? I'm not sure how to calculate this!
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Old 31st December 2013, 11:12 AM   #7
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You cant damage the DAC, cause the output is current source. It is not a voltage source that is sensible to the load (that is how I understand the theory) Any current source has its own internal resistance, so even if you bridge it, it still has load to work out the current.
But what you can do is overheat OpAmp and make his life very hard. But you no need it, do you? So put in place TPA152 or TDA6120 and thing is done=)
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Old 31st December 2013, 11:21 AM   #8
Mooly is offline Mooly  United Kingdom
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You won't damage anything but you might get better results if you include a small value series resistor, say in the range 30 to 100 ohms. Opamps aren't really suited to driving low impedance loads but that doesn't mean you can't get good results in the right circumstances.
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Old 31st December 2013, 03:23 PM   #9
Pano is offline Pano  United States
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I'll echo the comments above. You won't hurt the volume control, but the I/V opamp isn't going to be ideal for driving the headphones. The TPA152 suggest by Devilsdance seems a good, easy solution.

If you want to kick it up a notch, try the TPA152 headphone amp. I've used it with very good results. I've only used it with differential inputs, but it can be used single ended, too.
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Old 1st January 2014, 11:49 PM   #10
maxw is offline maxw  United Kingdom
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Thanks for the input folks.

This isn't to drive headphones permanently, I just sometimes use headphones for testing. So I'll stick with the opamp.

Now I have to look for other reasons why my DAC ICs seem to randomly stop functioning after a period of time
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