Headphones amplifier with floating output connected to line In ? - Page 1
Forum section: electronics / beginners

author: ratatax
author_rank: Regular Contributor
timestamp: August 25, 2021, 10:34:22 am
content: Hi,A lot of headphones audio amplifier chips have floating outputs (they don't require DC blocking caps and don't use GND either), like this one:But how do you make it safe if the user plugs the output into a Line In instead of headphones ? I guess the line in will force OUTL- and OUTR- to ground and will fry the chip. Or am I wrong ?

author: magic
author_rank: Super Contributor
timestamp: August 25, 2021, 10:57:31 am
content: There has to be more to it because typical headphone jacks have only one ground terminal shared by both channels.Perhaps it is meant to be included as internal amplifier in "active" headphones or for something else altogether.

author: Buriedcode
author_rank: Super Contributor
timestamp: August 26, 2021, 06:58:42 am
content: Quote from: ratatax on August 25, 2021, 10:34:22 amThey don't. And it may not "fry" the chip, but it certainly won't be good, and definately won't sound good, because they are driving the speakers differentially to allow for a larger voltage swing for a lower power supply voltage and to avoid large output DC blocking caps (that pop on power up/down). Since speakers require AC, any output driving a speaker that is referenced to ground, must be able to source and sink current - its output msut swing above and below that ground reference. With a single power supply, it cannot do that.Such devices are designed to be integrated into headphones, which is why they have differential outputs, not for devices that are designed to have stereo jacks to external headphones.Another method of avoiding the large output capacitor, that is employed by a number of manufacturers, is to have an on board charge pump to generate a negative voltage supply, so the outputs can swing above and below ground. These can be single-ended, ground referenced, and so will happily work with headphones that have commoned grounds. Off the top of my head, TPA6132 from texas instruments, is one example.Edit: I should probably poitn out you can drive a speaker with a single voltage supply, if you hold one node have VCC/2. So for headphones with a stereo jack the "ground" would be half your supply voltage, meaning the amplifier wouldn't have to go negative, just above and below that mid-point reference. However, driving differentially allows you to have an output swing that is almost twice the power supply voltage. Look up "half bridge" and "full bridge" drivers.

author: theHWcave
author_rank: Regular Contributor
timestamp: August 26, 2021, 10:17:14 am
content: Yes, connecting any of the outputs to ground will potentially fry the chip unless it has some over current protection built-in. I had the same problem with a TPA3116 power amp in a mono configuration. 100W at 2 Ohm on a floating output. I agree with the other comments that your circuit (as well as the TPA3116) are meant to directly connect to speakers and nothing else.On my TPA3116, connecting either output to ground (e.g. through a scope probe) blows the fuse of the power supply every time. I wanted to use the TPA3116 to boost my function gen. output for driving higher loads in circuits that are often ground referenced, so I was experimenting on how it could be made safe. I made sure the power supply output was not (mains) ground referenced and added an audio-transformer on the front-end to make the input floating as well. In that config, you can connect either output to ground without damage, but the isolation of those tiny (600 ohm) audio transformers doesn't look particular strong and then you have the problem of reduced audio quality because you are going through a transformer. Bottom line, I kind of gave up on using a TPA3116 as a function gen booster amp because of the floating outputs