Hello, i plan to build own DAC with PCM1794 and TPA6120 but nowhere i dont see how to control output volume. Between PCM1794 and TPA6120 i have I/V from OPA4134 as is in datasheet from PA6120. Can anybody help me with this maybe stupid problem? I am newbie, so slow on me please. 
I found solution, when i use variable resistor between hot and cold outputs. They stay balanced and it will be regulated. Or option number two is using PCM1794 only for his high dynamic range and not using balanced outputs.
I found solution, when i use variable resistor between hot and cold outputs. They stay balanced and it will be regulated. Or option number two is using PCM1794 only for his high dynamic range and not using balanced outputs.
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The TPA datasheet uses the PCM1792 where the volume can be controlled via the DAC chip using a micro controller and either the I2C or the SPI protocol.
In your case you'd have to sum the differential outputs of the left and right pairs of the OPA4134, with something like an OPA2134. Then you'd send the single ended output of the OPA2134s into a pot for volume control, then feed this into the TPA6120. The TPA6120 would appreciate being driven by a low impedance buffer though, so you'll need another opamp between the pot and the TPA.
In other words what you're doing here is building the analogue output stage as shown in the PCM1794s datasheet, putting a pot after it, then putting a buffer + TPA after that. Granted this is no where near as elegant as the PCM1792 + OPA4134 + TPA, but you don't have a lot of choice in the matter unless you're able to use a micro.
In your case you'd have to sum the differential outputs of the left and right pairs of the OPA4134, with something like an OPA2134. Then you'd send the single ended output of the OPA2134s into a pot for volume control, then feed this into the TPA6120. The TPA6120 would appreciate being driven by a low impedance buffer though, so you'll need another opamp between the pot and the TPA.
In other words what you're doing here is building the analogue output stage as shown in the PCM1794s datasheet, putting a pot after it, then putting a buffer + TPA after that. Granted this is no where near as elegant as the PCM1792 + OPA4134 + TPA, but you don't have a lot of choice in the matter unless you're able to use a micro.
If the DAC is well-implemented and hence does not show any appreciable amount of low-level nonlinearity, your main issue will be increased noise. As good as the PCM1794 is in terms of instantaneous dynamic range, the overall dynamic range (i.e. (maximum output) / (minimum output noise level)) of a headphone amp with a volume pot can be up to 20 dB greater still.
If you want to drive both super-sensitive IEMs and insensitive cans (600 ohm or isodynamics) with authority, you won't get around a "proper" headphone amp. That wouldn't be using a TPA6120A2 either, which requires an extra 10 ohms of output resistance for stability reasons, enough to upset the frequency response on critical IEMs, and does break into sweat when driving power-hungry low-impedance loads.
If you want to drive both super-sensitive IEMs and insensitive cans (600 ohm or isodynamics) with authority, you won't get around a "proper" headphone amp. That wouldn't be using a TPA6120A2 either, which requires an extra 10 ohms of output resistance for stability reasons, enough to upset the frequency response on critical IEMs, and does break into sweat when driving power-hungry low-impedance loads.
the digital "lost bits" heuristic is way out of date - you can set your sound sw to use 24 bit output even with 16 bit source
with 24 bit DAC you are simply reducing output into thermal noise - no meaningful difference from using a volume pot with rational system gain structure
headphone listening actually has ~ 10 dB worse noise floor than in a anechoic chamber - the headphone couples mechanical noise of muscle tremor, pulse, any motion to your ears
recording noise from mics, venue is always higher though - people really shouldn't be offering blanket advice without trying to educate on system gain structure, our hearing, recording practical limits
the TPA6120 can be safely operated with series lossy ferrite instead of 10 Ohm R - the ferrite can be over sized 10-100x on sat current to avoid audio frequency distortion - it only has to get to > |10| Ohms over a few 10s of MHz
sensitive IEM really deserve step down output transformers to maintain S/N
only a few exotic headphones need more V than the TPA6120 can deliver - even fewer if you bridge them - the 400 mA output is enough even for Orthodynamic headphones
with 24 bit DAC you are simply reducing output into thermal noise - no meaningful difference from using a volume pot with rational system gain structure
headphone listening actually has ~ 10 dB worse noise floor than in a anechoic chamber - the headphone couples mechanical noise of muscle tremor, pulse, any motion to your ears
recording noise from mics, venue is always higher though - people really shouldn't be offering blanket advice without trying to educate on system gain structure, our hearing, recording practical limits
the TPA6120 can be safely operated with series lossy ferrite instead of 10 Ohm R - the ferrite can be over sized 10-100x on sat current to avoid audio frequency distortion - it only has to get to > |10| Ohms over a few 10s of MHz
sensitive IEM really deserve step down output transformers to maintain S/N
only a few exotic headphones need more V than the TPA6120 can deliver - even fewer if you bridge them - the 400 mA output is enough even for Orthodynamic headphones
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