[AudioI] Volume levels with Octo

[Matt]

On 28/10/20 4:39 am, Gavin Hamill via People wrote:
> Hello,
>
> I'm using the RCA breakouts that come as standard with the Octo.
>
> This has been niggling at me for a while now so I need to discuss it. 
> When I plug my headphones directly into the dual-phono 'master output' 
> of my DJ controller, the sound is a little too loud for comfort as 
> you'd expect from line-level output.
>
> When I run a stereo-phono -> stereo-phono from the controller 'master 
> output' to ADC1 of the Octo, then launch JACK and connect ADC1 to DAC1:
>
> # /usr/bin/jackd -R -d alsa -r 44100 -p 512 -H -M -S
> # jack_connect system:capture_1 system:playback_1
> # jack_connect system:capture_2 system:playback_2
>
> ADC1 is at the default volume of '38' in alsamixer (described as 0dB).
> Finally I connect the same headphones using the same dual-RCA to TRS-f 
> to DAC1 (at 100% output volume) The sound is *much* quieter.
>
> Is this expected? I've had some complaints about the lower audio 
> levels and want to know if there's a way to crank it up without 
> introducing distortion.


Yes this is expected. You see the headphones are a low impedance device 
(16 or 32 ohms) and they require more power to drive them. I assume that 
the Octo DAC doesn't have enough power to drive them appropriately. It 
is probably better to use a headphone amplifier IC or something similar 
for that purpose.


>
> alsamixer exposes 'ADC1 Single Ended Mode' - when I set it to 
> 'Differential', the sound quality is excellent, but quiet. If I set it 
> to 'Single Ended' - the volume increases a little, but the sound is 
> badly distorted.

I assume that there are no other GND connections between the mixers and 
the Octo is being driven differentially ?


>
> Cheers,
> Gavin.
>
>
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