Preamp mode + headphones (Tone 2 Pro)

Hi all,

I’m just starting to use preamp mode to control volume when RCA is connected, but I also use it with headphones most of the time, I’m using Sennheiser HD660, should I switch to high/low impedance mode when I’m using headphones or I can stay all the time in preamp mode? (I thought volume control would not work for headphones in preamp mode for some reason but it works). What is the difference between low/high/preamp modes (except obviously that in preamp you can control the volume on RCA)?

You can see the topology of Tone2 Pro below.

From the topology you can see the volume can be controlled by two chips——DAC and NJU72315.
The difference between low/high/pre-amp modes is the volume of DAC and NJU72315.
You can see the relationship from the table below.

DAC Volume Analog Volume(NJU72315) Balanced RCA Output① Single End RCA Output① 4.4 Headphone Output① 3.5 Headphone Output①
Low Gain Fix to Maximum Adjust from -12dB to -62dB 4Vrms 2Vrms 0-5Vrms 0-2.3Vrms
High Gain Fix to Maximum Adjust from 0dB to -62dB 4Vrms 2Vrms 0-2.5Vrms 0-1.15Vrms
Pre-AMP Adjust from 0dB to -127dB Fix to Maximum 0-4Vrms 0-2Vrms 0-5Vrms② 0-2.3Vrms②

①Note: The results are built on the condition that the digital volume is fixed to maximum and the audio signal souce is 0dB.
②Note: The headphones’ outputs are varying from the RCA outputs.

In summary,if you want to use headphone Jack,I suggest you to choose the low or high gain mode according to the impedance of the headphone.If you want to connect to power amplifiter or active speaker,and the volume of the speaker can not be easy to adjusted,you can choose Pre-Amp mode.

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@Eric68 Thank you Eric, but in both cases volume controller not digitally? Because when I play DSD in pre amp mode volume adjustment work fine (which is impossible with digital volume control). From the point of sound quality if there any difference?


  • Yes,volume controller in both cases is not digitally. The digital volume of DSD format is locked,but you can control hardware volume(DAC) and analog volume(NJU72315).
  • We have measured the THD+N with the same attenuation by DAC volume controller and analog volume controller.The reasult is that THD+N of analog volume controller is lower than DAC volume controller.So the sound quality of analog volume controller is better.

Thank you for your answer, everything is clear now.