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Post by doug82 on Dec 25, 2020 8:37:51 GMT
Hi,
I'm new on the forum... I've just read the excellent analysis of the QC35-II (https://diyaudioheaven.wordpress.com/headphones/measurements/bose/qc35-ii/). I can see that with the NC on, the headphone creates a much nicer response to the wired input.
My question is: what happens to the analog signal on the wired input when the NC circuit is on? Does it get digitized > all the noise cancellation math done in digital > built in DAC > built in amp? Or does it go to the driver through an analog NC circuit that performs the noise cancellation form a (digitally computed) error signal? The reason I'm asking: is it fine to drive this through the wired connection from an external DAC/amp with NC on? It sounds fine (better than with NC off), but I don't understand what's going on inside the headphone.
Thanks for any insights!
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solderdude
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measureutternutter
Posts: 4,882
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Post by solderdude on Dec 25, 2020 15:48:33 GMT
Hi,
I'm new on the forum... I've just read the excellent analysis of the QC35-II (https://diyaudioheaven.wordpress.com/headphones/measurements/bose/qc35-ii/). I can see that with the NC on, the headphone creates a much nicer response to the wired input.
My question is: what happens to the analog signal on the wired input when the NC circuit is on? Does it get digitized > all the noise cancellation math done in digital > built in DAC > built in amp? Or does it go to the driver through an analog NC circuit that performs the noise cancellation form a (digitally computed) error signal? The reason I'm asking: is it fine to drive this through the wired connection from an external DAC/amp with NC on? It sounds fine (better than with NC off), but I don't understand what's going on inside the headphone.
Thanks for any insights!
I think the NC is not digital. There is nothing that points to this. In the Beats it is digitized first. Yes, it is fine to drive it wired from an amp but the driver itself is high efficiency and thus also low power rated 20mW. There is no problem using it wired with NC on but with NC off you can overload the driver (blow it up) with too much power. In active mode the input signal goes in to the internal amplifier. As 'feedback' it gets the signal from a microphone near the driver (in the space where your ear is) Because of this, for frequencies below 1kHz this means that the microphone 'dictates' the output signal as it compares it to the applied signal (headphone cable or BT receiver). So all other 'present' signals that are picked up as well as irregularities in the driver's frequency response are 'counteracted' by the internal amplifier giving the 'anti sound' needed.
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Post by doug82 on Dec 25, 2020 19:21:51 GMT
Thanks for the explanation! You confirm what I was intuitively thinking, that with the NC on, the sound will ultimately be determined by the microphones and the NC circuit - that's why it's surprising me that it sounds better with the NC on, even when driven by a nice dac/amp. Just out of curiosity: if the input goes in the internal amp (480Ω?), the driver (32Ω) will draw current from the internal amp and battery, not the external amp, right? So one would get the same result with a line level input directly from a dac?
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solderdude
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Post by solderdude on Dec 26, 2020 8:09:30 GMT
The reason why with NC the QC35 sounds better is because the rather substantial incorrect tonal balance between 20Hz and 1kHz is fully 'corrected' by the NC circuit. This makes a LOT of difference.
Of course the NC circuit won't play as loud as one can driving it directly but the 20mW rating of the driver means you wouldn't want any power on its input anyway.
Yes, the power is drawn from the amp in 'off' mode and when on drawn from the NC circuit.
Indeed, when using it with NC on you can connect to the DAc directly. Most DAC's won't have many problems driving an unusually low resistance of 480 Ohm. There will be no gain in SQ as the bottle neck is the internal amp and above all the microphone NC circuit. Bass distortion will be lower than (NC on) for certain compared to NC off.
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