Do amps with USB input like CXA61 allow changing main volume from PC

[justyn]

Hi

I'm looking for a stereo amp to connect directly to my computer via USB. I've been looking at the Cambridge Audio CXA61. It has built in USB + DAC, and motorised volume knob.

What I'd like to understand is whether on these (or other) amps when the volume on the PC is adjusted, does this adjust the master volume on the amp? OR are there still two separate volume levels to adjust, as with classic pc-audio-out->amp-audio-in connections.

Because as I understand it these amps will identify on the computer as a soundcard. If that is the case, then the soundcard volume on the computer ought to just be the volume level of the amp itself.

If not, and there are two separate levels, what is the optimum configuration here of both of them?

Thanks for any insights.

 

[Gray]

What I'd like to understand is whether on these (or other) amps when the volume on the PC is adjusted, does this adjust the master volume on the amp?

No Justyn, the PC will never have any control of the physical volume control of an amplifier.
It's best to have fixed (or full-up) volume set on the PC and control volume at the amp.
(Unless you really must have the convenience of volume control by PC).

 

[Vincent Kars]

these amps will identify on the computer as a soundcard.

More or less. Basically the PC sees a USB receiver, not to be mistaken for the amp.
Most of the time the volume control of the amp is analog so not affected by anything digital.
As it is USB audio, you can use digital volume control as well.
If you do, max out the bit depth.

The loudest a DAC could play is called 0 dBFS, say all bits are on.
If we lower the volume digital we ‘shift’ to the right.
If we play 16 bit program material on a 16 bit DAC and lower with 48 dB, we have only half of the number of bits left.

MSB LSB
1111111111111111
0000000011111111

Yes, we do loose resolution and with each bit chopped off, 6 dB of the dynamic range.

If we play 16 bits program material on a 24 bit DAC and lower with 8 bits (48 dB) we still have all 16 bits in the register of the DAC.

111111111111111100000000
000000001111111111111111

IMHO you can use digital volume control without issues as long as the data path between PC and amp is minimal 24 bits.

 

[justyn]

Thanks for your detailed replies.

I had not really considered that the volume on the device is probably actually analogue. Your comments on the bit depth, particularly with a 24-bit protocol, are very useful.

I've looked at the RS232 interface for the CXA61 to see if it supported volume up/down commands, in order to implement some PC control that way, but sadly it appears not.

So I shall probably leave the analogue volume level at a pretty high level and then adjust down digitally on top of that, while also bearing in mind the bit-depth.

Thanks again.

 

[Vincent Kars]

My advice would be to set the level slightly louder than comfortable.
Use the digital volume control to lower.
Technically this is better as analog volume control has a constant SNR. If you lower the signal, you lower the noise as well.
Digital volume control only affects the samples, not the noise so the more you attenuate, the worse the SNR become.
Probably I'm just splitting hairs as the noise of a modern DAC is extremely low, way below the noise floor of the power amp.