DAC and Tripath amps

abernardi · « **on:** 6 Jun 2012, 07:24 am »

OK, here's where I get to show off my ignorance, no teasing please...

I was discussing my Sensation 901 with someone yesterday and while I knew this is a digital amplifier, I hadn't really put 2+2 together. Could someone please explain to me in as close to layman terms as possible why we go to all the trouble and expense of getting the best DAC we can afford only to feed this hopefully pristine analogue signal into a T-amp which converts it back to digital, does it's thing and then back to analogue again. Isn't this redundant? Why isn't there just a USB, AES/EBU, ADAT, S/PDIF or some other digital input straight into the Sensation? The signal from the CD or server is already digital, so you save a D/A and an A/D conversion.
I'm sure I'm comparing apples and oranges here, it's probably a completely different format or paradigm, but can someone help me understand? THANKS!

srb · « **Reply #1 on:** 6 Jun 2012, 01:39 pm »

The Tripath and most Class T or D amplifiers aren't "digital" amplifiers, per se. They are analogue amplifiers that employ a digital circuit to switch the output transistors on and off, but there is no actual analog to digital conversion and digital to analog conversion of the input signal.

Steve

virtue · « **Reply #2 on:** 6 Jun 2012, 06:54 pm »

When people talk about digital amps, they are talking about a power stage that accepts a PWM input. What comes out of most source devices is SPDIF, TOSLINK (same underlying signal as SPDIF), I2C, DSD, etc. So... "digital" output is only half-way there.

Tripath takes an analog signal and outputs PWM. There are other controllers that take a digital signal and output PWM.

abernardi · « **Reply #3 on:** 7 Jun 2012, 12:21 am »

Ah, OK got it. So is it that much harder or complicated to convert the PCM, spdif, etc directly to PWM?

virtue · « **Reply #4 on:** 7 Jun 2012, 07:44 pm »

You need to do more than convert it...

You need to pulse the signal to the output transistors and use feedback to reduce noise from the power supply and switching artifacts.

The bottom line is that the quality of the implementation matters. There are DA (digital to analog converter) -> AD (like Tripath or IR or TI) -> PWM solutions that outperform DD (direct digital) solutions. There are AD (analog to digital converter) -> DD (like DDX) -> PWM solutions that outperform DD or DAC solutions.

That said, there are things you can do in the digital realm around signal processing that you can't do in an analog solution. Also, there are efficiencies that can be made in a closed-loop digital system (ie. DDX/FFX).