Direct Digital Amplifiers


I am very excited about the concept of direct digital amplifiers, such as the NAD M2 and the new NuForce DDA-100. There are others coming out too. I would think these amps are in their infancy. I would like to know if anyone has had direct experience with them and what are your impressions. I would like to see them accessible to digital equalization. Are there any models that incorporate this functionality?
peter_s
Peter,

Yes, Tact equipment employs this functionality. The tact S2150 digital amplifier paired up with the Tact 2.2XP preamp has really good digital equalization with a lot of flexability if you hook up the 2.2xp to a computer.
I am not sure if this is exactly what you are referring to but the NAD C 390DD has some form of digital EQ on board.

Bill
Good topic!
12-11-12: Willland
I am not sure if this is exactly what you are referring to ....
technically, a "direct digital amplifier" is a class-D amplifier that can accept, at its inputs, a digital pulse code modulated (PCM) data stream. Accepting a (digital) PCM data stream (which is the digital music stream) rather than accepting an analog version of the music stream is the real innovation of "direct digital amplifiers". Hence the "direct" in the name - you can directly connect the digital output of your computer or CD transport to this sort of amplifier & it will drive your loudspeakers. Internal to the direct digital amplifier, the PCM data stream is kept digital all the way & it is finally converted to analog at the output of the class-D output stage by the analog reconstruction filter/low-pass filter just before it drives the loudspeaker. The amplifier is still a class-D amplifier.
Today's class-D amplifiers accept an analog version (digital data converted to analog with an on-board or out-board DAC) of the music stream. The class-D amp converts this to a digital pulse wave modulated (PWM) signal & this is once again converted to analog by the reconstruction filter/low-pass filter just before it drives the loudspeaker.
Direct digital amplifiers do away with the initial conversion of the digital music stream to analog.
And, yes, like OP indicated, there are few direct digital amplifier products in the market today.
Hope that this helps some.
I share the OP's excitement about the technology. It seems to have so much potential for excellence by virtue of eliminating so much of the traditional signal chain.

Putting my money where my mouth is, I bought a C390DD to play with. I'm not ready to offer any final judgement, but I will say that it's very good sounding and it's probably not as good sounding as my traditional separates, which are much costlier. But its flexibility is mighty appealing. I still need to play with the analog input module, (RIAA applied digitally), and the subwoofer functions, as well as the room EQ, which I believe is still a pending feature. USB input is sub-par but is modular and likely to be upgraded in the future.
Bombaywalla, Class D amp doesn't convert anything to digital. It converts analog voltage to analog duty cycle of square wave and back to analog voltage thru filtering. Word "digital" would imply limit of resolution while it is unlimited - completely analog from input to output. People often confuse switching (where duty cycle is analog quantity) with digital.
Yes- class D amps operate in the analog domain. The 'D' is used because A, B and C were taken.

If you run a DAC directly into any traditional amplifier, the signal chain is actually simpler. Cass D amplifiers have considerably more processing to make things happen.
The tact s2150 amplifier is all digital with built in dac. Just plug in your transport via aes into the amp and you're ready to go.
Signal chain might be simpler but class D consist only of modulator and output Mosfets vs. multiple stages of gain in traditional amplifier.
I use a couple different implementations of direct digital and enjoy them immensely. A highly-modified Tact S2150 digital amplifier pair with the Tact 2.2X preamp into YG speakers and Lyngdorf subs allows me to digitally equalize a flatter room response and time align, while crossing over to the subs at a fast slope. Sounds great to my ears. Really opens everything up.

Another system in the family room uses a NAD M2 into Peak Consult speakers. No EQ, but very simple system, with just a music server feeding the NAD. The NAD is very good, very pure, clean and dynamic. Both amps are really power DACs that have the voltage and processing power to drive speakers directly: a great idea whose time has finally arrived. I've previously owned monos by Rowland, Pass, Bel Canto, etc. and while all have strengths and weaknesses, these are really good options, providing superb value compared to a separate DAC, preamp and amp.

Lyngdorf and DEQX also make great direct digital products that can do digital EQ. Go hear one and see if it floats your boat....
Seems as though only the TacT/Lyngdorf direct digital amps are capable of high current output. Are there any others?
Bombaywalla, Class D amp doesn't convert anything to digital. It converts analog voltage to analog duty cycle of square wave and back to analog voltage thru filtering....
you are right, Kijanki. I was trying to be simplistic in the interest of explaining the concept.
OTOH, Kijanki, a 2-level signal which is going from logic0 to logic1 could be considered a (very simple) digital (or binary) signal. Such a signal is both analog & digital. It has infinite resolution like an analog signal & at the same time it is discrete since it transitions from logic0<-->logic1 with no other state in between.
So, I do not think that I confused anything here. Both your comments & mine are correct.
But, yes, it's not a tradition digital signal like the one that is quantized with finite # of bits.
Kijanki, I know of a least one amplifier brand (tube) that only has one stage of gain. That's pretty simple- far more simple than any class D amp.
Atmasphere, that's true but output power is very limited while in class D it depends only on power supply and size of output Mosfets.

Bombaywalla, Adjusted analog quantity is duty cycle. For instance, when you adjust oscillator's frequency by turning knob it doesn't matter if output signal is square, triangle or sinewave - your adjustment is still analog. The fact that voltage jumps between two level's doesn't make it digital if adjusted quantity is duty cycle. Also, amplifier doesn't make any use of these levels other than converting to output voltage by taking average value (filtering).
Distorted electric guitar (square wave) dosn't make it digital. FM radio is not digital in spite of voltage moving between two levels.
Taken from the tact s2150 owners manual:

The digital input is taken to the central processor where it is reformatted into apulse width modulated signal of extreme precision. The pulse rate is measured atprecisely 384,000 pulses per second. Each pulse can have 256 different widths,with the narrowest pulse being a mere 10 nanoseconds wide. The clock frequencytherefore is 98 MHz. The central processor uses proprietary patented algorithms(Equibit) to arrive at exactly the right combination of pulse widths produce a highlyaccurate waveform. This is the most fundamental departure from conventional am-plifiers. TacT defines the waveform mathematically - we are not trying to follow oremulate a waveform by using feedback or feed-forward.Once the decision of the duration of the pulse is made the central processor con-trols FET-switches at the output with extreme precision. Voltage and current aredrawn from the power supply and fed to the speakers.The level of playback is controlled by adjusting the voltage of the power supply. Asthis voltage is switched directly to the speakers, it is of paramount importance thatthe power supply be totally free of ripple and noise. For TacT digital amplifiers, aswitch mode power supply of extreme precision with ripple rejection of more than135 dB has been developed. At full volume (voltage) the TacT M/S2150 delivers 58volts, equivalent to 150 Watts into 8 ohms. To reduce the volume the voltage of thepower supply is reduced. This means that the volume control is no longer part of anactive circuit.
Drubin, please keep us posted with your thoughts as you get to know the C390DD. I'm very interested in that piece! Thank you.

Cheers,

Scott
Bombaywalla, Adjusted analog quantity is duty cycle. For instance, when you adjust oscillator's frequency by turning knob it doesn't matter if output signal is square, triangle or sinewave - your adjustment is still analog. The fact that voltage jumps between two level's doesn't make it digital if adjusted quantity is duty cycle. Also, amplifier doesn't make any use of these levels other than converting to output voltage by taking average value (filtering).
what you wrote about osc freq adjustment being analog is correct but.........
when it comes to a class-D amplifier, it is well-known & well-documented that this amplifier is a discrete-time system. You cannot analyze a class-D amplifier using pure analog techniques. The switcher portion of the class-D amplifier is treated as a digital/discrete-time system & discrete-time + analog analysis techniques are jointly used to analyze the entire system.
Pick up any book on SMPS & you will see what I mean. Basics of DSP that you would learn from a text like Rabiner & Gold are used in the analysis of class-D/SMPS systems.
Even tho' the switcher's main function is PWM (which might be a analog quantity of duty cycle variation), this part of the class-D amplifier IS digital. Make no mistake about it. What I'm gathering from your comments is that I do not think that you have ever analyzed or designed a SMPS or class-D amplifier hence you are so vehement with your comments.
Open up a SMPS text book & read a bit of the analysis of the system. You'll quickly find out that it's a discrete-time system - part digital & part analog.

Distorted electric guitar (square wave) dosn't make it digital. FM radio is not digital in spite of voltage moving between two levels.
these were bad examples. Of course, a distorted electric guitar wave is not digital 'cuz there are stable operating points at the flat tops, flat bottoms & all points connecting the flat top to the flat bottom. This is way different from the class-D switcher output that is stable at only 2 points: logic 1 or logic 0 & nothing in between.
And, FM is phase modulation - a very analog concept. Today it is digitized in HD radio for more noise immune over-the-air transmission. So, while the over-the-air transmission is digital or discrete-time, FM radio is still analog.
we understand this.......
It is not discrete time system since discrete time system has countable number of states by definition. The states here are percents of duty cycle (quantity of interest) that changes with infinite number of states in similar fashion to change of frequency in analog Frequency Modulation. Yes I do design and use SMPS in my work. I'm surprised that you're confused by two levels in class D amp but not in FM radio signal. Again, for system to be "digital" at any point it has to have limited resolution (countable number of states) of quantity of interest - duty cycle, which class D amplifier isn't, being completely analog with unlimited resolution. In similar way FM radio signal has two states of voltage, but quantity of interest - frequency has unlimited number of states.

Very early primitive class D amps had digital modulator but these days almost everything is purely analog with unlimited resolution (Icepower, Hypex, NuForce etc.) Analog input voltage turns into analog duty cycle to become again analog output voltage by means of taking average value (filtering) of square wave (50% duty cycle representing 0V). Whole thing is continuous time system and has unlimited resolution unless, of course, amplifier has digital input which imposes resolution limit by definition.
I just read JA's review of the Devialet D-Premier. It's a different architecture and not "direct digital," but in a somewhat similar vein. I must hear it.
I'd like to see NuForce or NAD come out with a pair of monoblock direct digital amps with 150-200 watts per channel.