Can a 'power amp' be driven directly off a DAC

2005-11-19 18:43:18

Hi Everyone @ Hydrogen Audio,

I hope this isn't too off topic, not really typical hydrogen audio question...

I listen to all of my music, MP3s, OGG, MPC through a computer, connected to a DAC ( http://dv411.com/flyingcow.html ) and then into a normal/integrated 2 channel amplifier.

I have spotted a high quality NAD power amplifier, selling very cheaply and would like to know if it is possible to drive it directly off my DAC without going through a pre-amp - as I control all my audio settings, volume, input ect though the computer. This would save money and mean one less box, or have I missed the point of a power amplifier in the first place.

Thanks in advance for you time,

Peter

Can a 'power amp' be driven directly off a DAC

Reply #1 – 2005-11-19 18:50:02

Short answer:  yes.

Can a 'power amp' be driven directly off a DAC

Reply #2 – 2005-11-19 19:37:02

Thanks boojum, much appreicated.

Can I assume then that if this were connected, DAC -> Power Amp would the following assuption hold for volume/power output from the amp.

Digital output level 100% => amp producing 100% potential power output,
Digital output level 50% => amp producing 50% potential power output,
...

Essentially, I'd like to know - if I accidentally set the level on the digital output to 100%,  will the amp deliver maximum power ( 200watt ) and then destroy my speakers? Is this the compromise I would make by not using a pre-amp, and the volume control that would come with it?

Can a 'power amp' be driven directly off a DAC

Reply #3 – 2005-11-19 19:45:19

Your assumption is correct. Also the SNR will go down as you attenuate the signal digitally.

Can a 'power amp' be driven directly off a DAC

Reply #4 – 2005-11-19 19:52:47

You could do this, but there are a couple potential problems. The first is that since the PC volume would have to be turned pretty low for a comfortable level, you would have to be careful that no application ever turned up the volume all the way and you blast your speakers. Also, you would lose some of the dynamic range of your DAC by using it turned down so much (even digitally). You might end up with audible background noise when you listened at low levels.

I think I remember some NAD amps have some tiny gain controls, either on the front or on the back. I think it would be better if you could turn the amp gain down to the point where you got the loudest level you would ever listen to when the PC's volume was up all the way.

I wrote this before the other posts, but you get the idea.. And it might even be that when your PC volume was up all the way, the amp would be clipping badly. You have to check the maximum output of the DAC and input voltage spec for the amp to see.

Can a 'power amp' be driven directly off a DAC

Reply #5 – 2005-11-19 20:45:01

The level control in the DAC works by reducing the bit level. When you turn down the volume this way you are not making a smaller waveform as you do in the analogue domain, you are chopping off the lower levels of the signal. With 24 bit there is some wiggle room since there is no way to have actual signal level that low this side of cyrogenics, but by the time you get down to half way you are definitely degrading the output from the DAC. The same is true if you do the adjustments on the computer with software volume controls in the music player programs.

Can a 'power amp' be driven directly off a DAC

Reply #6 – 2005-11-19 21:04:40

The  relationship expressed above, between DAC level setting and amplifier power, is not at all the case. The amplifier power output varies with the music's dynamic range, instant to instant. Even at a modest volume level that produces fairly quiet audio, the amplifier may be driven to or near maximum on music peaks.

Likewise, the maximum input level would produce maximum output (or bad clipping) only on the louder parts of the music. For some kinds of music, with little dynamic range, that could be most of the time, but for other music, while it might be very loud at the DAC's 100% setting, amplifier output would be well below 100% much of the time.

The amplifier has some specification that expresses what signal level produces maximum output. In general this is intended to match the maximum preamp output. Different preamps and different line level devices can produce fairly diverse maximum output levels, however. For instance, I have two CD players. One is quite noticeably louder than the other for a given preamp volume control setting. It is possible that 100% output from the DAC could drive the amplifier into clipping more than 50% of the time.

Can a 'power amp' be driven directly off a DAC

Reply #7 – 2005-11-20 00:16:19

Lol, I imagine you surfing the internet, and ending on a website with sonic advertisement blowing your speakers out ! 

Can a 'power amp' be driven directly off a DAC

Reply #8 – 2005-11-20 00:55:45

Well, part of the reason for the unprecented (at the time) signal-voltage standard for CD-players and DACS  (>2volts)  was to make 'passive' preamplification easier. Cynics might say it was as much to do with 'stimulating' sales of amplifiers, as an olde worlde 250mV 'aux' input on a 70's integrated-amp just wouldn't work.

But anyway, a passive pre-amp is essentially just a source-selector and a volume pot, and that's actually the way a lot of 'purists' or , ahem, audiophiles do it.

Some even go as far as using variable 'transformer' pots, which can actually give you gain from a reasonably low-impedence source (which most CD players/DACS are).

My suggestion would to be to buy a good quality stereo pot (about 10Kohm) such as an ALPS, mounted in a small radio shack plastic box, in line between the DAC and the amp.

For general listening, you could keep this turned down and use your PC's digital volume controls, with no fear of, as PIO pointed out, giving yourself a heart-attack if a massively loud jingle or whatever happens to come down the pipe when you're online.

For those odd occasions that you want maximum 16-bit (or better) resolution (for 'serious' music listening) you would be able to max out the faders on your PC, and use the little box with the ALPS pot for setting volume.



>>the usual typo corrections

Can a 'power amp' be driven directly off a DAC

Reply #9 – 2005-11-20 12:00:06

PIO's 'imaginary' scheme has actually happened to me :S stupid firefox search noise!

That's why I have a second soundcard installed as the windows default these days. Only foobar and BSplayer are configured to play to my 'real' card.

The loss of resolution at lower levels I can live with(*), since my hearing perception also lowers with level and the noise of the power amp dominates at any level.

(*) with proper dithering of course.