DAC straight into power amp - not impressed!

[AlexM]

Hi,

My C-J pre-amp left for a service today, and while it is away I have been forced to plug my CD/DAC (with digital volume) straight into the power amp.

I am suprised to find that the sound is detailed with good stereo imaging and timbre, but a bit rolled off in the treble, and the bass is quite a bit less weighty and well defined and sounds somewhat muddy and opaque. Given that a pre-amp essentially buffers the signal and isn't supposed to add-much of itself, why should this be?.

One of the features of my pre-amp is a very low output impedance and lots of voltage drive - is the output stage of a DAC going to struggle driving the high-impedance input of a power amp? I wouldn't have thought it would be working any harder then when driving the pre-amp itself.

I suppose it could be that the digital volume control has to be significant attentuated to keep the volume sensible (-40Db).

Any ideas what is happening here?

Alex

[MartinT]

Some of us have advocated active preamps for the very reasons you state over the years. I don't like 'passive' setups as I find the sound lightweight and lacking in drive. It's that drive that a preamp is capable of that controls the power amp to within an inch of its life, giving drive, dynamics and bass weight. Attenuating via a digital control certainly wouldn't help as they work by throwing bits of resolution away.

[AlexM]

I really am quite suprised how much impact and bass drive is lost, as well as treble sparkle and detail. The resulting sound is a little mid-forward and insubstancial.

I am interested in why this is - I'm not driving long I/Cs, the input impedance of my power amp is 100Kohm, and the output stage only need to provide 1.2v to drive the power amp to it's full rated output.

As an experiment I will try adding some attenuators and running the digital volume control higher to see if the loss of resolution is caused in part by that. Cambridge Audio say that the volume reduction algorithm they use in the ATF2 doesn't result in bit truncation or loss of quality and in fact, they bill the component as a digital preamplifier. Whatever they say, I defintely think it sounds better with valve pre-amp in the loop!

[Yomanze]

I really am quite suprised how much impact and bass drive is lost, as well as treble sparkle and detail. The resulting sound is a little mid-forward and insubstancial.

I am interested in why this is - I'm not driving long I/Cs, the input impedance of my power amp is 100Kohm, and the output stage only need to provide 1.2v to drive the power amp to it's rated input.

As an experiment I will try adding some attenuators and running the digital volume control higher to see if the loss of resolution is caused in part by that. Cambridge Audio say that the volume reduction algorithm they use in the ATF2 doesn't result in bit truncation or loss of quality and in fact, they bill the component as a digital preamplifier. Whatever they say, I defintely think it sounds better with valve pre-amp in the loop!

I'm not surprised. On paper a passive or DAC volume should be absolutely fine, but I find a loss of dynamics, drive and a feeling that I need to turn the wick up a bit with passives / no preamp whereas a good active pre is the same at all volumes.

[John]

Passive pre can work fine in the right system No one would describe my system of lacking resolution but get it wrong and the sound is flat and lifeless
It not a good idea to control volume from the computer is not ideal

[Reffc]

Hi,

My C-J pre-amp left for a service today, and while it is away I have been forced to plug my CD/DAC (with digital volume) straight into the power amp.

I am suprised to find that the sound is detailed with good stereo imaging and timbre, but a bit rolled off in the treble, and the bass is quite a bit less weighty and well defined and sounds somewhat muddy and opaque. Given that a pre-amp essentially buffers the signal and isn't supposed to add-much of itself, why should this be?.

One of the features of my pre-amp is a very low output impedance and lots of voltage drive - is the output stage of a DAC going to struggle driving the high-impedance input of a power amp? I wouldn't have thought it would be working any harder then when driving the pre-amp itself.

I suppose it could be that the digital volume control has to be significant attentuated to keep the volume sensible (-40Db).

Any ideas what is happening here?

Alex

In theory, MOST modern DAC chipsets these days are perfectly capable of supplying a low distortion signal, at between 1.5 and 2V rms on full gain with less losses than analogue attenuation stages providing attenuation is within say -30dB and 32 bit digital attenuation devices are utilised. The problem with some is the amount of gain or where lower than 32 bit is used into high impedance passive attenuation circuit. That though is a bit of an over simplification so someone with more in depth knowledge may be better able to explain further. There's a good post on the subject (post#223) HERE .

In general though, I have always preferred digital sources played via an active amp stage even though in theory, S/N is going to be lower overall than direct from the DAC into power amp. This isn't usually noticed much as the power amp tends to have the lowest S/N ratio within the source/amplification stage. My CDP has an output level control, ostensibly for recording purposes, but can drive the power amps without any loss of resolution or apparent dynamics on its own. From what I've read on the subject, digital sources tend to work best into low impedance active pathways and not into passive high impedance pathways so I'm not surprised that some report very mixed results connecting DACs straight into passive pots.

[Yomanze]

Yes a lot of variables Paul as my DAC is around 60 Ohms output impedance (including output transformers) specified to drive 'any load you could care to think of' according to the manufacturer. Still, I do prefer active pres even with this DAC. Having said this something like a Bushmaster with a passive pre opens up more matching issues (even if this combo can work well with some systems).

[Yomanze]

Passive pre can work fine in the right system No one would describe my system of lacking resolution but get it wrong and the sound is flat and lifeless
It not a good idea to control volume from the computer is not ideal

Am sure those huge open baffles are plenty dynamic as it is so yeah as usual comes back to synergy.

[AlexM]

In theory, MOST modern DAC chipsets these days are perfectly capable of supplying a low distortion signal, at between 1.5 and 2V rms on full gain with less losses than analogue attenuation stages providing attenuation is within say -30dB and 32 bit digital attenuation devices are utilised. The problem with some is the amount of gain or where lower than 32 bit is used into high impedance passive attenuation circuit. That though is a bit of an over simplification so someone with more in depth knowledge may be better able to explain further. There's a good post on the subject (post#223) HERE .

Interesting. John Westlake seems to be saying that using a digital volume control on a source component running directly into a power amp is the lowest noise/highest performance solution. That as may be (from a theoretical perspective), but in practice an active preamp/buffer stage seems to be preferable to most. I thought that most preamp inputs were high impedance, and that it was important for the output impedance of the source to be low - is this not the case?. In what way does a power amplifier input differ from a preamp input? I mean this in terms of the load that the source component output buffers 'see'.

Regards,
Alex

[nat8808]

The output stage of the source still had to be able to drive the power amp well though. Essentially there is no difference between an active line stage in a pre-amp and the output stage of a source component... other than how it is designed and what it is specifically designed to drive!

I guess the 851 isn't designed in a way that drives your Conrad-Johnson sufficiently well.

Of course, err.. opening up ALL possibilities, free thinking and all that.. perhaps you're experiencing something more neutral and your previous set up was slightly top and bottom heavy?

I remember a famous speaker designer saying that that was many people's reaction to his speakers that he'd put so much effort into being completely flat responsed and neutral. As they came from other speakers accentuating certain extremes, they were hearing lack of sparkle and bass. Sticking with that neutral goal however, over time people adjusted their ears.. and now regard the designs as being neutral.