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Midrange DAC - how much should I spend? - Page 4

post #46 of 48
9/8/08 at 2:47pm
This Cambridge unit could make a nice companion to the iWadia 170i.
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post #47 of 48
9/10/08 at 6:24pm
Why not get the Zhaolu and Mod it yourself - it wil sve you a lot of money in the long run and compete with DAC's many multiples of its price.
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post #48 of 48
9/11/08 at 1:47pm
Quote:
Originally Posted by PFKMan23 View Post
From what I've seen no one really makes a DAC with a truly good output stage. There are some better output stages, but for the most part, you're dealing with stuff that just a reflection of the DAC chip's evaluation board or something slightly better. There are those with tubed output stages, but from what I've seen for the most part, they're decent at best. Given that, I really don't see the outputstage as a clear drawback because most DACs out there seem to ignore it as well.
i know i'm starting to sound like a fanboi, but... PS Audio DLIII has a discrete class a output stage... no op amps. from PS Audio's own propaganda:

Quote:
The output stage is the key
Once the sample rate has been converted, it is sent to the TI PCM1798 balanced DAC and its output is fed into our unique output stage.

DAC’s convert the digital signal into current. Unfortunately, to play music through our systems, we need voltage. So the first job of the all important output stage is to convert the current output of the DAC to a voltage, then amplify it so our preamplifier can play it back through the power amp.

99% of every DAC on the planet uses an op-amp to convert the output of the DAC to a voltage. We learned years ago that this is the worst thing you can do for the sound, because op-amps all have two major problems when used for this function: feedback and speed issues.

Without getting too technical, the greatest opportunity for a bright and edgy “digital” sound is caused by op-amp based current to voltage converters (IV converters) because the high speed of the DAC’s output causes transient or slewing induced distortion in the op-amps (SID). SID is a combination of feedback and transient response issues that some exotic high-speed op amps can come close to handling, but all have a problem with it.

PS Audio’s engineers avoid the SID harshness by eliminating the feedback of the IV stage. We do this by a unique single transistor IV converter with no feedback. Speed issues and SID issues are completely eliminated with this technique.

Active and passive filtering
Once the signal has been converted to a voltage, the next potential sonic weakness is found in the filtering. All DAC’s produce switching noise that must be eliminated before it reaches your preamp/amp combination. Most companies use a gain stage with active feedback filtering. The problem with this approach is somewhat the same as we just encountered with the IV converter, feedback and speed issues.

To solve this, PS Engineering uses a passive first stage filter to lower the noise before it enters the gain stage. This is a simple technique that completely eliminates the hardness associated with active filtering.

FET discrete output stage
Next to last, the gain stage itself which boosts the output of the DAC to the standard line level output required to play music.

For this challenge, PS Engineers stayed away from op-amps yet again, and built a 100% class A discrete FET based output stage. This high-end stage uses sweet sounding FET’s at the input and low noise powerful bipolar transistors for the gain and output stage.
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