Here's a possibly stupid question...

[tfarney]

If a DAC is built in to an integrated amp, does it need an analog output stage to bump the signal up to line level or can the signal from the DAC chip just be sent to the preamp stage of the integrated amp?

Tim

[lurcher]

Depends on the DAC, many (most) require a external filter/buffer stage,

[alb]

Tim.

Here's my basic answer, while we are waiting for someone to give you the whole story.


Some DACs for example 1541 are current output types. These have a very low output and need an I/V resistor to convert the output to voltage, so that the amp can use it.
The other type is voltage output - which means that an opamp is built into the DAC chip (one of the reasons these are not favoured by some.) However these produce an output which is much higher.

If you have a tube amp you could possibly choose the input tube to give the required amount of gain for a particular type of DAC. But most assembled DACs will have an analogue output stage to ensure compatibility with solid state gear.

I feed the output from my voltage type DAC straight into a valve gain stage, thereby doing away with the analogue opamp circuit altogether.
However, if your DAC has one of the better opamps fitted in the output stage and not too much circuitry, then the benefits of bypassing it may be reduced.

Hope that makes some sense.

[StanleyB]

If a DAC is built in to an integrated amp, does it need an analog output stage to bump the signal up to line level or can the signal from the DAC chip just be sent to the preamp stage of the integrated amp?
Tim

Before I offer my input, please note that I might be deemed to be biased in my answer since I designed and sell a well known DAC. So keep that in mind.

Am I to assume that you are thinking about the DAC chip itself, and the analogue signal coming out of the DAC chip audio pins? If that is the case, then it depends on the DAC chip number whether an amplified output stage is required. Some chips are current output types, which means you need a current to voltage conversion stage since the integrated amp will need a voltage at its input.
Some chips are low voltage types (0.6V in the case of the pCM1716), or high voltage (2V for the PCM1780). The integrated amp needs about 350mV on the AUX input, or 2V on its CD input.
In the days of the NOS (non oversampling) DAC chips such as the TDA1541 you needed an anti aliasing filter, which is a steep cut off filter to remove the 44.1KHz sampling frequency. Otherwise it could take your tweeters to the other world. Oversampling chips don't need a sharp 44.1KHz filter. It can be set higher than that in the case of an oversampling DAC. The benefit of this is that overtones and detail in wood and string instruments are easier to hear in the music.
Taking the voltage output from a voltage DAC chip directly to an external amp has its plus and minus points. The minus is that the signal can be full of sampling noise that can take out tweeters that have frequency responses running into 30KHz or more.
The plus side is that it takes out one extra piece of circuitry that could affect the sound.

[tfarney]

Before I offer my input, please note that I might be deemed to be biased in my answer since I designed and sell a well known DAC. So keep that in mind.

Am I to assume that you are thinking about the DAC chip itself, and the analogue signal coming out of the DAC chip audio pins? If that is the case, then it depends on the DAC chip number whether an amplified output stage is required. Some chips are current output types, which means you need a current to voltage conversion stage since the integrated amp will need a voltage at its input.
Some chips are low voltage types (0.6V in the case of the pCM1716), or high voltage (2V for the PCM1780). The integrated amp needs about 350mV on the AUX input, or 2V on its CD input.
In the days of the NOS (non oversampling) DAC chips such as the TDA1541 you needed an anti aliasing filter, which is a steep cut off filter to remove the 44.1KHz sampling frequency. Otherwise it could take your tweeters to the other world. Oversampling chips don't need a sharp 44.1KHz filter. It can be set higher than that in the case of an oversampling DAC. The benefit of this is that overtones and detail in wood and string instruments are easier to hear in the music.
Taking the voltage output from a voltage DAC chip directly to an external amp has its plus and minus points. The minus is that the signal can be full of sampling noise that can take out tweeters that have frequency responses running into 30KHz or more.
The plus side is that it takes out one extra piece of circuitry that could affect the sound.

Wow. A much more detailed answer than I expected. Actually, I was just trying to get to why the DAC in that little Glow Amp One I tested out last week sucked sooooooo bad, even compared to my very modest stand-alone DAC. I would guess, from the responses, that the answer is "it depends."

This is often the unsatisfactory answer experts give citizens. And I understand, having a few expert areas of my own.

Tim

[Togil]

Sensimilia, this is most interesting. So what exactly is meant when they say Mr. Yamada of Zanden does not use any filtering at all for his 1541 based CD player?
There are companies who remove the anti-aliasing filter in digital SLR cameras, the picture will be better in 90 % ( ? ) of cases but there will be strange effects on a few.

[lurcher]

Sensimilia, this is most interesting. So what exactly is meant when they say Mr. Yamada of Zanden does not use any filtering at all for his 1541 based CD player?
There are companies who remove the anti-aliasing filter in digital SLR cameras, the picture will be better in 90 % ( ? ) of cases but there will be strange effects on a few.

I think there may be a bit of a conflict of terms there, I had a look at the Zanden some time ago. While he doesn't use any digital filtering before the DAC (Increase the data rate, and then use a digital implementaion of the filter to join the original samples together) I think he does use filtering after the DAC. Looking at the picture,I would guess its possibly a constant k type of construction.

[Marco]

Oh, and look, there's a nice wee valve (ECC82 or something?) in there, too! I actually didn't realise that the Zanden had a valve output stage...

Marco.

[lurcher]

"His 'bridged-T fixed-impedance filter' is used predominantly in deep-sea telephone cables but here uniquely implemented for audio use."

Yep, must be a constant k filter.

[StanleyB]

I think there may be a bit of a conflict of terms there, I had a look at the Zanden some time ago. While he doesn't use any digital filtering before the DAC (Increase the data rate, and then use a digital implementaion of the filter to join the original samples together) I think he does use filtering after the DAC. Looking at the picture,I would guess its possibly a constant k type of construction.

Some designers are crafty lots, who use clever and careful words like marketing people in order to confuse us and lead us away from the embarrasing questions.
As you can see, there are a lot of big coils in that circuit. Anyone remembers what the frequency response of a 0dB 0Hz to 20KHz audio signal is after it passes through an inductor?
The frequently mentioned zapfilter uses the same trickery.

I won't say what I do in my DAC, but if you ever try non-disc based audio (including digital audio from SKY, DAB, Freeview) on my DAC and put that same music trough just about any oher DAC, you'll notice a peculiar difference. And that's is as much as I am willing to disclose. If you google my DAC together with airport express or SB3, you'll soon pick out a certain pattern of comments that people make. I thnk Gazgam in that tent of ill repute was the 1st person to ever mention it on the internet.