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Help me understand the analog stage of a DAC.

LegionOfHell

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My(incomplete) understanding of the analog stage: When the DAC chip converts the digital signal to an analog signal and outputs the analog signal through its pins, This analog signal is a bit weak, So there is a need for an analog stage, which(usually) amplifies the signal to line level and outputs it through RCA outs.

Therefore, there are three types of Tube DACS:

(1) The first type has only a tube for its analog stage.

(2) The second type has a tube at the beginning of the analog stage and solid state components after that.

(3) The third type has solid state components at the beginning of the analog stage and a tube afterwards.

Which one of these types gives you a better "tube sound" ?

Can you recommend any budget Tube DACs for under $200 USD ? Which type is the Aune T1 DAC/AMP ?
 

nsayer

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I made my own TOSLINK DAC. It's mostly the design from the datasheets for the major chips involved, but I think I can speak to at least some of your questions.

In the case of the PCM1793 DAC that I used, the output is differential. This allows the DAC to output a "negative" voltage even though it lacks a negative supply rail (analog audio is bipolar in nature - negative and positive - it's "fancy" AC). The analog stage that follows converts the differential signal into a single-ended (that is, relative to ground) signal. It also amplifies the result to line level (about 6v peak-to-peak) and implements low-pass filtering to get rid of harmonics and other crap that is outside of the audible range.

That's for this particular DAC chip. Others may have different output and require different conditioning. Some of the fancier ones I've seen have current outputs (rather than voltage outputs). In addition to any differential conversion they also need transimpedance conversion along with low-pass filtering.

And, of course, all of this has to be done with as little added noise and distortion as possible.

I don't have any input on your tube related questions, but wanted to contribute what I could to understanding the analog output stages of a typical DAC implementation.
 

DVDdoug

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I'm still waiting for an all-tube DAC... :D :D :D I think that would take 24 tubes (per channel) for a 24-bit DAC.

Some DAC chips don't need any additional circuitry. But it's usually just a buffer amplifier - No gain but low-impedance output and perhaps level shifting it the DAC chip doesn't have a negative supply. Or maybe just a little gain.

Which one of these types gives you a better "tube sound" ?
The tube can be anywhere in the signal-chain.

But I'm with nsayer... I don't want any tube distortion or any other kind of distortion.

Can you recommend any budget Tube DACs for under $200 USD
Tubes don't always introduce distortion but if something has "tube distortion" it will sound different than something else with tube distortion and you'll have to listen for yourself to see if you like it.

Guitar players often like the way their tube/transformer amps sound when overdriven. And, since every amp sounds different when driven into distortion they all have their favorites. In this case the "important" tubes are in the output stage into an impedance-matching transformer connected to the speaker. But they are usually all-tube with no solid state stages.
 

radix

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If I remember correctly, tubes are pretty sensitive to the circuitry after them. I'd think that tube to solid state buffer to output would give you a consistent, engineered circuit that has low sensitivity to the next stage.

I assume you are looking for a "tube sound", so want to introduce it somewhere?

Marc
 

Lambda

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Most modern high Performance DACs (the chip) output a Differential Current signal. But normally we use voltage not current to drive hifi gear like AMPs

To convert Current (i) to Volateg you need a trans impedance amplifier I/V Converter.
400px-TIA_simple.svg.png

Or better Differential version:
Buffering-High-Speed-DACs-Using-the-AD8138-Differential-Amplifier.png


You sure can Make a Differential Amplifier as well as a trans impedance amplifier with tubs.
But they won't be particularly good to moderns standards.
 

tw99

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If that adaptation stage is always needed, what's the reason it's not just integrated onto the DAC chip?
 

KSTR

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If that adaptation stage is always needed, what's the reason it's not just integrated onto the DAC chip?
Practical technical limitations, mainly. It doesn't make much sense to integrate the output stage from the viewpoint of semiconductor process parameters, in a high-end DAC chip (headphone dongle DACs are a different breed, though, and have an output stage on-chip). Further, there's a multitude of possible output stages and the product designer is offered the freedom to choose what he likes.
 

Robin L

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It's a buffer circuit. It's supposed to make impedance matching better. Anything else is distortion.
 

Robin L

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Let's not forget another main purpose of the output stage, the analog lowpass to filter out the unwanted signals (and noise) above the audio band.
Not really, that is much better implemented at the digital level.
 

KSTR

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Not really, that is much better implemented at the digital level.
Not quite, while in the digital domain the reconstruction filter can be realized much better you still want a filter for all the RF junk at >100kHz or so and every datasheet output stage shows such a filter. The digital reconstruction filter does nothing for switching spikes and general RF internal interference of the outputs because that happens after filtering, and it also does nothing for noise shaping above fs/2.
 

Lambda

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If that adaptation stage is always needed, what's the reason it's not just integrated onto the DAC chip?
Different Chips have different manufacturing processes and its not commercially viable to make a single chip good at everything.
So for the analog stage they use a sightly different semiconductor process hat allowed for the relativity large voltage change while maintaining linearity.
The DAC chip on the other hand is outputting a constant voltage as strange as as sounds. its just the current that changes (in an ideal world)

But off cause there are DACs that output voltage directly an fully integrated solutions needing minimal external components.
In general they are just not as good.


It's a buffer circuit. It's supposed to make impedance matching better.
It's also often a I/V converter.

Not really, that is much better implemented at the digital level.
This is not exactly how it works.
You still need Analog low pass filter (or you have some amount of imaging/HF content)
But thanks to oversampling and digital filters, the Analog high cut can be way outside the audible band.

The Ideal DAC always outputs "steps" this steps contain HF content. Oversampling makes the Steps smaller so the HF content is a way higher frequency and the smaller steps are smoothed out more easily.
 
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Vini darko

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My thought is get a normal high preformance dac. Then add a tube buffer modual in between the dac and amplifier. There are many boards and kits available on ebay and amazon. These allow you to customize to your hearts content while still having a reference source.
 

Lambda

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The thing is a good tube device dose not sound like a tube device.

What people identify with tubes sound is often more
Transformer Sound.saturation/soft clipping and Hysteresis
Asymmetric "soft" distortion. from a unbalanced amp or Tube class A. The distortion being diffident on the positive or negative halve wave.
Compression. do to thermal effects some tube power amps reduce Gain if they get hot and therefore act like a compressor.

All this is actually normally unwanted so if you want more tube sound maybe go with lower quality gear.

I have used a DI box in combination with ab cheap tube microphone preamp to get very nice distortion as a effect for my synthesizer.
 

garbz

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Not really, that is much better implemented at the digital level.
No you still need an analogue filter. The alias images are created as part of the digital to analogue conversion itself. You can't filter them out before hand. What you can do digitally is ensure the band between 20kHz and fs/2 is free from any data so that when the aliasing effects are created they are far enough away from the audio band that a simple 2nd order AA filter suffices in the analogue stage.
 
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