Some questions about audio DAC's

Hi. I've been using my simple knowledge to make simple headphone amps of different kinds. I've began etching PCB's with mixed results, but I think I can start making boards with SMD soon. My goal (in some future) is to make a really good headphone amp, like high-end stuff, but to do that I think I need an integrated DAC, if not then people would need a good sound card to get the best from the amp.

I know there are dedicated chips for this, but as I have understood it, I need loads of external components in order to make it work. I wonder why this is, the output of this DAC can be nothing else than an analog audio signal so why not just have a chip with 5 connected pins, gnd, +V, input and output? What purpose does these external components have?

I'm also wondering about the quality of the DAC. I remember buying the cheapest USB soundcard I could find a while back, but this thing had a sound quality that was unbearable. So what makes the sound from a DAC good?

I'm sure there are ics with very few external components but you need more than 5 pins. It may not be the best in quality, becuase you may wish to have an external clock, rather than an internal one (and maybe a PLL). These are all compromises for quality versus complexity. Inputs may be SPDIF or I2S. The clock may be from an external source or derived from the input. There may be some control inputs. There may be balanced audio outputs. The DAC will likely be stereo. There will certainly be power pins surrounding the ic for decoupling for analog and digital supplies. Read some Analog Devices datasheets, to get an idea of how to use them.

Hello Plecto,

to built a good headphone amp is not a problem. Therefore there are good ICs aviable.

Only you must think what Input signal you want to use. For analog input you need a pre amp with OPs and for digital audio sources you need a good DAC.

The external components set the favourable working point, the sampling sampling rates, filter components and frequency range of the output.

Sure it is easy to make a chip with only 5 connecting pins, but then you must make for every signal quality and sample frequency a new chips. This will be too expencive. :lol:

So what makes the sound from a DAC good?

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The sound of a DAC is depended from the sampling rate, the quality of the filter components, high bit resulotion and the signal - noise - ratio (SNR).

Regards

Rainer

Only you must think what Input signal you want to use. For analog input you need a pre amp with OPs

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Don't really wan't to go into the analog part, but why is a pre-amp needed for an analog input signal? Even an amp like the 25W LM1875 doesn't need a pre-amp, why would a 250-500mW headphone amp need one?

I think it will take a little too much energy to learn how a DAC works and what I have to do in order to improve one. I'm studying electronics at the moment and I will probably learn about this soon enough. Could someone link a schematic for a decent audio DAC? I know it's hard to define "decent", but alteast something that beats a regular on-board sound card from a cheap motherboard. I'm not sure if having a USB input will make the circuit more complicated, if so I could go for a spdif input instead (allthough I would consider USB more convenient)

Hello Plecto,

you need only a pre-amp for very low inputvoltage or for impedance matching.

Look for example to the HP of TEXAS Instruments. **broken link removed** Some of them have a headphone amp inside. Also you have many technical informations and support. Much of the ICs you can get as free samples or evalutions boards are aviable if you are a registered member..

Regards

Rainer

I clicked on a random DAC from your link, the DSD1702. The example circuit shown doesn't actually show alot of external components, it does however give block diagrams to a DSD decoder and a PCM decoder, not sure what those are.

Sure it is easy to make a chip with only 5 connecting pins, but then you must make for every signal quality and sample frequency a new chips.

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So it isn't possible to make a chip that can handle all common inputs? Something that adjusts itself, recognizes what input it is and act accordingly?

becuase you may wish to have an external clock, rather than an internal one

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This is because an external clock can be better than the internal one? Sort of like an MCU not beeing suitable as a clock so one would wan't a RTC connected to it? Why not include a brilliant clock inside the chip?

This might lead to an expensive chip though, I'm just wondering if it's possible

If you are going to put a DAC in your amplifier, regardless of the 'quality' of components or clock signals, you need to know what format the digital input is going to be. To be perfectly honest, there is very little difference in quality of audio DACs, after all they are only there to turn numbers into a voltage and if the numbers are corect and stable, so will be the voltage. The recovery errors in DACs are well specified in their respective data sheets but are n any case very small. Your biggest problem will be catering for the different digital formats. The most common is probably SPDIF for which there are dozens of DACs available but there is also HDMI, TOSLINK and several less common formats. These are not interchangeable, you will need different DACs for each. You will also need different connectors and interfaces for each.

The clock only needs to be of the correct frequency and preferably low jitter although at low sampling rates such as audio, the jitter will not cause significant problems.

To be honest, I think you are concentrating too much on a very minor issue and making a lot of work for yourself. I would forget the DAC and work toward a really good analog amplifier.

Brian.

To be honest, I think you are concentrating too much on a very minor issue and making a lot of work for yourself. I would forget the DAC and work toward a really good analog amplifier.

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I came to that conclution myself. The plan has not been to start making a DAC right now, I'm just preparing because I believe I will need one in the future. I would still like to have some more questions answered even though I won't need to understand it in great depths.

If you are going to put a DAC in your amplifier, regardless of the 'quality' of components or clock signals, you need to know what format the digital input is going to be.

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I'm going to go for USB. TOSLINK, HDMI and SPDIF are all attractive, but USB is far easier to deal with allthough I don't know what this has to say about quality or anything like that (probably nothing I hope). If I've gotten this correct, a codec in a computer does the job of converting whatever codec the audio is to PCM which it then sends out to the digital soundcard. If I stick to USB, can't I just make my DAC compatible with PCM and be done with it? I'm also curious about USB protocols and stuff, the computer have to recognize my DAC as a soundcard.

To be perfectly honest, there is very little difference in quality of audio DACs, after all they are only there to turn numbers into a voltage and if the numbers are corect and stable, so will be the voltage.

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So what about those DAC's that weigh 1kg and cost one million dollars, are they mostly for audiophools? I've allways been reading about those hifi geeks who say that your system needs a really good DAC to get the best out of your speakers. I have also been able to hear a difference between sound beeing sent from the analog output of a motherboard and sound beeing sent digital from the spdif to my receiver. I have actually done a blind test with some friends about this.

USB isn't an audio format, it's a connection system. The format of audio sent over USB can still be in one of many formats.
It is possible to mimic a USB CODEC, there are already several ICs on the market that do just that ( I use them here) but they require drivers on the computer to format the audio signal before it is sent to them. It means your amplifier will not work on other computers unless the driver software is also installed on them.

If you find a million dollar DAC, I'll make one that sounds exactly the same for $999,999.99 so you make a saving! I hear lots of complete nonsense about audio equipment and believe only what physics and acoustics says makes sense. For example, I know or someone who paid a considerable amount of money to have their amplifier power boosted by having 'gold' diodes fitted to it. They believed it completely because the output meters on the amplifier were pegged against the stop with even the quitest sound. It didn't occur to them that it wasn't actually any louder! The person 'boosting' the power had simply shorted out the resistors in series with the meters. I also witnessed first hand a salesman persuading a customer that speaker system A is better than system B by demonstrating them side by side while the customer listened to them over the phone!

True, the sound output of some motherboards leaves a lot to be desired but bear in mind they roll of production lines in China for pennies. It you did a test using the analog output of a good quality sound card it would have been much better. It's worth remembering that almost every sound source is analog and to be heard it has to be analog so ANY digital processing in the middle will degrade the quality to some degree. You can't put quality back in after losing it!

Brian.