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Old 18th August 2007, 09:26 PM   #1
_atari_ is offline _atari_  Germany
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Default DC coupling for DACs

Hi,

i am currently designing a USB DAC (I would consider it as entry level high end ).

Here you can find the project description.

The basic concept is quite simple:
- Using PCM2707 as USB receiver
- Reclocking and resampling using an AD1896
- WM8740 in mono mode as DACs
- separating analogue (DAC) and digital circuit (receiver, resampler) to reduce noise

I have now some questions which lay way beyond my knowledge and experience:
- Using transformers for output coupling seems quite usefull - the output can be easily configured balanced or unbalanced. But is it sonically wise? which transformers con be used (regarding sound quality and size footprint). Or is it better to use capacitor DC coupling?

- Are there any experiences regarding separating digital and (semi-) analogue circuits? My idea to separate it came from suggestions regarding high precision ADCs (I simply copied the principle of having separated VCC and GND with a buffer for transmitting data from the digital part to the DAC (which are the "analogue" part). Does it make more harm than good?

Thanks for your input.

_atari_
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Old 19th August 2007, 05:20 AM   #2
DougL is offline DougL  United States
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Quote:
- Using transformers for output coupling seems quite useful - the output can be easily configured balanced or unbalanced. But is it sonically wise? which transformers con be used (regarding sound quality and size footprint). Or is it better to use capacitor DC coupling?
I am using a transformer and am very pleased. YMMV.
IMHO transformers got an undeserved bad reputation simply because they are generally not suitable inside a feedback loop.
By the way, Capacitor coupling is not DC coupling.

Regards;

Doug
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Old 19th August 2007, 06:34 AM   #3
_atari_ is offline _atari_  Germany
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Quote:
Originally posted by DougL

I am using a transformer and am very pleased. YMMV.
IMHO transformers got an undeserved bad reputation simply because they are generally not suitable inside a feedback loop.
By the way, Capacitor coupling is not DC coupling.

Regards;

Doug

I know a lot of studio gear uses transformers for input/output coupling and especially the lundahl transformers have a very good reputation.

But as a übernoob i have two problems:
a) what transformer can be recommended
b) is this simple approach I took really feasable?

Here is a link to he schematics:

Click the image to open in full size.
The capacitors at the output are optional alternatives - therefore t looks a bit odd.
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Old 19th August 2007, 08:26 AM   #4
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What is the purpose of the transformers? LPF? Galvanic isolation?

A transformer somehow makes more sense in a current output dac.
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Old 19th August 2007, 09:28 AM   #5
_atari_ is offline _atari_  Germany
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Quote:
Originally posted by analog_sa
What is the purpose of the transformers? LPF? Galvanic isolation?

A transformer somehow makes more sense in a current output dac.

That is what I try to find out

The purpose of the transformer is twofold:
1. By using some configuration as sketched it should give DC protection - the same you can also achieve with some capacitors. My question is: Gives the us of a transformer some advantage?
2. The use of a transformer makes it very easy to configure balanced or unbalanced output - no need for opamps or stuff like this. I have a new versio of the schematics - I will post later which has jumpers to configure balanced vs unbalanced output.

The main aim was to get rid of DC which is always a problem with DACs.
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Old 19th August 2007, 10:57 AM   #6
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Quote:
Originally posted by _atari_

The main aim was to get rid of DC which is always a problem with DACs.

So do you intend using a gapped transformer then? This is already a significant compromise. Or do you think the differential offset can be ignored?
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Old 19th August 2007, 11:13 AM   #7
_atari_ is offline _atari_  Germany
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Quote:
Originally posted by analog_sa



So do you intend using a gapped transformer then? This is already a significant compromise. Or do you think the differential offset can be ignored?
oh sh*!

That is something I completely overlooked. You mean that I forgot to think about that both differential outputs can have different DC offset (e.g. positive on the positive and negative output) which can lead to a lot of problems if I just connect them with a wire (which goes through the transformer). So I would need a split transformer and connect OUTP and OUTN to ground?
Or would something like this help:

Code:
OUTP ---------
             )
             )
             )
GND ----------
             )
             )
             )
OUTN ---------
Are these kind of problems reasons that not very much people use transformers and stick to capacitors????
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Old 19th August 2007, 12:38 PM   #8
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Please don't get so excited. It's a Sunday

The offset may be mentioned in your dac's datasheet. For the AD1852 which i use it's about 50mV. I think this rules out most of the better transformers. Your dac may be more suitable. In any case look for a transformer capable of taking a few mV without collapsing.
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Old 19th August 2007, 12:53 PM   #9
_atari_ is offline _atari_  Germany
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Quote:
Originally posted by analog_sa
Please don't get so excited. It's a Sunday

The offset may be mentioned in your dac's datasheet. For the AD1852 which i use it's about 50mV. I think this rules out most of the better transformers. Your dac may be more suitable. In any case look for a transformer capable of taking a few mV without collapsing.
I had a week of holliday - so excuse my excitement
And thanks for your patience.

The DAC is a WM8740. It is a voltage output DAC. The datasheet tells nothing about DC offset. The output Voltage is said to be max 2.5V (VDD/2).

Anbody got experience with the WM8740?
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Old 19th August 2007, 01:25 PM   #10
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Quote:
Originally posted by _atari_


And thanks for your patience.


Nothing to do with patience. I am genuinely interested in your project.

At present i use a dac with a 2707 usb/i2s which is pleasant but certainly not spectacular. After a week of mulling over the 1896 datasheet i'm still not convinced that ASRC is the best way to improve upon the 2707 jitter-wise but it's certainly worth a shot.

The TAS1020 is probably a better solution if one manages to get eeprom contents and drivers off a commercial product. I haven't been successful.

With regards to the analogue stage, as already mentioned i use a DAC with voltage output which is obviously an easy cheap-skate solution. Once satisfied with the USB part i'll probably switch to a current output chip.

The transformer solution for your dac may very well sound great. Sadly, i am not in a good position to try it out. One of the transformers i have is a Lundahl LL1660/18mA and although it will easily swallow the offset it doesn't seem very suitable in other parameters. I also have an LL1544 which can take absoliutely no offset but will work as a parafeed. Don't know if it's worth trying it out.
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