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Old 7th June 2015, 08:58 PM   #1
surfstu is offline surfstu  United Kingdom
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Default Transformer Output Dac's - Yesterdays news?

With the help of the great minds on this forum I managed to implement Transformer output (UTC A-20's) to a CS4398 DAC a few years back.

I now have an AK4399 chinese (Weiliang) DAC and I like the sound but we always want to tinker and try to improve don't we...

So i started to search the web for transformer output on the AK4399 but there is much less of a fan base now on this topology... Very little info posted in the last 2 years!

Am I missing something or are transformer outputs no longer the way to go?

Stu
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Old 7th June 2015, 11:48 PM   #2
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Trafo outputs on DACs are definitely the way to go but driving trafos from opamps?
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Old 8th June 2015, 10:24 AM   #3
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Thanks for the reply abraxalito - probably me that is confused, are you saying that the AK4399 has opamps built in. and that they can not be avoided therefore making it unsuitable for transformer output?
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Old 8th June 2015, 10:42 AM   #4
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That may be what he is saying, but I have used several DACs of different brands - with opamp output stages tied to transformers. The results were always very good. Go for it.
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Old 8th June 2015, 11:18 AM   #5
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I'm saying that with opamps built in it would (as far as my experience goes) be desirable to either buffer the opamps first (to get the best PSRR from the opamp) or at a pinch, bias them into classA with an external CCS to avoid them creating more noise on the supplies from their classAB output stage. But on reflection perhaps you're using a trafo with a very high shunt inductance so the load current is insignificant, whereas I use el-cheapo ferrite trafos with lower inductance.

Just as a general perspective - with the attention to SQ that's implied by using trafos (meter readers in general don't have any kind of soft spot for trafos here on DIYA) why start out with a DAC that contains opamps (and is designed by the numbers rather than for SQ)? I'm curious to know the rationale.
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Old 8th June 2015, 12:53 PM   #6
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Does this mean that the AK4399 has opamps built in? I have UTC A-20 transformers and would like to use them to avoid the NE5344 opamp output stage...

Abraxalito, do you think i would not gain anything by doing this regarding SQ?

With the CS4398 chinese DAC board i Just found a place on the board to tap directly from the CS4398 before it used the opamp output stage, I was hoping to do the same thing with the AK4399 board from Weiliang.

Stuart
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Old 8th June 2015, 01:00 PM   #7
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Simply the isolation (hence common-mode noise reduction) that comes from using trafos makes them worthwhile in my experience. As far as I know, yes all voltage out DAC chips contain internal opamps.

I just took a look at the AK's DS - yes, see the block marked 'SCF' it means 'switched capacitor filter' - as far as I'm aware it has opamps, CMOS ones at that. Ditto the CS4398.
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Old 8th June 2015, 08:54 PM   #8
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Thanks for the encouragement, I checked the data sheet and see the switched capacitor filter. I presume that this then has pins

AOUTLP and AOUTLN for left channel
AOUTRP and AOUTRN for right channel

and I would simply connect the transformers to these pins using the diagram below?
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Old 8th June 2015, 10:39 PM   #9
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I would pull out all the resistors. You don't need or want series resistance between the DAC and transformer. Nor do you want the secondary loading resistor.

DAC data sheet specs regarding minimum resistance apply to DC conditions. With the transformer, you have no DC conditions, and the AC impedance is solely determined by the load. Ideally this load is very large, certainly not the 680 ohm resistors selected.

Been done many times before - take out all resistors.
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Old 9th June 2015, 12:00 AM   #10
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Definitely agree there - you don't want any series resistance. Perhaps they're included to ensure stability for the driving circuit - I'd substitute ferrite beads for the 1ks which add inductance but almost no series resistance. Also you don't want pure capacitance (the 4.7nF) but some resistance in series with this (say 10k) might do good as a primary snubber.
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