How to specify a transformer for dac output?

[sharpi31]

I have ordered a Behringer SRC2496 and plan to replace the AK4393 dac with an AK4396.

AK4396 data sheet

I am keen to bypass the SRC2496 active output stage and take the balanced outputs from the dac straight into a Lundahl (or similar) transformer to produce an unbalanced output.

I have read that others have had success doing this, but have no idea how I would select a suitable transformer or how I would connect everything together.

Can anyone direct me to a site that shows a similar schematic or one that explains how I should pick and implement the transformer output stage?

Any advice would be greatly appreciated

 

[sharpi31]

Would a Sowter 3575 be suitable?

Sowter 3575 spec

Would I wire DAC AOUT+ to pin 1 and DAC AOUT- to pin3 - output would then be taken from pin 4 with pin 6 connected to ground...?

 

[lcsaszar]

I suppose you need a 1:1 transformer since it has differential voltage output.

 

[sharpi31]

Or would a Jensen JT-10KB-D be more suitable? With it's 4:1 ratio, would it attenuate the total 5.6Vpp output of the dac to give a standard unbalanced line level signal....?

Jensen JT-10KB-D

 

[lcsaszar]

Most CD player has 2V RMS output, that is 2.8V peak, that is 5.6V peak-to-peak. This is why I wrote 1:1 turns ratio. Do you want to omit the analog filter at the output?

 

[sharpi31]

Yes I want to take balanced audio from the dac chip to a transformer and unbalanced audio from the transformer to my preamp. No filtering (other than the HF & LF roll-off and CMRR of the transformer). No coupling caps either.

Thanks for your help

 

[sharpi31]

Sowter have recommended that I use their 3603:

Sowter 3603 data

Does this look suitable? I'll place an order in the morning unless I'm given a reason not to!

Thanks again.

 

[lcsaszar]

At the first glance it seems good, but probably you will need to load the secondary with greater that 600 ohms (say 10 kohms) and there might be some peak in the frequency response at the top end because of underdamping. The 3575 seems to be a better choice, but I'd ask Sowter what they recommend.

 

[David Davenport]

Here is an example.

http://www.raleighaudio.com/passive_output.htm

Dave

 

[sharpi31]

Thanks all.

I found the thread below and plan on doing the following:

Sowter 3603 transformer
500ohm in series with each dac output to give >1Kohm with the 160ohm DCR of the transformer (The dac spec sheet asks for 1Kohm minimum load for DC)
No additional load on secondaries - the 100Kohm load at the preamp input only.

DEQ2496 direct transformer out

Sound reasonable?

 

[lcsaszar]

The load you mention is referred to GND and it is an issue if the center tap of the primary is grounded. If you leave it floating, there won't be any DC load (both output pins centered at Vcom i.e. 2.5V, no DC current -> no DC load).

 

[sharpi31]

I've done a lot more reading since my posts above, so I thought I'd provide an example of how to calculate minimum transformer dBu for use with a Vout DAC.

Dac: AK4393 (found in Behringer SRC2496/DEQ2496 etc.)

Sum of differential outputs = 4.8V peak-to-peak

4.8V peak-to-peak = 1.697Vrms

1.697Vrms = 6.812dBu

Now all you need to do is find a transformer with acceptable performance at this dBu level.

What counts as acceptable performance is another question altogether - I've found 0.1% distortion at 50Hz at the specified dBu level to be fine, although it's no doubt better to use transformers with even less distortion in the bass.

Doing the sums above for the AK4393, I'm tempted to try the two Lundahl LL1527 transformers I have lying around. These claim to offer 0.1% distortion at 50Hz at 6dBu, but are still below 1% distortion at 50Hz at 16dBu, so I'm sure they will be perfectly happy with the 6.812dBu coming from this dac.

Of course these calculations only cover signal level - there is lot's more than this to specifying a suitable transformer (phase, loading etc.). That's next on my list to learn...

 

[sharpi31]

This page is great for doing all of the calculations required:

dB dBu dBFS dBV to volts conversion - calculator volt volts to dBu and dBV dB mW - convert dB volt convertor converter calculation online attenuation loss gain ratio reference audio engineering dBFS dBVU 0 dB audio logarithm level converter peak to p