Transformer selection for DAC

oon_the_kid · 2013-09-20 4:07 am

Hi Everyone,

I am looking at out transformers for a DAC. Basically I am planning to modify a AKM 4396 voltage out DAC. I intend to feed the output voltage of the DAC chip into a buffer (direct without any capacitors) probably based on a Nelson Pass B1 design. There would be two output stage per DAC chip with regards to the differential output. The output will be fed a line output transformer.

I am looking out for a suitable transformer. Looked at a few websites, can't figure out which one to choose.

I am looking at a budget of about $100 for one pair. I could see Jensen being a good candidate but which model? I would like to have an amorphous core if possible, supposedly better, but I don't think it is really in the budget.

Any suggestions?

Oon

fandersen · 2013-09-20 5:01 am

I use Jensen JT-11-DMCF transformers on a CS4398 DAC. The Experience with this DIY DAC thread has a lot of discussion of various transformers from cheap to expensive.

oon_the_kid · 2013-09-20 8:39 am

Thanks,

That thread is too long to go through.. at over 400 pages... But found some nice references...

SOWTER DAC I/V TRANSFORMERS

Thanks.

Oon

petefrontiers · 2013-09-20 11:11 am

I use Jensen JT-11-EMCF on a AKM-4393 with very good results.

petefrontiers · 2013-09-20 1:05 pm

This is how I implemented it on my DAC. I used SE output circuit but it can be balanced if you want.

oon_the_kid · 2013-09-20 1:14 pm

Thanks Pete,

I was thinking of doing that too. But I reckon my load might be a bit heavy. So not sure if will overload the dac...

That is why I thought of using the fet buffer...

Oon

petefrontiers · 2013-09-20 1:22 pm

Correction. There is no connection to ground on the RCA output.

oon_the_kid · 2013-09-20 2:25 pm

Why did you choose this particular transformer. All the Jensen stuff looks so identical..

Ken Newton · 2013-09-20 3:11 pm

FYI - Nearly all amorphous core, or nickel alloy core transformer will require that you ensure there is essentially no (less than 1mV) D.C. offset applied to their primaries. If you will be utilizing a B1 style balanced JFET buffer, there will be too much D.C. offset at their outputs, necessitating either a hand trimmed compensation network or a D.C. blocking capacitor.

petefrontiers · 2013-09-20 3:16 pm

Ken Newton said:
FYI - Nearly all amorphous core, or nickel alloy core transformer will require that you ensure there is essentially no (less than 1mV) D.C. offset applied to their primaries. If you will be utilizing a B1 style balanced JFET buffer, there will be too much D.C. offset at their outputs, necessitating either a hand trimmed compensation network or a D.C. blocking capacitor.

Per previous testing, as been established that there is no DC offset at the output of AKM-439x DAC so it is optional to put a DC blocking cap before the transformer in this application.

petefrontiers · 2013-09-20 3:29 pm

oon_the_kid said:
Why did you choose this particular transformer. All the Jensen stuff looks so identical..

It was a decision based on price and which one was fitting inside a 1U case. JT-11-BMCF, or DMCF or EMCF or FMCF will do the job but each time you increase the first letter, the size and the price get lower and probably the performance in the lower frequency too.

Ken Newton · 2013-09-20 5:31 pm

petefrontiers said:
Per previous testing, as been established that there is no DC offset at the output of AKM-439x DAC so it is optional to put a DC blocking cap before the transformer in this application.

You're overlooking the following statement from the thread starter; "I intend to feed the output voltage of the DAC chip into a buffer (direct without any capacitors) probably based on a Nelson Pass B1 design."

The Pass B1 is a discrete JFET buffer. Variation in JFET Vgs parameter will be much greater than 1mV. So, even if there were zero D.C. offset from the DAC chip, there will be much more than a 1mV D.C. offset applied to the transformer primary unless cap. coupled or is otherwise nulled.

oon_the_kid · 2013-09-20 11:52 pm

Hi Pete,

I can understand that part about price point, but even within the same price point there are a few models sometimes. So is there a consideration in one of those graphs that makes you choose one over another?

Oon

oon_the_kid · 2013-09-21 12:34 am

Hi Ken,

Thanks for the tip. I originally wanted the transformer so as to avoid using a cap. Wondering which is the lesser evil. Looks like it can't be avoided a cap is a necessity.

For the load I will be having a passive ldr based voltage divided followed by a filter of sorts to compensate for the frequency response of my full range driver. Hence the heavy load. Probably to the tune of 1 to 2 kohm.

It's all still in the dreaming stage. I might put the transformer before the buffer and drive the buffer.. might be a better arrangement.. like pete's circuit.

Oon

abraxalito · 2013-09-21 2:07 am

To avoid needing a cap, you'd need to find a trafo with an airgap. I've made some myself using the kinds of ferrite cores used to make SMPSUs. The airgap is made by a single layer of sellotape between the two halves of the core. Works a treat as the output trafo on my DACs, despite having nowhere near the primary inductance of a 'real' trafo. Also much, much cheaper.

zigzagflux · 2013-09-21 1:33 pm

Lose the buffer. Go directly from the DAC output pins to the JT-11 series (I use the -DMPC). For optimal performance the higher in nickel content and the more core you buy (which means more $$$) the better the distortion and the more readily the transformer will pass 20Hz.
Do not use resistors between the DAC and transformer. Direct connect, no resistors or capacitors. I can provide some really impressive FFT plots from 20Hz to 20kHz showing this works extremely well.

petefrontiers · 2013-09-21 3:32 pm

oon_the_kid said:
Hi Pete,

I can understand that part about price point, but even within the same price point there are a few models sometimes. So is there a consideration in one of those graphs that makes you choose one over another?

Oon

Not really, I did not use the graphs to choose one over another. I just did what you are doing, I used the knowledge and the ears of other members here to choose a transformer that would work and sound good.

I would have chosen a DMFC but it was too tall for my enclosure. Cinemag transformers should work well also and are a little bit less expensive than Jensen's

http://cinemag.biz/output/PDF/CMOB-2.pd

Draki · 2013-09-21 4:40 pm

Do not use resistors between the DAC and transformer. Direct connect, no resistors or capacitors.

+ 1

petefrontiers · 2013-09-21 5:17 pm

zigzagflux said:
Do not use resistors between the DAC and transformer. Direct connect, no resistors or capacitors. I can provide some really impressive FFT plots from 20Hz to 20kHz showing this works extremely well.

Thanks for the tip. I will happily remove those resistors!

zjaakco · 2013-09-21 5:59 pm

output transformer

Hi, Oon
I have good experiens with the sowter 4383 trafo, used after the ak4396 dac.
First no power supply on the output.
and with this trafo a very high voltage output, adjusteble for a headphone.(or what else)
And indeed no risistors between the dac and the trafo.
I discover that for mine ears a 40K risister on the output of the trafo give
the best sound.
Ofcorse there are more possibilities, but
Sowter will help you with persenal wishes.
Good luck

Transformer selection for DAC

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