Transformer DAC Experiment

[pftrvlr]

I have this Cs8416-CS4397 DAC board for some time now. Among my 6 DAC, it is not the most listened one. I have read about using transformers with voltage output DAC and become interested. Here is what I have done over the weekend.

Actually this is the second iterations of the transformer DAC. The first one was a direct output from the transformer without opamp buffer, the second one is buffered. The direct output version has a very balanced and nature sound but lacking some dynamics and not as involving. The buffered version is much better when a right opamp is used. I tried different opamp for the buffer, and settle on LME49722. I am quite surprised that the sound of this DAC is very sensitive to the output opamp.

The DAC is installed in a DVD player case, all power supplies are from SMPS. The transformers are Edcore 10K/10K line.

I will let the DAC playing for a few days, then I will compare it to my other DACs.

[Pano]

Very nice! Good job.
I know very well those transformers with that chip. If you go with better transformers (Jensen, for example) you'll get a much more engaging sound and better dynamics. Maybe enough to drop the opamps - maybe not.

Also try about 3.3K across the primary of the transformers, if your circuit does not already have them. It presents a much flatter impedance curve to the DAC output.

Keep up the good work, have fun!
.

[pftrvlr]

Hi panomaniac,

I will try 3.3K resistors.

I have one question. Based on your experience, what impedence of th tranny is better? I guess it depends on the DAC chips. I want to try transformer on a PCM1704 chip also. Maybe a 600/600 tranny is better for current output DAC?

[WithTarragon]

A couple of questions on the Edcor transformers.

1. Why the choice of the PC 10K/10K? As opposed to the 600/600 for instance?

2. Are you letting the transformer's roll off serve as the anti-aliasing filter, or is there an RC filter in there somewhere?

[pftrvlr]

I happen to have a pair 10K/10K at hand, and thinking CS4397 likes a lighter load. I have seen someon using 600/600.
I don't have any filter in the circuit. My application is tube amp headphone system. I don't hear any digital noise even in NOS DAC without filter. Maybe it is just my ears.

[binspaul]

Do you have the schematics of this DAC ?

[Bill Fuss]

Originally Posted by pftrvlr

Hi panomaniac,

I will try 3.3K resistors.

I have one question. Based on your experience, what impedence of th tranny is better? I guess it depends on the DAC chips. I want to try transformer on a PCM1704 chip also. Maybe a 600/600 tranny is better for current output DAC?

600ohm trafos are not usable for I/V conversion, there are a number available that are made expressly for this purpose. They are generally 18-20/1 step up trafos with very low primary impedence. If you try some let us know how they sound.
The 600s are not ideal for voltage out dacs, but they are much better than higher impedence trafos such as 10k. A little series resistance will keep from overloading the chip outputs.

Best, Bill

[Pano]

The impedance of a transformer depends on a number of things. I don't understand them all, but do know some of them.
AFAIK, the Edcor 10K:10K and the 600:600 are both wound on the same core and same bobbin. They are both 1:1 transformers. There are just some slight differences in the windings to optimize them for different impedances.

The impedance seen on the primary mostly depends on the impedance on the secondary x the turns ratio and the inductance of the transformer. Just because the transformer is labeled 600 or 10K or whatever, does not mean that is its impedance. I will try to post some impedance curves later.

Most of the voltage output chips like a load of about 1K or a little higher. They seem to work OK over a vast range, but are usually spec'd at 1K.

Because you probably have a load of 20K-50K on the transfo secondary (your amp or preamp) that's close to what will be seen on the primary of your 1:1 transformer. I've found that putting about 2.2-3.3K across the primary flattens the impedance curve nicely to near 1K when the transformer is loaded with the next device - amp, preamp, etc. 1K is not essential, just the target neighborhood.

With transformers, you don't need any low pass filters. Not enough of the digital hash gets thru to bother anything. That's what is so nice about them.

Have a look at the Jensen JT-11-YMPC. Or for a few dollars more, the JT-11-HFMPC rated for higher voltage. These are the best off the shelf transformers I have tested. Very nice. Won't break the bank.

There you are, far more that you ever cared to know!

[AdamThorne]

Originally Posted by Bill Fuss

600ohm trafos are not usable for I/V conversion, there are a number available that are made expressly for this purpose. They are generally 18-20/1 step up trafos with very low primary impedence. If you try some let us know how they sound.

Hmmm I'd be interested to hear about this also...

[revintage]

I happen to have a pair 10K/10K at hand

Hey pftrvlr,

Those together with a parallelled resistor is the way to go. Also try different secondaryloadings. If you like it you may invest in a pair of Lundahls or Slagles. Using 600/600 with series resistors is not recommended.

What differs in the 10k:10k and 600:600 is inductance. The 600 has the smaller inductance, normally in the ballpark of 2-4H which means a reactance of ca 50 ohm at 20Hz. A 10k might have up against 100H, 13k at 20Hz.