Ok some more thoughts on installing coupling transformers.
First let’s recap:
Kevin states the Dac requires 200 to 400 ohms of load for I/V (Current to Voltage) conversion. The higher the load resistor the higher the voltage output. To high and you risk the Dac chips becoming unstable.
Now coupling caps: A coupling cap is used to block DC and interface with the input impedance of the pre/amp. If we drop the value to 1uf this will start to roll of bass in "certain systems". Both Kevin and Steve Nugent told me 4.0uf will be sufficient. In "stock form" we have a 10uf Mica cap, this may be "uf" overkill, but sonically it sounds great after break-in. So there! Changing this balance we get into what I will call fiddling with "Quantum Things”. (Ok for the most part these things are electrically explainable but trial an error provides quicker results than even the most perfect scientific modeling). The selection of a coupling cap and I/V resistor are also dependent on the pre/amp load … so depending on your gear, mileage may vary.
And if you think that is interesting…..
Enter the transformer:
First off we still need is to keep the I/V load below 400 ohms. If we add a 600:600 transformer, we are adding 400 ohms to the 600 ohm primary load of the transformer. That puts a 1000 ohm load on the Dac chips. Not good! Now you add in the fact that with transformers the secondary (pre/amp) “will” affect the already sub-optimal primary load of the transformer. Let the problems begin.
With a coupling cap, the load on the Dac chips stays constant (more or less). With the transformer it will change depending on the secondary load. This is the major difference. There are simple formulas to get us in the ball park “if” we know the pre/amp load. Assuming a 50k secondary load, math predicts a 50 ohm I/V resistor with a 50 ohm:50 ohm coupling transformer will keep the load on the Dac chips in their stable zone and not affect frequency response in the 20-20K range. However with transformers we do all this with a tradeoff = loss of output voltage! This may/may not be and issue depending on the pre/amp. We could counter this voltage loss with a 1:5 or 1:10 coupling transformer. These new transformers of course will react even differently with the secondary load. And round and round we go.
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Don’t get me wrong, I am all for continuing on with trying coupling transformers. However as you can see transformer coupling adds in a whole new can of worms. If we “know” the secondary load and the amount of voltage required than we can ball park the proper transformer? I imagine that a 1:1 50 ohm transformer will work for the majority of us, we simply play with the I/V resistors to dial things in. (If you don’t have the stomach for this kind of thing than I suggest sticking with cap/resistor swapping).
Let the games begin………………………………….
Lots of good info on transformers here:
Un title page
Check under the input transformer link. Silk offers an exceptional 1:1 50 ohm transformer for DAC’s. $72 each.
A group purchase may bring this price down substantially.
May be this will be my next step to do something with it.
