Pcm4222 evm experimentations with an input transformer

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hop.sing

Well-known member
Joined
Jun 21, 2005
Messages
160
Hi

I have a question regarding the analog input of the pcm 4222.
Just for the fun of it I placed a HQ 2:1 Pikatron transformer right into the pcm 4222 analog input. I bypassed the opamp in front of the converter and lifted the secondary of the pikatron with two 1k resistors 2.1v. I connencted the junction of the two resistors with output of vcomr of the opa227. Then I terminated the transformer with a 470 Ohm resistor for best frequency response. The 2.7n filter capacitor is still in the signal chain in front of the converter input. Measuring and listening I find no drawback from my breadboarding compared to the original configuration (still in place on the other channel). I have a little more 2nd and 3rd harmonics in the bass region, which is expected, but 500Hz upwards everything is very close to the other channel. the frequency respons up to 100k is very similar with the exeption of a very slight ringing at ca. 60k. In the audioband erverything matches nearly perfect.
Most interesting to me is that the sound of the transformer input seems to be slightly more natural for my ears. The signal sounds more integrated and the transients a little less harsh.
Now I need to know if there are serious drawbacks to that arrangement I am not aware of. I saw that on the jensen site http://www.jensen-transformers.com/as/as086.pdf that there is a similar cirquit, but with another converter chip. Of course the the transformer performance depends on the source impedance, but most of the time it will be coupled with the one preamp, I am testing it right now.
Thanks for help
Tobias
 
Great post! I'm thinking about adding a transformer to my  PCM4222EVM.

Can you elaborate why you connected the vcomr of the opa227 to the transformer secondaries? From reading the manual it seems to me the bias is only necessary for the differential op amp, so why connect it to the transformer?

Thanks!

Input  schematic attached for reference.
 

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