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Old 27th May 2003, 10:37 AM   #1
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Default transformer out

I've been fooling with a transformer output and have been having some problems. Suggestions?

The DAC is the no-oversampling, no output stage Convertus/Kusunoki 4x TDA1543 style, which outputs about 5mA current in the "no signal" state. Presently this has a 680R I/V resistor, a first-order lowpass filter cap in parallel to the resistor, and an output cap of 5uF.

I have some nice line transformers that I thought might replace the output cap. It's a nominal 600R input 1:1.5 line transformer, with a high signal rating, something like +27dbu. I thought that the high power rating would allow the transformer to accept some DC in the core, namely the 5mA of the DAC.

I left the resistor and filter cap, but replaced the output cap with the transformer. Result: no sound! Reason????
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Old 27th May 2003, 07:29 PM   #2
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Quote:
I thought that the high power rating would allow the transformer to accept some DC in the core, namely the 5mA of the DAC.
I might be wrong but as far as I know they don't like DC at all. At www.amplimo.nl I read this:

Never try to check the windings with an ordinary multimeter. The DC-current of the meter will magnetise the core!
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Old 27th May 2003, 08:29 PM   #3
tiroth is offline tiroth  United States
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These nice line transformers often have very high permeability (=> easily saturated) cores...does it work if you keep the output cap?
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Old 28th May 2003, 01:44 PM   #4
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It seems that trannies designed for DC are appropriate here. Thanks for the kind replies.
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Old 1st June 2003, 01:14 PM   #5
DWong is offline DWong  Hong Kong
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What is the DC supply voltage of your Dac? If you have a 8v supply, then 680ohm I/V would be fine but the DC would definitely saturate the tran's core.
An alternative is to use 5V supply with 221ohm I/V. Use a 1k variable resister for Vref. Then connect one of the primary to the output of the chip and the other to the Vref pin. Adjust the 1k variable resistor so that the DC offset across the primary of your trans is min. With this set up you would probably get musical sound out from your Dac. However, the bass would be a bit shame (due to DC). That's my experience with EDcor WSM 600/600 impedance matching transformer.
You might also try another alternative similar to parafeed configuration to block DC. Simply connect the output of the Dac to one of the primary with the other primary connect to a parafeed cap (I use 5uf) which is then connected to ground instead of Vref pin. The bass will be better in this configuration. I am now using this configuration. It works for me! I could also correct the phase of the Dac with the transformer.
Worth to try and see whether you like it or not.
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Old 1st June 2003, 02:48 PM   #6
DWong is offline DWong  Hong Kong
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Sorry Paul! I don't know what I have done that make your message deleted.

The reason I still keep the trans in the Dac is I found it has better resolution from top to bottom. The output cap was originally a 100uf non-polar BG.

It is however my personal preference only. I think Peter Daniel didn't like the results with Lundahl transformers. Would Peter also share with us your experience and how you connected the Lundahl to the dac?
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