Kim Hardee
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- Dec 5, 2001
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I recently purchased a Lite DAC-AH and enjoy the sound except on loud complex passages where it seems to distort very audibly. I've read elsewhere in this forum that the recent Lite DAC-AHs (maybe all of them) have 6V going to the TDA1543s instead of 8V as shown in the schematic and on the PC board. As stated elsewhere, if 6V is used then the 270 ohm resistors (R35,R36) on the outputs of the TDA1543s are incorrect. They should ideally be about 163 ohms. Using 270 ohms will cause the outputs to exceed the 1.8V to 4.8V maximum output voltage swing of the TDA1543s for loud music. With about 163 ohms the maximum output voltage swing will not exceed the limits.
To test this I ran RMAA on my DAC-AH as is and measured about 2.2% harmonic distortion minimum after tweaking the Vref adjustment pot. I then added a 470 ohm resistor in parallel with R35 and R36 to effectively change the 270 ohms to 171 ohms. Then I re-ran RMAA and found that the harmonic distortion was reduced to about 0.1%. I also found that the audible distortion on loud passages was gone. I side effect is a slight reduction in output voltage.
I haven't done any extended serious listening yet, but so far the change sounds as good, if not better, than before for soft to medium-loudness music and much better for loud music.
I strongly recommend that other owners of the DAC-AH consider this change, which is very easy, or to replace the LM7806 voltage regulators with LM7808s and run the TDA1543s at 8V. Then the 270 ohm resistors will be fine.
The full change is to solder a 470 ohm resistor across R35 and a 470 resistor across R36. Then adjust the VREF trimpot until the outputs of the TDA1543s are at about 3.3V. That's it.
If you decide to change the LM7806s to LM7808s, then don't add the 470 ohm resistors and adjust the outputs to about 4.3V.
I hope the dealers that are selling the DAC-AHs take note of this and make the change on units that they ship.
To test this I ran RMAA on my DAC-AH as is and measured about 2.2% harmonic distortion minimum after tweaking the Vref adjustment pot. I then added a 470 ohm resistor in parallel with R35 and R36 to effectively change the 270 ohms to 171 ohms. Then I re-ran RMAA and found that the harmonic distortion was reduced to about 0.1%. I also found that the audible distortion on loud passages was gone. I side effect is a slight reduction in output voltage.
I haven't done any extended serious listening yet, but so far the change sounds as good, if not better, than before for soft to medium-loudness music and much better for loud music.
I strongly recommend that other owners of the DAC-AH consider this change, which is very easy, or to replace the LM7806 voltage regulators with LM7808s and run the TDA1543s at 8V. Then the 270 ohm resistors will be fine.
The full change is to solder a 470 ohm resistor across R35 and a 470 resistor across R36. Then adjust the VREF trimpot until the outputs of the TDA1543s are at about 3.3V. That's it.
If you decide to change the LM7806s to LM7808s, then don't add the 470 ohm resistors and adjust the outputs to about 4.3V.
I hope the dealers that are selling the DAC-AHs take note of this and make the change on units that they ship.