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Circuit error in Lite DAC-AH: Easy fix

post #1 of 210
Thread Starter 
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.
post #2 of 210
Awesome, I had heard they used the wrong resistor values for the passive I/V. This post explains an easy way to remedy the problem. It explains why my DAC-AH fails the undial clipping test miserably.

Could you go into more detail on adjusting the VRef? Do you measure the output voltage just infront of C38 and C39 ?

Thanks for the great post!
post #3 of 210
Thread Starter 
Regal,

Yeah, just measure the DC voltage (with no sound input) on the top of C38 and C39. That's also AOR and AOL of the DACs. 3.3V will put the outputs in the "sweet spot" which is halfway between 6V-1.2V and 1.8V (the max spec output swing). You will probably see a slight difference between the left and right channels. Just set the average to 3.3V. If you want more margin use 330 ohm resistors instead of 470 ohms across R35 and R36. This will give an effective resistance of about 150 ohms, but will decrease the output level slightly.

I wasn't so happy with the sound of this DAC until I made this change. Good luck.
post #4 of 210
That's what you get for using passive I/V
post #5 of 210
Good news, thanks for the update Kim. In other threads people were reducing volume in foobar, or using dsp limiters, in an attempt to reduce distortion. Nice to have proper fix for it.

Edit: why did you use 2 resistors in parallel instead of removing the stock resistor and replacing with one ~160 ohm?
post #6 of 210
Quote:
Originally Posted by Garbz
That's what you get for using passive I/V
But I do not think that Kim Hardee did the passive mod on his Lite Dac-Ah.
post #7 of 210
Thread Starter 
So far I've just added the 470 ohm resistors in parallel with R35. R36. You can certainly replace R35,R36 with resistors in the 150 to 170 ohm range, but that would require getting to the backside of the board. Adding parallel resistors is much easier.

Actually, I plan to replace the LM7806 voltage regulators with LM7808s to provide 8V to the DACs, add DAC heatsinks and then bypass the opamps with 4.7uf Auricaps. That will give me almost 2V RMS max output (the standard).
post #8 of 210
Thanks for the heads-up. Do you think the cheap radio shack 1/4 watt resistors would be sufficient?

I have also considered switch out the 7806s for 7808s, but I worry about durability even with heat sinks. Let us know how it sounds when you make the changes.
post #9 of 210
Thread Starter 
I don't have any experience in how various types of resistors affect the sound, but there are opinions in various forums on this site. I plan to try Vishay resistors at about $6 each, but I'm sceptical that I'll hear any difference compared to other types. The 470 ohm resistors that I added were from Radio Shack, and I think the DAC now sounds fantastic. I'm sure you'll like the result.
post #10 of 210
Thread Starter 
I'm not too worried about the durability of the TDA1543s running at 8V. The data sheet specifies operation from 3V to 8V with a maximum allowable voltage of 9V. If a manufacturer specifies an operating voltage range, that means the part will be reliable over that range. That typically means the part can operate for 10 years or more if it's powered up continuouusly. Of course 6V operation will provide even longer use.
post #11 of 210
Ok so lets say i have a dac-ah with the 3 7806s... all i would have to do is switch those out to the 7808s ? Is there anything else i have to do other they swap those out?
post #12 of 210
Quote:
Originally Posted by pelayostyle
Ok so lets say i have a dac-ah with the 3 7806s... all i would have to do is switch those out to the 7808s ? Is there anything else i have to do other they swap those out?
The only thing would be to fine tune the trimpot to get the proper output voltage. All the other components (I/V resistor) are originally set up for 8V regulators on the schematic.

Edit: there are two 7806/8 regulators, not three.
post #13 of 210
Thread Starter 
That's right. Just remember that the correct DC output voltage for the DACs is 4.3V when LM7808s are used for maximum clipping headroom (although some think the best sound is at 3.85V).
post #14 of 210
Quote:
Originally Posted by TheMarsVolta
The only thing would be to fine tune the trimpot to get the proper output voltage. All the other components (I/V resistor) are originally set up for 8V regulators on the schematic.

Edit: there are two 7806/8 regulators, not three.
Where on the board is the trimpot that needs to be adjusted?
Also, where would i measure the voltage to make sure im getting the 3.85v as suggested?
post #15 of 210
Thread Starter 
The trimpot is located near the output jacks on top of the PC board. Mine is a blue rectangular shape with the trim screw on top, scured with some sort of white goop. Just turn it with a screwdriver and the goop will break free.

You can measure the DAC output voltages on one side of R35, R36 (the other side is ground, or you can measure on the DAC chips directly. I believe it's pins 5 and 7. I would suggest trying 3.85V and 4.3V and see if you hear a difference, especially on very loud complex sections of music.
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