Valab NOS DAC - subjective listening and modifications - Page 108

Thanks marcelnl,

Yes pat put one in the digital signal path, spidf input, based on his positive report I have ordered a precision, to do likewise.

In the long run, I think the Texas are more seductive.

Guys I am having an issue with adjusting the output Voltage. The best I can do is about 1.5 - 1.6 VAC, but according to all reports the output should be 2.0-2.1 when playing a 1kHz 0DB sine wave.

I have replaced the VR500R thinking it was defective - got same results.
I went from 390R IV to 340 IV resistors and still got same results.

Any ideas for a fix?

it could be due to measuring technique, what are you using ?
Some older devices only show ~ 1.6 times the full peak to peak while measuring a sinus. I favour a scope for this sort of measurement.

If the output really is low it could be due to the supply voltage of the DACs as that affects the output (as well as a one or more DACs being defective could affect the output current and through that the output voltage).
Can you measure the bias?

Just my two cents, maybe there are real electronic buffs here that have a more precise hint.

Just curious, did he also exchange the 2k7 at the 5Vdc and 1k resistor to ground and did it affect quality?

Strange, the 390r should give you higher output voltage, but at the risk of some distortion. I think the rating of 2.0 is for peak output.

What is the problem with your measured voltage, are you having difficulties driving downstream gear?

No nothing like that he replaced the 110r coax impedence resistor. Don't know why this is not 75r?

Oh yes it gave more clarity in this position

thanks for that info!
I was aiming at the 2k7 2k7 and 1k resistors between the 74ac74 IC and the DAC carrying the data signal and the 5Vdc supply.
That a resistor in the the s/pdif network would make a difference seems to be expected (in the diagram it shows as 75 Ohm, not sure why they would deviate..... perhaps they ran out? it should not matter that much anyway if it is 110 vs 75 Ohm imho as hardly any cable is exactly 75 Ohm)

I am getting what appears to be distortion and I am wondering if that is the issue. I need to troubleshoot more.

Older devices...you mean a 1st gen VALAB like I have?

The LT1048 has been dialed in to 7.65 VDC.

mrdon, Try this,

When you adust the trimmer, turn it clockwise until it just distorts, they turn it anticlockwise and this time count the half rotations, until it distorts again.

Then turn it clockwise again exactly half the number or half turns you counted. This way you will know when it is exactly at optimum.

I was aiming at the measurement equipment, especially an analog voltage measurement device is no good at measuring peak to peak voltages and will display a much lower voltage than the actual peak to peak value.

the 7.65 Vdc sounds allright, perhaps one of the DACs is defect?

Wood, just curious, what equipment do you have hooked up when you did your comp?

hifi2001, yes this is very relevant, my gear tends to lean to the bright detailed, very fast side, with a charlize2 amp with full blackgate fk mod, all solid silver speaker and interconnect wire. Perhaps the PRP are too much of a good thing there. Though I will try them again in the Valab and the Chameleon.

Pat, of course this is a direct comparison, of 2 resistors that excel in there own rights, its hard to be very critical of either. Its good to have the choice of the two, for system balancing. The PRP peps the Valab up in a natural way, Texas, way more fluid, good if the rest of the set up is already very energetic IMO. Will try them again but give them a longer burn in. Just so important to get that IV thingy right!