Haha, i wonder if he will...
From his website:
"NO SNAKE OIL REQUIRED: Many audiophiles want to believe more elaborate or exotic DACs offer higher fidelity. The ODAC demonstrates you do NOT need any of these for 100% transparent performance:
- Elaborate and/or High Current Power Supplies"
Roger that on the NC4MB and resistor. Thank you.
Debugging the Salas BiB
Hi,
I've built the +5 and -5 version of the Salas BiB as per the "BibGuideRev2.pdf".
The +5 is working great. The -5 however...is a problem. What I am having a bit of difficulty figuring out due to the closed loop feedback is what's causing what. Here are the symptoms:
Input voltage is -16.6
Maximum output voltage is -3.7 (range of pot is -3.2 to -3.7 at R203)
D202, D203, D204 are normal brightness
D205,D206 are quite dim
Voltage at R203 is -3.55
Voltage at anode of D206 is -70mV
Voltage at collector of Q204 is -3.7
Adjusting the 2K pot brings no change in voltage. It appears the excess voltage at the gate of Q206 (same as collector of Q204) is causing the problem? I've replaced Q204, Q205 and Q203. D205 and D206 are functioning fine. To test, I clipped a 2.2K with 5V supply across each and they are at normal brightness.
I should also indicate that the load resistor is 75 ohms. I'm trying to achieve about 130mA current for a load of about 65mA. The R201 used is 10 ohms. These same values work fine on the +5 supply, and I'm trying to get the same shunt and load current here.
Thanks for your help!
Gary
Hi,
I've built the +5 and -5 version of the Salas BiB as per the "BibGuideRev2.pdf".
The +5 is working great. The -5 however...is a problem. What I am having a bit of difficulty figuring out due to the closed loop feedback is what's causing what. Here are the symptoms:
Input voltage is -16.6
Maximum output voltage is -3.7 (range of pot is -3.2 to -3.7 at R203)
D202, D203, D204 are normal brightness
D205,D206 are quite dim
Voltage at R203 is -3.55
Voltage at anode of D206 is -70mV
Voltage at collector of Q204 is -3.7
Adjusting the 2K pot brings no change in voltage. It appears the excess voltage at the gate of Q206 (same as collector of Q204) is causing the problem? I've replaced Q204, Q205 and Q203. D205 and D206 are functioning fine. To test, I clipped a 2.2K with 5V supply across each and they are at normal brightness.
I should also indicate that the load resistor is 75 ohms. I'm trying to achieve about 130mA current for a load of about 65mA. The R201 used is 10 ohms. These same values work fine on the +5 supply, and I'm trying to get the same shunt and load current here.
Thanks for your help!
Gary
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Salas;
R201 voltage drop is 0.5V
Both Q201 and Q206 are the types you stated
I've reviewed my schematic over to ensure consistency with your design, and see no problems. The PCB was made from the schematic, so there shouldn't be any issues there.
Gary
Improved, better, but still not quite there.
I changed R201 to 5 ohms. The ref LED's are now at "normal" brightness. I can adjust the voltage from -4.85 to -5.4, which is perfect. However, R201's drop is just 370mV, which means I am getting a meager 70mA, a bit short of the target of about 130mA. Here are some other voltages:
Q202 Vs = -7.47V
Q204 Vb = -0.63V
Q206 Vg = -4.7V
Gary
Okay. When you say "4th LED position", is that a specific LED?
If the LED drops total across D302, 303 and 304 for 5.1V, it means that I need to add another LED to create a greater difference, and drop across R201?
Please elaborate. Thanks!
Hi Salas,
I did the schematic capture and made the PCB. I have only 3 LED positions.
Looks like I need 4 ?
Gary
Okay.
I guess it's wrong to assume that the Vgs of the power FETs are close. Looks like they can vary by enough margin that one has to accommodate for this in the design.
Thank you for your kind assistance Salas,
Gary
Salas,
Using green LEDs has fixed the problem on the -5 supply. I paralleled 2, 10 ohm resistors and there is 140mA total current, 67mA to the load.
Now the -15V regulator is giving me grief. LEDs D205 and D206 won't even light up, and the maximum voltage I can get is -2.9V
The -15V PCB is identical to the -5 now, except that the -15 uses a 5K trimmer and a 1.8K for R203. Across the shunt resistor I am getting 0.73V, so with 5 ohms this is 150mA.
Can you please provide a suggestion as to what the issue might be?
Thanks.