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Old 3rd September 2013, 11:47 PM   #3411
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Quote:
Originally Posted by agdr View Post
What DC offset voltage do you measure on the output of each channel with nothing plugged into the input jack? Should be around 3mV or so.

If the offset voltage looks OK one thing to try would be new output NJM4556A chips, however they are not likely to both be bad. Is the amount of distortion about equal on both channels? Also take a DC voltage reading from the metal shell of the gain switch (ground) to pin 4 of either NJM4556, then again from ground to pin 8 and see what you get (the power supply rails).
Hi,

finally I had the chance to measure DC offset and op-amp output: everything is OK, as specified in NwAvGuy web site.

I found something strange to me: distortion is almost gone if I removes the 9V rechargeable batteries and I drive the amp by power plug.

I'll try to replace the op-amp

northernsky
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Old 4th September 2013, 12:15 AM   #3412
agdr is offline agdr  United States
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Originally Posted by northernsky View Post
I found something strange to me: distortion is almost gone if I removes the 9V rechargeable batteries and I drive the amp by power plug.
A bad battery or a bad charging circuit on one battery could cause that situation. Make sure you get +/-12Vdc on pins 4 and 8 of either NJM4556A to ground while on A/C, and +/-8.5 to +/-10.15Vdc on those pins while on batteries.
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Old 23rd September 2013, 12:18 AM   #3413
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Ok, first of all, hi to all, first post here.

I have a weird issue with my O2 + ODAC + batteries. The thing is that it works flawlessly but I cannot turn it on only on batteries, it means, I have always to plug the charger first. Then I can remove the charger and it works on batteries as it would do with the charger...

Any insights? Thank you in advance!
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Old 23rd September 2013, 03:49 AM   #3414
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I figured it out. The batteries I chose are giving around 7.8V which apparently is not enough to power up the amplifier, because it works perfectly with some alkaline ones I found around which are providing 8.8V. I will search for another option.

Just in case somebody wanted to know.
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Old 23rd September 2013, 09:42 PM   #3415
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Help! I used a different LED, MV5364MP4A (yellow), as listed in the NwAvGuy's bill of materials. It is on order and soon to be en route for delivery. However, I just stumbled on the fact that it has a forward voltage of 2.1 V, which apparently has a large effect on the power management circuitry. I scourged through the O2 guide for a formula of some sort to see it is still within spec to no avail. What will happen if I do use this part?
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Old 23rd September 2013, 11:52 PM   #3416
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Originally Posted by sonichedgehog36 View Post
Help! I used a different LED, MV5364MP4A (yellow), as listed in the NwAvGuy's bill of materials. It is on order and soon to be en route for delivery. However, I just stumbled on the fact that it has a forward voltage of 2.1 V, which apparently has a large effect on the power management circuitry. I scourged through the O2 guide for a formula of some sort to see it is still within spec to no avail. What will happen if I do use this part?
False alarm! I misread the datasheet. I mistook the first red, RED, MV5064.MP4A, in the list for the standard one used in the O2 circuit, H-E RED, MV5764.MP4A. Luckily, this yellow one is still safely within the threshold, 2.1 V forward voltage, vs 2.0 V forward voltage.

Out of curiosity, though, how exactly does the circuit reference the LED for voltage? Is it simple multiplication, meaning I will have a slightly higher threshold voltage in my case, or something else?
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Old 23rd September 2013, 11:54 PM   #3417
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Originally Posted by sonichedgehog36 View Post
Help! I used a different LED, MV5364MP4A (yellow)
Try changing R5 from the original 270k value to 249k, Mouser part #270-249K-RC:

270-249K-RC Xicon | Mouser

The math details of how that power management circuit works are worked out here

O2 amp CRC, diode, cap, and heatsink mods

The original red LED actually runs at a measrued 1.8Vdc due to the low current RocketScientist/NwAvGuy is putting through it, even though the datasheet shows Vf = 2.0Vdc at the full test current of 10mA:

http://www.mouser.com/ds/2/143/ds300477-42398.pdf (opens PDF)

Similarly I'm guessing that your yellow LED will probably run at around Vf = 1.9Vdc when current limited by that series 40.2K resistor in the O2 circuit, rather than running at the datasheet value of 2.1Vdc @ 10mA.

Working through the math then the 249K R5 will yield a per-battery PM "on" voltage of around 8.28Vdc (vs. 8.33Vdc for the red LED and original R5 value) and an "off" voltage of 7.09Vdc per battery (vs. 7.07Vdc for the red LED and orignal R5).


If instead you leave R5 and the other resistors as they are with the original values, but use your yellow LED instead of the red LED, the net result would be the O2 shutting off sooner than it normally would as the battery runs down. Essentially not using all the charged battery and giving you less run time. The power managment "on" voltage would be 8.89Vdc and the "off" voltage would be around 7.54Vdc per battery. Might not be all that much of a problem.

Last edited by agdr; 24th September 2013 at 12:23 AM.
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Old 26th September 2013, 06:34 AM   #3418
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Quote:
Originally Posted by agdr View Post
Try changing R5 from the original 270k value to 249k, Mouser part #270-249K-RC:

270-249K-RC Xicon | Mouser

The math details of how that power management circuit works are worked out here

O2 amp CRC, diode, cap, and heatsink mods

The original red LED actually runs at a measrued 1.8Vdc due to the low current RocketScientist/NwAvGuy is putting through it, even though the datasheet shows Vf = 2.0Vdc at the full test current of 10mA:

http://www.mouser.com/ds/2/143/ds300477-42398.pdf (opens PDF)

Similarly I'm guessing that your yellow LED will probably run at around Vf = 1.9Vdc when current limited by that series 40.2K resistor in the O2 circuit, rather than running at the datasheet value of 2.1Vdc @ 10mA.

Working through the math then the 249K R5 will yield a per-battery PM "on" voltage of around 8.28Vdc (vs. 8.33Vdc for the red LED and original R5 value) and an "off" voltage of 7.09Vdc per battery (vs. 7.07Vdc for the red LED and orignal R5).


If instead you leave R5 and the other resistors as they are with the original values, but use your yellow LED instead of the red LED, the net result would be the O2 shutting off sooner than it normally would as the battery runs down. Essentially not using all the charged battery and giving you less run time. The power managment "on" voltage would be 8.89Vdc and the "off" voltage would be around 7.54Vdc per battery. Might not be all that much of a problem.
According to the Radioshack discharge diagram you referenced in your guide, it looks like these voltage thresholds would be more ideal. http://support.radioshack.com/suppor...h-9v-lodis.gif

In light of this, I would be of the opinion that raising the voltage range of the power management would allow the batteries to be drained within their optimal ranges, and maybe even improve the battery life by operating within the designer intended higher ranges. I certainly not an expert on this subject as you are, and your later comments in that article seem to conclude that a lower voltage floor and ceiling could be ideal with the necessary modifications.

And yet, I am now wondering if perhaps using this yellow LED has these additional benefits to bring to the table.
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Old 26th September 2013, 08:16 AM   #3419
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sonichedgehog36 - You are right, I calculated that 249K resistor to match up with RocketScientist / NwAvGuy's original high/low battery voltage points. The modification though with the wider high/low voltage range should give longer runtime and less chance of oscillation.
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Old 26th September 2013, 10:39 PM   #3420
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Another quick question regarding the negative voltage regulator: Could I simply drop in an LM7912CT in the place of a MC7912ACTG without worry about an increase of noise or stability? (There was an order error made by the Mouser customer service representative.) These seem to be competing products just from different manufacturers, but I am a little unsure.
Linear Voltage Regulator, Negative, 12 V, 1.0 A
Linear Regulator (LDO) - Single Channel LDO - LM7912 - TI.com
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