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Old 11th February 2012, 05:09 PM   #2081
agdr is offline agdr  United States
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Quote:
Originally Posted by Tawz View Post
Running the amp from batteries, however, nets me 9.1 Volts on both output channels (measured from p2).
Make sure that the orientation of the band on D2 and D6 matches up with the symbol on the PC board. Also try re-heating the solder joints on those even if they look OK.

With the AC adaptor and headphones unplugged, batteries in and the O2 power switch on, what voltage do you measure from ground (shell of the gain switch) to the battery terminal closest to the edge of the board and to the terminal farthest from the edge of the board?
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Old 11th February 2012, 07:26 PM   #2082
jfrana is offline jfrana  United States
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I do know that a click and thump is expected on turn off and on (as per documentation), and while it is not startlingly loud, it is a solid click and thump. Voltage measurements are close to expected (if a tad low throughout, especially U2-p7 at 8.5v). Hopefully it is nothing to worry about. Voltage at output is under 4mv, with ac, no batteries, and no input. Measuring voltage at on and off, the most extreme value I have seen was -7 volts at turn off which quickly comes below a volt and decreases to < 10mv within a minute.

All good? Would you risk $1000 headphones without further testing?
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Old 11th February 2012, 07:42 PM   #2083
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Originally Posted by evanft View Post
Is there a way to tell given an output V for my source and my headphones specs if the O2 will clip at gain level X, Y, etc.? I did some calculations using that spreadsheet that's floating around and it looks like I need a gain of 3.2 for my source (2.16 V) and my headphones (DT880 600 ohm).
The calculation is quite simple:

The NJM2068 cannot swing to more than about 1.2..1.5 V of either rail (datasheet says 1.5 V), so the maximum allowable gain stage output amplitude is approximately
Vmax,p-p = (V+ - V-) - 3 V
with V+ and V- being your supply voltages.

For sine output this means
Vmax,rms = Vmax,p-p / 2.83

The maximum allowable gain is
Gv,max = Vmax,rms / Vsource,rms ,
with Vsource,rms being maximum source RMS output (2.16 V in this case). (Note that for digital sources like CD players, very "hotly" mastered material may produce peaks up to 1.5..2 dB above nominal 0 dBFS amplitude, so 10..30% of extra headroom can't hurt.)

With a gain of 3.2, you'd normally be fine during mains operation (+/- 12 V) but would run into clipping when running on batteries (+/- 8.4 V .. +/- 9.45 V typ). I'd recommend a low gain of 2. High gain would depend on whatever you have in terms of portable devices; 6x is fine for a Clip+ or similar (you won't get more than 0.80 Vrms out of that whatever happens), "Euro-capped" players may require more than the maximum 8x (18 dB).

Last edited by sgrossklass; 11th February 2012 at 07:47 PM.
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Old 11th February 2012, 08:01 PM   #2084
Tawz is offline Tawz  Canada
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Thanks agdr and sofaspud! I had soldered D6 in backwards. DC output levels while running from batteries are below 6mV.

I did notice whilst performing the low voltage shutdown tests that output DC spiked to around 1.5V, but then quickly settled down around 164 mV. Is this within acceptable limits?
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Old 11th February 2012, 08:56 PM   #2085
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Originally Posted by Tawz View Post
I did notice whilst performing the low voltage shutdown tests that output DC spiked to around 1.5V, but then quickly settled down around 164 mV. Is this within acceptable limits?
Seems quite high (the figure quoted in another thread was 50 mV max). Such a peak would be about 40 dB above my normal listening levels.

Are people plagued by dead regulators lately or what? (Shouldn't appear when running on batteries then, however, so I don't know what's up here.)

Last edited by sgrossklass; 11th February 2012 at 09:03 PM.
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Old 11th February 2012, 09:06 PM   #2086
agdr is offline agdr  United States
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Originally Posted by jfrana View Post
Measuring voltage at on and off, the most extreme value I have seen was -7 volts at turn off which quickly comes below a volt and decreases to < 10mv within a minute.

All good? Would you risk $1000 headphones without further testing?
Whoa! No, that doesn't sound right, if that -7.0V is on the output jack going to the headphones. I would expect maybe 100mV at most, dropping down to 3mV is a few milliseconds. I have to run right now, but best to hold off plugging in the phones until me or someone else here can help you with that.
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Old 11th February 2012, 10:29 PM   #2087
jfrana is offline jfrana  United States
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The -7V was the most extreme I saw on P2, from square to farthest, when holding black probe to black and red to farthest of P2 -- turning off with headphones UNplugged. After a few seconds it falls below 200mv and within a minute of holding probes is below 10mv.

With headphones plugged in it is an entirely different story. The highest I saw was -170mv and dropping to 0 within a second of turning off. The turn on max was less than -100mv and dropping to 3.4mv within a second.
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Old 12th February 2012, 02:03 PM   #2088
agdr is offline agdr  United States
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Originally Posted by jfrana View Post
The -7V was the most extreme I saw on P2, from square to farthest, when holding black probe to black and red to farthest of P2 -- turning off with headphones UNplugged. After a few seconds it falls below 200mv and within a minute of holding probes is below 10mv.

With headphones plugged in it is an entirely different story. The highest I saw was -170mv and dropping to 0 within a second of turning off. The turn on max was less than -100mv and dropping to 3.4mv within a second.
I better make sure I'm understanding your test correctly. Is it the case that with the amp on and the headphones unplugged the voltages you measure on P2 are around 3mV, but then after the power switch is turned off you got that -7V, which then dropped down to zero? On my first read I was taking that -7V to be on P2 with the amp turned on.

The voltage you would want to measure is the voltages on P2 with the amp having been on for several seconds and the headphones unplugged. Should be somewhere around 3 - 8mV. If you are getting that on both channels then your O2 is passing the output DC bias check. RocketScientist has the 8mV figure in his instructions. I've been lucky and seen around 3mV on mine. That comes down to differences in op amp batches.

-7Vdc on the headphone jack continuously with the amp on could be enough to damage headphones if applied for several seconds.

Try reheating the solder joints on both ends of R12 and R13 regardless of the voltage measurements. Couldn't hurt.

Here is another test to try. Ideally the power supply rails after the mosfets should drop at about the same rate and in the same way. With the headphones unplugged take a look at the voltage from ground to pin 4 of U3 and compare what it does with the voltage between ground and pin 8 of U3 when the power switch is turned off. Then make the same comparison when the power switch is turned on. The polarities will be opposite, but the rate at which the voltage rise or drop on the two pins should be about the same. You shouldn't see something like the voltage on pin 4 coming up almost instantly but the voltage on pin 8 taking 5 seconds, etc.

Last edited by agdr; 12th February 2012 at 02:26 PM.
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Old 12th February 2012, 03:38 PM   #2089
agdr is offline agdr  United States
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Originally Posted by Tawz View Post
I did notice whilst performing the low voltage shutdown tests that output DC spiked to around 1.5V, but then quickly settled down around 164 mV. Is this within acceptable limits?
I would agree with sgrossklass, that 1.5V sounds on the high side. I don't have any ideas there at the moment though.

Just curious - what part numbers do you have in for Q1 and Q2? There were several options listed in the BOM.
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Old 12th February 2012, 07:05 PM   #2090
jfrana is offline jfrana  United States
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Yes the -7v was a surge for a moment. On turning off, with probe on P2 and headphones unplugged, the DMM has -5v (or -6.5 or 2.1) immediately on turn off and is below 200mv within a couple seconds. On turn on, P2 comes to 3.4mv within a second and holds steady, never to rise above 4mv until the surge at turn off.

U3, pins 4 and 8 on turn on nearly immediately rise to -11.7 and 11.7. At turn off, both drop below 1v within a few seconds. (Same observation from U4, pins 4 and 8).
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