The Objective2 (O2) Headphone Amp DIY Project

[dewardh]

Originally Posted by RocketScientist

lots of interesting stories

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Personally, I'd just run a 12V transformer and avoid the issue altogether. Don't see the point of overvoltage on the regulator inputs . . .

[dewasiuk]

Originally Posted by RocketScientist

@dewesiuk, you are correct about the unloaded voltage. And the regulators may outlive you running at a 60C junction temp for a few hours a day. A lot of commercial gear runs parts much hotter than that. You don't even want to know what some of the junction temps are in a typical 55 inch LCD TV. Pioneer had an entire series of stereo receivers that ran the regulators so hot the PC board (over several years of the receiver being stacked with other gear and stuffed in a hot closed equipment rack or similar) degraded, turned black, and eventually the pads on the PC board failed before the regulators did.

For those wondering, I took a class on DFR--design for reliability. It was taught by the head of engineering for one the biggest DFR consulting firms in the country. He had lots of interesting stories to share.

Those things are rugged bastards haha. Another thing I noticed is that the electrolytic caps beside the regulators also warm up too(I'm guessing that's either because they're quite close to the regulators or their voltage is more than half the rated 35 volts?

[RocketScientist]

@dewardh, that's why the WAU12-200 is the primary part number in the BOM. But, for those with low line voltage, or who want to do sine wave testing of the O2, the WAU16-400 does have some slight benefits. Because there's a lot of ripple from the half wave supply under load, if you heavily load the amp or have low line voltage, the regulators fall out of regulation on the "troughs" (low points) of the ripple. This is true even when a DMM still reads a high enough unregulated DC voltage.

The good news is the O2 has such high PSRR even when the regulators drop out of regulation under adverse conditions with a 12 VAC transformer, it barely changes the measured performance of the amp and hence is very likely inaudible. But it's still not ideal.

The caps get warm just from the regulators. You can run a 79xx into thermal limiting and boil water with it and it will survive. Really.

[DubiousMike]

Quick BOM question - I placed my mouser order based on the BOM effective 11/16. I got an update from mouser a day or two ago that one the parts is not expected to be available until early January. (Specifically, the 270-2.74M-RC 2.74Mohms 1% Xicon Metal Film Resistor)

I noticed you have a new BOM up as of 12/2. Should I go ahead and cancel my pending order and reorder per the new BOM? I poked around a little, but didn't find anything explaining the BOM update. Was this just based on part availability? Thanks!

[dewardh]

Originally Posted by RocketScientist

But, for those with low line voltage, or who want to do sine wave testing of the O2 . . . Because there's a lot of ripple from the half wave supply under load, if you heavily load the amp or have low line voltage, the regulators fall out of regulation on the "troughs" (low points) of the ripple.

I'd address low line voltage at the source, since it would negatively impact everything else (except incandescent bulb life) as well. And I confine sine wave testing to the bench, where I know I have adequate power. The "troughs" are most easily addressed with bigger caps (the physical size of which should not be an issue for an AC powered application).

[agdr]

Originally Posted by DubiousMike

Quick BOM question - I placed my mouser order based on the BOM effective 11/16. I got an update from mouser a day or two ago that one the parts is not expected to be available until early January. (Specifically, the 270-2.74M-RC 2.74Mohms 1% Xicon Metal Film Resistor)

I noticed you have a new BOM up as of 12/2. Should I go ahead and cancel my pending order and reorder per the new BOM? I poked around a little, but didn't find anything explaining the BOM update. Was this just based on part availability? Thanks!

RocketScientist reduced R25 from 2.74M to 1.5M and R9 from 40.2k to 33k. He wrote that the old values work OK but the new values provide slightly better performance of the power management circuit in low battery conditions. So yes, nuking your pending order and re-ordering from the new BOM would be a good way to go.

Originally Posted by Turbon

It thought 20VAC was the max...

Brgds

AC vs. DC there. 20VAC coming in from the transformer will be somewhere around [20Vac * sqrt(2)] - 0.7Vdc[diode drop] = 27.5Vdc for a half wave rectifier, with no load, since the caps charge up to more-or-less the peak of the AC waveform. That is the DC voltage coming into the rectifier then, of course. Add a load to that and you will get somewhere around (27.5Vdc * 0.9) = 24.7Vdc for an average value, depending upon load current.

[RocketScientist]

@DubiousMike. You don't need to cancel the entire order but you can cancel just the 2.7M resistor if you want and have a source for any resistor in the 1.2M - 1.8M range--even a 1/4 watt (stand it on end). It's not part of the audio circuit so it's not a critical part. You can also leave R9 at 40K if you want. I explained the 11/30 posts here that same day in this thread. The 12/2 changes were also explained here on 12/2 and involved increasing C6 and C7 to 1.0 uF just as a precautionary measure. I don't have any evidence there's any benefit to that change. You can use the same cap as for C1 in the C6 and C7 locations by re-bending the leads or the part in the 12/2 BOM with the correct lead spacing.

[cheapskateaudio]

Originally Posted by RocketScientist

I have both contact and IR thermometers--both by Fluke and very accurate.

I don't know what "draws 35ma, and the regs have 15ma at the outputs" means. They should be the same number. And if your amp is drawing 35 mA idling with no batteries, something is wrong or your mods have really messed up the amp.

At 35 mA and 24 volt unregulated, you get 24-12 = 12 volts drop and .035*12 = 0.42 watts not 0.615 watts.

The junction temp rise would be, worst case, 0.42 * 60 = 25C. So even with a warmish ambient of 30C that gives 55C junction temp. By definition the case temp has to be lower than the junction temp. Your 77C is way out of line. So either something is wrong with your measurements, your regs are damaged and dumping a bunch of current through the ground terminal (very unlikely), or you need a better thermometer.

As they have posted, many others are using the O2 with the HD650 and similar headphones, including me, and are very happy with the sound and nothing gets uncomfortably hot or overheats.

I suspect one or more of your mods has messed up your amp. It may well be oscillating at ultrasonic frequencies which would explain your bad sound complaints and high current draw. I would suggest restoring it to the default "stock" build and then you should get the same great performance as the rest of us. If not, there's still an error with your build.

Do you not know how your own amplifier works?

Not trying to be an *** but I put some numbers up that were basically inline with other numbers. So I found your response a tad smarmy.

The reason I ask about the case temps is that if you aren't calculating the specific heat capacity for the volume of air in the case, along with the thermal conductivity of the case then you're just assuming what the temperature of the case will be.

[RocketScientist]

I'll let others comment on that one.

[sofaspud]

Somewhere it has been previously written that the O2 is "a datasheet circuit with a Wikipedia output." So detractors have therefore chosen to focus on the relatively unconventional split supply to attack the design and designer.
CSA, it really does lean toward clueless. The reference is to the case of the regulator, the plastic part with leads and tab sticking out, not the B2 or whatever is used to enclose the amp.

I think these detractors would like nothing better than to make the O2 a banned topic here or anywhere else.