The Objective2 (O2) Headphone Amp DIY Project

[Davey]

Originally Posted by sofaspud

Whoa. These should each measure about 0.7 volts across.
I think you are measuring wrongly.
Have you done the inspection? Are the components oriented correctly? This has bitten more than one O2 builder.

Those are rectifier diodes. They won't measure 0.7 volts constantly since they're not always forward-biased.

With a 12VAC transformer you'd probably measure somewhere near 20VDC across either diode.....but it's actually not a constant DC you're measuring.

For troubleshooting the power-supply section of the O2, best to attach a DVM black lead on the ground point and move around with the red lead. (Power switch in the off position.) It should be easy to find the malfunctioning component.

Cheers,

Dave.

[archro]

Hello everyone, I got a problem when I try to build my second O2 AMP, when I plug the new O2 with AC power, the voltage across BT1 is 12V, but the voltage across BT2 is 21.3V I read these numbers while the SW1 is off.
Does this means there are some issues with U6?

[sofaspud]

Originally Posted by Davey

Those are rectifier diodes.

Okay. I don't want to disagree with anything in your post.
But you're directing it to the wrong guy.

[randomlf]

Originally Posted by ericj

My self-built O2 was working quite well. I am an experienced circuit builder so this came as no surprise to me.

And then i took it on a trip. And the wall transformer i brought with me turned out to be 12vdc, and i didn't realize that mistake until I'd tried to charge and/or power the O2 with it for some hours.

Woah. I was lucky, I did the same thing with 12vdc adapter, but nothing went broken with the O2. Still works 100% with batteries. Haven't tested with AC adapter though.

[Davey]

Originally Posted by sofaspud

Okay. I don't want to disagree with anything in your post.
But you're directing it to the wrong guy.

No, I didn't....I was directing it at you since your comment needed the correction.

Those rectifier diodes will not measure 0.7 VDC across.

Cheers,

Dave.

[agdr]

Originally Posted by ericj

Thanks. I'll debug this weekend.

Whoops. randomlf's post above has a good point. I was assuming the O2 didn't have batteries, but re-reading your post I see it does. Things get more interesting with batteries. I'll change my guess from a dead regulator chip to a damaged battery and possibly one or more dead op amps. Would be good to test your headphones too on a known good source to make sure there is no DC damage.

With batteries installed a tip-positive DC adapter would still power the positive rail by the action of the O2's diode logic, and charge/maintain the positive rail battery, with the U5 regulator running in dropout. But now the negative rail would be held up by the negative battery until it discharges to the point where the power management circuit trips the mosfets and shuts the whole thing down.

But the PM circuit wouldn't pick up on the fact that positive rail is on AC and the negative rail is on batteries, since it just looks for a minimum of 14Vdc across both rails combined. So the first problem here is that the O2 PM design assumes both batteries are discharging more or less equally. In this situation if the positive rail is at 11Vdc or so, the negative battery would get down to 14Vdc - 11Vdc = 3Vdc which is enough to permanently damage it. Generally 0.9Vdc per cell is considered the minimum for NiMH it seems, and this would be 3Vdc/7 = 0.42 per cell.

The second problem is that a pretty unbalanced set of rails would be presented to the op amps. Maybe starting out at +11vdc/-9.5Vdc and going down to +11Vdc/-3Vdc at the point where the PM circuit trips. For small imbalances the maximum AC swing on the op amps is just affected, clipping against the lower voltage rail first. But for that much rail imbalance (+11/-3) it could start affecting the DC bias of the op amps. I kind of doubt it would kill them although it is possible if the common mode input voltage range gets exceeded and they latch up in some fashion, but it could very well produce a high(er) DC offset on the output, which may have made out to the headphones. All of this is reversed, of course, for a tip-negative DC adapter. Then the positive rail battery might be damaged.

The good news is that the regulator chips probably did survive if there were batteries in place. The negative rail battery would have prevented the negative regulator from seeing a reverse voltage. I also don't see any path in all of this for damaging the mosfets, unless one of the output NJM4556 chips or the NJM2068 chip happened to dead short.

So the first thing to check is still those voltages out of the regulator chips on D1 and D5 to make sure the AC supplies are working OK. Then I would be inclined to just put a finger on each of the op amps to see if any are heating up excessively. Then measure the DC offset on the O2 outputs and see if it is much higher than the typical 3mV.

[cruncynutcornflakes]

I finished my O2+ODAC build last night. Many thanks to Stefan at Headnhifi for supplying the parts and answering all my queries promptly!

At the end of my build, when I was wiring up the ODAC inside the O2 case, I had a lot of trouble with the wiring. My initial wiring was too bad, but upon realising that the CAT5 I had cut was too short, I had to remove them and make new lengths. When I removed the wire from the ODAC, the holes were filled with solder and nothing I could do would remove it. As a result, I had to solder them 'on top' of the holes. It's a bit of a hack job but it works (but): is it possible that a poor connection would result in a slight channel bias? I can't be 100% sure but I am fairly certainly that music playing on the left channel on my headphones is louder than the right? If one ground was less secure, could this also cause it?

I've tested with two headphones: Sennheiser HD580 and Takstar HI2050. Using the left cup on my right ear on both headphones rules out that I have slightly poorer hearing in one ear.

As a side note, any tips for removing the solder from the holes? I might delve back in and try and redo the holes.

I would really appreciate any photos that anyone has of the ODAC -> O2 connections. All the photos I have seen of build leave this step out for some reason!

[keithostertag]

Hey guys-

In my upcoming O2 build I want to have a pass thru to another device. The O2 will be part of my stereo/tv setup, between my source (dvd player) and EQ. I don't use a preamp. So, the signal needs to go into the O2, but it needs to also go to the EQ. This way we can choose to use the O2 or not, or use it in addition to the rest of the stereo equipment. This happens sometimes because, for instance, one of us likes to watch movies using the headphones while the other person listens to the speakers.

So, is it OK to just parallel connect a second pair of RCA connectors with the O2 inputs to route the signal to the EQ? Without additional circuitry or negative impact?

Also, on occassion we will want to use two (very different) headphones at the same time. Can you recommend a simple circuit to add that will enable me to use two very different headphone simultaneously with the O2 output? I would switch this splitter/buffer so it's isolated when only one pair of headphones are being used (the normal case), since obviously I can't optimize the O2 for more than one set of headphones

Thanks, Keith Ostertag

[counter culture]

Originally Posted by keithostertag

So, is it OK to just parallel connect a second pair of RCA connectors with the O2 inputs to route the signal to the EQ? Without additional circuitry or negative impact?

Yes. The O2's input impedance is 10k. The EQ should have a reasonably high input impedance. They should have only a small effect on each other.

You can listen and see if you can hear any difference, but the only difference should be a slight drop in volume.

Originally Posted by keithostertag

Also, on occassion we will want to use two (very different) headphones at the same time. Can you recommend a simple circuit to add that will enable me to use two very different headphone simultaneously with the O2 output? I would switch this splitter/buffer so it's isolated when only one pair of headphones are being used (the normal case), since obviously I can't optimize the O2 for more than one set of headphones

You're better in this case to use a second O2 in parallel with the first (and the EQ). Again, this should have a small impact on the overall volume. A fussy person might consider it necessary to have a buffer to drive all three circuits in parallel, but in all probability the only impact will be on the overall volume. I don't recommend running 2 sets of phones off the headphone amp output and anyway different phones are likely to have radically different volumes.

Suppose the EQ has an input impedance of 10k (a common enough value), this will make the parallel input impedance (of all three devices) 3k3, which most sources will still drive without problem. Input impedance in audio is usually targeted to be 10* output impedance as a rule-of-thumb, but that is a rough rule-of-thumb. As long as the input impedance doesn't fall below the output impedance, it'll be pretty much OK.

[keithostertag]

OK, thanks very much!