The Objective2 (O2) Headphone Amp DIY Project

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All you need do initially is choose a physically suitable high brightness red LED. That will work 'out of the box'. If you then want to trim the low battery cut off voltage (say it cut off before the batteries were discharged, or didn't cut off at all) then you just tweak the value of R9 up or down.
 
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Yes, a high brightness part is very bright on as little as 1 milliamp and below. The forward voltage of the LED depends mainly on the colour (red are the lowest) and then with smaller variation from differing devices from various manufacturers.

A typical red LED drops around 1.7 volts at low current. The O2 runs the LED at around 300 to 400uA (MICRO amps). Using a different LED is a non problem in the overall scheme of things.
 
Does it mean the prupose of the LED is to control the voltage going to U2 while the R9 resistor is to provide a good amount of current to the LED to make it work? I got a bit confused about the current thing, in the data sheet of the proposed LED, the If is 0.01A while you said O2 runs the LED around 300 to 400 uA, which is 0.0004A. Does it mean that even I use the purposed LED in O2, I will get a darker light from it than what the data sheet suggest how bright it will be?
 
Silly me! I have actually found the answer of my own question in the threat (#4312). So basically what I need to do is to matain a Vf=2V voltage for the LED I have choosen while having a If simillar to 300-400uA. If the If is a bit higher than 400uA, the LED will get dimmer, while if the If lower than 400uA, the LED will get a bit hotter. Am I correct?
 
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The current little LED's run at won't generate noticeable heat.

W=I*V which for 10ma at 2 volts Vf would be 0.01*2 which is 0.02 watts. Not a lot ;)

The forward current in the LED (If) is determined by R6 in the 02 You take the total supply voltage, say 18 volts, then subtract Vf which leaves 16 volts. Now work the current out. R6 and the LED are a simple series circuit across the supply.

I=V/R which is 16/40000 which is 0.0004amps or 0.4ma or 400uA.
 
Hello,

I want to build retro looking desktop enclosure for O2. Here is the sketch of the front plate (height slightly over 4 cm):

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If you like the design I can post the BOM candidate here.

I have no knowledge of electronics, and all I can do is connect black boxes and follow well-written instructions. Please forgive me the simplicity of my questions...

1. Gain switch:
Can I replace gain switch with another bistable switch, assuming the same number of poles and proper contacts for signals? Can I use wires to lift it from the board? Are CAT5 wires sufficient? Should I shield the connecting cable? What is the maximum length for such connection?

2. POT:
Same as above regarding cabling... Also, could you point me to the replacement potentiometer but for plate mounting?

3. Power switch:
Can I use power switch to just simply disconnect mains connection to the transformer? I plan to move PSU into the enclosure and use it to simultaneously power the driver for UV meters (uses 15VAC). I tried to "analyze" the NwAvGuy's schematics, but the description next to the switch scares me a bit, as it says "Do not switch AC". I was thinking about closing the connection permanently in place of S1.
 
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Nice design :)

1/ If you permanently wire the highest value gain setting resistor in place on the board (no switch needed) then you can use a simple switch arrangement to just switch another resistor in parallel to the first to give higher gain. Simple and imo a better arrangement than the original.

Wires can be unshielded, its all low impedance anyway. No problems there.

2/ 10k dual gang log pots are widely available. You will have to check suppliers in your area.

OMEG - PC2G16ECO10KB - POT, DUAL GANG, LOG, 10K | CPC UK

3/ Yes, you can switch the transformer mains supply. Be sure to use a switch rated for this type of use.

(no idea what that description of do not switch AC is about. There is no AC in that area. Maybe he means that particular switch is not suitable for AC. Its meaningless though)
 
Thank you very much :). Which version do you prefer, wooden or black?

I plan for Galaxy enclosure (GX247) and Nissei TR-35 meters.
Volume knob is 40 mm diameter anodized aluminium.
Bistable, black switches for 16 mm hole, collar diameter 18 mm.
Aiming at standard equipment, available in diy audio shops - hopefully keeping costs under 150 euro.

I'll try posting links and proper technical drawings when I get more solid design. For example, now it uses non-existing black Neutrik NYS2203 jack plug without pattern.

Anyway, going back to O2 itself. Another thing that bothers me slightly is grounding. Since I'm introducing mains power into the chassis, I should ground it properly using mains ground. Also, there will be another board for the vu driver connected to output, before switched RCA outputs and jack. In it's original state O2 is connected to chassis only at J2, and chassis isn't grounded at all.
Any tips or pitfalls to avoid in this particular case?
 
That's a nice can of worms you're opening there. I am very much assuming that you aren't planning to use your O2 exclusively with sources that are floating?

Since referencing circuit ground to protective earth tends to wreak havoc with unbalanced audio connections, traditional commercial hi-fi devices tend to be IEC Class II a.k.a. "double insulated", with a 2-conductor mains lead. These have to be built more carefully than the more DIY-friendly Class I (safety earthed) ones but it is doable. As a rule of thumb, any single insulation fault must never have the device go live. You are looking at stuff like generous heatshrinking, plastic standoffs for PCB mounting, maybe an extra plastic insulating sheet under the power supply board or insulated mounting of the transformer, stuff like that.
 
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Thank you very much :). Which version do you prefer, wooden or black?

The two tone looks good if you can carry it off and get a good finish :)

Grounding... pretty much as sgrossklass says really.

One possible option is to securely ground the chassis and to use isolated input jacks (standard practice anyway) to keep the circuitry from contacting the chassis. Then you can tie the O2 audio ground to mains ground via a resistor such as a 10 ohm 1 watt. There are also ways to make that into a high current connection using diodes such that they would only come into play under a fault condition.

The resistor alone is simply to reference the 02 to ground. It does not provide any protection in the event of a mains fault to 02 board.

Ultimately you will almost certainly not be able to make a double insulated design that would comply with the regulations. The transformer would need to be approved for the purpose and the wiring correctly implemented. Always ask 'what if' ? What if the switch fell apart (it happens) and the live wire was free to touch something. If it touches the grounded chassis then that is OK, if it touches anything else it is not OK.

The O2 should also be fused with a small rated fuse, perhaps as low as a T63ma (time delay) for a very small transformer.
 
One possible option is to securely ground the chassis and to use isolated input jacks (standard practice anyway) to keep the circuitry from contacting the chassis.
Insulated jacks have actually not been "standard practice" since about 1990.
Ultimately you will almost certainly not be able to make a double insulated design that would comply with the regulations. The transformer would need to be approved for the purpose and the wiring correctly implemented.
It's not rocket science either though, just fizzicks. Make sure everything primary side is insulated twice and kept well away from anything secondary side, also the transformer housing should be insulated if it's not potted anyway (popular small transformers in DIY often are). I would very much hope that any good-quality (name brand) transformer would withstand at least a few kV pri/sec. The difference between Class I and Class II comes in elsewhere - larger ones to be used for Class II need a shield winding in order to keep capacitive coupling to mains to a minimum. At the 10 VA max that an O2 might need, this should be pretty much a non-issue though.

If you're a chicken you might also wire in the mains switch secondary side, a fair few devices from the '70s did it like that. The downside is that the transformer is always exposed to whatever is on the mains. I have an old radio where the orientation of the mains plug matters (we're talking Europlugs here), the wrong one results in hum and can give you a bit of a tingle (I also managed to blow the input protection diodes like that). There has to be some subtle insulation fault in it, even though there is no ohmic connection to be found. I can only presume that it's either a manufacturing defect or it took some high voltage spikes at a time.
The O2 should also be fused with a small rated fuse, perhaps as low as a T63ma (time delay) for a very small transformer.
Something on the order of 10 watts nominal is about the smallest you can even protect with a primary-side fuse on 220-240 V~. T 63 mA sounds about right, and maybe T 100 mA in 110-120 V land. Real small-fry stuff (think 5 VA and lower) tends to rely on the transformer's thermal fuse instead, coupled with secondary-side fusing (maybe something self-resetting like picofuses).
 
The two tone looks good if you can carry it off and get a good finish :)

I was originally planning to mill a groove like 1,5 mm deep to accommodate piece of real wood, or any nicely textured material like leather or even stone, so it can match kitchen sink look of my HD600. But I won't be able co carry it off, since front panel screws screw me over. No way to move them, no way to get rid of them. And it will look ugly and chipped as holes go over the edge of wood insert, also without any lip to cover imperfections. I would have to switch to Galaxy 2U height with 10mm thick panel, which has no screws. Also, costs twice as much and it's just too big. This will have to wait till my kids are bigger, so I can start thinking about a proper amp for proper speakers.

As for the grounding and safety issues, thank you all very much for feedback. Now I think the most reasonable thing to do is to close the transformer along with the relay in it's own plastic case, like Hammond 1591XXLFLBK. No 230V at the switches or anywhere outside that box. I might even ditch the plans to move PSU into chassis, since I can use original O2 supply for everything.
 
O2 DIY build issue need help (getting mono sound output)

Hi guys,

I am new to DIY and I need help.
Been trying to figure this issue out and scratching my head and feeling helpless and lost over here and need advice from experienced people out there.

I am building my own O2/ODAC combo and this is my first DIY project.

So excuse the poor job of soldering but let me explain the issue to you with photos and hopefully one of you might know how to solve it.

1) So I built a standard O2 amp (with the exception that I am using 1x/3x gain) and it tested fine and everything worked perfectly.

2) Then I removed front power and added rear power, everything again worked perfect.

3) Then I added ODAC, and everything worked perfect.

4) I removed the 3.5mm output and added a 1/4" jack. The jack is switched so that I can also have rear RCA output for speakers and when i plug headphones in to the front the speakers will cut out and sound to my headphones.

5) This is where I had the issue, with the 1/4" jack I am getting mono sound.
i.e. Left ear gets sound for both left and right, and also right ear gets sound for both left and right.

6) I thought I might have a faulty 1/4" jack so I ordered a new one and still the same issue.

7) I tried soldering to P2 using bottom hole for right and the one just above it for left

8) in the photo below I haven't soldered earth yet, but that is also connected on the pre-tinned solder lug on the jack you can see and in the second hole from top of P2

I'm hoping someone can help me get this correct.

Thank you.

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Hi guys,

I am new to DIY and I need help.

Welcome to the forum! Congratulations on your O2 build.

You are correct that the bottom hole of P2 (toward the batteries) is right and the one above it is left. And the two above that are ground. Both are connected together, so either will do for headphone ground.

Your imgur photo links are not working. Try using the "manage attachments" button below the edit window here, it will upload your images directly.

I'll bet your problem is in here:

8) in the photo below I haven't soldered earth yet, but that is also connected on the pre-tinned solder lug on the jack you can see and in the second hole from top of P2

Most headphones share the ground wire between the left and right headphone drivers. So if that 3rd headphone ground wire isn't connected you get both the left and right signal going through both drivers. Somehow your connection of the headphone ground wire to P2 hole #2 isn't making a good connection.

EDIT: got your photos to load by forcing the links to open in a new window. I'm curious why there are two ground wires coming off that 1/4" jack. Should just be one ground connection on the jack that goes back to either hole 1 or 2 on the O2 P2. Do you have a part number for that jack?

Had to think for a minute why you have C8 lying on its side, but you did that to allow the 1/4" jack to fit. :) OK as long as you insulate the leads, which it looks like you have done. Couldn't hurt to put a piece of heat-shrink around the whole cap, if you have a piece that large, to make sure nothing cuts through the thin label and touches the can. Let the heat shrink hang off past the back of the cap a little bit (like 1/4") to keep anything from touching the end, too.
 
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EDIT: got your photos to load by forcing the links to open in a new window. I'm curious why there are two ground wires coming off that 1/4" jack. Should just be one ground connection on the jack that goes back to either hole 1 or 2 on the O2 P2. Do you have a part number for that jack?

Thank you for the welcome, the part number for the jack is NJ5FD-V the same one recommended by HeadN'Hifi, JDS, Mayflower.