The Objective2 (O2) Headphone Amp DIY Project

Hi all,

I am close to ordering the parts for the O2/ODAC. The recommended LED indicator (MV5764.MP4B) is out of stock, as is the yellow alternate (MV5364MP4A). The other alternate (HLMP-1301) is available, however I notice that it has a different forward voltage (1.9v) than the MV5764.MP4B (2.2v). Will I have to make any changes if I use the alternate LED?
 
Thanks Mooly. Is the 47k multiturn still necessary when using NwAvGuy's alternate LED's? As far as I can tell he doesn't mention anything in his documentation about using the alternates so I'm unsure if they are "plug and play" or require some modification along the lines of what you suggest.
 
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I wouldn't like to say tbh. The circuit relies totally on the LED used (specifically the voltage it develops across its terminals) and as such seems far to variable between devices to use as a reference voltage.

Its nothing to worry over though, just see how the circuit behaves when one of these alternatives is fitted and then decide on your options such as fitting the preset. Just make sure the circuit cuts off at the required point such that the batteries aren't over discharged. If it allows over discharge or cuts off prematurely then you need the preset to allow the trip point to be set exactly.
 
Okay, more time to work on this again, started over with all the "step-by-step voltages" tests. I clear everything up to step 11, U2 Pin 2. I see +8.56 VDC on that pin, not negative volts. R5 closest to batteries shows +11.77 VDC, but end of R9 closest to C6 shows +7.96 VDC.

I also tested pins 4 and 8 on U2 and U4 sockets, those read 7.2 VDC on pin 4 and 103 mV pin 8. I'm reading the suggestion again and realizing that was probably supposed to be tested with the ICs in. Derp.

Suggestions show that the U2 problem could be R5 or R6, but I wouldn't have cleared the previous resistance tests if that was the case, would I have? The other suggestion is U2 is wrong or damaged. Could this part failing over the course of testing cause my other problems, or is there something further along that might also need replacing. I'd like to reorder as few parts as possible, obviously.
 
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Lets work with what is obviously amiss.

I also tested pins 4 and 8 on U2 and U4 sockets, those read 7.2 VDC on pin 4 and 103 mV pin 8.

Recheck this. Are you on batteries or AC power ?

Put the black lead of your meter to ground and read the voltage on pin 8 of U2. It should be either positive 12 volts or positive 'battery voltage' depending on whether you are on AC power or not.

Keeping the black lead on ground now measure the voltage on pin 4 of U2. That should be negative 12 volts or negative 'battery voltage'.

Lets start with those and see what you have. If you really have a positive voltage on pin 4 then we need to start checking basic continuity of what goes where.
 
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Have you looked at the typical sensitivity of the most common low impedance headphones ? Its around 100-105db for 1mw. 600 ohm types will need more voltage applied to reach that 1mw figure but that's easily achievable here with opamps having no high value series output resistor.
 
It's sensitivity / efficiency indeed.

With a headphone type sporting a 96 dB SPL / 1 mW efficiency spec (like typical Beyers), the ~4 Vrms maximum output on batteries would still be good for up to 110 dB SPL, even if 26 mW into 600 ohms doesn't sound like much!

Even the notorious AKG K340 clocks in at about 93 dB SPL / 1 Vrms sensitivity wise, and would still provide up to 105 dB SPL.

Now an HE-6, that would be a bit of a problem indeed (~90 dB / 1 Vrms, 53 ohms), and it's officially considered part of the few headphones that the O2 cannot drive with authority. Still, even that one should work decently if you're not a notoriously loud listener.

The fact that most headphones actually require little power gives amp designers a great degree of freedom. You'll find literally everything from tiny ICs running on +3.3 V at less than 5 mA quiescent current draw (but still decent performance), to concepts with single-ended Class A output that still have no more than a few watts of idle power dissipation. The large variation in sensitivity due to differences in general construction (from BA IEM to "head speaker") and impedance tends to be a far bigger headache.
 
Hi all,

I am close to ordering the parts for the O2/ODAC. The recommended LED indicator (MV5764.MP4B) is out of stock, as is the yellow alternate (MV5364MP4A). The other alternate (HLMP-1301) is available, however I notice that it has a different forward voltage (1.9v) than the MV5764.MP4B (2.2v). Will I have to make any changes if I use the alternate LED?

I had used non standard BOM , red LED with fwd drop of 1.74V & R6=33K. Little less efficient than the standard BOM led
CAUTION-DO NOT USE ANY OTHER COLORED LED
For detailed answer,as posted by agdr

"...Here is a long-winded answer that should help to clear some of the terminology up.

LED forward voltages drop with decreasing current. The datasheet is saying that at 10mA through the LED its Vf will be around 2.0. That is the "test current", something the manufacturer has picked to be a common usage condition. But with the the O2 LED current dropped down to 0.5mA we are getting Vf = 1.8V. Turns out you can drop a LED's current down so low the light isn't even visible to the eye anymore (way below that datasheet test current number), but the LED is still doing its thing and maintaining a (slightly lower) forward voltage. People use LEDs in circuits all the time as voltage references, like in the O2, but with the LED current so low no (visible) light is coming out.

So for the O2 the LED current is set by the 40.2k series resistor R6. The good news is that you can make that resistor anything you need it to be to get the 1.8V from the red LED. Changing R6 won't mess up anything else in the circuit but may draw a bit more current from the batteries if you make it smaller. For example, with the O2 on AC power, the power rails are at 11.8Vdc each (12V minus two diodes), for a total of (2 x 11.8Vdc) = 23.6Vdc across R6 and the LED. Subtract out the 1.8V for the LED and your LED current through the resistor is (24V - 1.8V) / 40200 ohms = 0.54mA.

Most standard LEDs would not produce much, if any, visible light at that low of a current. Your supplier is right, 5mA would be more typical rating, with 1mA about the smallest rating you will find on datasheets - but remember, you can run them at lower currents even if no light comes out. The "high efficiency" label just means the LED will actually produce enough light to be visible even at this smaller (than the 10mA data sheet test current) low 0.54mA number. When the O2 is running on batteries that are almost dead, the LED and resistor have only about 14Vdc across them. So the LED current then is even smaller - only (14 - 1.8) / 40200 = 0.3mA.

Red LEDs have a forward voltage between 1.6Vdc and 2.0Vdc (see the Vf table herehttps://en.wikipedia.org/wiki/Light-emitting_diode). If you are willing to give up actually seeing the LED light up you can use a "non high efficiency" one and just run it at a lower current. If your local supplier has one that is labelled on the package as "2.0Vdc", you will probably find that if you run it at a small current that Vf can be dropped to 1.8V. Works great, even though you can't see much, if any, light coming out anymore. Just make R6 slightly bigger or smaller as required. Different Red LEDs will have slightly different Vfs in the range, too. Try other models if needed to find one that runs around 1.8V.

If you have some resistors and a DMM handy, put the led and a 39k resistor in series with two 9V batteries (like my avatar, lol!), to get 18Vdc (roughly midway between the O2 high and low power rail numbers), and measure the voltage drop across the led. Then just change that resistor up or down to get the Vf = 1.8Vdc, then solder that resistor into the O2 in place of R6.

The other way around is if your supplier has a red LED that has Vf=1.8Vdc, but it needs a bigger current than 0.5mA to get there, just make R6 smaller as needed. For example, say you've measured that your supplier's LED will hit Vf=1.8Vdc at a current of 5mA. Using the highest case 23.6V rails for the O2, your R6 would then need to be (23.6V - 1.8V) / 0.005A = 4360 ohms = 4.3K standard value. You will loose a small amount of O2 run time on batteries increasing the LED draw to 5mA from 0.5mA, but at least the O2 will work.

You can also just use a red LED and (R6) resistor combination with a forward voltage of 2.0Vdc. The only effect would be the O2 turning off slightly later, when the batteries drop to around 6.94V each rather than 7.07V, if using R25 = 1.5M and R9 = 33k. But you want to use the lowest LED current you can that will produce a stable Vf since the batteries will run down faster with the increased LED current.

If your supplier has a "high efficiency" or "super bright" LED with a rating of Vf=2.0 at 10mA or 20mA, it will be more likely to still produce some visible light when the current is dropped down to 0.5mA or 1mA. That is why RocketScientist used that type of LED in the design, so he could run the LED with as small a current as possible that would still produce light, to minmize battery loading.

This modification allows the O2 amplifier batteries to run bit longer before the power management circuit cuts them off. RocketScientist’s original values for R25 and R9 did a similar thing. This mod puts the values roughly in the middle between his existing O2 values for R25 & R9 and his original values.

The modification drops the trigger threshold for the O2 amp's power management circuit to turn “on” by about 0.75V per battery, from around 8.33Vdc to around 7.59Vdc. The threshold to turn “off” is dropped by about 0.35Vdc, from 7.07Vdc to 6.72Vdc. RocketScientist’s original values of R9 = 40.2k and R25 = 2.74M put the “on” threshold at about 6.95Vdc and the “off” threshold at about 6.33Vdc per battery.

This mod was inspired by a recent post in the main O2 there here:

The Objective2 (O2) Headphone Amp DIY Project

The change involves increasing the value of R9 from 33k to 36.5k and the value of R25 from 1.5M to 2.1M. Suitable resistors from Mouser are part 270-36.5K-RC and 270-2.1M-RC:

270-36.5K-RC Xicon | Mouser

270-2.1M-RC Xicon | Mouser


Details:

In RocketScientist's existing O2 power management circuit, when the MOSFETs are "off", U2 pin 7 is near the negative supply rail. That puts the 1.5M hysteresis resistor R25 essentially in parallel with the 33K R9, to give a parallel resistor value of around 32,290 ohms. This voltage goes to one input of the U2 comparator, pin 2, to be compared to the LED voltage on U2 pin 3.

The photo below shows the measured voltage across the O2's LED at about 1.78Vdc. The photo is taken with the O2 on AC power, but I also tested it on batteries and the LED voltage is nearly the same, as it should be. RocketScientist is using the LED essentially as a voltage reference.

So with the power management circuit just barely "off", the voltage at U2 pin 2 from the R5/(R9 || R25) voltage divider must be right at 1.78Vdc to match that reference voltage from the LED on pin 3. That gives a current through the R9 || R25 (parallel combo) and into R5 of 1.78Vdc / 32290 ohms = 55uA. Multiplying that current by the value of R5 gives a voltage drop across R5 of (55uA)(270K) = 14.88Vdc.

Now adding that voltage back to the 1.78 volts across R9 gives the rail-to-rail voltage at the point where the power management circuit activates and turns "on": 14.88Vdc + 1.78Vdc = 16.66Vdc. Assuming the two batteries are approximately equal, dividing this by two will give the voltage of either battery at the point where the PM circuit turns on as the batteries "recharge" once their load is removed (PM circuit had turned off): 16.66Vdc / 2 = 8.33Vdc.

That voltage matches up with the 8.30 volts measured in the linked post above in the main thread. The batteries in that case were charging back up to 8.30Vdc, just a hair below the 8.33Vdc level need to turn the PM circuit on again, and have it oscillate on and off to signal the batteries are in need of charging.

To find the other trip point to turn the PM circuit "off", with the power management circuit already "on", just work through all the math above but this time with the hysteresis resistor R25 in parallel with R5, the 270K resistor, instead of R9 since the comparator output U2 pin 7 will be near the positive rail when the PM circuit is on. 1.78Vdc across the 33K R9 gives 54uA through R9 and into the 228.8K parallel combo of R5 and R25, producing a (228.8K)(54uA) = 12.34Vdc. Adding 1.78Vdc and dividing by 2, for the two batteries, gives the "turn off" voltage trip point of around 7.07Vdc per battery.

Going through similar math with RocketScientist’s original values of R9 = 40.2K and R25 = 2.74M result in a power management turn “on” voltage of around 6.95Vdc per battery and a turn “off” voltage of around 6.33Vdc each.

8.4V NiMH batteries have a useful charged voltage range of around 8.5Vdc - 8.7Vdc from here:

http://support.radioshack.com/suppor...h-9v-lodis.gif

NiMH 9-Volt Battery Engineering Data Sheet

after which the voltage across the battery drops off rapidly. But as those curves show the discharge rate has a lot to do with it, and various brands of batteries may have slight differences in their per-cell voltage (8.4Vdc is a 7 cell battery x 1.2V per cell). So a different "8.4Vdc" battery may have its useful charged voltage range a bit lower.

This modification changes R25 to 2.1M and R9 to 36.5K to produce a turn “on” voltage of 7.59Vdc per battery and a turn “off” voltage of 6.72. Putting all of this in a table gives, per battery:


R25 = 1.5M, R9 = 33K (current O2 values): turn on = 8.33Vdc, turn off = 7.07Vdc

R25 = 2.1M, R9 = 36.5K (this modification): turn on = 7.59Vdc, turn off = 6.72Vdc

R25 = 2.74M, R9 = 40.2K (original O2 values): turn on = 6.95Vdc, turn off = 6.33Vdc

The 6.72Vdc per battery trip point to turn the power management circuit "off" in this modification matches up with the bottom of the discharge curve for the "8.4Vdc" NiMH batteries in the link above. That works out to be around 6.72Vdc / 7 cells = 0.96Vdc per cell. RocketScientist's initial value of 6.33Vdc to turn off works out to be 0.90Vdc per cell, which is about as low as a NiMH cell should probably be discharged without affecting its lifespan. Note that the discharge graphs for the battery in the link don't even go that low - probably a hint from the manufacturer not to go there. "
 
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For detailed answer,as posted by agdr

Thanks for the re-post! Most folks are probably not aware that O2 modification thread exists anymore.

I also have the original O2 red LEDs for sale in my thread in the Vendor Bazaar forum here. See posts #31 and #32. :) Looks like Mouser is supposed to have them back in stock in mid-June.
 
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First of all, Hi to everyone, and sorry for making my first post here an SOS one!

I don't know if I'm in the right forum, but I need help figuring out what is wrong with my O2 amp. I bought a kit from JDSLabs and assembled it, making sure every part was the right value. I started testing it by measuring resistor values as in NwavGuy's documentation and all values are dead on except R5 wich shows 157KOhms instead of around 100KOhms, and R25 showing 312KOhms instead of around 330KOhms.

I've tried measuring what voltages I get at the battery terminals as per the next step in the document, but it looks like the negative rail is not functionning. I get 12 volts on the positive, but only -800 mVolts on the negative.

Any ideas on where I should look for a fault? I've replaced U6, Q1 and Q2 after realizing I had soldered U6 the wrong way around.

I've done troubleshooting before, but I'm quite perplexed by this issue.

Thanks in advance for any pointers or ideas anyone might have.