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Old 29th June 2012, 12:46 AM   #101
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@agdr, if my numbers are correct your LME48720 mod lets you run your K550's up to a very loud 112dB before it runs out of current at 25mA. If you keep 'em paralleled like the O2 runs the NJM4556, then you can get to a whopping 118dB. It only takes 1.58V to get there.
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Old 30th June 2012, 11:08 PM   #102
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Originally Posted by ethanolson View Post
@agdr, if my numbers are correct your LME48720 mod lets you run your K550's up to a very loud 112dB before it runs out of current at 25mA. If you keep 'em paralleled like the O2 runs the NJM4556, then you can get to a whopping 118dB. It only takes 1.58V to get there.
I agree, that all sounds right. I've left them paralleled due to the only hiccup I see on the data sheet, a (only) 100pF direct-drive (no output resistor) capability per each chip half, probably not enough for a typical headphone cable. Leaving them in parallel should double that to around 200pF, then leaving the 1R balancing resistors in is the old trick (series resistance) to increase stability in the face of load capacitance. So in short, just leave the O2 output section the way it is (paralleled) and swap the NJM4556s for LME49720s. Perfect for the AKG K550s.

One interesting note on the DC offset issue. I heard back from AKG and their answer is essentially the same as what RocketScientist said, above. A DC offset voltage just moves the diaphram slighly one way or the other, meaning that at maximum excursion it would run into the mechanical limit of travel in that particular direction a bit soon. But normal listening levels are no where near maximum excursion, so small DC offsets won't matter AKG says. I specifically asked about 1 - 5mV and they said that is fine.

Putting numbers to that, I measure about 80mV(rms) to each channel of the phones as being about as loud as I can take it for normal listening. In the case of the O2, with that 3mV typical DC offset on the output, with an 80mV swing the transducer would wind up with 83mV in one mechanical direction and 77mV in the other. But the "hearing damage 94dB" level on the box is 135mV, and as you've noted above max excursion is probably somewhere around 158mV. So that extra 3mV of DC offset is still well within the mechanical travel bounds.
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Old 2nd July 2012, 04:46 PM   #103
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I don't think a measly 1 ohm series resistor will do much in terms of capacitive loading isolation. I'd expect something in the order of several ten ohms to make things bulletproof.
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Old 6th July 2012, 02:41 AM   #104
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Originally Posted by sgrossklass View Post
I don't think a measly 1 ohm series resistor will do much in terms of capacitive loading isolation. I'd expect something in the order of several ten ohms to make things bulletproof.
Definitely something to try (higher output resistance) if someone starts experiencing oscillations with LME49720s in for the NJM4556s powering the AKG K550s. I've scoped it and no HF oscillation that I can see so far. The LME49720s themselves are most likely stable with the amount of headphone (cable) capacitance involved, at least in this parallel configuration.
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Old 8th October 2012, 04:58 PM   #105
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Default Bass boost mod won't work for gain = 1 (no R17 & R21)

I've been meaning to post a correction for awhile to the bass boost mod I posted earlier in this thread. Thanks go out to OPTiK for running into this issue and letting me know.

The bass boost mod won't work if the O2 amplifier is set up for a gain of 1, ie with R17 and R21 clipped out of the circuit on the low gain switch position (or if R19 and R23 are removed in the high gain position).

The reason is that the bass boost modification increases the voltage gain of the O2 first stage for low frequencies, which means there must be voltage gain there to start with. A gain of 1 means no voltage gain by the non inverting op amp formula [1 + (R16/R17)] = [1 + 0] = 1, essentially a unity gain buffer, so altering the value of R16 and R22 with the boost circuit will produce no effect.

My apologies to anyone else who has run into this. I should have made the effect clearer in the writeup!
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Old 8th October 2012, 10:08 PM   #106
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Default O2 amplifier increased battery run time modification

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


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:


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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Last edited by agdr; 8th October 2012 at 10:27 PM.
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Old 13th October 2012, 04:39 AM   #107
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Default O2 amp longer runtime #2, low power op amps from BOM

RocketScientist's writeup on the O2 amp includes an option of using a set of low power op amps he specified for increased runtime. It seems the option has been largely forgotten or unused, since as RocketScientist wrote that the distortion and noise floor can be a bit higher, depending upon what one is doing.

However, RocketScientist did not include the effect of headphone sensitivity in his writeup about the low power chipset. This modification shows that for very sensitive headphones, like the AKG K550s at 114dB/V, the distortion levels are well into the "great" range from the dScope measurements he has posted with the low power chips. He shows "very low" distortion up to about 0.7Vrms output swing at 15R output impedance. The K550s only require a maximum of abour 80mV (rms) swing at 32R, well into the low distortion part of his dScope curves.

The writeup is here on his blog,

NwAvGuy: O2 Details

and search for the heading "low power option" and the heading "low power distortion & output" for his dScope distortion graphs with the chipset. The chips involved are listed on any of his BOM's for the O2 amp, under the gain resistor section. The output NJM4556 chips are replaced with TLE2062CPs and the NJM2068 gain op amp replaced with the OPA2277PA.

These two low power chips also have much better input bias current numbers (pA vs. nA) then the NJM chips, resulting in lower DC offset at the output of the O2.

The first photo below shows the new chips. The TLE2062CPs are at the top and the OPA2277PA at the bottom. The next two photos show the quiescent (idle) current of the O2 with the original NJM chips still installed. 22.2mA on one channel and 22.1mA on the other, which matches up perfectly with RocketScientist's mention of around 22mA idle for the NJMs in his writeup. The batteries are freshly charged for all this, both running at about 9.1Vdc.

The next two photos show the new idle current with RocketScientist's low power chipset installed. 4.8mA on both channels, representing a 78% reduction in idle current. RocketScientist says "below 8mA" in his writeup. I'm at a loss as to where he came up with the extra 3.2mA of current usage. The idle current is significantly more than the current used by the headphones, as RS notes, so this idle current reduction represents a huge decrease in current draw. Assuming around a 30mV normal (rms) listening level on the AKG K550s, that is just (30mV / 32R)(2 channels) = 1.9mA, a drop in the bucket next to that 22mA - 4.8mA = 17mA idle current reduction!

Assuming Tenergy is lying about the 250mAhr rating on the 8.4Vdc cells (I've been working with high power LED flashlights too long, lol) and they are really 220mAhr or less, a 17mA drop in current per battery represents 220mAhr/4.8mA - 220mAhr/22mA = 35 hour increase in run time! That means charging your O2 once every 4 days rather than every day.

The next two photos show the reduction in output DC offset into the headphones. 380uV and 1.03mV vs. about 3mV per channel with the NJM4556 output chipset. That is better than a 3:1 reduction.

I have left the output resistors at 1R, rather than reduce them to 6R as RocketScientist mentions in his low power write, since the very low voltage swing needed for the AKG K550s would still result in low chip to chip balancing currents even at 1R each. For headphones requiring larger voltage swings, closer to that 0.7Vrms "knee" in his dScope distortion curves, using the 6R output resistors would probably reduce distortion slightly.

All measurements aside, the final photo is some actual listening time. Sounds great! In this case the O2 amp is being fed with the HiFimeDIY ODAC clone here, into the K550 headphones, O2 running on batteries.

I should re-iterate that this mod is assuming very sensitive headphones or IEMs, like these 114dB/V AKGs. The mod would probably work very well down to a sensitivity of 110dB/V or so. Lower than that you will run into all the issues that RocketScientist originally wrote about for this low power chipset, namely significantly increased distortion due to the higher voltage swing requirements and higher noise. This low power chipset would not work well at all with my 100dB/mW Shure SRH940s, for example. Those need closer to 1.8V rms max. From RocketScientist's dScope curve for the low power chipset you can set the distortion starts rising exponentially (at 15R) around 0.7Vrms output.
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Last edited by agdr; 13th October 2012 at 05:04 AM.
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Old 18th October 2012, 03:17 AM   #108
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Default O2 amp power mgmt latch modification - no oscillate

This O2 headphone amplifier modification adds a latching circuit that keeps the power management circuit "off" permanently - until the amp power switch is turned off and back on to reset - regardless of any re-charge behavior by the batteries.

The battery re-charge issue is described in the posts above about hysteresis. Once the load is removed from the batteries when they get low, when the power management circuit turns off, the battery voltages tend to rise on their own which can turn the circuit, and hence the whole O2 amp, back on again. The O2 then just oscillates on and off like that making a noise in the headphones.

This modification adds a couple of mosfets across the 33K R9 resistor in RocketScientist's schematic. When the power management circuit turns off, the gate of mosfet Q1 goes high (to the positive rail, whatever voltage that is at the moment). That signal now goes to the gate of a new mosfet in this modification that in turn "shorts out" R9, bringing pin 2 of the comparator U2 to the negative power rail. Lowering that pin voltage forces the gate of Q1 high, which is already is, thereby creating a latching feedback loop. If the battery voltage subsequently rises it will have no effect, since that would ordinarily increase the voltage at U2 pin 2 by the action of the R9 / R5 voltage divider. With U2 pin 2 now latched at the negative rail voltage the power management circuit will stay off permanently, regardless of battery voltage, until the O2 power switch is turned off and back on.

The one trick to the circuit is that RocketScientist has the power management circuit turn off briefly when the O2 is first turned on to prevent a turn-on pop in the headphones. C1 C16, and C21 are part of that function. This action would trip the lock circuit and lock out the O2 from ever turning on in the first place without the addition of an initial time delay, which is formed from the second mosfet in series with the first across R9. The time delay RC circuit feeding the gate of that mosfet produces an initial power-on delay in the latch circuit of 1 -2 seconds (1 sec = 24Vdc rails on AC, 2 sec = 14Vdc with nearly dead batteries). The two mosfets in series logically "AND" the power-on time delay and latching signals as a condition to short U2 pin 2 to the negative power rail.

The first circuit below shows how the new parts are wired into the O2 power management circuit. The second circuit is just an LTSpice test circuit for function.

Suitable parts from Mouser are:

Two n-channel mosfets: these MUST be +/-30Vdc gate mosfets. A 2N7000 with a +/-20Vdc gate will not work.
FQN1N60CTA Fairchild Semiconductor | Mouser

Resistor: 4.7M 1/8W 1%
RN55D4704FRE6 Vishay/Dale | Mouser

Capacitor: don't use an electrolytic for this. The leakage is too great and will prevent the circuit from working correctly with the tiny currents involved. Use a multilayer ceramic capacitor (MLCC), 2.2uF, X7R, 10%, 50V
FK20X7R1H225K TDK | Mouser

The bad news is that I don't see any way to fit this modification into the standard B2-080 O2 chassis, at least using these through-hole parts. The slightly taller B3-080 case would have to be used. All 4 parts in the circuit have surface mount versions though. It may be possible to fit a SMD layout for the the circuit underneath the O2 PCB in the B2-080 case, and wire the board into the bottom of the O2 PCB, although I haven't tried it.

For a power managment circuit "on" indicator, wire a small 3mm led in series with a resistor to run at 1mA or so, across C8. Then just drill a small hole in the O2 front panel and glue the LED into the hole. When the PM circuit is on the LED will be on, and when off and latched the LED will be off. The same thing could be done with a second LED across C9 (careful to observe polarities!) to show the same for the negative rail and Q2.
Attached Images
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Last edited by agdr; 18th October 2012 at 03:44 AM.
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Old 18th November 2012, 04:53 PM   #109
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Default O2 power mgmt latch mod does fit under the PCB

It turns out that the RocketScientist O2 amp power management circuit latching modification that I posted above does fit under the O2 PCB in the standard B2-080 case just fine - if you are really good with a soldering iron adn some DIY work.

The purpose of the latch circuit, again, is to prevent the natural voltage rise that happens with the batteries when the power management circuit cuts the load off from reaching a level where the pwr mgmt circuit turns back on, hence oscillating

I added the circuit to the bottom of the 8-cell AAA NiMH O2 modification that I posted in another thread to test it. Works great, with one thing that I'll call a feature rather than a bug. Once the latch circuit trips, the O2 amp must be powered off for a full 2 minutes to reset it. It takes that long for the RC time delay lockout circuit in the latch circuit to bleed down. If the O2 is turned back on in less than two minutes the mosfets will just remain off.

All of this should be done on a antistatic mat with grounded wrist strap, grounded tools, grounded soldering iron tip, etc. to keep from blowing the mosfet gates.

The first two photos below shows the resistor and axial capacitor that form the RC time delay circuit soldered across the outer pins of R5 and R9 under the O2 PCB. When R9 was soldered in I left the leads long so I could solder the mosfets to them.

The mosfets source and drain leads are bent at right angles, then the drain of one soldered to the source of the other. Then the mosfet pair are soldered onto the free R9 leads and the leads trimmed.

Finally one mosfet gate lead is soldered to the connection point of the RC delay circuit, while the other is bent around and soldered to the nearby gate connection of Q1, as shown in the 3rd photo.

The whole thing clears the bottom of the B2-080 case just fine, as the last photo shows.
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Last edited by agdr; 18th November 2012 at 04:57 PM.
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Old 27th November 2012, 10:07 AM   #110
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I have 2 headphones, an Audio Technical CKS77 and Sen HD650. Using the O2 to power both, I found that it is a bit soft and muddy with HD650. I have been waiting for the desktop version of O2, I think it will take more time... so I done a little test by supplying the O2 with Salas SSLV shunt regulator. I don't have tools to measure but I tell it is cleaner, louder and bass is better.

Here is the setting:
+12.16 320mA
-12.19 290mA
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