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Question about power supply for the O2 headphone amplifier.

LegionOfHell

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So I recently purchased an O2 headphone amplifier that comes with a WAU12-200 power supply.

Some quotes from NwAvGuy's blog:

In North America the Triad WAU12-200 from Mouser is rated at 12 volts but is really about 13.5 VAC with no load, and on normal 120 volt line voltage works fine for anything but full power sine wave testing or driving rare low impedance power hungry cans. If your line voltage is below 117 VAC or 235 VAC, and/or you plan to drive difficult low impedance headpones (i.e. HiFiMan planars), I would suggest a 14+ VAC transformer at 400+ mA.

In real world use, the WAU12-200 is the least expensive option and works fine at normal line voltages playing music (not sine waves) into 99% of headphones.

The power supply, with a 13.5 VAC (no load) wall transformer, is right on the edge of letting some ripple though under worst case conditions. If the O2 is used with low line
voltage, and for sine wave testing, or using very power hungry low impedance headphones, a higher voltage transformer is recommended (14 – 20 VAC).

Now I have more powerful amps, but I just wanna make my O2 compatible with all my headphones. The only planars I have are the HE-400 , T50RP MK3 and I have plans to buy either the Deva pro or the Sundara. Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?
 

DVDdoug

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That's probably impossible to answer...

If you have the amp and the headphones you already know if it's going loud-enough without distortion or not...*

You'd need to know how much voltage the amplifier puts-out to the headphones with a given power supply voltage. And you need to know the sensitivity of your headphones (usually published if you know what headphones you'll be using), and you need to know how loud you want to go (dB SPL).

...It's really unusual to find any audio equipment without a regulated power supply. The regulator not only holds a constant DC voltage with varying input power voltage (usually that's the AC power from the wall) and with varying loads, but holding the DC voltage constant provides a strong filtering effect to reduce any AC hum or higher frequency whine from a switching supply. (Most switching supplies operate above the audio range but it's still good to minimize the noise.)


* That may not be 100% true. With very loud sounds, sometimes I get the "feeling" that my ears are distorting. But that's way too loud anyway and bad for your hearing!
 

richardm

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Is this JDS labs now? They sell a 15v adapter https://jdslabs.com/product/15vac-power-adapter/

Edit: I see JDS Labs was making a version of the O2 (licensed?) and they sell a power supply 15VAC 500ma for it still

Seems like nwavguy still hasn't resurfaced after... eleven years? Did everyone decide to ignore the "no derivative works" part of the open source license he selected for the O2?

I've been out of the loop for a while.
 

solderdude

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Now I have more powerful amps, but I just wanna make my O2 compatible with all my headphones. The only planars I have are the HE-400 , T50RP MK3 and I have plans to buy either the Deva pro or the Sundara. Should I buy a 14V AC+ adapter or is my adapter good enough for the headphones that I have / plan to buy ?

The only 2 interesting and deviating aspects from the O2 were the battery management circuit and the idea to do amplification first and the volpot after that followed by a buffer.
The idea of doubling the output current was not novel.

The battery management was a bit simple in charging circuit just a small constant charging current even when batteries were full. As long as that current is small (overnight charging) the batteries can handle that. Nothing novel in that aspect.

When feeding the amp from an AC the power rails went up to + and - 11.6V DC and could not get higher if a higher input voltage was used. The regulators might get a little warmer/hotter with a higher AC voltage but there will be no increase in output power. Output power was limited by the regulator voltage and output current by the output opamps and not the 12V regulators. On battery the supply rail voltage was about max 8.2V (or 7.1 if the wrong batteries were used).

So increasing the AC input voltage only heats up the regulators a bit more and increasing the current rating of the transformer also does not do anything as the output current is determined by the output devices.

The novelty was in the fact that this amp did not use a rail splitter to create a symmetric power supply voltage (when battery fed) but the + and - voltage rail monitored each other and switched both off at the same time when the voltage of one of the batteries dropped below a certain level, or the power switch or one of the regulators failed.
It also helped when powering down the amp when the mains was switched off and no batteries were present and prevented DC on the output and large 'thumps'.

The other thing was the construction of applying gain before the volume control and using opamps just to 'buffer' (1x gain) the output of the volume control.
That had the advantage of being able to use a bit noisier opamps in the gain stage without having to worry about noise at the output of the amp because when you turn down the volume the noise from the amplification stage also went down. So you could use it with sensitive IEMs and high impedance headphones.

Of course it had a downside as well as nothing is free. The downside was that when used in higher gain the input stage could easily be clipped and that would sound clipped regardless of the volpot position. This was the biggest drawback.

These days the performance is not that great compared to what's on the market these days and performance have been surpassed at low price levels. It basically has become an irrelevant device these days.

You can increase output power though but this requires using output opamps that can supply more current and you might need to use 15V regulators to increase the output voltage. You have to make sure to use opamps that can handle the 15V rail voltage. Most opamps can handle +/-17 to +/-18V.
You would have to use a higher AC voltage transformer though, about 18V AC in that case.
Those opamps that can handle 18V (all of them have to) you can even be adventurous and use 7818/7918 regulators and increase the AC input voltage to 20V-22V AC.

The good thing about NwAvGuy was that he created awareness, showed measurements, discussed the design at length, promoted DIY, also made it available as a finished product and had to take all the flack from 'the better hearing people' just as Amir is scorned today.

Awareness is there, measurements are there, there are plenty DIY sites, 'better hearing people' still make the same claims, and the performance has been surpassed at low cost.
 
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LegionOfHell

LegionOfHell

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I contacted JDS Labs and MayFlower, here is what they said about the stock WAU12-200 power supply(12V) :

JDS Labs:
we observed an elevated number of issues from the 12V supply. When too much current is consumed for an extended duration, the transformer becomes saturated and voltage drops far below 12V, resulting in major audible distortion. Second, the failure rate was too high under heavy loads.

MayFlower:
You're completely fine with the stock supply.
 
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LegionOfHell

LegionOfHell

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JDS Labs:
we observed an elevated number of issues from the 12V supply. When too much current is consumed for an extended duration, the transformer becomes saturated and voltage drops far below 12V, resulting in major audible distortion. Second, the failure rate was too high under heavy loads.

I guess JDS labs meant only for "extended duration" and under "heavy loads" I need to get a new power adapter...otherwise I am ok ? I will use the unit(O2) max 2 hours per session.
 
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