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Review and Measurements of Massdrop Alex Cavalli Tube Hybrid (CTH) and JDS O2 Headphone Amps

Wombat

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NwAvGuy license. Will he reappear to defend it
e3363.jpg
 

Timbo2

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Just buy the JDS Labs version, its build quality is great.

https://www.jdslabs.com/products/35/objective2-headphone-amplifier/

At the time I purchased JDS had "B stock" available. B stock is only cosmetic - electronics are new and can be customized. So the lettering on my O2 is somewhat faint and uneven, but the price was US$116 shipped. No regrets - it's worked just fine for me.

My one complaint was that because it is essentially hand made, the circuit board rattled inside the case. On something like this that isn't unexpected. I put a few small pieces of sticky backed foam on the two 9V batteries and it completely solved the issue.

The board is hand made and my example was constructed properly, but I wouldn't say I was in awe of the soldering. So I'd simply say the device is "properly made".
 

flipflop

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Bhargu

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I thought the massdrop O2 only changed the connectors and the LED. If I remember correctly, the LED change only affects the drain rates of the battery. Since it is a desktop design, it should not affect the performance. Also, they didn't change the board design, only connected the changed connectors using cables. JDS and Mayflower does the same.

The power adaptor is not a properly rated one. Even though it says 12V AC, it is actually around 14V.
 

TungstenC

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The power adaptor is not a properly rated one. Even though it says 12V AC, it is actually around 14V.
It should output higher with no load since it's an unregulated power supply(transformer), the specified 12V rating is with the load drawing the rated current.
 
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έχω δίκιο

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It should output higher with no load since it's an unregulated power supply(transformer), the specified 12V rating is with the load drawing the rated current.
You beat me to it. I'd add that it's one example of where bigger isn't necessarily better. Many people make the mistake of buying grossly over-rated (current-wise) unregulated wall warts and then end up with too high a voltage.
 

TungstenC

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FYI I measured the output of the transformer supplied with the MD O2+SDAC (240V 60Hz, 12VAC 220mA)

Unloaded: 15.21 VAC
loaded(60 ohms): 12.85 VAC
Mains voltage during measurement: 230 VAC 50Hz
 
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amirm

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As it has already been requested, can you please double-check your output impedance measurements?
Other measurements show the O2 output impedance to be much lower.

0.54
http://nwavguy.blogspot.com/2011/07/o2-headphone-amp.html

0.71
https://www.innerfidelity.com/images/JDSLabsO2ACPowered.pdf

0.47
https://www.seeko.kr/zboard4/zboard...word=o2&select_arrange=headnum&desc=asc&no=13

<1
https://docs.google.com/spreadsheet...9VnTpkpjapOXt-4nFZ5YkGFIYs/edit#gid=292276897
reddit user slowsound
I have addressed this point before. My measurements usually show somewhat higher impedances than published or what others report. I use the same dummy load setup I use for THD+N vs power and such more cables and connectors are involved than someone just sticking a resistor right at the headphone socket. Since we are measuring such small values, these add up to to higher impedances as reported. I can switch to what others do but I think my test is more realistic as headphones also have thin and long cables that contribute the same increase.

Also, I do my testing with an audio analyzer at 1 Khz. This is much more accurate than anyone using a voltmeter. Impedance also changes with frequency so if someone is using a lower frequency, then they would get different values.

Bottom line, look these measurements as a comparison rather than absolutes. And that small differences of half an ohm or whatever is not material.
 

έχω δίκιο

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FYI I measured the output of the transformer supplied with the MD O2+SDAC (240V 60Hz, 12VAC 220mA)

Unloaded: 15.21 VAC
loaded: 12.85 VAC
Mains voltage during measurement: 230 VAC 50Hz
According to NWAVGUY's website:

NWAVGUY's blog said:
AC Wall Transformer - There has been some confusion over the wall transformer and it’s a critical component. Most wall transformers put out DC but the O2’s power supply requires at least 13.5 volts AC. The ideal transformer would be rated at 14 - 16 VAC and 400 mA or higher. 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.
 

TungstenC

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Hi. I found someone tore massdrop version down. So, can it make some differences from original nwavguy version
https://www.massdrop.com/buy/massdrop-o2-amplifier/talk/241801

I couldn't find the Reddit thread that had the pictures of the O2 internals from MD's product description with those "substitute" components but here's the response from MD in that thread.
MD Will said:
"That said, letting our contract manufacturer sub in some alternative parts to get us a prototype for pictures was definitely a poor choice we won't be making again. Think of how much pain and suffering could have been avoided if we'd waited the additional couple weeks it would have taken for sample quantities of all the proper parts to reach the manufacturer. Lesson learned there for sure.

Do you think it would be best to remove the PCB photos up right now to avoid confusion? Obviously it'd be best to post pictures of a PCB from our contract manufacturer with all the parts according to spec, but there's probably not enough time in this drop to make that happen... Actually, probably easier to add some notes on the pictures to avoid confusion instead. Let me know your thoughts on this."
https://www.massdrop.com/buy/massdrop-o2-amplifier/talk/242344

…and a peek on the MD O2 with a missing solder joint from a user that received a defective unit.
kCmj04v3RMm2jwVzbzCZ_20180730_233819533_iOS.jpg
https://www.massdrop.com/buy/massdrop-o2-amplifier/talk/2155941
 

Bhargu

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FYI I measured the output of the transformer supplied with the MD O2+SDAC (240V 60Hz, 12VAC 220mA)

Unloaded: 15.21 VAC
loaded: 12.85 VAC
Mains voltage during measurement: 230 VAC 50Hz

Someone else on MD measured 13.8 no-load and 13.2 loaded. One MD operations guy posted pics with 14.9 no-load and 14.2 loaded. So these adaptor units seems to vary a lot by unit.
 

Timbo2

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For those that have an O2 or are planning on using one on AC I’d suggest removing your audio source before removing it from mains AC.

Backfeed on input

I’d imagine it is minimal current, but it is enough to scramble the brains on my Topping D10.
 

TungstenC

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Someone else on MD measured 13.8 no-load and 13.2 loaded. One MD operations guy posted pics with 14.9 no-load and 14.2 loaded. So these adaptor units seems to vary a lot by unit.
Oh sorry, I didn't specified the load. I used a 60 ohms resistor during testing so It will be lower than what they tested with the O2 itself.
NwAvGuy: O2 Details said:
AC WALL TRANSFORMER: 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). Some 12 VAC transformers and/or a lower AC line voltage may cause the amp to exhibit higher distortion during low impedance sine wave bench testing as the power supply will fall out of regulation and let AC ripple through the regulators. A 14 - 20 VAC transformer solves the problem (like the WAU16-400 in the parts list). 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 On Semi regulators reach their drop out voltage at about 11.5 VAC loaded. If the amp is driving a low impedance load at high current levels, you need at least 12 VAC loaded (200 ma or about a 60 ohm 5 watt resistor if you want to test a transformer). This doesn’t leave much room for using a 12 VAC transformer if the line voltage is low (below about 117 VAC) hence the primary 14 – 20 VAC recommendation
 
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I couldn't find the Reddit thread that had the pictures of the O2 internals from MD's product description with those "substitute" components but here's the response from MD in that thread.

https://www.massdrop.com/buy/massdrop-o2-amplifier/talk/242344

…and a peek on the MD O2 with a missing solder joint from a user that received a defective unit.
View attachment 14537
https://www.massdrop.com/buy/massdrop-o2-amplifier/talk/2155941
It seems like that he changed his answer. Anyway, I see now Mayflower, MD have QC problem. I considered O2 + D50. But it seems better to wait topping's new amplifier( A50?).
 

Wombat

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Someone else on MD measured 13.8 no-load and 13.2 loaded. One MD operations guy posted pics with 14.9 no-load and 14.2 loaded. So these adaptor units seems to vary a lot by unit.

Or their mains voltage was different.
 
OP
amirm

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Veri

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It seems like that he changed his answer. Anyway, I see now Mayflower, MD have QC problem. I considered O2 + D50. But it seems better to wait topping's new amplifier( A50?).
I can recommend JDS too though. Workmanship is OK, customer service is stellar: if you end up with a problem they will help you however they can :)
 
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