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

Blank Verse

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Thank you Amir for your excellent reviews. I just got my first headphone amplifier recently (a JDS Labs O2) and in great part I did it thanks to your endorsement. I don't plan to power anything too demanding (like HE6). I'm planning to use this with HD600 mostly, though I was thinking about the K240 Sextett or K240DF. I know these cans are demanding, and I wonder if any people have tested them with the O2.

I am no audiophile, but to my ears the O2 is a golden amp. Extremely clear and detailed. I can't believe the amount of resolution and details I can hear, even with my lowly MDR-7506. On the negative side, a lot of my material now seems to have glaring flaws I didn't notice before.
 

Jimster480

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Thank you Amir for your excellent reviews. I just got my first headphone amplifier recently (a JDS Labs O2) and in great part I did it thanks to your endorsement. I don't plan to power anything too demanding (like HE6). I'm planning to use this with HD600 mostly, though I was thinking about the K240 Sextett or K240DF. I know these cans are demanding, and I wonder if any people have tested them with the O2.

I am no audiophile, but to my ears the O2 is a golden amp. Extremely clear and detailed. I can't believe the amount of resolution and details I can hear, even with my lowly MDR-7506. On the negative side, a lot of my material now seems to have glaring flaws I didn't notice before.
It is always a slippery slope as more quality = more flaws revealed.
I also have a JDS O2 (mine is a custom unit) and I loved it for a few years. Now I don't use it anymore ever since I got the THX AAA789 because i just have one amp on my desk. However I will likely use it again when I build a second setup for my other desk.
I never had any issues with it and JDS Labs really stands behind their products!
 

Blank Verse

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It is always a slippery slope as more quality = more flaws revealed.
I also have a JDS O2 (mine is a custom unit) and I loved it for a few years. Now I don't use it anymore ever since I got the THX AAA789 because i just have one amp on my desk. However I will likely use it again when I build a second setup for my other desk.
I never had any issues with it and JDS Labs really stands behind their products!
Yes, you are absolutely right. It makes me wonder what kind of equipment some engineers master with HAHA. But it is like cleaning a window which hasn't been cleaned in years, everything is more clear and if, say, it allows you to see the hobo down the street taking a leak in a corner more clearly, too bad for you I guess. ;)

Yes, even though I bought my unit second hand I contacted JDS a few weeks ago and I have never had such a pleasant interaction with a retailer/manufacturer since I was a kid with the icecream truck man. A really standup company.

Using the O2 as a secondary unit makes a lot of sense because it is so compact. Mine has the AC and line input in the front which makes it a little more unwieldy, though. But it is still worth it for the results. BTW, what cans have you used the O2 with?
 

Jimster480

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Yes, you are absolutely right. It makes me wonder what kind of equipment some engineers master with HAHA. But it is like cleaning a window which hasn't been cleaned in years, everything is more clear and if, say, it allows you to see the hobo down the street taking a leak in a corner more clearly, too bad for you I guess. ;)

Yes, even though I bought my unit second hand I contacted JDS a few weeks ago and I have never had such a pleasant interaction with a retailer/manufacturer since I was a kid with the icecream truck man. A really standup company.

Using the O2 as a secondary unit makes a lot of sense because it is so compact. Mine has the AC and line input in the front which makes it a little more unwieldy, though. But it is still worth it for the results. BTW, what cans have you used the O2 with?
Yea my customizations are to have everything on the back like a traditional amp and just have the headphone jack in the front (I did a 3.5mm because that is what I have the most of) and I have a 1x/3.3x gain instead.

I used it with my Denon AH-MM400, 1More Triple Driver Over-ear & Quad Driver IEM & Triple Driver IEM (the last 2 are lost now at an event) and Panasonic RP-HD10K, Oppo PM-2 and MrSpeakers (now Dan Clark Audio) Aeon Flow Closed.. we have tested out various other headphones like Hifiman Ananda & HE6 and LCD2, Aeolous etc at headphone meets in the past too. It didn't have an issue driving basically anything we connected it to.
 

Blank Verse

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Yea my customizations are to have everything on the back like a traditional amp and just have the headphone jack in the front (I did a 3.5mm because that is what I have the most of) and I have a 1x/3.3x gain instead.

I used it with my Denon AH-MM400, 1More Triple Driver Over-ear & Quad Driver IEM & Triple Driver IEM (the last 2 are lost now at an event) and Panasonic RP-HD10K, Oppo PM-2 and MrSpeakers (now Dan Clark Audio) Aeon Flow Closed.. we have tested out various other headphones like Hifiman Ananda & HE6 and LCD2, Aeolous etc at headphone meets in the past too. It didn't have an issue driving basically anything we connected it to.
Thank you for the details. That's quite some variety. What I heard before is that HE6 were not driven very well from the O2, but it's good to know that the results are at least acceptable, because those have to be some of the hardest cans to drive.

I'm looking forward to getting a few headphones (HD540, K240DF, etc) with the security that I won't have trouble driving them.
 

Jimster480

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Thank you for the details. That's quite some variety. What I heard before is that HE6 were not driven very well from the O2, but it's good to know that the results are at least acceptable, because those have to be some of the hardest cans to drive.

I'm looking forward to getting a few headphones (HD540, K240DF, etc) with the security that I won't have trouble driving them.
The HE6 required my 3.3x gain and the knob to be pretty cranked, however it was definitely possible to listen to.
 

Garlucky

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Can I leave a headphone jack adapter in the o2 while It's turned on, without having a headphone connected?
 

rongon

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am still waiting to see an example of a tube amplifier that uses a switching power supply to drive high voltage rails for amps.

It's an exaggeration to ask for a "tube amplifier" using this power supply configuration. When one reads that, one thinks of a big old push-pull tube *power* amp like a Dynaco Stereo 70, when here we're talking about a single little preamp tube (a 6922).

1. Tube *preamps* do not require very much current. A 6922 (equivalent to 6DJ8 or ECC88) will require at most 20mA (0.02A) for a pair of triodes running full tilt. In this design, it could be drawing as little as 6mA (0.006A).

2. The 6922 can work pretty well with as little as 65VDC plate (anode) to cathode. If there's a constant current load in the plate of the 6922, the high voltage power supply rail could be as low in voltage as 100VDC with no penalty. Getting 100VDC good for 20mA of current from a (for example) 24VDC 1A supply is not at all difficult. There are switch-mode boost converters that can do that. You can buy them from sellers on eBay, even. Certainly also from Mouser, Digikey, Aliexpress, Taobao, etc.

3. A guy who goes by kodabmx on diyAudio.com uses one of these boost converters for his tube preamp B+ supplies. The one he uses can accept 12VDC and boost that to over 200VDC. Of course the current is stepped down in proportion to the step up in voltage. As mentioned above, that is not at all a problem in a design like the CTH.

I hope that helps clarify the matter once and for all.
 

Roen

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This is a review and detailed Measurements and comparison of Massdrop Alex Cavalli Tube Hybrid (CTH) and JDS O2 Headphone Amplifiers. The CTH is on kind loan to me from a forum member. I purchased the JDS Labs O2 last year but have not had time to review it until now.


As the name indicates, the CTH is a hybrid design meaning it uses a tube as a buffer/pre-amp which then drives transistors for output stage. This solves the problem of needing a transformer on the output of the tube amp to bring its impedance down. Like countless designs like it, it uses a low voltage external power brick meaning the tube is not really operating in the region it is designed for. So likely provides little to no gain.

The CTH came beautifully packaged with custom foam inserts in a pretty large package. The unit itself is very attractively finished and gives a feel of quality.
There is a protection circuit which cuts out the output in the case of excessive distortion/DC which is nice.

The JDS O2 is a traditional solid-state design using op-amp integrated circuits. It was designed by now missing but famous, NWAVGUY. He went through a large scale evaluation of ideal op-amps for such use and the design is incredibly simple. A buffer op-amp feeds two others in parallel to increase power handling. A 1 ohm output resistor is used for better load sharing.

View attachment 14452


Protection circuit is afforded by the built-in mechanism of the output op-amps. The most complex part of the design -- if you can call it that -- is the power management circuit which is there because the unit uses a couple of 9 volt rechargeable batteries. So the unit is portable and can be used for up to 7 hours depending on volume.

The design is offered as open source for the world to use but unfortunately with restrictive licensing which doesn't allow modification. The change of a minor circuit element could violate the license, leading to really strange designs as the amp is paired with DACs in the same box.

This is a unit that is designed from ground up with attention to clean, clean design, verified in every step using instrumentation (Prism Sound dSound analyzer). All of it is extensively documented on NWAVGUY's website (http://nwavguy.blogspot.com/2011/08/o2-details.html).

Pairing such a design against a tube one may seem unfair. But I had no other tube amps to test against. A tube amp is supposed to come and I may compare them against each other. For now, I am told if the O2 is configured similarly to CTH, they will be retail for $200 so it is "fair" battle from that point of view.

So let's get into the measurements and see how they do.

Measurements
I wanted to start with our usual dashboard I show for DACs but ran into a problem of what voltage input to use. With DACs the input is digital so I set it to max. Here it is an analog input so I had to pick something. Furthermore, I had to pick something for output voltage. I looked at the specs for massdrop's CTH and it said that their distortion rating was at 1 volt RMS. It doesn't say if it is input or output but I assumed the latter. So for simplicity, I picked the same 1 volt input which means the unit is not amplifying anything (acts as a "buffer"). So it ought to provide some of the best response it can.

Here is CTH:
View attachment 14453

Massdrop specs CTH at 0.0014% THD+N with this output. As you see, what I get is about five times higher at 0.009%. SINAD which is our signal above noise and distortion is as a result a lackluster 80 dB. What you don't see is that the number was jumping up and down constantly to tune of 3 dB or so.

Just as well, the significant amount of noise in the 1 kHz FFT on top right was changing in every frame displayed by my Audio Precision Analyzer. To the left, we see significant amount of mains leakage which is most likely due to emissions mitigation in the switching external power supply used.

To the right, we see a single second harmonic and nothing else with respect to harmonics. This is very good in the context of what this device is supposed to do: provide that "tube sound." Alas, there is no excuse for the ton of noise and mains contributions which do nothing euphonic and only rob resolution from the device. Fortunately our hearing in low frequencies is poor so the mains noise was not audible.

Here is the JDS Labs O2 Dashboard using the same settings:
View attachment 14454

See that using similar external switching power supply, the mains leakage at 60 Hz is impressively low at -125 dB. We see a regular train of harmonics of our 1 kHz tone. Our SINAD numbers are nearly 20 dB better, which means we get to hear what our good DACs can produce (the best of which go up to 115 dB of SINAD). There is an unfortunate rise of higher up harmonics above 5 kHz which would have been nice to not see.

I thought for fun and to dig further into 1 kHz noise and distortion, I put a "wide angle lens" on my Audio Precision APx555 analyzer and show the response all the way up to 1 Mhz with 1.2 million point FFT. This is what we get from the two products:

View attachment 14455

Now we can clearly see how much higher the mains noise is in CTH if you look to the left of our main 1 kHz tone. The op-amps used on O2 provide great rejection of power supply noise (major advantage over discrete designs). We also see far less high-frequency noise from the switching circuits of the external power supply in O2 in the > 100 kHz region than the CTH. Neither product comes with regulatory certification so one would wonder if the CTH can even meet the emissions standards at those levels.

Most important graph in headphone amp is power vs distortion and noise (THD+N) so let's review that at 300 Ohm:
View attachment 14486

The sloping down part of the graphs is the "noise dominated" segment and the JDS O2 naturally shines. Its performance advantage is massive over CTH. Alas, it runs out of power sooner, delivering half the power compared to CTH.

We see similar results with 50 ohm load:
View attachment 14487

If one picks 0.01% distortion+noise as the worst case limit, the CTH fails that at all power levels! In my experience, things get pretty audible above 0.1% so with that much lower bar, the CTH is good for 1 milliwatt to 1.3 watts of power. Full spectrum of JDS O2's power can be used until clipping.

BTW, massdrop rates the CTH at 50 ohm to have 1 watt of power which per my measurements above, it beats easily. So good on them for being conservative!

Frequency response is flat to 45 kHz limit I set on my analyzer:
View attachment 14458

We see some channel imbalance in both (1 volt output) so let's drill into that. First up is CTH:
View attachment 14459

Until we get to pretty low output levels, deviation remains below 0.6 dB or so.

Here is JDS O2:

View attachment 14460

Similar level of deviation but at other regions.

I think that is it. Let's get into subjective listening results.

Listening Tests
I adjusted the levels on both amps relative to 1 kHz tone and then used my AB passive switching box to alternate between the two amps. I used the Topping DX7s to drive both units using an RCA splitter cable. Listening levels were good to somewhat loud levels.

The immediate signature difference was slightly accentuated highs in CTH. This makes sense due to higher distortion levels in this unit. The high frequency emphasis gave a slight sense of wider soundstage as it usually does. In contras the JDS O2 produced what I call a more natural high frequency response with no exaggeration. Note that those of you younger than me with better high frequency response will hear more of a difference here.

The other noticeable difference was the better resolution of JDS O2. Decaying notes would vanish sooner in CTH, producing a more dull and lower fidelity. Again, this is a minor thing and would most likely require training to hear. Otherwise, both reflect the fidelity of what is being played.

I also thought the bass response was ever so slightly better in JDS O2 as levels went up.

I heard nothing that was "euphonic" or that "magic tube sound." Whatever magic there is, is taken over by higher noise levels and distortion.

All of this is consistent with previous tests of these hybrid amps or amps with high levels of distortion.

Conclusions
The JDS O2 labs is what its designer said it would be: a very well executed, simple headphone amplifier with very nice response. Its weakness for some people, of which I am one, is not having extreme amount of power. To that end, I like the iFi iDSD Black better. Click to see the review of iFi iDSD Black Edition. If you don't need the extra power and built-in DAC, then the JDS O2 Labs gets my recommendation.

The Massdrop CTH amp is beautifully put together and attractively priced. Alas, despite its heritage, it misses on design excellence such as suppression of mains noise. It doesn't seem to have benefited from proper design verification -- certainly not to the scale of O2. It does have more power and if you are not super critical about your listening, it does the job of producing good sound. Fundamentally I don't understand or see any value in these hybrid tube amps so I can't recommend it based on engineering.

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As always, questions, comments, recommendations, etc. are welcome.

If you like this review, please consider donating funds for these types of hardware purchases using Patreon (https://www.patreon.com/audiosciencereview), or upgrading your membership here though Paypal (https://audiosciencereview.com/foru...eview-and-measurements.2164/page-3#post-59054).
Do you remember what the gains were in the O2 that you purchased? Was it 2.5x and 6.5x or was it 1x and 3.5x?
 
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