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.
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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:
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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:
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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:
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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:
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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:
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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:
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We see some channel imbalance in both (1 volt output) so let's drill into that. First up is CTH:
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Until we get to pretty low output levels, deviation remains below 0.6 dB or so.
Here is JDS O2:
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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.
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