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

amirm

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#1
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.

Alex Cavalli Tube Hybrid (CTH) and JDS Labs O2 headphone Amplifier Review and Measurement.psd.jpg

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.

1533147959992.png



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:
Alex Cavalli Tube Hybrid (CTH) headphone Amplifier dashboard Measurement.png


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:
JDS Labs O2 Headphone Amplifier Dashboard measurement.png


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:

Alex Cavalli Tube Hybrid (CTH) and JDS Labs O2 headphone Amplifier 1 kHz FFT Comparison Measur...png


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:
Alex Cavalli Tube Hybrid (CTH) and JDS Labs O2 headphone Amplifier THD vs Power at 300 ohm Com...png


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:
Alex Cavalli Tube Hybrid (CTH) and JDS Labs O2 headphone Amplifier THD vs Power at 50 ohm Comp...png


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:
Alex Cavalli Tube Hybrid (CTH) and JDS Labs O2 headphone Amplifier frequency response at 300 o...png


We see some channel imbalance in both (1 volt output) so let's drill into that. First up is CTH:
Alex Cavalli Tube Hybrid (CTH) headphone Amplifier Channel Match Measurement.png


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

Here is JDS O2:

JDS Labs O2 Headphone Amplifier Channel Imbalance measurement.png


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.

-------------

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).
 
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mi-fu

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#2
Thanks Amir. The CTH review is one that I have been waiting for.

Sorry for the newbie question. You mentioned "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."

Do you mean that the tube does not really play a large role in the sound where it should?
 

amirm

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#3
Sorry for the newbie question. You mentioned "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."

Do you mean that the tube does not really play a large role in the sound where it should?
Yes, a "real" tube amplifier would use much higher voltages to get the tube to work in the ideal operating point. They would have internal power supplies to generate the high voltages they need. Using an external switching power supply generating DC does not allow that.
 

mi-fu

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#4
Yes, a "real" tube amplifier would use much higher voltages to get the tube to work in the ideal operating point. They would have internal power supplies to generate the high voltages they need. Using an external switching power supply generating DC does not allow that.
Got it! Thanks!!
 
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#5
Yes, a "real" tube amplifier would use much higher voltages to get the tube to work in the ideal operating point. They would have internal power supplies to generate the high voltages they need. Using an external switching power supply generating DC does not allow that.
Emphasis mine ...

How so?

It's entirely routine to generate all manner of different voltages from a single DC input, switching or otherwise. As long as the total internal power consumed isn't greater than that supply can deliver (including allowing for conversion losses) its a non-issue. In fact it's super common to do this to yield multiple different voltage levels in a single device from a single SMPS supply.

That's not to say it's necessarily the BEST way to do it, but it is absolutely doable.
 

amirm

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#6
t's entirely routine to generate all manner of different voltages from a single DC input, switching or otherwise. As long as the total internal power consumed isn't greater than that supply can deliver (including allowing for conversion losses) its a non-issue. In fact it's super common to do this to yield multiple different voltage levels in a single device from a single SMPS supply.
It is for generating different low voltages. Generating hundreds of volts though for a tube circuit is a different animal than just buying some off-the-shelf switchers to generate 5 volt and such.
 
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#7
It is for generating different low voltages. Generating hundreds of volts though for a tube circuit is a different animal than just buying some off-the-shelf switchers to generate 5 volt and such.
It's done all the time for generating significantly higher voltages (multiple kV if you like) as well, even from a trio of AAs.

No big deal at all to take a 28v input and get 250v out of it.

Again, may not be the best approach but that's a long way from "won't allow".
 

garbulky

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#8
Amir, it's nice to see you finally write about one of your products. :p
(I kid!)
It looks like the CTH has quite a good bit of horsepower. And its distortion is appropriately low though its lower power output distortion figures aren't that stellar.
 

amirm

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#10
Forgot to note that in CTH one channel creates static as you change volume. Likely DC running through that one channel's pot. It was audible with both my sensitive IEMs and Sennheiser HD-650.
 

έχω δίκιο

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#15
There is a little channel inbalance with the O2. Is it audible?
I believe (someone correct me if I'm wrong) that the imbalance is likely to be explained by tolerances in the resistors and/or the analog, dual-ganged, audio taper pot used for the volume control. That means that there is probably a unit-to-unit variance in channel balance, and we don't know if what amirm measured is worse than average, typical, or better than average for this unit. I've had an O2 for several years that I built from parts from JDS Labs and I have not perceived a channel imbalance.
 

amirm

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#16
There is a little channel inbalance with the O2. Is it audible?
I will need to test it with my sensitive IEMs to be sure. With normal headphones and at my listening levels, no I di
t's done all the time for generating significantly higher voltages (multiple kV if you like) as well, even from a trio of AAs.

No big deal at all to take a 28v input and get 250v out of it.
If you want dirty power, at very low amps, sure. Have you seen a tube amplifier powered such?
 

Timbo2

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#18
I believe (someone correct me if I'm wrong) that the imbalance is likely to be explained by tolerances in the resistors and/or the analog, dual-ganged, audio taper pot used for the volume control. That means that there is probably a unit-to-unit variance in channel balance, and we don't know if what amirm measured is worse than average, typical, or better than average for this unit. I've had an O2 for several years that I built from parts from JDS Labs and I have not perceived a channel imbalance.
I measured mine with an old Fluke 73 meter and got a delta at various positions of around 0.0004 and 0.0010 volts with a maximum voltage of 1.970/1.974 from my D10 on 1kHz sine via YouTube.
 
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#19
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, each of which handles one half of the cycle.
That statement totally intrigued me, but the schematic didn't back it up. It looks like the op-amps are used in parallel to increase the current output of the amp. Which NWAVGUY talks about here: http://nwavguy.blogspot.com/2011/07/o2-design-process.html

Thanks for the link to the design! It's great to read all the design tradeoffs that go into something like this.

--chris
 
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#20
If you want dirty power, at very low amps, sure.
There's no reason for it to be any dirtier than any other switching supply; that's just a question of what you do after the raw DC-DC conversion.

And yes, of course, if you're boosting voltage it has to come at the expense of a commensurate drop in available current. Just as it would with a conventional transformer/linear supply. Last time I checked I wasn't getting 600 volts out of a standard US AC socket.

Have you seen a tube amplifier powered such?
I believe the MCTH is one such example.

I can think of a couple of well-regarded, currently-product, battery-powered tube amplifiers even (though those may be using tubes with lower voltage requirements than the 6922 - I'd have to check), including units from Woo and ALO.

Again, I'm not saying it's the BEST way to do it ... but if you're shooting for accuracy and an objective persuasion then absolute/definitive statements about things not being "allowed", when they're actually extremely common, is probably a bad way to go about it.
 
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