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

[amirm]

@amirm

I think you mixed up the loads on the thd+n curves. The lower thd+n values should be on the 300 ohm one, right?

Good catch. The curves are correct as is the large notation on top. I just forgot to rename the graphs in the legend. Let me see if I can fix that.

 

[amirm]

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.

Extremely common? The most common power supply for a tube amp is a built-in AC power supply with the appropriate transformers. It is not remotely using a DC input and trying to step that up to hundreds of volts.

 

[amirm]

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.

Ah, you are correct. Somehow I thought one of them was in inverting mode. I will correct the review.

 

[Xulonn]

Mat at Techmoan pretty much agrees with your opinion about tube headphone amps, although he only did a non-technical review of three really cheap units. (He also has many cool reviews of other off-the-wall tech gadgets on his YouTube channel.)

However, in another review - this one of a tube buffer - Mat liked the subtle changes made to the sound by the Yaqin CD3, a 6SN7 (sorry, no blue LED's lighting up the tubes from the bottom) cathode-follower unit. In fact, Mat liked it enough to keep the Yaqin in his system, and the below review tells that story. The CD3 seems to be a pretty popular system "add-on." and I might get one to get my "fix" of tube nostalgia upon occasion. I like the idea of a relatively inexpensive but good quality tube buffer to let me play with "tube sound" by rolling tubes and perhaps even "upgrading" capacitors!!! (They've even published a schematic for the CD3!)



When researched tube tube buffers recently on the internet, if appeared that Yaqin seems to have the only decent good quality unit in the lower price range currently. And the Yaqin actually uses a transformer-based power supply to drive the drive the tubes to "normal" operating levels.

I had to chuckle at the description of the interior components when the bottom plate was removed, and when I saw the below comment in the video...(what could possibly be more meaningful here at ASR that referencing "an extensive scientific study" from a respected university?)

 

[Randem Tox]

Extremely common? The most common power supply for a tube amp is a built-in AC power supply with the appropriate transformers. It is not remotely using a DC input and trying to step that up to hundreds of volts.

I said converting one level of DC from a low-voltage SMPS to higher voltages was extremely common. Doesn't matter whether it's for tubes or not - that's by no means the only application - nor is it even close to the most extreme example of such DC-DC conversion.

You're the one that said such a supply "wouldn't allow it", which is absolutely not the case.

But not to worry ... instead of pointing out basic, factual, errors, or asking questions about statements that read as such, here so they can be addressed, I'll let others on the sites that like to attack you, your methods/measurements and your apparent general lack of attention to detail, find them instead.

 

[amirm]

I said converting one level of DC from a low-voltage SMPS to higher voltages was extremely common.

That is not the question I was asked. The question was in the context of the review and tube amplifiers in general. I am still waiting to see an example of a tube amplifier that uses a switching power supply to drive high voltage rails for amps.

But not to worry ... instead of pointing out basic, factual, errors, or asking questions about statements that read as such, here so they can be addressed, I'll let others on the sites that like to attack you, your methods/measurements and your apparent general lack of attention to detail, find them instead.

You are taking a specific answer and saying for general applications it is not true. I hope the difference is clear now. A proper tube amplifier as I was addressing is simply not designed using an external power supply.

 

[Xulonn]

A proper tube amplifier as I was addressing is simply not designed using an external power supply.

At least not a switching model - the typical "brick"'
Here's the HiFi Man EF5 headphone amp (now out of production?). Rather than a "brick" type switching PS, it used a transformer-based external PS.

 

[amirm]

At least not a switching model - the typical "brick"'
Here's the HiFi Man EF5 headphone amp (now out of production?). Rather than a "brick" type switching PS, it used a transformer-based external PS.

View attachment 14470

Now that is a good correction. Yes I was meaning having a little external switching supply. "Audiophile" amps do exist both for headphones and speakers that use external power supplies generating high voltage.

 

[derp1n]

The lads at head-case.org were pretty harsh on the CTH's engineering.

 

[restorer-john]

It's done all the time for generating significantly higher voltages

Perfectly true. It's been done for 50 years with switching inverters for VFDs (which are essentially a triode).

In the early 1970s we had vacuum fluorescent displays in pocket calculators and they all used a miniature inverter to get the voltages up around 150v from 2xAA batteries.

This in the Cavalli:

 

[amirm]

The lads at head-case.org were pretty harsh on the CTH's engineering.

Here I thought I was rough on manufacturers.

 

[mindbomb]

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.

I have an o2 amplifier (from massdrop), and this is something that often strikes me when using other headphone amplifiers. Nwavguy said this doesn't happen on the o2 because he had the gain stage before the volume control, and that protected the volume control from dc, as well as allow him to lower the thermal noise.

 

[cjfrbw]

Another interesting review. I was under the impression that Cavalli wasn't making headphone amps any more. Maybe he is just lending brand name and circuitry to other manufacturers.

 

[maverickronin]

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.

The O2's external power is just a plain AC transformer, not an SMPS.

 

[amirm]

Another interesting review. I was under the impression that Cavalli wasn't making headphone amps any more. Maybe he is just lending brand name and circuitry to other manufacturers.

Definitely looks like a revised design:

I wonder if he is getting anything for lending his name this way.

 

[maverickronin]

Another interesting review. I was under the impression that Cavalli wasn't making headphone amps any more. Maybe he is just lending brand name and circuitry to other manufacturers.

He's licensing designs now. There's another from Monoprice which is supposed to come out some time this year.

 

[έχω δίκιο]

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...
...
This is a unit that is designed from ground up with attention to clean, clean design, verified in every step using instrumentation...

The latter explains the former. NWAVGUY didn't want his carefully engineered design, which he verified through instrumentation, to be messed up by some hack who decided to cut costs or "improve" it without measuring the effects of the changes. It's intended as proof that you can build a top-notch headphone amp at a modest cost using good engineering verified through objective measurements, one that outperforms much of the Schiit out there on the market. The effort is all for naught if some Chinese vendor offers a $28 "version" that substitutes the cheapest possible components, or if someone sells one with "class A audiophile discrete" boutique op amps that underperform the carefully chosen parts he used.

 

[Raimei Templar]

Glad I did not purchase the CTH, wonder if the Liquid Carbon X is also flawed?

I very nearly purchased that a few weeks ago.

Its interesting that you note that the O2 seems to increase/improve bass response subjectively, I also thought the same thing when I tried it and compared it to the Schiit MAgni. I got told I was wrong on some forums, that it was magically neutral! It is a wonderful product and JDS labs are great to deal with, I recommend it over the Magni. Just doesnt look fancy.

 

[alex]

I own a CTH. There are a couple pretty bad things about it, but I enjoy the sound it has over my laptop, phone or dragonfly red.

I do think the build quality is quite bad, because I had one for a couple of days and it must have shorted because it stopped working. They sent me a new one no problem and I still have the old one in my closet. Not sure what I'm going to do with it.

The hole for the tube, as mentioned in the headcase teardown, does not line up with the tube. Its kind of hilarious how little attention to detail they have. It isn't just a cosmetic thing as it makes it harder to get the tube in and keep it in. The slightest knock of the amp will push the tube out of place because there is constantly force pressing against it.

Aside from these facts it is relatively cheap and I do like the sound, even though I haven't used a lot of desktop sized amps in the past so I don't have much to compare it against. It seems it has higher definition than my LGV30 or my dragonfly red.

Unfortunately I can't use it anymore because I bought some absurdly sensitive IEMs and am pretty much exclusively listening to them. They are so sensitive they hiss out of my laptop. On the CTH there is a really high level of hiss and its unlistenable.

 

[Wombat]

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

The link above is a great article on designing a headphone amp. Essential reading for those interested in headphone amplifiers. It will answer questions one hasn't yet thought to ask.

I sure knew what I was getting when I purchased my O2 - from Mayflower electronics.