Disclaimer: ALO Audio graciously supplied the CDM for the purpose of a Headfonia review. I paid nothing for it.

Subject: ALO Audio Continental Dual Mono (CDM)

I got into valve amplification late. The first valve amp I really latched onto was the Woo Audio WA3. As you know, I'm a firm believer in ALO Audio's Studio Six (reviewed here) and the lovely Bender, Pan Am (reviewed here).

CDM is somewhat of an enigma. It comes with batteries, it is drilled with vents, and it even packs in a tote sack; you'd think it was a portable amp. It is. And it isn't. Firstly, it gets mad hot. Second, it wont' fit in your pocket. In fact, putting inside anything isn't a good idea, at least while it's on. Use that tote bag to safely transport it from listening venue to listening venue.

But while it's on (and not inside anything), and while you're plugged in, and while Dire Straits is rubbing a hole in your ears, CDM is phenomenal. It's got a great DAC that decodes DSD, a powerful, a current-strong headphone amp that snubs loads, a low-noise output, and that warm, sometimes-fuzzy valve sound.

And, you can roll those valves. You'll need to watch out for ESD and be handy with an allen key, but you can roll those valves. Wonderful.

The biggest drawback of valves is that they output higher absolute levels of distortion. Even ALO's flagship, Studio Six, maintains a high amount of both THD and IMD distortion that all audio analysers interpret as aberrations. And for good reason: they are. But that's par for the course. Valves distort. As long as that distortion isn't exorbitant, some people (myself included) love how it sounds. Certain amounts of IMD and THD distortion are acceptable; under the right circumstances they sound good, or preferable to certain people.

But, to others, they are unacceptable.

Aberrations such as sudden upticks in any distortion vis-a-vis unloaded signals, or the loss of frequency information in certain bands is a bad thing and the sign of a bad amplification circuit. To see what I'm talking about, check out RMAA results for COZOY's Astrapi, which falls apart under loads that the iPhone battens down pretty well. CDM's THD and IMD numbers are not good. But no valve amp I've tested posts good IMD and THD numbers. That CDM's numbers neither escalate under load, nor drop frequency information when under load, is proof of a good amp circuit.

In every metric, CDM outperforms Jaben's Porta Tube+, an amp/DAC onto which I heaped praises back in 2012.

Another thing to note: CDM employs the Wolfson WM8741's low-pass filter, which gently rolls off high-frequency information when being fed through USB. Some will like this. Others will not. Finally, CDM's line out from the DAC is very good.

Later today, I will publish a review of the CDM at Headfonia. The below RMAA and square wave results will be referenced in that review.

NOTE: I chose the iPhone 6 to supply the analogue signal for the CDM for several reasons:
1. I own it and so do many other people
2. The quality of its output more than satisfies the requirements of the CDM
3. Other analogue outputs, such as from the Ryuzoh-modified AK240 perform only marginally better the iPhone 6 when connected to the CDM. I chose not to use it because it is not mine. I want to establish a suite of standard-tested devices to which I have long-termm access.

The following Rightmark Audio Analyzer tests were conducted through this equipment.
Source: ALO Audio CDM through Apple iPhone 6 and iMac 27" (2012)
ADC: Lynx Studio HILO LT-TB
Computer: 2012 27" iMac
Cables: 1,5m Hosa Pro 3,5mm stereo to dual 3-pin XLR (around 8$)

Loads:
NL - no load
SM2 - Earsonics SM2
ES7 - Audio Technica ES7
DT880 - Beyerdynamic DT880/600

16-bit & 24-bit TARGETS no load