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The Naked Truth Audio Calla Mono Amps, $5,000 pr. Naked Truth Audio, PO Box 1353, Ojai, CA 93024. ph
805.646.8383; fax 805.646.9683; e-mail info@nakedtruthaudio.com; www.nakedtruthaudio.com.

It's been a while since I've delved into tube power amplification. Had a bad string of fiery burnouts a few years back, and I
honestly have had little desire to get the fire extinguisher back out just in case. After the last amp that flamed a tube, I had this paranoia about leaving the listening room with the amps on; couldn't even go into the kitchen to eat without turning them off. It seems that every tube amp I've had in has had something go wrong during the testing period, save the Wolcotts, and as a result, one could say that I'm not a big fan of such devices.

It was over a year ago that I heard the NTA mono amps at the CES in Las Vegas (they were at T.H.E. Show actually), and had been pretty impressed with what I heard. I talked to a guy named Bill Sibner, and felt that he was a pretty straight shooter. His amp was probably a good product. Some time later I received an e-mail from Bill wanting to know if I was still interested in a review. Against my instincts I said yes, and the following resulted.

Tale of the tape. Due to the requirement that the amps be returned a little sooner than I would have liked, some of the details that I wanted to include here cannot be. However, I did have the amps long enough to listen to them thoroughly, and they were completely broken-in when they arrived, so that helped immensely. The Calla mono amps are OTL triodes in push/pull configuration. The amps deliver approximately 60 wpc into an 8 Ohm load, same for 4 Ohms. Each amp is assembled upon a super sturdy 12 awg chassis, upon which there are 8 6AS7 output triodes, and 3 6SN7 input triodes. Each amp has an RCA input for single ended operation and an XLR for balanced. On the back there are two pots with which to adjust bias and push/ pull balance. For adjustment there is a digital meter on the rear which eliminates the need for a multi-meter on the user's part. There is one set of speaker terminals for each channel. Coupling caps are either Relcaps, Hoveland, or the paper and oil AmpOhms -you choose. The amp sent to me used the Relcaps. All power supply caps are bypassed with small value polypropylenes, and all internal wiring is four-nines silver coated, Teflon insulated. Silver bearing solder is used. Inputs and output terminals are WBT, and chassis feet are Black Diamond Racing. Pots on the rear are from Bourns. Power supply capacitance equals 180 joules. Diode bridges are custom built using Intersil 4A, 1200V Hyperfast low noise soft switch diodes. Input impedance is 100 kOhms, and power draw is 250 watts. I didn't get to measure the chassis to ground potential, but did find that the amps sounded best in standard, or positive, polarity.

Set-up. By means of the meter on the back of the amp, and the provided instructions, setting the bias and balance was easy. This amp had some hours on it before getting to me, so tube drift was minimal during operation. I basically set it up per the
instructions, and didn't have to adjust it until it was time to send it back. One thing that I did find inconvenient was the meter
itself. For some reason in my room, the light struck it wrong and the digital readout was hard to read easily. Finding just the right angle to view it had me bent over and pinched between it and the rear wall. Fortunately, the amp proved rock steady and resetting was not necessary. I used the amp in balanced and single ended, but most of my listening was done in RCA single ended using the Symfonia Opus 8 preamp. Given the high input impedance of the amps, and being inherently easy to drive, every preamp in my collection drove them easily including the Aronov and Audire.
For listening, I used the 4 Ohm Osboms Oust to be mean) and the very friendly 8 Ohm Merlin TSM. I was surprised that the OTL was able to even do 4 Ohms, but it did and the sound did not suffer, at least not notably. But then again, I didn't drive it real hard into 4 Ohms either (my paranoia you know). I am compelled to say that the operation of the Calla was faultless the entire time that I had it... it didn't even burp.

The power cord used was essential to optimized operation. The manufacturer sends a standard Belden-type cord that really did have nice highs, and imaged pretty well. But I didn't like the midrange with it (disjointed), so my stable of cords were called upon to give the Calla a fair chance. Audience, no. TG, no. JPS, no. Walker, yes. Electra-Glide, yes. See a trend? The two ribbon based cords brought out the midrange and properly incorporated the bass into the sonic scheme of things. In the case of the Calla, the Walker was extremely dynamic and just a little bit dark chocolate sounding, but not much. The E-G was lighter sounding, but the soundstage was simply amazing .... a real 3d-BB. Actually, the two ribbon cords sounded very much the same in most important respects, but there were some minor differences as delineated here. I could have lived with either cord on these amps, outstanding they were, and neither changed the inherent character of the amps, which is what I am going to report upon. Throughout the audition period I used the TG Audio HSR speaker cables.

Sonics. Never played with an OTL amp over an extended time before. I enjoyed this experience though, as nothing caught fire, and the amps seemed very stable... I needed that. This was not however, my first experience with a high-end tube amp. I've owned an ARC 75, and I've reviewed sophisticated amps from Bruce Moore, Wolcott and VAC. And every year I spend time with valves at shows and in the homes of friends. So, a "newbie" I am not.

What did -I like best about the sound of the Callas? The midrange, what a midrange! In some respects, I've never heard anything like it ... not tube or transistor. It was the CD from Buddy Guy and Junior Wells called "Last Time Around - Live at Legends" that tuned me on to what was happening. It's an incredible CD when inflection, low level detail and dynamics are the issue. In other words, this is one super live and organic sounding recording. Track #7 was called up, and it sounded like I've never heard it before. I've had the amps on the Walker cords, and there was immediacy in the recording that had my ears seemingly at the mic inputs. It wasn't the detailing, the Moon W3 had as much or more of the detailing right, and the Clayton M100 mono amps had the tone, but what no other amp in this survey did was get the dynamic immediacy of the vocals so amazingly close to right. In that range it strongly reminded me of the dynamic rightness possessed by the CTC Blowtorch preamp. If you've been to live shows, even amplified ones, there is an explosive "thereness" to close miked vocals and instruments. It's like the sound skips your ears and goes straight to your musical core. It's fast, it's immediate, it's upon you with a suddenness and presence that rivets your attention to it - it's the ultimate in immediacy. In this case it was more than Junior Wells sounding like he was in the room, it sounded like I was getting a live feed of his vocals from the
club. Now you understand.

From that golden window in the midrange, the sonics paled somewhat going in both directions from it, higher and lower. Not to say that the frequency extremes were bad; they were not, but then again, the super dynamic transparency displayed in the midrange made any deviations from near perfection all the more audible everywhere else. Highs were well balanced and airy even with the provided power cord, cymbals had a nice metallic shimmer to them, and various bells and triangles could be differentiated easily by tone, color and sheen. Not the no-holds-barred best I've ever heard, but pretty dam good, and very much more than respectable.

The bass could be problematic, but aren't tube amps supposed to be that way? With power cords other than the ribbons, the middle and deep bass lacked some integration with the upper bass, and the emphasis in the lower regions was boomy. Enter the ribbons and my complaints disappeared, or at least were greatly lessened. A beater of Krells in the deep bass now? Nope. But, more in balance with the rest of the presentation. And when I added the ACI Jaguars with a pair of mono Titan powered subs (tricky to get right, but great once you do), the bass coming from the Callas was no longer an issue, and the listening could commence in all its glory. If this is what happens when you take the output transformer out of the equation and go OTL, I'm all for it.

Conclusion. Use the "right" power cords with this amp or die .... they be ribbons captain! There may be other cords that are not ribbons and cost less than the going rate for a set of quality ribbon cords (you'll need two you know) that sound good. I just don't know of them; educate me. Once past that, the midrange is the star attraction with this OTL, and a true star it is .... more lifelike, more dynamically present and solid than just about anything made for any money. Come to think of it, I heard a couple of ARC Reference mono amps a few years back have similar midrange impact and power, but if memory serves me right, they seemed more tonally washed out, less dense. But that's speculation from an old memory.

What power amps would I compare the Callas to from recent history? Some of the Premiers from CJ, 1987 vintage or thereabouts, but without the hazel burnish in the mids.

Oh well, this is not a universal power amp meant for everyone looking to energize a system. It has some incredible strengths, which if they are to be fully appreciated must be tended to with the utmost care. Don't ask it to do prodigious bass like the 500 wpc switching amps in some pimply faced teenager's Nova with 15 " woofers. If possible, take the bass out of the equation, and let the Calla sing an incredible song in the mids and highs. But if you must go full range, choose a loudspeaker of reasonable efficiency (though you don't need horns), and high enough impedance to help tighten things up a little,, 8 Ohms or better will do the job. Then sit back for the midrange ride of your life!

Martin G. De Wulf
Bound for Sound #134

Upcoming Reviews

Soundstage is scheduled to have John Potis review a pair of Calla amps in April.

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