Expert review and test of the Nagra Classic Preamplifier and Classic Amp  by Australian Hi-Fi Magazine. Free pdf download included.

If your tastes lean to the unusual and the exotic, and you’re in the market for a pre-power combo, you need to investigate Nagra’s ‘Classic’ Series… in particular its Classic Preamp and Classic Power amplifier. They’re a world away from looking mass-produced, because they’re not. They’re hand-built in Switzerland to exacting standards—which you might have guessed simply by looking at the photographs above of them—and they combine the magic of valve sound with the might of solid-state.

Nagra Classic Preamp
Nagra’s ‘Classic Preamp’ is a hybrid valve/solid-state design that uses a pair of 12AX7 dual triodes and a 12AT7 high-frequency twin triode. These are miniature valves with a very long service life, so will rarely need replacement. And, when it does come time to replace them, you’ll have no difficulty obtaining them because they’re extremely popular and quite common valve types… unlike the valves used by some UK manufacturers, for example, which are available only from the manufacturer!

The strange-looking meter at the left side of the Classic Preamp is what Nagra calls a ‘modulometer’. It’s basically an old-fashioned moving-armature voltage meter but Nagra has rather cleverly turned it ‘back to front’ so what looks like a knob at the front of the meter actually houses the operating parts, enabling access for calibration or repair without having to disassemble the chassis. Not that this is likely, however… moving-armature voltmeters are so reliable and robust that one built 30 years ago will likely outlast an LCD or LED display that was built yesterday. Nagra uses its modulometers for different purposes on its various products. 

The one fitted to the Classic Preamp shows voltage at the pre-amplifier’s output terminals (though in decibels referenced to a 1 volt output, rather than directly as volts).
The toggle switch to the left of the modulometer is not a power switch, as you might have guessed, but a spring-loaded multi-select actuator which allows you to adjust the brightness of the modulometer’s backlighting through six levels as well as turn it off.

Just to the right of the modulometer is a LED display that shows the input source you’ve selected by rotating the electronically-actuated rotary control to the right of the display. In its default mode it cycles through the five available inputs showing in turn XLR, RCA-1, RCA-2, RCA-3 and RCA-4, with relays clicking with each selection. As you can see, all inputs are line-level types—there is no phono input, so if you want to connect a turntable, you’ll need an external phono preamplifier.

Despite the source switching being controlled completely electronically, the controller does not cycle continuously in a loop: once you have reached RCA-4, you have to counter-rotate the control to access the other inputs. You can rename all these inputs if you like, though you’re limited in the number of characters you can use for each one.

 The display also allows you to check how many hours are on the valves, whether you’ve set the preamplifier for mono or stereo operation (as well as set it for either!), and indicates whether you’ve adjusted the balance of the left and right channels away from the default (0dB) for each. All of these operations are accessed by fairly arcane processes that involve alternately pressing and turning the controller dial: This is a bit tricky to master, but strangely satisfying once you have. The rotary volume control is partially recessed into the front panel and its range of action can be modified using the small toggle switch to its left, which offers a choice of gain modes (0dB or +12dB).

Just to the right of the volume control is a small toggle switch to select which of the Classic Preamp’s outputs you want to use: XLR (balanced), RCA (unbalanced) or the headphone socket on the front panel. The only issue I had with this arrangement is that if your headphones are inefficient you’ll have to turn the volume up when listening to them, which means you’ll have to be very careful to make sure you turn the volume down before resetting the switch to either the XLR or RCA position, otherwise you’ll blast your speakers with high-volume sound. Although the headphone socket looks quite unusual, it’s a standard 6.5mm stereo socket—it just has a protruding shroud around its periphery.

The large control to the right of the volume control is a power switch… well, actually, it’s more than a power switch. In addition to using it to turn the Classic Preamp on and off, it also selects ‘Mute’ plus has a special position ( R ) to which it has to be switched if you want to use one or more of the several different remote controls that are available to control Nagra components.

Although the Classic Preamp can be connected directly to the 240V mains, it can be optionally powered by an external power supply, such as the Nagra MPS (Multiple Power Supply) or the Nagra ACS II. One reason for using the MPS might be to provide total isolation from the mains power supply, because the MPS can be fitted with an Li-Ion battery.

Also available for the Nagra Classic Preamp are optional input and output transformers (hand-wound in-house by Nagra) to create a fully symmetrical floating signal.

Nagra Classic Amplifier
Nagra’s Classic amplifier can be user-configured either as a stereo power amplifier, in which configuration it’s rated by Nagra as having an output of 100-watts per channel into 8Ω, or as a mono power amplifier, in which mode it’s attributed with a 200-watt output power rating.

The single bar control switch on the front panel of the Nagra Classic power amplifier has positions for ‘Off’, ‘Auto’, ‘Mute’ and ‘On’. The ‘Auto’ setting enables the Classic to switch itself in and out of Standby mode, so it switches itself ‘On’ when it detects an audio signal at its input and ‘Off’ when no signal has been detected for a period of time.

Nagra’s distinctive moving-armature modulometer makes an appearance here as well, though in this case it’s a ‘double modulometer’ because the meter has two needles rather than just the one. The black-tipped needle shows the output of the left channel and the red-tipped needle shows the output of the right channel. There are different calibrations for stereo operation into 8Ω loads and for mono operation into 8Ω loads. To the right of the modulometer is a momentary-action toggle switch that can be used to dim the display lighting through six levels or turn it off completely.

Although the double modulometer can be a guide to how much power is going to your loudspeakers, the needles’ response time is not fast enough to show if the amplifier goes into clipping (which and amplifier will do if you accidentally overdrive it), so Nagra has helpfully included a chameleon LED that flashes when the amplifier goes into clipping to alert you to turn the volume down. It flashes yellow when momentary clipping is detected (which is harmless and usually undetectable by the human ear) and flashes red when continuous clipping is detected (which will certainly be audible and could certainly harm your speakers if allowed to persist). In cases where continuous output stage clipping is detected, the LED doesn’t merely glow red, the amplifier will also automatically shut itself down to protect both itself and your loudspeakers.

The rear panel has both balanced (via gold-plated XLR) and unbalanced (via gold-plated RCA) inputs, with a toggle switch to select between them. The sensitivity of each input can be adjusted between 1V (for rated output) and 2V (for rated output) using a toggle switch. The amplifier can also be set (using a three-way toggle switch) for ‘Bi-amp’, ‘Normal’, or ‘Bridged-mode’ operation. Nagra has very sensibly recessed all these toggle switches so there is no possibility of moving one accidentally. Remote in/out terminals and an earth terminal are also fitted.

Only a single set of speaker outputs is fitted, using rather unusual high-current connectors made by French firm Cardas, where a single screwed fitting tightens the posts of both positive and negative speaker terminals simultaneously. Whilst admiring the facility of this clamping system, it best suits those whose speaker cables are terminated with either spade or ring connectors. However, if you are not using the amplifier in bridged mode (in which case you should not install the ‘bridge links’ supplied into the appropriate places on the rear panel) you can use these bridge link ‘holes’ to insert speaker leads terminated in banana plugs.

So far as the internal circuitry of the Nagra Classic is concerned, both the power supply and the output stage are linear, with the output stage using a single pair of MOSFET output devices in each channel that Nagra says: ‘operate in pure Class-A over a very large power band and beyond in Class-AB.’ Using only a single pair of output devices is fairly unusual in a high-power amplifier, but Nagra says it has a reason for this approach: ‘Generally, assembly of an equivalent output stage requires several pairs of transistors, which is difficult to achieve precisely,’ says the company. ‘The simplicity of the Classic Amp’s design is the key to its musicality.’

In Use and Listening Sessions
Although the Classic Power amp is always ready to go from the get-go, the Classic Preamp has to be warmed up for optimal performance, which takes around four minutes depending on the ambient temperature in the room. Because you can’t see the valves, Nagra’s front display shows the word ‘Heating’ while the valves are doing that… very helpful! Because the Classic Preamp does use valves, I would recommend that you turn it off whenever you’re not using it, in order to extend the life of the valves. Nagra reckons that if you leave the amplifier on all the time the valves will last for at least 5,000 hours and that getting 10,000 hours from the valves ‘is not unusual’. My advice is do the maths, price the cost of replacement valves and decide how you want to play it. Personally, I’d switch it off whenever I wasn’t using it, and have the Classic Amp set for ‘Auto’ operation.

I discovered one operational trap for young players right at the outset, which came about because of my habit of muting components when switching from one input to another in order to avoid switching transients. That trap is that when Nagra’s Classic Preamp is switched to ‘Mute’, the input selector is disabled, so you can’t switch inputs. I can can see the sense in this, because it means you can’t accidentally switch from a source component with a low-voltage output to one with a high-voltage output (potentially getting an unwanted blast of high-volume sound), but I still found it a bit quirky.

Right from the very first notes that issued from this Nagra combo I was hooked by their sound. Those notes were from one of Joni Mitchell’s underrated classics, ‘For The Roses’ from ‘way back in 1972 (not that I was around to hear it when it was first released… I’m a late-comer to Mitchell’s art, but am now a card-carrying follower.) The Nagras made the sonics on this album totally cohesive, weaving all the strands into a seamless and totally satisfying whole. The clarity of the sound I was hearing when I listened was revelatory, one great example being the echo to Mitchell’s voice on Blonde in the Bleachers, which sounded more real than I’ve heard it from most other amplifiers. Russ Kunkel’s intro on drums is a searing, sonic blast… wonderful! Then there’s the sound of Mitchell’s piano on Lesson In Survival, which is first a lesson in how Nagra’s ‘keep it simple’ approach to amplifier circuitry pays off in improved sonics and musicality, and second a reminder that a great many of the recordings made back in the 1970s are beautifully clean and wonderfully dynamic and ‘way better than those that are being made today, despite the so-called ‘improvements’ in technology. (Digital Audio Workstations have a lot to answer for, IMHO.) Just listen to the string sound of the piano on this track… it’s sensationally authentic. It’s a wonder to me why For The Roses isn’t in everyone’s library, though one reason may be it’s one of those albums that has to be listened to from start to finish for it to make sense—it’s a story with 12 chapters—yet strangely enough, its best-known track, You Turn Me On, I’m A Radio, was included solely to try to get airplay for the album, so it’s really the odd track out, and you’re probably better off to skip it.

The Nagras’ ability to convey dramatics was demonstrated to me when playing Maya Fridman’s arrangement for piano and cello of Prokofiev’s occult opera ‘The Fiery Angel’ (TRPTK TTK 0009). On this version she plays a 200-year-old cello and her collaborator, Artem Belogurov, an 1880 Erard. The Nagras reveal the darkly wooden sound of the old Erard to great effect—you can easily hear how completely different it is from that of a modern grand piano. The sound of Fridman’s cello is also wonderfully reproduced by the Nagras, and you can be certain you haven’t heard the cello sound so many ways before in the one work… it’s as if Fridman has deliberately included every playing technique she could. Don’t let the ‘opera’ bit put you off, by the way, this work is absolutely hypnotic… you won’t be able to stop listening.

I found the clarity and illumination the Nagras gave to the reproduction of the midrange worked particularly well when I was auditioning choral works, such as Mozart’s famous ‘Requiem’ where I could separate out individual singers from the choir and even position them across the soundstage. When the choir was in full voice, the sense of scale and richness of sound from the Nagras literally sent chills down my spine. I truly love this work, and to me it matters not who actually wrote it, because no-one is certain who did… but one thing about which we can all be absolutely certain is that it made Count Walsegg-Stuppach more famous than he ever would have been if he hadn’t financed it, so his hundred ducats were well spent!

I tested out the Nagras’ ability to deliver grunge at the highest possible volume levels with what I think is one of the best heavy-metal tunes ever written—Black Sabbath’s Paranoid. No problems whatsoever… the duo performed like the professionals they are! The modulometers on both the preamp and the power amp were flailing around like the baton of a crazed conductor, and the volume was such that I was fearing for the safety of my loudspeakers, but despite this I could not hear even the slightest inkling of any compression or distortion… and yes, that yellow LED was blinking.

Conclusion
The name Nagra has been synonymous with high-performance professional audio equipment ever since the company developed the first professional recorder more than 60 years ago. Since then the performance of Nagra’s professional components has gained the company three Oscars and an Emmy. 

The Swiss company’s involvement in consumer audio is much briefer, having commenced only in 1997 with the release of the Nagra PL-P Preamplifier, but in that brief time it has made an indelible impression on the high-end audio scene, and these Nagra Classic components show why.  # Eric Duchamp

For more information, contact: Advance Audio Australia

Readers interested in a full technical appraisal of the performance of the Nagra Classic Preamplifier and Nagra Classic Power Amplifier should continue on and read the LABORATORY REPORT published on the following pages. Readers should note that the results mentioned in the report, tabulated in performance charts and/or displayed using graphs and/or photographs should be construed as applying only to the specific sample tested.

Laboratory Test Results

Newport Test Labs measured the power output of the Nagra Classic amp at 105-watts per channel at 1kHz, both channels driven into 8Ω, which is just above Nagra’s specification. Interestingly, it delivered a little more power at 20Hz (110-watts per channel), however, as you can see from the tabulated results, it only managed to equal its 100-watt specification at 20kHz. At 1kHz the Nagra Classic didn’t go anywhere near the theoretical ideal of doubling its power output when load resistance was halved, with Newport Test Labs measuring the both-channels-driven output as just 132-watts per channel, only 0.9dB more than the output into 8Ω. It did, however, get close to this theoretical ideal at 20Hz, where the lab measured output at 190-watts per channel. Channel separation was excellent at low and midrange frequencies, but a rather poor 44dB at 20kHz.

The frequency response of the Nagra Classic duo was measured as extending from 2.5Hz to 82kHz –3dB, and from 5Hz to 46kHz –1dB, as you can see in the tabulated figures, so there’s plenty of bandwidth on tap. This bandwidth is reflected in the appearance of the square waves in the accompanying oscillograms.

The 100Hz square wave exhibits tilt, which shows the 2.5Hz –3dB downpoint, but there’s no bending that would indicate any phase errors.

The 1kHz square wave is not quite perfect, with some rounding on the leading edge, as I’d expect from any amplifier whose bandwidth doesn’t stretch beyond 100kHz.

This rounding is even more evident on the 10kHz square wave, but it’s still a very creditable result.

The same is true of the performance into a highly reactive load. There’s an initial half-wave-height overshoot, but the ringing is damped quite quickly, suggesting that the output stage of the Classic Amp will be stable into even the most difficult of loudspeaker loads.

Distortion at an output of 1-watt is shown when the amplifier was driving 8Ω loads (Graph 1) and 4Ω loads (Graph 2). The distortion ‘signature’ is such that it would appear that Nagra is not using much negative feedback, presumably to improve ‘musicality’. The graph shows second harmonic distortion at –73dB (0.0223%), third-order at –92dB (0.0025%), fourth-order at –101dB (0.0008%), fifth-order at –96dB (0.0015%), sixth-order at –108dB (0.0003%) and seventh-order at –118dB (0.0001%). Except at very low frequencies, the noise floor is maintained down below –120dB. Distortion increased when the Nagra was driving 4Ω loads, with second harmonic increasing to –67dB (0.0446%), the third and fourth harmonics to around –76dB (0.0158%), the fifth to –84dB (0.0063%) and the sixth and seventh to around –89dB (0.0035%) plus there are additional higher-order harmonics in the output as well.

Distortion at 100-watts into 8Ω and 100-watts into 4Ω is shown in Graphs 3 and 4. Distortion has increased, as I’d expect, but perhaps more so into 4Ω loads than into 8Ω. Into 8Ω, the second-order component was at –80dB (0.01%), the third at –88dB (0.0039%), the fourth at –104dB (0.0006%) and the fifth at –74dB (0.0199%). The next four components are all hovering either side of –90dB (0.003%) and most of the higher-order harmonics are around 100dB or more down (0.001%).

Into 4Ω loads we can see distortion increases further again, and that the odd-order harmonics start dominating in the overall output. Although the overall THD+N measured by Newport Test Labs is low (0.02%), I would suggest the Nagra will be happiest with loudspeakers whose nominal impedance does not fall below 6Ω or so. You can see that the noise floor across the audio spectrum has not dropped much further down than when it was referenced to a one-watt output: I would have expected a more significant reduction. The actual overall signal-to-noise results are good though, with the Nagra combo returning figures of 84dB A-weighted referenced to one-watt output, and 97dB A-weighted referenced to rated output.

Measured at just 0.1Ω, output impedance was not as low as I might have expected for a solid-state design with a simple output stage, but it’s still low enough to result in a respectable damping factor (Newport Test Labs measured damping factor as 80 at 1kHz), and also high enough to ensure the frequency response is not overly load-dependent, as you can see from Graph 6, which shows the frequency response across the audio band into a standard resistive 8Ω laboratory test load (black trace) and into a load simulating a two-way bookshelf loudspeaker (red trace). As you can see, the two traces are often identical and, where they diverge, the difference is not great enough to be audible. #  Steve Holding