Review: Four desktop amplifiers/DACs

If you’re looking to build your own iPod or iPhone stereo, AirPlay audio system, or desktop audio system, you could buy self-powered speakers, such as computer speakers or studio monitors. But with a small integrated amplifier, which combines a preamplifier’s volume-control and source-switching features with a power amplifier for driving speakers, you can take advantage of the vast selection of passive (non-amplified) bookshelf speakers, which range from inexpensive, do-it-yourself speaker kits to slick-but-pricey audiophile models. Such speakers are often found as part of a small home audio system, but they can also be right at home in your bedroom, kitchen, or office—even on your desk.

Small, inexpensive amplifiers usually produce around 20 watts per channel when paired with 8-ohm speakers (the most common impedence for compact speakers). While a walk through your favorite big-box store might suggest that amplifier power ratings in the hundreds of watts are necessary, a 10- or 20-watt amplifier is usually more than enough power to fill a normal-sized room with sound.

While I’ll be looking at a few dedicated small amplifiers in a future review, I recently had a chance to test a number of models that also include a built-in digital-to-analog converter (DAC). This feature makes these models particularly useful as part of a computer-based audio system, or when paired other digital components such as Apple’s AirPort Express, as the DAC extracts the source’s digital audio signal—avoiding the source’s electrically noisy internal components—and uses higher-quality circuitry to convert this signal to an analog signal that the amplifier can send to the speakers. Each of these models also includes a built-in headphone amplifier separate from the main amplifier—this dedicated circuitry should do a better job of driving headphones than the built-in headphone jack on a computer, iPhone, iPod, or other device, and also allows use of the built-in DAC when listening to headphones.

We tested four small integrated-amplifier/DAC combinations from three different companies. Each is smaller than a standard stereo component and can provide enough power to drive a pair of bookshelf speakers to reasonable listening volumes (and, depending on the combination of amp and speaker, even to unreasonable ones) in a small- or medium-size room. However, the four models vary widely in price, design, features, and sound quality.

Topping TP30

The least-expensive model I tested is Topping’s $180 TP30 Class T Digital Mini Amplifier with USB-DAC ( ), distributed in the United States by Parts Express. The TP30 is approximately the size of paperback book or an external hard drive, at 8.1 inches deep, 4.1 inches wide, and 1.8 inches high. The black, extruded-aluminum enclosure sports a silver, brushed-aluminum faceplate—a look that’s clean but generic. Two switches on the front control power and input selection (the latter allowing you to choose between the unit’s analog- and USB-audio inputs). There is also a sturdy volume knob with an overly bright blue LED encircling it, and a 3.5-mm (1/8-inch) headphone jack powered by the built-in headphone amplifier. The rear of the device hosts an input for the included power supply brick, small binding posts for connecting speaker wire (which can be bare or terminated with banana plugs), and the aforementioned audio inputs: left and right RCA for analog and USB type B for digital. When using the USB input, the TP30’s DAC can decode audio up to 16-bit, 48-kHz.

The TP30 incorporates a Class T amplification chip designed by Tripath (now part of Cirrus Logic), a high-efficiency amplifier similar in some ways to switching Class D designs. Parts Express specifies that the amplifier produces 10 watts of power per channel when connected to 8-ohm speakers. (The Topping TP21 is also available. For $150, it eschews the USB-audio input but ups the power rating to 14 watts per channel.)

NuForce Icon-2

NuForce is a company that began as a maker of high-end audio equipment, but later branched out into personal-listening gear such as headphones and compact stereo equipment. The latter category includes the versatile $349 Icon-2 Integrated Desktop Amplifier ( ), a small, stylish, integrated amplifier available in red, blue, black, or silver aluminum—the silver Icon-2 the company sent Macworld has a Braun-esque minimalism and looks great next to an aluminum MacBook. The unit is slightly smaller than the TP30 at 6 inches tall, 4.5 inches deep, and an inch wide (when positioned vertically using the included silicone base).

The front of the Icon-2 features two knobs: one for turning on the unit and controlling the volume level, and the other for choosing the audio input. The front panel also hosts a 3.5 mm headphone jack powered by the built-in headphone amplifier, and a soft-white LED that indicates power. On the back panel are the Icon-2’s three audio inputs: two analog (a 3.5-mm stereo-input jack and and left/right RCA jacks) and one digital (a USB type B jack). Also on the rear panel are a connector for the device’s power adapter, a 3.5-mm line-out jack (for use with a subwoofer—the company says the Icon-2 can be damaged if this jack is used without speakers attached), and left and right speaker connectors.

When using the digital input, the Icon-2’s DAC can decode high-resolution audio up to 24-bit, 96-kHz, and a company representative told me the Class D amplifier provides up to 18 watts per channel into 8 ohms.

Returning to the speaker connectors, most amplifiers use traditional speaker connections such as binding posts or spring clips—connections that work with basic speaker cable. NuForce, however, uses ethernet jacks on the Icon-2, which means you use ethernet cable in place of traditional speaker wire. Indeed, NuForce includes with the Icon-2 a pair of the company’s unusual speaker cables, which have a standard RJ45 ethernet plug for the amplifier end and a banana plug on the speaker end. ethernet cable is well shielded for audio applications and can handle the Icon-2’s power levels, according to a NuForce representative. If your speakers can accommodate banana plugs, and if the included cables are sufficiently long, this unorthodox solution is very convenient, letting you avoid the traditional process of cutting and stripping speaker wire. If you need longer wire, you can construct it yourself from ethernet cable using a wiring diagram on NuForce’s website, or you can use an RJ45 coupler to attach additional, standard ethernet cable and extend the included speaker cables.

However, NuForce’s banana plugs are not compatible with speakers that use spring-clip connectors, though a NuForce representative said that the company would modify its cables for users that need bare wire. It’s also relatively easy to modify the included cables, or any ethernet cable, yourself, but this defeats the purpose of NuForce’s easy-to-connect cables. Compatibility aside, I did find that the relatively stiff cable was a bit unruly. I think the use of ethernet cable will be convenient for most people, but somewhat of a pain if you’re in the minority that needs a more flexible solution.

NuForce Dia

NuForce’s newest model is also an integrated amplifier with a built-in DAC. The $299 Dia Digital Input Amplifier ( ) has the same enclosure and amplifier circuitry as the Icon-2, but eliminates the USB and analog inputs, replacing them with two optical-digital inputs and one coaxial-digital input. These inputs are connected to an improved DAC that’s capable of decoding 24-bit, 192-kHz high resolution audio. The company also added a wireless remote control and eliminated the RJ45 speaker terminals in favor of spring-loaded binding posts that accept bare speaker wire (but not banana plugs or spades). Because these binding posts take up more room on the Dia’s rear panel, the subwoofer output now shares a jack and circuitry with the headphone amplifier. (Simultaneously pressing the On and Input buttons on the remote toggles between these two functions. The firmware in early Dia units allowed only line-out functionality on this jack, but can be upgraded to the latest firmware for the cost of shipping.)

The Dia has only a single knob on the front. Pushing the knob once turns on the amplifier, while pushing it again cycles between the Dia’s three inputs (the current input is indicated by white LEDs on the front pannel). Pushing and holding the knob turns off the amplifier. Rotating the knob adjusts the volume, with the volume level indicated by a column of blue LEDs on the front panel. The Dia includes the same silicone base and power adapter as the Icon-2, but does not include speaker cable.

Put simply, if your source equipment has optical- or coaxial-digital output, and you can get along without USB and analog inputs, the Dia offers the same amplifier as the Icon-2 and a better DAC for $50 less. Except for the MacBook Air, every recent Mac includes an optical-digital-audio output, as do many HDTVs and optical-disc players, the Apple TV ( ), and the Airport Express ( ). iPods, notably, do not.

Neuhaus Laboratories T-1 Amplifier

At $495, Neuhaus Laboratories’ T-1 Amplifier ( ) is in many ways the opposite of the Topping TP30: The T-1 is significantly larger (although still small compared to full-size stereo components), more expensive, and more feature-packed than the TP30. And while the TP30 uses a cutting-edge switching amplifier design, the T-1 incorporates old-school vacuum tubes.

At 10.6 inches wide, 8.7 inches deep, and 5.9 inches tall at its highest point, the T-1 has about the same footprint as an iPad or a sheet of letter-size paper, and is, on average, about as tall as an iPhone. It looks sharp with a polished-stainless-steel body and a glossy-black face plate. (A white model is also available and looks similarly lovely.) The T-1’s two 6N2 vacuum tubes are proudly displayed near the front of the device, behind a black cage that discourages—but doesn’t prevent—touching (the tubes can get quite hot and are somewhat fragile). Those who have never used tubed gear will probably love the faint glow emitted by the tubes when the device is powered on. The T-1 also feels dense, implying solid construction and quality components.

The unit’s front panel features two knobs, one for volume control and one for source selection, as well as a 3.5 mm headphone jack powered by the built-in headphone amplifier, a power button, a soft-blue-LED power indicator, and another button I’ll get to shortly. When it comes to sources, the Neuhaus has the most diverse set of inputs here: USB type B, which can handle audio up to 16-bit, 44-kHz; optical digital, which will decode up to 24-bit, 96-kHz audio; and two sets of analog RCA inputs. The T-1’s back panel also has a socket for the included power cord (the internal power supply means there’s no power brick), and a post to which you can attach the included antenna.

Antenna? The T-1 also supports Bluetooth audio, allowing Macs, iOS devices, and other A2DP (stereo-Bluetooth) gear to stream audio wirelessly to the amplifier. The aforementioned front-panel button lets you disable Bluetooth (to keep your devices from automatically pairing with the T-1), and it also turns off the overly bright Bluetooth-status LED.

The T-1 ships with a USB cable, an optical-digital cable with a 3.5-mm plug, a wireless remote control for adjusting volume, an Allen wrench for removing the tube cage, and gloves for handling the tubes.

Speaking of those tubes, audiophile factions have long debated the use of tubes versus solid-state components. (The T-1 uses tubes for pre-amplification, while Class AB, solid-state circuitry is responsible for most of the 18 watts of output power. Class AB designs are less efficient but potentially better-sounding than Class T and Class D designs.) Solid-state factions argue that transistors have better measured behavior (including lower distortion) and are generally more powerful than tubed gear. Tube lovers claim that transistors sound sterile while tubes have a smooth, natural, musical sound. I corresponded with a Neuhaus representative about this design choice, and Neuhaus is clearly in the latter camp—the company’s representative extolled the usual virtues of tubes, but also pointed out that tubes help cover artifacts found in poorly compressed music files. And while tubes do have a limited lifespan, the Neuhaus representative told me that they should last at least ten thousand hours. When they do kick the bucket, the tubes cost $10 each to replace, and the representative compared the process to changing a light bulb.

Sources and speakers

To test these amplifiers, I used a few different devices as sources: an Airport Express (using both its analog and optical outputs), a MacBook (using USB, optical, and Bluetooth connections), an iPad (using Bluetooth and USB) and an Apple TV 2 (using its optical connection). I used a range of bookshelf speakers, including the $104-per-pair Dayton Audio B652 and the $199-per-pair Energy CB-5 (each of which can be found for substantially lower street prices), as well as the $460-per-pair NHT Absolute Zero and an NHT SuperZero XU/SW1P satellite/subwoofer system (discontinued—an approximate current equivalent, the Super Stereo, costs $499). Finally, I used the Etymotic mc3 ( ) and AKG K 701 headphones to evaluate each amplifier’s headphone output, comparing performance to a $149 HeadRoom Total BitHead headphone Amplifier.

Listening tests

I began my listening with each amplifier’s digital inputs. The Topping TP30’s amplifier had no problem playing loudly enough with any of the four speakers to fill my small office or bedroom. I do feel the TP30’s audio quality is lacking in absolute terms, as it sounds somewhat “hard” and obscures some instrumental detail. The effect is more noticeable with better speakers, like the Energy and NHT models. In relative terms, though, given the size and price, the performance is impressive.

Next I tested the NuForce Icon-2. The Dayton speakers minimized the differences between the two amplifiers, but when using the Energy and NHT Absolute Zero speakers, the Icon-2 lacked the TP30’s harshness, brought out some musical detail, gave a better sense of silence between notes, and tightened up the bass output. Overall, it’s a better match for the more-expensive NHT speakers, although whether that improvement is worth almost double the price is debatable.

I then switched to the NuForce Dia. With the Dia’s improved DAC, I noticed tighter, stronger bass; crisper, more-distinct highs; quieter space between notes; and improved detail, top to bottom. In fact, the Dia coaxed better bass performance from the Energy speakers than I thought possible. I initially attributed this solely to the Dia’s better DAC, but a NuForce representative suggested that the removal of analog circuitry (including the analog pre-amplifier stage) was the primary source of improvement.

When I first fired up the Neuhaus T-1 and played music from its optical input, the T-1’s “tube sound” was subtle but apparent. Instruments and voices were smoother and sounded more—for lack of better words—round, rich, and pretty. This portrayal of music seems to smooth over some instrumental detail and thus sacrifices some accuracy, but there was much to like about the T-1’s romantic presentation. Indeed, it does improve the sound of low-bit-rate MP3 files (although it won’t work miracles). In direct comparisons to the Icon-2, I preferred the T-1’s performance, although I’m not sure whether that’s because of the tubes or in spite of them. (My money’s actually on the T-1’s DAC.) Although the T-1 and Icon-2 offer comparable resolution of detail, The T-1 has noticeably stronger bass output and a warmer sound. Strictly speaking, the T-1’s sound probably isn’t neutral, but it sounds great with small bookshelf speakers, such as the Energy model, that generally lack bass impact.

When I compared the T-1 to the Dia, I found the two units offered similar levels of performance. The Dia was more crisp, clear, and detailed, and thus probably more accurate overall, but I can see some listeners preferring the T-1’s presentation, which was at times more engrossing. The biggest difference was in bass output: the Dia offered tighter, better-controlled bass, while the T-1 had more overall bass volume. (When I used either the Dia or the T-1 with the Apple TV and the NHT subwoofer/satellite system, neither amplifier had trouble providing a dramatic home theater experience.)

DAC or no DAC?

I did my initial listening tests using a digital signal to test each amplifier’s built-in DAC. However, I was curious how much of each amplifier’s sound quality was due to its DAC, so I also tested each with an analog source. When I switched from the TP30’s USB input to its analog input, connected to an Airport Express, I noticed decreased bass volume and depth, and a decrease in resolution—unless you have a good source, the Topping definitely sounds better via its USB input. Using the Icon-2’s RCA inputs, again connected to the Airport Express, I noticed a similar decrease in performance between the digital and analog inputs, with the Icon-2’s performance dropping closer to that of the Topping (although the Icon-2’s analog inputs still sound better than USB audio through the TP30). The Dia, of course, does not have any analog inputs.

Evaluating the T-1’s other inputs was more complex, due to both their number and some odd issues I encountered. First, I switched to using the Airport Express as an analog source, and I noticed similar effects to those I observed with the TP30 and Icon-2: a loss of detail, looser bass, and less of a sense of momentum to the music. The T-1’s DAC paired with the AirPort Express’s optical output offered a definite improvement over analog.

Desktop amp/DACs compared

	Price	Rating	Power (watts/channel @8 ohms)	Digital Inputs	Analog Inputs
Neuhaus T-1	$495		18	USB, optical, Bluetooth	RCA (2)
Nuforce Dia	$299		18	optical (2), coax	none
Nuforce Icon-2	$349		18	USB	RCA, 3.5mm
Topping TP30	$180		10	USB	RCA

When I switched to the T-1’s USB input, though, I noticed a slight decrease in quality, with sustained musical notes taking on an odd vibrato, and a slight echo to the sound. (This was most noticeable with headphones, but it occurred with speakers, as well.) The effect was apparent only with “louder” recordings, such as rock mastered with compressed dynamic range, and it occurred regardless of the actual playback volume; quieter recordings such as classical music displayed this issue only in their loudest passages. Overall, this issue was subtle through speakers, but much more pronounced with headphones, giving the impression that the left and right channels were slightly out of sync. A Neuhaus representative claimed that this was not normal behavior, and sent me a second review unit, which displayed the same behavior. The representative also provided a third unit from the latest production run, which again had the same odd, slightly distorted sound through the USB input. Sound through the USB input wasn’t awful in most cases, but I preferred even the T-1’s analog inputs to its USB input.

I also tested the T-1’s Bluetooth connection. With my Mac streaming music over bluetooth, I noticed a significant reduction in dynamic range and musical detail compared to the optical input, and even compared to the Airport Express’s analog output. Bass sounded muddy, and high frequencies lacked energy. I could also clearly hear some intermittent buzzing (similar to interference caused by older GSM cellular phones) when no music was playing. This was annoying when the system was otherwise silent, but as long as music was playing at moderate volume, the music obscured this sound. Overall, the Bluetooth input may be fine for background music, and its wireless nature makes it a nice convenience, but for serious listening, the optical or even analog inputs yield far better results.

I noticed one other problem with the T-1’s DAC: On the first two units I tested, the optical and Bluetooth inputs swapped the left and right channels when listening to speakers, though the USB and analog inputs worked as expected. Through headphones, this was reversed: The left and right channels were correct when using the optical and Bluetooth inputs, but backwards through the USB and analog inputs. I requested a third review unit, and on that unit, the result was slightly different, though similar: Both analog inputs had correct left and right channels when listening through speakers, but the channels were swapped when using headphones; the digital inputs had correct channels when listening through headphones, but the channels were reversed when using the speaker outputs. In discussions with Neuhaus, a representative told me that they had investigated the issue, identified the problem, and would fix it future production runs. (The company did not provide us with a unit from those production runs.) We asked Neuhaus whether customers who are experiencing the swapped-channel issue could get their T-1 replaced with a new model; the company had not responded by the time of publication.

Headphone jack of all trades

Each of these units features a discrete headphone amplifier, letting you take advantage of the built-in DAC when listening to quality headphones—a feature that should be particularly handy in a desktop audio system. I tested this component by comparing it to my Total BitHead, a relatively inexpensive, but solid, standalone headphone amp with a USB DAC. First I compared the BitHead to the Topping TP30 using AKG’s K 701, which I found readily reveals differences between headphone amplifiers. The BitHead provided stronger low-bass performance, better detail, and a better sense of momentum—usually an indicator of better resolution of transient response. However, the TP30’s headphone output was entirely listenable, and it sounded less harsh and offered better resolution than my MacBook’s built-in headphone port.

The Icon-2 got closer to the Total BitHead’s performance, with better control of the K 701’s bass and a more-detailed, less-crowded sound than the TP30. However, the Icon-2’s headphone output emphasized high frequencies and gave them a slightly harsh edge, which doesn’t make for a great pairing with the K 701, a headphone that can sound harsh at times. Given that the Dia’s headphone jack features the same circuitry as the Icon-2’s headphone amplifier, the Dia’s sound through headphones was very similar to the Icon-2, except that the Dia’s better DAC offered improvements similar to those I heard through speakers. Still, as with the Icon-2, I found the Dia a little harsh with headphones. Finally, turning to the T-1, I noticed only marginal differences compared to the BitHead: The T-1 had weaker low bass and a comparative lack of detail and space between notes. But these were marginal differences, and the T-1’s tubes gave the sound a beautiful sweetness. The BitHead’s presentation was better overall, but as with speakers, the T-1’s reproduction of music via headphones was charming and lovely, and some people might prefer it.

My observations were similar when using the Etymotic mc3, but far less dramatic. With easy-to-drive headphones such as most inexpensive canalphones and canalbuds, any of the models here do a great job as headphone amplifiers. There is one notable exception to this, which is that sensitive headphones such as the mc3 will make more noticeable any electronic noise. With the TP30 and Icon-2, these noises were low enough in level to be easily covered by music, but when using the T-1’s Bluetooth input, the interference I noted earlier made the mc3 unlistenable, and it was loud enough to be annoying even with the K 701.

Macworld’s buying advice

Any one of these products, paired with a set of quality bookshelf speakers, makes a solid computer-based audio system. Given the range of prices, features, and sound quality, choosing between these amplifiers comes down to your budget and exactly how you’re planning to use them. If you want to bring sound to a small room as inexpensively as possible, and you have a set of unused speakers gathering dust (or plan to buy something inexpensive like the Dayton or Energy speakers), the Topping TP30 will allow you to do so for a great price. In fact, the Topping’s street price is considerably lower than its suggested retail price—at the time of publication, it was only $109.

The NuForce Icon-2 offers solid performance, versatile features, an attractive design, and the perfect size for a computer-based desktop audio system. But at about twice the price of the TP30, you’ll want to be sure your speakers are good enough to take advantage of the Icon-2’s superior sound quality. And if you have the space, $349 can get you a full-size stereo component that might provide better performance, although built-in DACs can be difficult to find in full-size amps at this price.

At $50 less than the Icon-2, and with a better DAC, more-compatible speaker connections, and a remote, the Dia is a real winner, although the lack of USB and analog inputs could kill the deal for some people. Sonically, the Dia was my favorite of the bunch, demonstrating the impact of the design changes introduced since the Icon-2. And as with the Icon-2, there are some great full-size integrated amplifiers in this price range, but few (if any) will have a built-in DAC, and none will have such a tiny footprint on your desk. I recommend the Icon-2 over the Dia only if you need USB or analog inputs.

At $495, and with a remote control and a full array of audio inputs, the T-1 is less a budget desktop amplifier and more a solution for a moderately priced compact stereo system. A full-size component might give you better amplifier performance, but, again, you’ll likely be sacrificing the T-1’s built-in DAC, Bluetooth functionality, good looks, (relatively) small footprint, and luscious tube sound. However, I don’t recommend the T-1 if you plan to connect to a source via USB (and you should consider Bluetooth connectivity as a convenience rather than a quality audio connection). Also, while I’m glad that the swapped-channel is being addressed, it’s disconcerting that the flaw wasn’t caught until I tested the T-1. These issues aside, the T-1 is a beautiful-sounding amp and DAC in an attractive package.

R. Matthew Ward lives in St. Louis and looks forward to reclaiming the closet space occupied by a stack of empty amplifier boxes. He writes about audio, Apple, and other cool stuff on his personal blog.