There has not been a conventional preamplifier in my main audio system for quite a while, because no multichannel preamp is available that's of high enough quality. Instead, I use the high-precision digital volume controls in my players and DACs and choose sources with a relay-based multichannel analog switch. Plus, I tell myself that no preamp can be more accurate and transparent than no preamp.
However, I recently installed Dirac Live 3, which effectively reduces the available system gain by almost 20dB! In my salad days, I would have designed and built a balanced driver with gain for each channel; today, I require an off-the-shelf solution. After weeks of research, I found one: the Topping Pre90 ($599)
I didn't really want a preamp in the system; I just needed more muscle. My DACs, which have a specified maximum output of 3V, were being pushed to the limit. The Pre90 was enticing me with 16dB of maximum gain and potential output of up to 50V peak-to-peak.
"I see [a preamp] as a high-quality volume control plus input selector," said Topping's John Yang. "The main goal is to achieve [the] lowest noise and distortion possible. Digital volume controls in DACs are in the digital domain, before the D-to-A conversion, so the noise of the DAC remains the same after attenuation. So, you always lose S/N ratio this way, regardless of the precision of the digital volume control. An analog volume control conserves SNR, because it attenuates the noise as well as the signal from upstream." However, a careful design is required, he noted, to avoid adding more noise. "The noise needs to be low at all volume settings. Low-impedance internal circuitry allows this low-noise performance."
A high-quality preamp can serve other functions, too, and it's precisely what I'm seeking: Adding an active, buffered gain stage can add gain and drive capability and reduce distortion.
Internally, the Pre90 is the very model of a modern analog preamp. It is fully balanced. It increments gain in 0.5dB steps via relays that switch among resistor arrays. Its active stages are based on nested feedback/composite amplifiers (NFCAs), an increasingly popular construct that can deliver extremely low noise and distortion without resorting to complex, bulky circuits or esoteric semiconductors. On paper, it seemed perfect, with two channels of balanced or unbalanced input and output, adjustable gain, and eye-popping specifications. Despite its low cost and small package, the Pre90 shares many characteristics with larger, more complex, more expensive products, including the recently reviewed Benchmark Media LA4/ HPA4 and the Pass Laboratories XP-32.
One limitation to the naked Pre90 is that it provides just one RCA and one XLR input. Topping, though, has a solution in the form of a matching accessory: The Ext90 costs just $249, connects to the Pre90 with an included umbilical, and adds one more RCA and three more XLR input pairs. The Pre90 doesn't provide any adjustment for interchannel (LR) balance, so if you need that capability, you'll need to find a place for it elsewhere in your signal path.
The Pre90 arrives in a nice cardboard box. Its chassis is compact, sleek, well-finished and substantial. The front-panel display is easy to read, with an on/off/function button on the left and the knob to control volume and other settings on the right. The rear panel bears a power switch, an IEC power connector, the RCA stereo output pair, the XLR stereo output pair, the two input pairs (RCA and XLR), and, finally, a multipin connector for attaching the Ext90.
The Ext90 comes in a matching box with the same chassis as the Pre90. It has no front panel controls or indicators; its rear panel bears, from right to left, a single RCA stereo input pair, three XLR stereo input pairs, and a complementary multipin connector for attaching to the Pre90. When attached by the provided cable, it is automatically recognized and integrated into the Pre90's operations.
With just the base Pre90, you can toggle between the two inputs with either the front-panel button or the included remote control. When the Ext90 is installed, the front-panel button steps through the inputs in one direction. Remote-control buttons permit stepping through the inputs in either direction, but neither permits direct input selection. You can also set level settings independently for each input and output, set the volume at turn-on or for input-switching, and set the brightness of the display. All these settings can be saved.
One peculiarity: You can set the Pre90's output to "XLR only," "RCA only," or both "RCA and XLR." As is typical in devices that offer both, balanced output is 6dB higher than unbalanced output. When you choose XLR output only, the display reflects the XLR outputas you would expect. Similarly, when you select RCA output only, the display shows the RCA output. But when you choose "RCA and XLR" output, the XLR output will be higher than the RCA output by 6dB, but the display will indicate the gain at the RCA output. So, the Pre90 will display 0dB when the RCA is outputting the same level as the input signalbut the XLR output will be 6dB louder than the display indicates. These phenomena are not affected by the choice of input.
I inserted the Topping into my setup between the output from my Coleman 7.1SW source switch and my power amps using the balanced connections. I chose the XLR outputs, inserted the AC cord, flipped the power switch, and configured the Pre90 with the remote control.
The remote control is frustrating. One must aim it carefully at the IR sensor, which is situated at the right side of the display panel but hard to see. The RC is sensitive to poor lateral aim in particularless accommodating than most other RC's I've used.
I found the knob on the Pre90 chassis somewhat small for comfortable use. It is lightly stepped and smooth, but it lacks the inertia of larger controls on more expensive devices. Assuming you aim well, the remote control works better: Just click the volume buttons for 0.5dB steps or hold them down for continuous linear stepping. As with other volume controls of this type, volume changes are accompanied by quiet relay clicks.
Since my original use for the Pre90 was as a low-noise gain stage, I needed to see if it was as quiet and transparent as required for such a self-effacing role. For a quick check, I set it on top of one of the power amps at the speaker end of my 25' balanced interconnects, where it would be easy to swap it in and out of circuit. At unity gain, no change in the sound was immediately discernible. Moreover, at any level setting from 99dB to +16dB, there was no audible noise, even when I pressed my ear to each of the eight drivers in my speakers. So far, so good.
I relocated the Pre90 to the main equipment rack and inserted it into the signal chain. The gain controls in Roon and JRiver were set to "Fixed," and those in the sources (Okto and exaSound DACs and my Oppo player; see the Associated Equipment sidebar) were set to 0dB. I quickly confirmed the essential transparency of the Topping Pre90 across a wide range of attenuation and gain settings. I began listening carefully but not in an organized fashion. I played whatever struck my fancymy usual procedure as I gradually latch on to the salient characteristics of the sound.
Sooner or later, I stumble on a selection that allows me to define more exactly what a new component is contributing. This time, it was Sol & Pat (Alpha Classics Alpha 757, 16/44.1 PCM download), which pairs violinist Patricia Kopatchinskaja and cellist Sol Gabetta playing works from the 17th century through the present. The first track is startling: Tambourin in C Major by Jean-Marie Leclair opens with a tambourine. Sol and Pat's strings dance along while someone stamps out the beat on the floor. The perspective via the Pre90 (with the other components in the system) is fairly close, tightly presented within a modest ambient frame so that, at a realistic level, the duo is right there in the space between my left and right front speakers. I was hooked and so listened to the rest of this delightfully varied program, ranging from the simple and familiar to the challenging and fascinating.