Solid State Preamp Reviews

In recent years, Adcom has carved an enviable niche for themselves in the entry-level category of high-end audio. Their excellent GTP-400 tuner/preamplifier, which I reviewed in September 1989 (Vol.12 No.9), has further enhanced their reputation for musically satisfying sound at affordable prices. The GFP-565 is Adcom's newest preamplifier and their most expensive to date. The GFP-565 was designed to offer more than simply excellent performance for the price asked. This new arrival is Adcom's attempt at manufacturing a preamplifier which can compare favorably to the most expensive state-of-the-art products offered by other high-end manufacturers. As such, its $798 price tag is still reasonable, especially when the 565 is compared with other preamps in the under-$1000 price range.

I'd like to correct a potential misunderstanding (a dealer I know was even confused) regarding Adcom's current product line. Although the physical appearance of the GFP-565 is virtually identical to Adcom's older GFP-555 preamplifier, the 565 bears absolutely no internal resemblance to the 555, and as such it is not an updated GFP-555. Adcom is preparing a completely redesigned GFP-555, called the GFP-555 II, which will retain the physical appearance and $500 price tag of its predecessor. By retaining the same case, front, and rear panels used in the GFP-555 for both of their new preamps, Adcom has been able to put all of their money into the circuitry of these new products, rather than spending some of it retooling the mechanical parts. I firmly agree with this philosophy.

Like the GTP-400 tuner/preamp, the GFP-565 combines the design talents of Walt Jung and Adcom's product-development director C. Victor Campos. The 565's main amplification circuitry and power supplies were designed by Walt, while component selection was a collaborative effort. Walt's contributions involve far more than the fine-tuning of existing circuitry, since the phono preamp and line-level stages employ unique circuit topologies which he developed. Walt's reputation as the foremost authority on the use of IC op-amps in high-performance audio circuitry is further reinforced by the GFP-565 preamp, in my opinion.

Victor Campos brought his rare talents for realizing a first-class design through the final stages of production without compromising the performance along the way. In fact, the production version of the GFP-565 is, if anything, slightly better than the pre-production prototype I first evaluated. As has been the case with other Adcom products, the 565 is completely American in design, and although the main printed circuit board is manufactured in the Far East, final component assembly and testing are done right in East Brunswick, NJ.

The GFP-565 was designed to satisfy purists, who normally demand as simple a signal path as possible, as well as those audiophiles in need of a full-featured preamp. These two seemingly contradictory requirements were addressed by supplying the 565 with three sets of outputs. The bypass outputs are intended for the purists (among whom I include myself). No tone controls or filters corrupt the signal path when the bypass outputs are used, but the balance control is still operational. The bypass outputs are also direct-coupled, eliminating any possibility of sonic degradation normally associated with coupling capacitors.

The bypass outputs still allow the use of an external processing loop, so they are not absolutely purist in the sense of minimizing every possible switch contact. A mono switch and loudness contour are also available when the bypass outputs are used, but the contour switch and its associated filter components are connected to a tap on the volume control, so these components and switch contacts are not in the signal path. Like the GTP-400, the loudness contour's characteristics are based on the studies of Robinson and Dadson. The contour characteristics are gradually reduced as the volume control is advanced, but the circuit should be switched out when listening at normal volume levels.

Similarly, the mono switch does not actually place contacts in the signal path; it simply connects the left and right channels together. A few purchasers may wish Adcom had included separate left/right mono switching, helpful for playback of the double-duration mono CDs from Rodolphe Productions. However, if you purchase the 1953 Clemens Krauss performance of Wagner's Ring (footnote 1), which you should if you love this music, you'll receive a free L/R mono switchbox. Although the switchbox is a cheap, flimsy affair that you'd never want to leave in your signal path, the box could easily be connected to the 565's external processing loop, to be switched in only when necessary. Plans for constructing such a switch have also appeared in Audio Amateur (footnote 2).

For those in need of the tone controls, two additional outputs providing those features are included. The normal output is capacitor-coupled with a high-quality film device, whereas the lab output is direct-coupled. The tone controls provide boost or cut below 300Hz and above 3kHz, with a minor side effect on information between those two frequencies. A high-cut filter is also provided via the main and lab outputs. The filter characteristic is very gentle, being only 2.7dB down at 20kHz. This might be just enough high-frequency cut to remove some of the "edge" from a cheap CD player, but I would hope that anyone purchasing the GFP-565 would be using sources worthy of its own performance.

Six inputs are provided on the GFP-565, including phono, CD, tuner, tape 1 and 2, and video/aux. Separate selector switches are provided for listening and recording, and since two tape outputs are also included, tape copying in either direction is possible. The 565 does not have an infrasonic filter for the phono preamp. I asked Victor Campos why he had dispensed with this feature when it had been included on previous preamps, including the GTP-400 and the GFP-555. Victor said that he believed that any infrasonic filter compromised the sound quality of the phono preamp. The GTP-400 and GFP-555 contained the filter because of the fact that these less expensive preamps would probably be used with cheaper turntables which generate more infrasonic noise.

The 565 will probably be used with more expensive turntables which do not suffer from this problem. Of course, this still does not address the record-warp issue, but the GFP-565 does not become unstable when fed a large amount of infrasonic information from a warped LP. When I compared the 565 to the Audio Research SP-11 II (more on this later), I found that warped LPs played through the SP-11 II caused visibly greater infrasonic movement of my woofers than the same discs played through the 565 at the same playback level. The SP-11 II contains a switchable infrasonic filter.

The circuitry in the GFP-565 is based on IC op-amps, but their implementation is rather sophisticated. The ICs used in the 565 are specially selected devices bearing Adcom's own part numbers. Both the phono preamp and the line-level stage employ high-current, low-output-impedance IC buffers within the feedback loop. These IC buffers are not op-amps, instead being unity-gain devices designed to work with an op-amp to form a complete gain stage. The op-amp's output feeds the input of the high-current buffer, which in turn drives the feedback loop and the next stage of amplification. This configuration allows the op-amp to provide the necessary gain without being affected by the loading of the next stage. The IC buffers are externally biased for pure class-A operation, and each device is mounted on a small aluminum heatsink.

The implementation of the buffers in the phono preamp is rather unusual. The gain of the phono preamps is accomplished with an ultra-low-noise IC op-amp. A low-noise op-amp will not, however, produce a silent phono preamp unless the RIAA equalization network has very low impedance at high frequencies. In any feedback-based RIAA preamp, the noise level will ultimately be limited by the impedance of the RIAA feedback network. Adcom uses an RIAA network with an impedance of approximately 250 ohms at high frequencies, whereas most other IC-based phono preamps have an impedance of 1k or higher, which in turn results in a higher noise level. So why do other manufacturers use a high-impedance feedback loop when they could optimize the noise performance by lowering the impedance? Very simple. Few, if any, IC op-amps can supply enough current to drive a low-impedance RIAA network while still maintaining low distortion.

Adcom's high-current IC buffer makes the use of a low-impedance RIAA feedback network possible. The op-amp provides the necessary voltage gain, with its output feeding the input of the buffer. The buffer, in turn, drives the RIAA network, with plenty of current to spare. The result of all this is the quietest phono preamp I have ever encountered. The low output impedance of the IC buffers in both the line-level and phono stages also improves dynamics by an order of magnitude over what the op-amps could achieve by themselves. The output stages of the IC op-amps are also operated in class-A, not with the common pull-down resistor technique, but using a more sophisticated current-source arrangement. All integrated circuits are soldered to the PC board, eliminating contacts in the signal and power-supply lines which result when IC sockets are used.

The phono input is a fixed-gain circuit designed for use with cartridges whose outputs are 2mV or higher. The designers deliberately avoided making an all-purpose phono preamp with switchable gain for low-output moving-coil cartridges. I can't argue with this approach. A phono preamp circuit should really be designed for a single gain setting; doing otherwise will invariably result in compromised performance. You can't optimize a phono preamp for high-output cartridges, throw in a switch and resistor, and expect the preamp to perform equally well with low-output cartridges. An optimum preamp for low-output cartridges requires a circuit topology tailored for that purpose. My experiences with audiophiles who use low-output cartridges is that most have their favorite step-up devices anyway. [This runs counter to my experience.—Ed.]

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Footnote 1: Reviewed by RL, Vol.11 No.8, August 1988.

Footnote 2: Galo, Gary A. "New Compact Disc Technologies" in "Ask TAA," Audio Amateur, August 1989.