ADCOM GFP-750 Preamplifier
Martin Appel
2 August 1999

gfp750.gif (27429 bytes)Specifications

Output Level (Rated) 1.0V
THD + N @ Rated Output (20 Hz to 20kHz)
Balanced, <0.025%
Unbalanced, <0.095%
THD + N (1 kHz)
Balanced, <0.015%
Unbalanced, <0.017%
Output Impedance:
Balanced, <1200 ohms
Unbalanced, <600 ohms
Input Impedance:
Balanced, 94k ohms
Unbalanced, 47k ohms
Frequency Response: 20 Hz to 20 kHz, +0/-.25dB
IM Distortion (@ Rated Output):
CCIF from 4kHz to 20kHz (Balanced) <0.009%
CCIF from 4kHz to 20kHz (Unbalanced) <0.035%

SMPTE (Balanced) <0.05%
SMPTE (Unbalanced) <0.05%
Signal to Noise Ratio (Ref. To 1V)
"A" Weighted (Balanced) >102 dB
"A" Weighted (Unbalanced) >102 dB
Input Sensitivity (@ Rated Output)
Balanced 183mV
Unbalanced 365mV
Power Consumption: 30 VA
Chassis Dimensions: 3.5" × 17" × 11" (H × W × D)
Weight: 15 lbs.
Price: $1250

Manufacturer: Adcom
10 Timber lane
Marlboro, NJ 07746
Tel: 732/683-2356
Fax: 732/683-2358
Web: www.adcom.com

Preamplifiers
   What do they do, what shouldn’t they do and why do we need them?

"It must amplify the smallest of musical signals and depending upon the complexity of its switching circuitry, direct the signal without altering it in any way to the power amplifier--ideally, the proverbial straight wire with gain."

Most people who develop an interest in audio grow from a one box mass market receiver into a system of separates usually containing a stereo preamp, power amp, and various source components.

Preamplifiers provide a control center for your system, switching between the various sources and stepping up their lower output voltages to levels that are able to drive most power amplifiers to full rated output and beyond. Each piece in the chain from the source component to the speaker influences what you finally hear. The preamplifier has a very difficult and vital job. It must amplify the smallest of musical signals and depending upon the complexity of its switching circuitry, direct the signal without altering it in any way to the power amplifier--ideally, the proverbial straight wire with gain. For the most part this is impossible. Even preamplifiers in the price range of three to four, or even ten times the price of this review unit, can not fully achieve this.

The Adcom name is one familiar to most people in the high end audio world because many of us on real world budgets started out with Adcom equipment. It was relatively inexpensive, solidly built with good sonic attributes and the door through which we entered into the high end.

Fortunately, my system is comprised of the Uther 3.0 DAC by Camelot which has its own volume and balance controls and two sets of outputs. My power amp, the David Berning ZH 270 OTL has two sets of single-ended inputs, switchable on the front face of the amplifier with an A/B switch. Input A was fed directly by Camelot DAC and input B was connected from the DAC to the Adcom 750 preamplifier and then to the power amp. System conduit consisted of Harmonic Technology Truth-Link interconnects, Pro-11 power cords, and Pro-9 Bi-wire speaker cables. For my evaluation, only the unbalanced RCA inputs and outputs of the GFP-750 were used.

Other associated gear included a Sony DVP 7000 DVD player as the transport; the speakers were TMS Adiabat 8.5’s and Phantom 5.2’s. Vibrapods and Black Diamond Racing Cones were used as well.

"With any new piece of equipment, the GFP-750 included, I set aside a period of about two weeks playing the unit five to six hours a day, using a variety of sources, to burn it in. No critical listening was done but a positive impression was surfacing."

Before starting the review process I want to mention a few facts about this fine product and then we will see how close the Adcom GFP-750 comes to that straight wire with gain, or in other words, to no preamplifier.

The GFP-750 operates in two modes, passive or active. In the passive mode, a toggle switch is thrown which activates a red LED on the face plate. The signal only sees the four gang volume control which is really nothing more than a line level controller. The passive mode greatly reduces the chance for signal alteration by eliminating active circuitry from the signal path. Since any distortion introduced by the preamplifier will be magnified by the power amplifier, maintaining the purity of the signal is critical.

In the active mode, only one active gain stage is used and it’s class A at that. It offers five unbalanced RCA-type inputs, and one set of balanced XLR inputs with true differential balanced circuitry. There are two pairs of RCA outputs plus a tape output, as well as one balanced XLR output.

Adcom has built a unit with a hefty power supply and short signal paths, with high quality HEXFETs used in the active gain stage. A HEXFET is a type of MOSFET manufactured by International Rectifier. For you tweakers, there92s a detachable power cord. An external processor loop is provided for connection to an external surround sound processor, bypassing the volume control when activated. In keeping with contemporary styling trends, the remote control has a rather slight profile. One can adjust the volume and balance, and select inputs as well as mute. It does not allow you to change from stereo to mono, or stereo reverse, or from the active to the passive mode, which can all be done via the front panel controls. As you can see, this is a feature-laden unit.

With any new piece of equipment, the GFP-750 included, I set aside a period of about two weeks playing the unit five to six hours a day, using a variety of sources, to burn it in. No critical listening was done but a positive impression was surfacing.

Listening impressions

My first evaluation would be to compare the preamp in its bypass mode against a direct feed from my digital source. The Berning OTL power amp is a tube unit with amazing abilities (see my previous review, May 1999). When switching in the preamp I did not expect what I heard. The soundstaging and timbre of instruments as well as voices were extremely--and I do mean extremely--close. The preamp did not introduce any coloration or artifacts altering one’s appreciation of the music. All the clarity was there but something was missing (which is not always bad).

At various listening sessions, my son Marc, a trained musician for both trombone and guitar, (not an audiophile) would join me and add his input. One CD we listened to was Keith Jarrett’s At The Blue Note: Saturday, June 4, 1994, First Set (ECM1577 78118-21577-2). The trio was beautifully recorded and it became evident that what was missing was the air and extension in the high frequency range, which manifested itself on Jack DeJonette’s delicate and varied cymbal work. To paraphrase Marc, it was as if the decay of the cymbals was cut short. In the bypass mode, this was slight but noticeable and to a lesser degree with Keith Jarrett’s piano. The bass resolution, timbre, and resonance of Gary Peacock’s bass playing were right there with no apparent differences in achieving the full body of the instrument.

Through the Adcom, the dynamics of the piano and the full body of the instrument almost matched the direct feed, but that slight loss of full note decay or ambience was just barely noticeable. And I do mean barely. I feel this is still quite an achievement and more an act of omission than addition of coloration or artificiality. Consider also the fact that an additional set of interconnects and connectors are in the chain. This in itself will cause some losses to the sonic protrait.

" …the comparisons were made comparing the Adcom against no preamp. I dare any preamp to fully pass that test without exhibiting some deficiencies."

In the active mode, the sound in general is still very high caliber, but some of the very slight problems evinced in the bypass mode grew more noticeable. The sound took on a slightly darker character, which might work better with other harder-edged electronics that have strident high frequency reproduction problems.

Playing Chesky’s The Ultimate Demonstration Disc (Chesky UD95), these qualities once again manifested themselves. Rebecca Pidgeon’s voice on her rendition of "Spanish Harlem" was beautifully presented through the Adcom and was eminently enjoyable. But when I switched to the direct feed, it became apparent that a small amount of air was missing and through the active mode, a slight softening occurred accompanied by a further loss of decay. Even with these slight differences, the sound was always extremely listenable and musical.

After all, the above comparisons were made comparing the Adcom against no preamp. I dare any preamp to fully pass that test without exhibiting some deficiencies.

In order to be fair, I wanted to compare the Adcom to another preamp with a worthy reputation. I had on hand an Audible Illusions Modulus 3, a well-respected tube preamplifier. I placed the AI 3 in the system. The AI 3 is a value-laden preamp costing about $1000 more than the Adcom, but it includes a phono section. The RCA Living Stereo CD, Stokowski: Rhapsodies (RCA 09026-61503-2) was very instructive in portraying the strengths and weaknesses of each preamplifier. Both gave beautiful performances, but the air and warmth of tubes were clearly evident through the AI 3, with the addition of a little euphony. The Adcom’s bass reproduction again proved to be the superior of the two, a little quicker, cleaner, and more accurate.

That air and decay proved to be a notch more there through the AI 3 but images were a little better focused with the Adcom. Both units provided an enjoyable listening experience with different strengths and weaknesses. The Adcom, having a remote control, something the AI 3 doesn’t have, as well as a single stereo volume control, made life a lot easier than making adjustments through dual mono volume controls of the AI 3.

Phono—No-Go

When discussing my upcoming review with Mark Rooyakers of Adcom, I inquired whether or not Adcom intended to make a phono section, either internal or external, to go along with their preamplifier. Sadly, the answer was no.

I hope they reconsider. Admittedly, the company has made a business decision to focus on home theater as well as digital sources and didn’t see fit to include vinyl as a profitable area. A pity. If their design skill was directed towards a phono section at a price point comparable with their fine preamplifier, it would be a can’t miss opportunity. Vinyl, as we all know, is making a comeback and even with a small market, in this writer’s humble opinion profitability could still be realized.

"How many of us spend time listening to the real thing? I mean acoustic instruments without thousands of watts of amplification. As a reviewer, my reference will and should always be live music…"

My Bottom Line

All in all I must give the Adcom high marks. After all, the idea of paying an exorbitant amount to hear nothing except a volume control is not Adcom’s way. For those of you out there wanting a preamp with a solid footing in the high end, the Adcom GFP-750 gives it to you and still leaves you enough cash to spend on those other necessities of life--like hearing live music.

The Adcom in general showed itself to be a much more than entry level performer, with super bass control and reproduction that was detailed, with excellent imaging and smoothness. At the same time, a good sense of pace and rhythm was evident. The spatial presentation was 3-dimensional and far from the flat, hard, 2-dimensional one that some earlier Adcom offerings produced. Adcom should feel quite good about itself on two fronts: one, for producing a winner at this price level comparable to other preamps at two to three times its price, and two, for making the decision to employ the design skills of Nelson Pass.

I’ve often wondered when I read other reviewer’s work as to what they use as a reference. Often times, in discussions with colleagues, we go on and on talking about this piece of equipment or that speaker with incredible midrange, using all sorts of audiophile lingo to describe what we’ve heard. How many of us spend time listening to the real thing? I mean acoustic instruments without thousands of watts of amplification. As a reviewer, my reference will and should always be live music, not someone’s idea of the ultimate system with all sorts of audiophile values. Keep listening.