Adcom GFP-750 Line Stage Preamp Redux
|Adcom GFP-750 Line Stage Preamp Redux|
Frank J. Alles
2 February 2000
10 Timber lane
Marlboro, NJ 07746
Tel: 732/683-2356; Fax: 732/683-2358
"Suffice it to say that the Adcom throws an expansive 3-dimensional soundstage that is very precise in locating instruments and performers."
Martin Appel originally reviewed the ADCOM GFP-750 line stage preamp here at The Stereo Times in August 1999.
Meanwhile, my own review of the GFP-750 was published in Vol. 5, No. 6 of The Audiophile Voice magazine. This article will serve as an adjunct to both of those reviews. It contains additional information about my listening experience, concerning the use of sound-enhancing audiophile feet and a budget home-brew tweak that will tend to raise some eyebrows…
I will forego the usual description of the unit’s features and its circuit topology. That information can be found in Martin Appel’s review (see above).
Reading through the published specs I was happy to see that the "A" weighted signal to noise ratio was extremely low, greater than 102 dB down, which is about as quiet as it gets in a consumer audio preamp. My highly efficient InnerSound Eros speakers signified their thanks by the lack of audible hiss emanating from the electrostatic elements. The S/N and distortion figures are a bit lower for the balanced circuitry than for the unbalanced, indicating that the unit’s performance is optimized for balanced operation. This was more obvious in looking at the graphs that accompanied my test sample than from the published specs. Both THD and IM distortions are very low in either case.
The output impedance is less than 1200 ohms at the balanced outputs and less than 600 ohms at the unbalanced outputs. This is sufficiently low to ensure negligible high-frequency roll off with long cable runs driving most modern amps. However if you’re using primarily the passive mode, it’s best to keep the interconnects as short as possible.
To put the GFP-750 through its paces I used it in two different systems. I tried both the passive and active modes of operation and I used the unbalanced as well as the balanced inputs and outputs.
My source components were the same for either system. I used the Townshend Audio Mk III Rock turntable with a modified Rega RB-300 tonearm and a Transfiguration low-output MC cartridge. This fed a custom AHT/P phono stage, which I connected, to the "Tuner" input on the Adcom. My digital source was the Parasound C/BD-2000 transport coupled to a Parasound D/AC-2000 processor via a Harmonic Technology digital cable. In this system, the GFP-750 was substituted for an AHT tube line stage using two 5692 tubes. The amplifiers were the Monarchy SM-70s, used as monoblocks to feed the electrostatic panels of my InnerSound Eros speakers. The InnerSound bass amp drove the woofer sections. I used this configuration off and on in conjunction with a Paradigm Reference Servo-15 subwoofer.
In my alternate reference system, the Adcom fed a Sonogy Black Knight amplifier, connected to Eminent Technology LFT-8a speakers. Custom made Walsh-type tweeters were switched in and out with the ETs, so that I could better gauge the Adcom’s high frequency performance.
"To the far right and left at the outside edges of the speakers, the harmonious strings of the Pop Arts String Quartet came to life with noteworthy precision. Chris Spedding’s bouzoukia, a long-necked mandolin-like instrument, added its own unique flavor to the sound."
My initial impression of the GFP-750 was that it was a touch bright throughout the treble spectrum. However after a few weeks of break-in, this slight brightness seemed to diminish. Later, when I switched from the WireWorld Equinox III interconnects to Kimber Kable Hero balanced interconnects, the high frequencies abated a bit more to the point where, if anything, the treble spectrum became just slightly reticent (comparatively). This had the effect of showcasing the beauty of the Adcom’s grainless midrange reproduction, while retaining slightly soft detailed and focused highs. Hmmm.
I can recall playing the late Harry Nilsson’s "Remember," (Warner Sunset/Atlantic 83153-2) from the You’ve Got Mail soundtrack album. At that time I was using the Eminent Technology speakers. Harry’s vocal was locked into center stage, a few feet behind the speakers, perfectly focused--I mean rock-solid, without the slightest tendency to wander. To the far right and left at the outside edges of the speakers, the harmonious strings of the Pop Arts String Quartet came to life with noteworthy precision. Chris Spedding’s bouzoukia, a long-necked mandolin-like instrument, added its own unique flavor to the sound.
In fact, a couple of days later, I had a non-audiophile friend over to listen and I played him the same cut. After listening attentively for a short time, he turned to me with a puzzled look on his face and asked where the other (surround) speakers were! I explained that there were none and he looked at me like I was pulling his leg. Suffice it to say that the Adcom throws an expansive 3-dimensional soundstage that is very precise in locating instruments and performers.
Moving the unit to my primary reference system with the InnerSound Eros speakers afforded me an opportunity to play with a suspension system for the Adcom. Originally I set the unit atop a Townshend Seismic Sink which was sitting on my concrete floor. My impression of the sound was quite favorable.
In looking back through my notes I see a recurrent theme. The sound was superbly focused. Lyric comprehension and inner detailing (especially true in balanced) were among the best I had experienced in the system. It appeared that the passive mode of the Adcom held a slight performance edge over its active stage. The soundstage dimensions were very close, but I thought that the bass went a little deeper and that the sound was even more focused and transparent--though not by much.
The only thing I could point to as being slightly off the mark was that the presentation was just a bit dry--devoid of the harmonic lushness that tubes can impart, perhaps related to a slightly recessive lower midrange region. This was true of both passive and active configurations.
The Way It Sits
Just when I was musing something along the lines of: "Excepting the slight dryness, this preamp would be mighty-fine," an idea struck. On a hunch, based on a demonstration I had witnessed during a recent meeting of the NJ Audio Society, I took a common 12.5" × 1.75"butyl bicycle inner tube, pumped just enough air in it for it to take its shape and support the weight of the preamp, and then inserted it between the unit’s chassis and the Seismic Sink. I also put a Shakti Stone on the cover over the power transformer and that seemed to help a bit too--but not nearly as much as the bike tube.
This seemed to allow the active circuitry of the GFP-750 to more closely match its passive prowess. The bass seemed to delve deeper with more impact and this added a bit more space to the soundstage, but more importantly, the piece sounded somehow sweeter and more harmonically engaging--more ah, tube-like.
Going to FOURPLAY’s album of the same title (Warner Bros. 9-26656-2) I was surprised at how dynamic and lifelike the instruments sounded. Playing track 9, "October Morning," the kick drum almost blew me out of my seat toward the finale where the band’s really kickin’ it. I swear I got the same kind of a rush I get when I hear a live band. It was that dynamic and forceful.
My vinyl sounded great too. Playing L’Histoire du Soldat, from Igor Stravinsky Conducts 1961 (Columbia MS 6272), I was very taken with the natural timbre of the violins and the brass. The interplay of the instruments from their respective positions in the soundstage was somehow more involving than I could recall from my past listening sessions. I think that this was due to a combination of factors, such as an improved sense of dynamics, a lower noise floor, low distortion and timbral accuracy.
I also tried using a set of Black Diamond Racing’s Mk4 Pyramid Cones under the preamp, which resulted in a more extended and detailed top end and tighter, but less ample mid-bass reproduction.
On George Gershwin’s Rhapsody in Blue, from Dayful Of Song (Delos DE 3216), the nimble finger stabs of Andrew Litton on the grand piano were clearer, with less homogenization than before. One could easily hear the sharp initial attack of the hammer hitting string, which was impressive in itself, but combined with the full glorious decay of the notes it proved to be very involving. Going to An American in Paris,from the same CD, the cymbals on the left channel were very distinct and airy and reached way back to the rear of the soundstage. This was clearly the best I had ever heard them sound. I was definitely hearing many of the subtle nuances of this recording that I had not detected previously. BDR Mk3 cones were tried as well, but this resulted in a sonic portrait that was more vague (or less precise) and for that reason I preferred the Mk4 cones with this particular piece of gear.
I spoke to DJ Casser, president of Black Diamond Racing to glean some insight as to what was happening. In his opinion, air-bladder suspensions have the effect of rolling off high frequencies and boosting the mid-bass/lower midrange area. My observations would generally support his views, but in my estimation, the alleged high frequency roll-off was more subtle than severe. Casser also feels that some degree of dynamic compression and blurring occurs and I did detect a slight loss in clarity in the midrange and upper frequencies, and perhaps a smidgen of compression there as well. But in the lower frequencies, I felt that the extra mid-bass plump offered by the bike tube suspension might be preferred, by some listeners, to the tighter and more controlled yet leaner sounding presentation offered by the BDR Mk4 cones. This would be especially true if one’s speakers are also bright sounding, because the air-bladder approach tames the high frequency spectrum a bit—in concert with the fuller mid-bass reproduction.
Although I don’t have many nits to pick with the Adcom’s sound, I can find something to criticize about some of its control features. For openers, my InnerSound speakers are very efficient and the Monarchy SM-70 amplifiers have unusually high input sensitivity. What happened, is that turning the GFP-750’s volume control to the 8 o’clock position (practically off) resulted in moderately LOUD listening levels in my room. The passive (no-gain) mode worked a little better--I could get past the 9 o’clock point with that. In most (less sensitive) systems, higher volume settings should be the norm.
Also, I found the unit’s remote to require a bit of practice, dexterity and patience. The rotary volume knob is motor-driven and it doesn’t start turning the moment you press the button, nor does it stop rotating the moment you release it. For me this resulted in a series of up and down adjustments to hit the volume level I was aiming for. As I recall, the Krell KAV-250p with its electronic volume control and sequential LED readout had a greater range of adjustment toward the lower volume settings and was easier to operate with its remote.
Additionally, you can not see the setting of the volume or balance knobs from across the room. There is no mark or indicator light that can be seen from distances of more than a few feet. My solution to that was to cut two small strips of Peter Belt’s ‘phile-foil and stick one to the face of each knob to show the location of the indicator groove. This worked well and who knows, the mystic rainbow foils may have improved the sound a bit as Belt claims.
Further, you have to flip a switch on the face of the preamp to switch between the passive and active circuitry. Since the position of the switch is indicated by a LED on the front panel, it would have been nice to have that capability on the remote control as well.
The Final Analysis…
The renowned design skills of Nelson Pass combined with Adcom’s no-nonsense approach have produced an affordable winner. Despite my nit picking on the ergonomic shortfalls of the GFP-750 I must admit that this is a very exceptional preamplifier. It is solidly built, includes infrared remote control and both passive and active operating modes, not to mention balanced and unbalanced circuitry. For the low price of only $1250, one would expect a preamp offering all these features to be sonically compromised in some way. I’m happy to report that this is NOT the case.
A slightly recessive lower midrange was my only real complaint; and no, it is not as lush sounding as some tube preamps. In my view, experimenting with the bike-tube suspension and the BDR cones helped an already great sounding product to perform even better.
Another point that I should make regarding the use of audiophile feet and suspensions under preamps is that my findings are not particular to the Adcom unit. In fact, I have observed similar sonic improvements to a few other preamplifiers in my audio system, regardless of whether they were solid-state or tube devices. In other words, my comments shouldn’t be construed as being indicative of any design flaw(s) in the Adcom preamp—rather, they should be looked upon as an endorsement for the effectiveness of different feet and suspension systems.
I am quite confident in asserting that the GFP-750 is an outstanding piece of audio gear. If you’re in the market for a line stage preamplifier and you’re thinking, "Perhaps a nice Mark Levinson, or maybe a new Krell..." do yourself a favor and couple the Adcom name to that train of thought.