Product Review - Adcom Model GTP-600 Surround Sound Tuner/Preamplifier - November, 1995

By Bill Yee


Adcom GTP-600

Frequency response 20 - 20 kHz +0 -0.5 dB, input impedance 25 kOhm, output impedance 475 Ohm, harmonic distortion 0.05%, size 4"H x 17"W x 11 1/2"D, weight 17 pounds. $1000.

Adcom, Inc., 11 Elkins Road, East Brunswick, New Jersey 08816. Phone (908) 390-1130.

I have been living with the Adcom GTP-600 Surround Sound Tuner/Preamplifier unit for about two months now, and I can say that I am glad to have it. It features Adcom's own proprietary version of Dolby Pro-Logic Surround Sound as well as the regular Dolby Pro-Logic mode.

The GTP-600 has replaced my old Dolby Pro-Logic Surround Sound decoder, the Audio Source SS Three/Series II. The Audio Source SS Three/II has served me well for the past couple of years and is definitely a notch above the many Surround Sound all-in-one stereo receivers for decoding surround sound. As a matter of fact, I would still recommend this unit to anyone looking for a Dolby Pro-Logic decoder on the cheap. It has since been dis-continued but can be found used or left over in dealers stock.

I am also using an Audible Illusions Modulus 3 tube preamp which is a bare bones purist unit without bells and whistles and without remote control. This is nice for the times one wants to torture oneself just to squeeze out every last sonic nuance, but I sometimes go through sane and lazy moments where I would like to control it all from my listening chair.

Thus I decided I wanted something kinda high-endish that provided remote control and Home Theater Dolby Pro-Logic decoding capabilities. I looked around for what was available that fit my desire. I chose the Adcom GTP-600 based on its performance, features and looks. It lists for $999.99.

The real driving force was that I wanted remote control again. But sound quality is also very much at the top of the list. I've lived with and listened to several mid-fi SONY receivers, and I even had the SONY top of the line ES Digital DSP Home Theater all-in-one model TAE-2000ESD preamp for a year.

Although the SONY TAE-2000ESD preamp was a gem in all other respects, it just did not match the Audio Source SS 3/II in terms of Dolby Pro-Logic decoding or in presenting a convincing sound space. None of them really did the trick for me in the way of Dolby Pro-Logic surround sound decoding and steering logic presentation.

The Audio Source SS Three/II did surpass all of them, however, and remained in my system up until recently. The Adcom GTP-600 has replaced my manual preamp with Audio Source SS 3/II combination for home theater applications (I still employ the Audible Illusions Modulus 3 preamp as my main rig for real serious audio moments). The Adcom unit seems to be a real nice piece of work. It definitely has a great Adcom Cinema enhanced Dolby Pro-Logic decoder and it is better at presenting a 3-dimensional illusion of space than the Audio Source SS 3/II.

The Adcom GTP-600's Cinema Dolby Pro-Logic mode is the mode that I like best over conventional Dolby Pro-Logic. The conventional Dolby Pro-Logic mode just didn't open up the illusion of space as did the Adcom Cinema enhanced mode. Most of the times when I compared the two Dolby Pro-Logic modes, the enhanced mode made things come alive. Going from enhanced to conventional mode, things just seem to collapse as if some of the life was taken out of it. From what I understand, the Adcom Cinema mode processes the rear channel information in a way that makes it very similar to THX processing. And it sounds just as good as THX, maybe even better.

For stereo listening, the Adcom GTP-600 provides three DSP modes for Stadium, Concert Hall and Night Club effects and a special 5-channel stereo mode. I personally never pay much mind to these types of DSP modes on any piece of equipment. On the Adcom I found that they could enhance some recordings sometimes, but I would just as soon by-pass these modes and not even use them. But if you do use them, they do provide minimal sonic coloration as compared to many of the mid-fi receivers that I have listened to.

The Adcom GTP-600 provides four audio/video inputs and four audio only inputs. The audio/video inputs are labeled video 1, video 2, video 3 and video 4. The audio only inputs are labeled CD, tuner, tape 1 and tape 2. One thing missing that I wish wasn't is an input for phono. That is a shame, because previous Adcom preamps had one of the best sounding phono stages available today. (I also owned a top of the line Adcom model GFP-565 preamp for a year; it had a superb phono stage.)

For the video in/out jacks, Adcom provides both the standard RCA phono jacks and the S-video connectors. Video 1 and video 2 have input jacks only. Video 3 and video 4 have both input and output jacks. I did not test out the S-video jacks, but the RCA jacks seemed to work just fine for all my video needs. Video switching was okay with no detectable video degradation when I passed my SONY model SLV-686HF VCR's signal through it to my SONY 25XBR monitor. Oh, take note that you can choose to use only one type of connector for your video needs even though there are two types of jacks. That is because there is a switch on the rear panel that switches usage between S-video jacks or RCA phono jacks only but not both. If you use RCA jacks for connecting one VCR, you must use the RCA jacks for connecting all of your other video sources. Likewise if you use the S-video jacks for one, then you must use S-video jacks for all the rest.

There are preamp outputs for all the channels, front left, front right, center, rear left, rear right and of course a subwoofer output jack.

The subwoofer output has a two position switch to choose between adding a low pass filter or running the subwoofer output jack full range. You would use the full range setting to feed a powered sub-woofer system that already has its own built in low pass filter. That's a nice touch.

The only complaint I have about the subwoofer output is that, in the Home Theater Cinema or Dolby Pro-Logic modes, the subwoofer output signal level was just barely acceptable feeding my Adcom GFA-585 power amp driving my Audio Concepts Sub-1's. Adcom could have provided a little more gain at this sub-woofer output to allow for us bass freaks. In contrast, the Audio Source SS 3/II subwoofer output provided lots of reserve gain.

In regular stereo mode, however, the subwoofer output level was better. It at least allowed me to pump up the bass volume to match my main speakers' sound levels and then some. I guess Adcom designed this subwoofer output to be used with higher gain amplifiers provided by powered subwoofer systems. It could be that the Audio Concepts Sub-1's are not that efficient or that the Adcom GFA-585 amp input sensitivity is lower than my main channels Adcom GFA-5800 amp. I found that the Adcom GFA-5800 input sensitivity is a bit higher than the older GFA-585 amp. Nevertheless, I think that the subwoofer output jack ought to have enough of an output range to take care of various amplifier gains that the unit could be mated to.

Okay, not to make you think this unit doesn't cut the mustard. Quite the opposite really. Overall this is one fine enjoyable unit. If there is one criterion that I measure a product on, it is customer satisfaction. I bought this unit, and I have no regrets about it. Did you ever buy a product and then feel that you could have made a better choice? I did not get this feeling at all with the Adcom GTP-600. It has been a very satisfying addition to my living room. It has given me many hours of refreshed listening experiences both in regular stereo and surround sound.

It is a great regular stereo preamp unit also. For those of you who are not the fanatical purist type and want tone controls, the Adcom provides bass and treble contours that are very subtle yet very effective. The Adcom GTP-600 does provide a tone-defeat switch which by-passes the tone control circuitry for a more purist approach. A Hi-Filter switch is also provided.

The only minus here is the loudness control. I found that switching in the loudness switch colored the sound a wee bit too much in the boomy bass region for my tastes. But then again, this is all an interaction of the speakers, input source, power amp I use, and my room acoustics.

Something that is not mentioned in the Adcom manual but that I noticed while using the unit is that there seems to be a signal phase reversal of the main channels when you switch the tone-defeat switch in and out. When I have the subwoofer playing along with the main channels in regular stereo mode, I notice a bass cancellation interaction between the main channels and the subwoofer when I switch in the tone controls. This could be disturbing to those who think that they are going to adjust the tonal characteristics of the system by turning up the bass control, switching in the tone control circuit and finding out that there seems to be less bass than before (due to out of phase cancellations) rather than more.

The tuner seems to be decent and of high quality. It did not quite match the quietness of my Broadcast Reference Denon TU-680NAB tuner, but when the signals were high enough, the sonics were close. The Adcom tuner seemed to be more prone to static and noise. Its AM rejection was not that good.

Sonically, the Adcom seemed a little brighter than the Denon tuner. This sometimes made the Adcom tuner sound more up front and in the room than the Denon tuner. The Adcom AM tuner section sounded like an average "limited in fidelity" AM radio. It was better than most of the average mid-fi receivers, but nothing to rave about. It certainly was no match for the Denon TU-680NAB tuner's AM Stereo section. The Denon TU-680NAB tuner is probably one of the best if not the best AM tuner sections around. It was designed to be used by AM radio stations to monitor their own Stereo AM signal quality. It is capable of wideband AM sound that at times sounds just like FM stereo.

But then the Denon tuner by itself costs over half the price of the Adcom GTP-600. Overall the GTP-600 tuner section is an excellent unit and its sonics are top-notch and very much acceptable. You probably would not hear a difference between it and costlier tuners unless you do a direct A/B comparison, and even then you might prefer the more forward Adcom sound.

The Adcom GTP-600 provides 3 AC power outlets to plug other system components into it. Two AC outlets are switched and one is unswitched.

Well that's about it for my review of the Adcom GTP-600 surround sound tuner/preamplifier. I give this unit a big thumbs up even with its minor quibbles that I've mentioned above. I am happily satisfied with its surround sound decoding performance, its surround sound sonic performance, its regular stereo performance, and its real serious cool looks.

In Summary I will comment on and rate the unit as follows:

  • 1) As a Home Theater Dolby-Pro Logic Surround Sound control center unit.
    2) As an entry level high-end audio Tuner/Preamp unit.

Here's the straight dope on it.

As a Home Theater Control Unit


  • Nice proprietary surround sound decoding mode does sound better and more convincing than the regular Dolby Pro-Logic mode.
  • Use of an Analog Decoder chip instead of a Digital Decoder chip accounts for the better than average sonics.
  • Has very good to excellent sonics on all channels.


  • The remote control is large and clumsy, I keep dropping it still while trying to adjust things. Also hard to tell which end is front and which is back. I've picked it up several times aiming the wrong end at the GTP-600 front panel.
  • Subwoofer line level output is not high enough to drive my subwoofer setup to strong earthquake levels when in the home theater modes. Subwoofer output drive was adequate on regular stereo and audio modes though.

As an Audiophile Tuner/Preamplifier Unit


  • Excellent sonics.
  • Has Bass and Treble tone controls that have a mild but nice tonal effect on the upper highs and lowest bass frequencies.
  • The tone controls can be bypassed for a more purist approach.
  • Five channel Stereo mode can really add a nice impact to music when driving high quality power amplifiers and full range speakers for the center and rear channels also.
  • Black front panel cosmetics look seriously nice, unlike many over-done Japanese mid-fi receivers these days.


  • Loudness contour button adds too much mid-bass boost making music and especially vocals sound boomy and artificial. Not at all like the mild but more useful and pleasing boost provided by the BASS tone control.
  • Tone Control defeat switch reverses signal phase polarity when switched in and out of circuit. This interacts with the subwoofers signal phase and can cause bass cancellation when tone controls are switched in circuit.
  • No PHONO stage or input for good old fashioned LP record playing.
  • Tuner scan misses stations sometimes needing several passes to lock onto a station exactly.
  • Tuner prone to static noises and pops on FM Stereo medium signal conditions.
  • Tuner station memory system can only memorize 14 stations in two banks of 7 stations each. It could use more memory capability.
  • Surround sound mode switch sequences through the seven modes as opposed to allowing user to directly select the mode that is desired. To get back to regular stereo mode from enhanced Dolby Pro-Logic surround mode, you must press the mode select button six times. Each button push takes some time to settle so that you cannot rapidly switch through the modes.

Other System Components Used During Evaluation

My evaluation system consisted of:

  • Home brew modified Aria-3 speaker systems used as the two main left and right front speakers. Each speaker built of top-notch, high quality, no-compromise parts consisting of Accuton C211 1" ceramic dome tweeter, Accuton C277 5" ceramic dome mid-range and a Focal 8K415S 8" woofer. Cabinet built like a brick outhouse out of 1.5" thick to 1" thick MDF.
  • Adcom GFA-5800 power amplifier driving the front left and right speakers.
  • Adcom GFA-585 stereo power amplifier. One channel driving a pair of Audio Concepts Sub-1 subwoofers and the other channel driving a vintage 1968 original DYNA-A25 speaker system as the center front channel.
  • Sansui TU-717 DC-Coupled integrated stereo amplifier (year 1979 vintage) driving the rear left and right speakers.
  • Rear speakers are also a home brew job each consisting of a Vifa 1" aluminum dome tweeter and an Audio Concepts AC8 8" woofer. Medium to high quality parts used for the crossover networks.

Bill Yee

Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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