Adcom GDA-700 D/A processor

The availability of the Pacific Microsonics High Definition Compatible Digital (HDCD®) PMD100 decoder chip, manufactured by San Jose's VLSI Technology, has brought about a minor revolution in Compact Disc playback. It brings sonic improvements in imaging, soundstaging, and resolution of detail. In the past six months, Stereophile has published a number of reports on the HDCD decoder's operation, what HDCD recordings are available, and the improvements brought by the HDCD chip to specific digital audio processors (footnote 1). High-end manufacturers are incorporating the $40 HDCD chip in their newest decoders, including the $4695 Sonic Frontiers SFD-2 Mk.II D/A processor, the $15,950 Mark Levinson No.30.5, and the $8195 Spectral SDR-2000 Professional HDCD D/A Processor (reviewed in Vol.18 No.5).

"The difficulty is that conquering the top end of the global hi-fi market is hardly a way to get fabulously wealthy," said one observer ("Pacific Microsonics—Before the Gold Rush," The Economist, April 15, 1995). For HDCD-encoded CDs to be enjoyed by more listeners, relatively inexpensive processors need to be produced, like the $799 Audio Alchemy DDE v3.0. Similarly, the $1495 Enlightened Audio Designs (EAD) DSP-1000 and the $1000 Adcom GDA-700 are equipped with the PMD 100 HDCD decoder and are considered more "affordable" D/A processors.

The good news is that, by the end of March 1995, six of the 25 manufacturers licensed to use the HDCD process were offering units selling for less than $1500 (Adcom GDA-700, Audio Alchemy DDE v3.0, Electronic Visionary Systems DAC-2, EAD's DSP-1000, Parasound's D/AC-1600 HD, PS Audio's SL Three), and three were offering price-effective upgrades (Counterpoint's D/A-10 HDCD Upgrade, Pink Triangle's DaCapo HDCD Upgrade, and Theta Digital's DS Pro Generation III HDCD Decoding Module).

Another aspect of "getting the technology downmarket," states Michael Ritter, Pacific Microsonic's President (in The Economist) is to produce more HDCD-encoded CDs. Only a limited number of HDCD CDs had been issued by late summer 1995. Pacific Microsonics' just-announced summer 1995 shipments of production versions of the HDCD professional encoder to major producers should increase the availability of encoded discs. Even if the HDCD discs are slow to appear, both Robert Harley and Lawrence B. Johnson of the New York Times have found that all CDs sound better on HDCD-based processors. Other commentators find that HDCD-encoded discs sound better when played over standard non–HDCD-equipped processors. For that reason, this review was carried out with both types of CDs.

I selected an Adcom GDA-700 for this review because it is the HDCD upgrade of the company's GDA-600, which RH praised (in Vol.17 No.3) for its "terrific bass, a great sense of pace, and open and spacious soundstage...wide dynamic expression and smooth...refined treble." Yet he found that the GDA-600 playing HDCD-encoded CDs "wasn't even close" to a price-matched HDCD-equipped Audio Alchemy DDE v3.0 (Vol.18 No.7, p.137), which "has firmly established itself as the processor to beat in the under-$1000 price category." He hinted that an HDCD-equipped GDA-600 (enter the GDA-700) could help Adcom regain its proper place in the decoder pantheon.

For comparison with a previously reviewed "affordable HDCD decoder," I used an Audio Alchemy DDE v3.0, reviewed by RH in Vol.18 No.7 (p.137), with many of the Audio Alchemy accessories also recommended by RH. (See RH's reviews in Vol.18 No.7 and Vol.18 No.9 for details about the operation of these units.) That meant setting up its remote option, and attaching the company's digital transmission interface (DTI) v2.0 jitter attenuator via its I2S bus.

The Adcom GDA-700 is housed in a standard U-shaped chassis, with a top cover held on by Mark Levinson–style countersunk Allen screws in front and Philips screws in back. The inside of the cover is fitted with a layer of damping material. Even so, the chassis makes a hollow sound when the front-panel switches are operated, just as RH had noticed with the now-discontinued GDA-600 (see Vol.17 No.3, p.109). Although this should have no effect on the unit's signal-processing functions, it gives the impression that the Adcom is lightweight, not particularly solid.

The front panel is beveled at the top and bottom. Controls include an On/Off power switch. At center panel, a rotary control selects between "Optical" (TosLink EIAJ), S/PDIF coaxial, and AES/EBU data inputs. There is a second rotary switch that controls digital-domain polarity inversion. A row of LEDs indicate the digital signal's sampling frequency. Also included is an HDCD indicator that lights whenever an HDCD-encoded datastream is detected.

The rear panel features a number of inputs and output connectors. From left to right are a balanced XLR jack next to an RCA jack for each channel's output, and then an AES/EBU input on an XLR connector. The manual states that this is the unit's "premium grade input, with significant electrical and sonic advantages over other digital inputs." Next, two RCA-type inputs are provided, then a TosLink input comes next, and is described by the manual as the "optical equivalent of the coax S/PDIF inputs described earlier." Unlike the GDA-600, the Adcom GDA-700 does not have a digital output on an RCA output jack. The detachable AC line-cord jack completes this tour of the back panel.

As the GDA-700 is an HDCD upgrade of the GDA-600, its circuit layout is just as neat, organized, and efficient as its predecessor. Each unit features a large power supply contained on a separate circuit board mounted at the left edge of the inner chassis. The power-supply and signal-processing boards are separated by 2.5" and connected by a 12-conductor cable which plugs into sockets on each board. The 700's power supply features two transformers: an EI-core type for digital and a large, low-noise blue (made in India) toroidal for the analog circuits. I counted five large regulator heatsinks for the seven three-pin regulators—all TO-220 types, as in the '600. Incoming AC is filtered with over 13,000;uF of power-supply filter capacitance. RFI and EMI filtering are handled by an integral AC filter/IEC inlet for the detachable three-wire AC cord. The high quality of components and the extensive regulation make this a first-class power supply for a decoder.

The digital circuit board is solder masked and firmly suspended off the chassis floor with standoffs and supported at chassis front and rear. The input receiver has been upgraded from the GFA-600's Crystal CS8412 to a "potted" (all internal components fixed in black epoxy) UltraAnalog AES21. The UltraAnalog chip contains as many as 60 components, including the equivalent of an '8412 and a second phase-locked-loop circuit. The effect of these two components would be to reduce jitter to a tenth of that found with an 8412 alone. This means that the GFA-700 has a corrective PLL circuit similar to the "Digital Flywheel" found in the EAD DSP-1000.

The decoded digital data signal is then applied to the Pacific Microsonics HDCD decoder chip, the output of which feeds a pair of 20-bit Burr-Brown PCM1702 D/A chips. The Adcom's output stage features class-A–biased, direct-coupled, proprietary op-amps that achieve low levels of noise and distortion.

Compared with the Enlightened Audio Design DSP-1000 Series III and the Audio Alchemy DDE3, the Adcom GDA-700 has the best build quality of the three units featured in this review.

There seemed to be minimal requirements for installing and setting up either the Adcom or EAD decoders, other than moving manual rotary switches to the correct positions for proper input and polarity. By contrast, setting the Audio Alchemy units involved toggling pushbuttons on either the DTI's front panel or the DDE's remote to turn on the correct LED settings on the front panels. The Audio Alchemy Remote Wand allowed me to control volume, mute, and balance without moving from my seat, some 15' away. A remote definitely helps in fine-tuning the system while remaining involved in the music.

Comparisons among the three decoders were complicated because their output voltage differed markedly. The Audio Alchemy DDE v3.0, with its remote volume adjusted to maximum for best resolution, delivers 3.6V fixed output, considerably more than the GDA-700's 2V maximum. This voltage difference favored the Audio Alchemy decoder, making it sound much more "lively and transparent," but this difference was lessened when the gains were matched (see JA's recommendations for level matching in the sidebar of his Mark Levinson No.38S preamplifier review, Vol.18 No.7). Because two of the three decoders have fixed outputs, I matched the gains for all three by adjusting the preamplifier's volume control before each listening comparison. I set the voltage output from the amplifier at the Quad loudspeaker's terminal to 3.2V AC on a Micronta Digital Multimeter when driven with the 1kHz warble tone on Stereophile's first Test CD.

Of course, the Test CD is not HDCD-encoded. Therefore, the DSP-1000's switchable attenuation for non-HDCD CDs was left in the default position (attenuation on) to match the two other decoders. Each musical selection was auditioned on all three converters, with the order of converters used switched (both input and output cables) every two selections. As a result of these time-consuming procedures, the listening comparisons took much longer than other reviews. If I'd had a preamp that "remembered" gain settings, such as the Audio Alchemy DLC or the Mark Levinson No.38S, my job would have been much easier!

Inserted into my system, the Adcom GDA-700 created a deep sonic soundstage with more specific layering and clearer instrumental outlines. It was the most transparent of the three units and yielded better resolution of low-level detail. In addition, it seemed faster, with a strong sense of dynamic pace and bass impact.

Bass dynamics were the GDA-700's forte. When a solid pipe-organ note was played, the Adcom helped create a tight, pressured, sustained bass note, as heard during John Rutter's "A Gaelic Blessing" on his Requiem and Five Anthems (Reference RR-57CD). I've heard this same "room lock" effect during a live performance of the pipe organ in the Princeton University chapel, a huge acoustic space with a 50' ceiling and stone walls, modeled on a medieval cathedral.

Other bass passages were reproduced well, especially low-level musical detail. Listening to Bruce Yeh's HDCD-encoded Ebony Concerto (Reference RR-55CD), I found the deep tom-tom slap had good, solid impact and heft on Artie Shaw's Clarinet Concerto. The Adcom was able to render the image of a large acoustic space when playing bass; it was almost the equal of the EAD processor, particularly on the "Lux Aeterna" selection from the Requiem CD. Yet the male chorus's words were more distinct over the Audio Alchemy system. Perhaps the DTI's jitter attenuation helps decode speech!

Transparent midrange reproduction was another asset of the Adcom GDA-700's performance. Woodwinds, particularly Bruce Yeh's clarinet on Ebony Concerto, were open and effortless; the natural timbre of each instrument was brought out. Other details sprang into relief, such as snares in the drum heads just behind the clarinet during Shaw's Clarinet Concerto when the orchestra played. The Adcom was faster reproducing musical transients. While the EAD was liquid and smooth, the Adcom was detailed and fast.

The Adcom was more involving, particularly in rhythmic pieces. The cymbals and percussion section sizzled and were more alive on the Chorus Line selection. The Adcom conveyed a strong rhythmic drive in Pepe Linque, an instrumental piece on Oregon's Beyond Words (Chesky JD130). Even greater pace and drive were realized on the "Broke, Raggedy and Hungry" selection on Strike a Deep Chord. Dr. John (vocals), Brian Stoltz on guitar, and Freddy Stubble on drums really got cooking when the Adcom GDA-700 was running in my system. Both the Adcom and Audio Alchemy created a sonic texture and dynamic pace that drew me into the music.

In his recent review of the full Audio Alchemy system connected by the I2S bus (Vol.18 No.9), RH reported a "newfound sense of air and bloom around instrumental outlines." The DTI jitter filter, and HDCD-equipped DDE v3.0 unit played with considerable soundstage depth, tight bass, and natural instrumental timbres. I also heard these same qualities when the Audio Alchemy system played in my listening room. The Adcom GDA-700 also rendered a good sense of the musical acoustic, and, at times, matched the EAD DSP-1000—the two decoders were that close. But it went beyond the EAD unit, for it could resolve musical textures. For example, the harp and violin could be placed in space in front of the chorus in Rutter's Lux Aeterna, and discernible as separate. I could hear the reflections from the walls as the sound of the struck cymbal fades away in the Chorus Line overture on Beachcombers. The Adcom GDA-700 was particularly good at resolving low-level detail.

Summing up
All three of these HDCD-equipped decoders have wide dynamic range, which lends pace and impact to digital music. Now it's possible to have large dynamic contrasts without the glare and harshness associated too often with the CD medium. These improvements are available in all three of these under-$2000 decoders—good news for audiophiles. The Adcom GDA-7000 offers HDCD decoding and excellent sonics, and should join Audio Alchemy DDE v3.0 and the DTI v2.0 on Stereophile's "Recommended Components" list.

The Adcom GDA-700 has the best build quality of the three units. Sonically, it was more dynamic, more transparent, and more adept at retrieving low-level detail, and thus more musically involving than the EAD DSP-1000. If transparency of sonics, retrieval of low-level detail, and bass slam and heft are critical, the Adcom GDA-700 may be for you.

Footnote 1: "HDCD Overview" (Vol.18 No.3); an interview with HDCD's inventors ("Reinventing the Digital Future," Vol.18 No.5); HDCD's implications for the audio enthusiast ("Perfect Sound Forever," Vol.18 No.5); HDCD's impact on the sonic quality of high-end digital decoders (in a review of the Spectral HDCD D/A processor, Vol.18 No.5); and news of recent recordings ("Industry Update," Vol.18 No.11).
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tonykaz's picture

Ok, its an Adcom.

Bob Katz just revealed ( in his Innerfidelity Article ) how DACs are becoming "transparent".

His most recent DAC is even more transparent that his previous DAC, which he seemed to think of as perfectly transparent.

However, it seems to require a superb STAX headphone system ( acting as an Audio Microscope ) for even professional Sound people to hear these differences.

The important word of wisdom here: "transparent".

So, that's the quality we should be considering in DACs : they should be "clear windows".

Of course a person would need to know what the music "should" sound like. How is anyone supposed to be able to know this??

Tony in Michigan

Dakmart's picture

... Isn't HDCD actually owned by Microsoft these days?

John Atkinson's picture
Dakmart wrote:
Isn't HDCD actually owned by Microsoft these days?

Yes it is, since 2000, but they don't appear to have done anything substantive with it. See

John Atkinson
Editor, Stereophile

JRT's picture

This article about the old Adcom GDA-700 mentioned that it used Pacific Microsonics' PMD100. That PMD100 was a flawed design that audibly colored the sound. The design flaws were later corrected in the PMD200.

The PMD200 was Pacific Microsonics code running on an off the shelf DSP, and was not backward compatible with the flawed PMD100 which was a device specially manufactured for Pacific Microsonics.

Regardless any of that, for better HDCD playback than you will get from a device using the PMD100, dBpoweramp can be used to transcode HDCD data to 24_bit, 44.1_ksps PCM digital audio, and can convert that to suitable FLAC. You can play that back from a variety of devices and software through a modern DA converter without need for the now obsolete Pacific Microsonics DSP.