It is déjà vu all over again. Lately, it seems that I have become a magnet for Marantz modified CD players from Holland. First it was the AH! Tjoeb 99, followed by the Njoe Tjoeb 4000. Now the stage is set... drum roll please, for the Heart CD 6000, which you have probably guessed by now is based on the Marantz CD 6000. Heart actually offers a trio of modified players, with the CD 6000 representing the top of the line. The other two, based on the Marantz CD 4000 and CD 5000, respectively, are currently not distributed in the US. The CD 6000 is already a pretty sophisticated platform to modify, as the stock unit is recognized in its own right as a decent mass market player. Start off with a stable Philips transport (much nicer than the short tray type used in the CD 4000), decent power supplies and passive parts, and two good quality 1-bit Digital to Analog Converters (DAC) and you have got the makings of a good player.
The concept of modifying a mass market unit makes a lot of sense in the under one kilobuck price point, since it allows the combination of mass market economics with deft high-end audio touches. The result can be a major sonic transformation that transcends the limitations of the price point. What I'm trying to say in plain English is that no small high-end company (and they're all small) can build a CD 6000 from scratch in limited quantities for a few hundred dollars and stay in business for more than a few hours. High-end prices are "inflated" partly because of the cost of doing business in production runs of twenty units or so. On the other hand, would Marantz include a tube output stage in a mass market player - even if they could do it cheaply? The answer is a resounding no!
Many consumers apparently find it quite strange to accept a mix of digital and tube technology. Some of the mail I have received following my review of the Tjoeb players was pretty illuminating. Basically, some readers wanted to know whether the tube stage was really a good idea since it would be adding distortion to what would otherwise be a pure signal. Heck, tubes have been catching flack for the past 40 years for being higher distortion devices than transistors. Tube line stages, typically operated class A, are intrinsically low distortion circuits, easily beating a spec of 0.1% total harmonic distortion (THD). Sure, you can get a solid-state OpAmp to measure even better by application of global feedback, but in the end while THD is lower into a test load like an 8-ohm resistor, such a device sounds worse on music because of transient distortion products. The beauty of tubes is that they are inherently linear devices and don't need feedback to measure decently.
Should you read specs or listen to music and let your ears decide? I have to confess to being sufficiently dyslexic about this issue to the point of actually giving preference to listening tests. Is this really such an heretical viewpoint? After all, for a product that is intended for musical enjoyment, it is only right to let your ears be the final arbiter of sound quality. My advice is to let your ears decide because ultimately you should enjoy the music. By the way, the average speaker produces several percent THD, yet very few audiophiles seem to be aware of this, as slick audio magazines and marketing agencies deflect attention away from this fact. The good news is that many consumers are in fact listening. Several of our readers who have purchased the Tjoeb have written to let me know how much of a sonic difference it has made - even in the context of old electronics and speakers.
The Heart Modifications
Every Marantz CD 6000 undergoes "open Heart" surgery at the factory. I say this not merely to score another point in a long list of potential puns (let us see, Rhythm, Beat, Double-Beat, and Attack are already assigned to Heart power amplifiers), but to emphasize that choices made during the mod process are critical to the musical health of the patient.
Starting at the output of the DACs, the stock NJM4560 OpAmp, which is used as a low-pass audio signal filter, has been replaced by a passive network with better transient response. The loss in gain is made up by the addition of precision line-level transformers that feed the tube line stage - the replacement for the stock solid-state output stage. Each channel of the line stage uses a 6922 dual-triode (Sovtek brand) connected in parallel. Of course, the line stage requires additional power supplies for the anode and filaments. These are fitted onto an added circuit board. Finally, the muting transistors at the output have been removed, which is said to significantly improve sound quality. Oh, I almost forgot, damping material has been added to the cover of the player to minimize vibrational energy from affecting the active circuitry.
The Sound: Chapter One
This is one review that underwent a difficult gestation period. The first review sample was DOB - dead out of the box. I could not get the front panel to power up - not exactly the sort of first impression the US distributor, Frank Stuppel, was hoping for. A replacement unit was promptly delivered to my doorstep and the initial installation was completed without a glitch.
Part of my evaluation protocol is to treat a new piece of gear much like a bottle of red wine. There is a strong analogy between the initial break-in requirements of most high-end audio gear and the needs of a good bottle of red. Pop the cork, and fail to properly aerate the wine, and the palette gets hit with a bit of harshness. Given enough time to aerate, the wine gains in smoothness and bouquet. My first sonic impression was of a wide soundstage, slightly veiled, with a warm and romantic midrange, and lots of loose bass. That's the good news. The bad news was a gritty treble range, fuzzy image outlines, and a lack of dynamic punch. Male voice sounded fine most of the time, but female voice left a lot to be desired. All in all, the Heart painted a much more exciting soundstage than a mass market player is capable of. I haven't specifically auditioned the stock player, but I have heard enough sub $500 mass market players to reach the firm conclusion that they're nothing more than sonic zombies, sleep walking their way through the music. All the notes are there, but the emotional intensity is missing in action.
I fully expected at least the upper octaves to smooth out over time. In truth, the longer I listened the less enamored I became with the sound. The upper octave grit was rubbing my neurons raw. And the lack of image focus was the final straw. It was time to take matters into my own hands. When it comes to tubed players… when it doubt , start tube rolling.
Choice of tubes often makes for a huge sonic difference and can spell the difference between success and disaster. My experience with the AH! Tjoeb and Njoe Tjoeb was no different. The unfortunate thing about tube substitutions is that it's difficult to predict before hand which tube or tube type are ideally suited to a particular application. Most of the tube-guru prognostication is based on limited auditions in the context of a particular product and system context. It is human nature to generalize on the basis of limited experience, and even a quick search of the World-Wide Web turned up a large number of far reaching opinions. For example, I have seen the Sovtek 6922 bashed as a yawner, not worthy of serious audio use. Buyer beware: often, such opinions are spouted by folks who are eager to sell NOS vintage tubes at inflated prices. Well, while the ubiquitous and inexpensive Sovtek 6922 is not the dog that some make it out to be, it is clearly not the right tube for the Heart CD 6000.
The Sound: Chapter Two
The 6DJ8/6922 family of high transconductance dual triodes includes a lesser known member - the 7308/E188CC. This is an industrial, long-life version that I'm just willing to crown as the undisputed heavy-weight king of its class. It is generally more ruggedly build and tends to be less microphonic. Both Amperex and Siemens produced outstanding versions of this tube. There is also a gold-pin version of this tube, though the steel-pin version is more readily available and hence relatively cheaper.
First in was the Amperex 7308 gold pin, branded as JAN. This moniker stands for Joint Army navy and simply denotes tube production earmarked for military procurement and meeting a tighter spec than commercial production. Even so, there may be as much as a factor of two variability in tube parameters between JAN samples. It is important to source tubes from a dealer who carefully tests tubes for noise, microphonics and matches for transconductance. This tube made for a shocking difference in the sonic demeanor of the Heart. The upper octaves smoothed out considerably, image outlines snapped into focus, and dynamic shadings improved dramatically. Tonally, this tube sounded drier and less romantic relative to the Sovtek 6922. The romantic glaze of the Sovtek was replaced by a lovely treble detail, and to-die-for clarity and imaging precision. This proved to be a very linear tube and that weaved a seamless presentation from soft to loud. Bass lines were tight and pitch perfect without the bloated emphasis of the Sovtek. The only ingredient missing was that last measure of harmonic richness. The sweetness and sheen of soprano voice was slightly diminished, and as you can imagine the timbre of a violin was similarly impacted.
No problem. To fatten up harmonic textures, I substituted the Gold Aero 7308 gold-pin, made in Germany and probably of Siemens manufacture. This tube seems to be the king of schmaltz. It gave the Heart a rich and slightly dark presentation. It was a though the Tube Gods had slam dunked the Heart into a vat of dark chocolate sauce. Textures were positively dripping with creamy and richly layered nuances; absolutely yummy! Of course, I simply lapped it all up. High-calorie sound is my idea of enjoyable sound. As with the Amperex 7308, this tube is also blessed with stunning dynamics. Transients were unfolded with the speed of a photon torpedo. Microdynamics were give full scope of expression - a complete surprise for a player priced at under $800. Resolution of low-level detail also competitive with any player I've heard to date priced under $3,000. The sound was integrated from top to bottom; no one octave vied for my attention. The grain and grit I have come to expect from inexpensive players were completely absent. Finally, a Heart with soul!
There's no prestige in owning a Heart CD 6000 player. Most of your audiophile friends will simply mistake it for a mass-market Marantz. But you can laugh all the way to the bank, knowing how much they paid for comparable sound quality. With its captive "cheap" power cord, you can sleep easy at night knowing that a major system decision such as "what exotic power cord to buy" is not a pertinent issue for you. Their collective jaws, however, are likely to drop to the floor once they sit down for an extended audition.
The catch, of course, has to do with the choice of 6DJ8/6922/7308 tube. The sound varies from OK to spectacular depending on whether you roll in a substitute such as a Siemens 7308 or stick with the stock Sovtek 6922. I hope that the at least the US distributor decides to offer a premium tube upgrade option direct to the consumer. That would save you the hassle of trying to shop around in the shark-infested waters of the Internet for a fairly priced premium tube. Note that the table below assumes use of the Heart with a premium 7308 tube type.
Frequency Response: 20Hz - 20kHz (+/- 0.3 dB)