Product: Octal 6 tube preamplifier
Manufacturer: Mapletree Audio Design
Retail price: $345.00 USD kit, $445.00 assembled
Reviewer: Richard George
Reviewed: June, 2002

In the Golden Age of audio, kits were available that allowed the hobbyist to build almost anything electronic. One of the most successful of the kit makers was Dynaco, whose ST70 power amplifier set a standard for quality and value that made it one of the most successful products of its day. The secret to its success was simple: decent quality components, straight forward construction, very good performance, and great value. The ST70 is still sought after today, 25 years after the last ones were manufactured. Dynaco, Heath, and many others also built preamplifier kits using similar criteria.

Recently, there has been a rebirth of the audio equipment kit market. While it does not yet approach the massive market of 40 years ago, the products that are available have helped to revitalizethe tradition of great value in a market where over-inflated audio equipment prices seem to be the norm rather than the exception. In these days of $1,000 speaker cables, products that stress quality and value are more than welcome. Manufacturers of kits tend to be small companies and include Asusa, Decware, and Bottlehead, to name just a few of the many North American firms. Mapletree Audio Design (MAD) is a Canadian firm that produces several audio kits including preamplifiers and headphone amps.

Preamplifiers, more accurately called control amplifiers, come in two basic types: a passive preamp acts only to control the level of a source signal and pass it along, with no additional gain, to the power amplifier; and, an active preamp has a separate gain stage that allows it to amplify a signal that might otherwise have insufficient voltage to efficiently drive a power amplifier. Active vacuum tube preamps can use different topologies to accomplish the goal of signal gain; each type has advantages and disadvantages. The MAD Octal 6 kit, a standalone, line-level preamp, is the subject of this review.

What Is the Octal 6?

The MAD Octal 6 is a single unit, line-level preamplifier that uses vacuum tubes for the gain stage. The power transformer, located on the back of the chassis, is hidden beneath a decorative plastic cover. Six tubes occupy the remainder of the chassis top in front of the transformer. Four control knobs, a power switch, and a power LED are located on the front of the unit. One knob selects the circuit topology, one selects the input, one controls volume, and the remaining knob controls channel balance. Three pairs of input RCA jacks and two pairs of output RCA jacks are located on the back panel along with a detachable power cord and fuse. The Octal 6 is assembled using entirely point-to-point wiring - no circuit boards are used.

The Octal 6 incorporates a few unique features that differentiate it from its competition. While most preamp designers select a circuit topology and tout its exclusive use as being superior to any other, the fact remains that each circuit topology is a compromise; one type would work better in one system, another would be best in a different system. The Octal 6 incorporates three circuits into one design. The three topologies, which include passive, shunt-regulated push pull (SRPP), and common cathode/cathode follower cascade (CF) are selected by the left knob on the front of the unit. This feature gives the Octal 6 the flexibility to have three distinct sonic signatures. Which is best depends on the source, the power amp, the speakers, and the listener's preference.

Most designers of vacuum tube audio gear base their products on a narrow range of well known and 'acceptable' tubes. Through marketing, public perception that these select few tube types are the 'best' or perhaps only viable choices for audio equipment has been carefully cultivated. That perception is simply not true. Even worse, tubes that are in the most demand become scarce and very expensive. Instead, Mapletree Audio's designer, Dr. Lloyd Peppard, selects tube types based on actual rather than perceived benefits. While the tube types used in the Octal 6 were primarily chosen for their performance in the circuit, not all the benefits of the Octal 6 tube choices are audio; some are economical. The Octal 6 uses three pairs of octal vacuum tubes. None of the three types are in current production, but all are easily obtainable from new old stock (NOS) sources. Even more importantly, the tube types used, 6R7G, 12J5GT, and 12SN7GT, are very inexpensive: a few inquiries with tube dealers found that a complete set of new tubes for the Octal 6 would cost about $30 USD for all six tubes. Thirty dollars is less than the cost of a single 6SN7GT (the 6-volt version of the 12SN7GT), which is a very well-regarded octal preamp tube. With the long tube life typical of most preamps, an Octal 6 owner with one or two extra sets of tubes should be set for many years. The chief benefit to this type of availability and pricing is that it allowed Mapletree Audio Design to include very high quality NOS tubes without having to raise the price of the Octal 6 to an unreasonable level.

Kit Assembly, or, How Many Thumbs Do I Really Have?

From the Hammond power transformer to the Teflon-insulated, silver-plated copper signal wire, the components in the Octal 6 kit were generally very good. The only apparent exceptions were the two plastic, 3-position, rotary switches. They felt lightweight and cheap. However, in use they function very smoothly and reliably. With at least 150 hours on the unit, the switches continue to function properly. Most resistors were 5% carbon film type. However, critical signal path resistors were 0.5% Holco metal film. Coupling caps were Solen, polypropylene film capacitors. A bridge rectifier and fast recovery diodes were used in the power supply.

Assembly instructions contained a complete list of all parts to be used, and, all parts listed were included in the kit - there were no errors of omission on the part of the manufacturer. One nice feature was that a little extra wire of each type was included in the kit - this is quite handy in case a mistake is made in measuring or cutting. The instructions were clear, succinct, and easily understandable. While this kit might not be the best choice for a first-time kit builder, neither would it be a poor choice. The additional complexity of the dual preamp circuits is largely offset by the high quality of the assembly instructions. The only caveat here is that a newby kit builder should practice soldering prior to beginning assembly. Fortunately, concise soldering instructions are included with the kit as an aid for a complete novice. Assembling kits can be great fun; everyone should be encouraged to give it a try.

Kit assembly took place over a four day period, taking about 9 hours total. All assembly was done according to the instructions. In each phase, wiring and component assembly were checked against illustrations to ensure compliance with directions. Assembly was fairly easy and quite enjoyable. All parts fit as expected, and no unanticipated problems cropped up during assembly. In fact, the MAD Octal 6 presented fewer assembly difficulties than any other kit I have worked with recently.

Everything went very well until I reached the checkout stage after assembly completion. On powering up the unit, the right channel was completely silent; the tubes glowed but no hum, no noise, and no signal reached the right speaker. The diagrams for each stage of assembly were checked against the finished product. Everything checked out; nothing was miswired. A mulitimeter was then used to check for unintentional grounding of some component. In the end, the culprit was located. A tiny ball of solder had rolled down behind one RCA output jack and grounded the hot lead. The bit of solder was in a location that could not be seen until the RCA jack was removed. As per instructions, all RCA plugs were initially tinned prior to assembly. This mistake appears to have occurred when the output lead was soldered into place. So, how many thumbs do I really have? I'll let you know after I assemble my next kit.

Once repaired, the unit checked out properly according to the remainder of the checkout instructions. The Octal 6 was then placed into a system for a burn-in period of about 60 hours.

How Does it Sound?

The Octal 6 provides three separate topologies that each have advantages and disadvantages. In either active mode, this preamplifier reproduces the classic, warm, tube sound that is so musical and desirable, but stays clear of the dark, muddy character of some older vacuum tube products. The passive mode allows the signal to pass through the preamp without significant sonic alteration, delivering output to the amplifier that is virtually as detailed as the source signal.

As with any high quality audio equipment, synergy of components is crucial. During testing of the Octal 6, three power amps were used: a Decware Zen SE84-CS, a Fi X, and a pair of nOrh Le Amp monoblocs. After much experimentation I found that the Octal 6 was a poor synergistic match for the Zen amp, but it worked very well with the nOrh monoblocs, and it worked exceptionally well with the Fi X amplifier.

Positive Impressions

Having three separate, circuit topologies is beneficial to the audio enthusiast who has multiple amplifiers, speakers, or sources. Also, the three circuits give versatility to the preamp allowing it to be 'tuned' to be a better match for musical material and listener mood. When using the Octal 6 for casual listening the difference in active modes is subtle and usually not noticeable (provided volume levels are approximately equal). Both active modes seem to have a slightly richer sound, with more rounded notes than does the passive mode. The passive mode seems to have a somewhat thinner sound. For casual listening, either active mode is more enjoyable, if only slightly so, than the passive mode. During critical listening, the differences between the two active circuits were clearly defined.

The common cathode/cathode follower (CF) topology is a classic circuit design. In the Octal 6, it results in relatively high gain, but pays the price with greater noise and higher measured distortion. The most important aspect of this mode is its rich, warm tube sound. Highs are smooth and pleasant, rather than sharp and gritty, as has become the norm with audio equipment in the 'digital age.' Mids are clean and natural. Some similarly-priced, modern, solid-state preamps may deliver better detail than the CF mode, but do so with exaggerated harshness that distracts from the music. The Octal 6 in CF mode renders budget-priced CD-players much more pleasant by smoothing out the grit and taking the edge off the sharpness. Compared to the Octal 6 passive mode, musical notes seem to bloom in the CF mode with a rounder presentation that generally is more full and substantial. The soundstage is larger, the instruments seem larger, and the music moves out into the room in front of the speakers, rather than being located between them. The CF mode can contribute to improved dynamic range when combined with low-powered amplifiers by giving more gain on the front end.

The shunt regulated push pull (SRPP) circuit is somewhat more modern and is designed to produce a 'cleaner' output with lower distortion and noise. The tradeoff is somewhat reduced gain compared to the CF topology. While sounds still 'bloom' compared to the passive mode, the SRPP has a slightly harder edge to notes and transient sounds, and it has somewhat greater detail than the CF mode. The SRPP seems to be an excellent compromise between the softer tube sound of the CF mode, and the more accurate but thinner presentation of the passive circuit. When combined with a tube-output CD-player and the Fi X amplifier, the SRPP mode produces excellent sound quality. Either active mode works very well with the nOrh Le Amps, allowing a slightly smoother and more enjoyable sound in the mid- to high-frequencies.

The passive circuit does not add any significant noise or distortion, and it produces an output signal that is truest to the input. The passive circuit has the cleanest sound with the greatest detail. If the source is good enough, the passive circuit is often, though not always, preferred. Transient response, from low to high frequencies, is quicker and sharper, and attacks are cleaner than with the other two circuits. During evaluation, this mode was best when matched with the Dynavector DV20X, as the analog source, and the Fi X power amp.

The Octal 6 bears a strong family resemblance to the rest of the Mapletree Audio family. Apart from other MAD products, it doesn't seem to copy the appearance of any previous audio device - it is an original. It has an elegantly simple appearance, as though it was built in an earlier age. This is not a criticism; it is a compliment. Sometimes, audio equipment designers try too hard to make their products look antique. Their efforts often lack the elegance of the best that an earlier age had to offer, and instead look clumsy and ungainly. Other designers try to make their vacuum tube equipment look ultramodern, like a space-age, solid state device with vacuum tubes added as a styling exercise. Such equipment carries its anachronistic aesthetics to an unpleasant degree. However, the Octal 6, with its old-fashioned, black, control knobs, understated black and brass highlights, small wood accent panels on the chassis sides, and dark red finish is pleasing to the eye. These features complement the dominant feature of the Octal 6, which is, of course, the six octal vacuum tubes. Octal tubes are substantially larger and more visually commanding than the smaller, nine-pin, miniature tubes frequently used in modern preamplifier applications. The combination of the large tubes and simple chassis design is quite attractive without being gaudy or overdone.

Negative Impressions

When using high efficiency speakers in a quiet listening environment, the Octal 6 emits a noticeable hum. The hum is not loud, just noticeable, and it is different for each of the three topologies. There is no significant hum associated with the passive mode; in SRPP mode, hum can be heard a few feet in front of the speakers; in CF mode, the hum is slightly louder than in SRPP mode. From my listening position, approximately 8 feet in front of the speakers, the hum in CF mode can barely be discerned if the environment is very quiet, and it cannot be heard at all if there is any background noise. This is not a major problem, as it can be with some vacuum tube products. It is simply there.

The CF circuit seems to smooth out some detail and round the 'edges' off notes. Attacks seem softer than the other two modes, despite having the most power available. There is slight loss of detail, particularly when comparing the CF mode to the passive mode. High frequencies are somewhat rolled off - not extremely so, just noticeably. Bass is not very quick; it tends to sound slightly soft and sometimes a bit sluggish. This aspect of low-frequency reproduction is subtle, but is noticeable when compared to some other preamps. The sound is 'classic tube', which is not in itself a negative aspect. The 'classic tube' sound was also listed under 'positive impressions', because this is a very pleasant and enjoyable sound. It is listed under negative for only one reason - if an audiophile wishes to obtain a preamp with the lowest possible distortion, greatest detail, and least alteration of the signal, neither the CF nor SRPP circuits of the Octal 6 will suffice.

The only negative aspect of the passive mode is loss of signal strength. When using a Decware Zen Triode stereo amplifier (1.8 wpc) to drive 92dB speakers, the volume knob had to be turned to over three-quarters of maximum to achieve the desired listening level of about 85dB. A significant loss of signal strength occurs in this mode, if it is used with very low powered amplifiers. When the Fi X amplifier (3.5 wpc) was connected to the Octal 6, the situation improved substantially. When using the passive mode with the pair of 100-watt monobloc Le Amps, the signal loss was insignificant relative to the maximum practical volume.

All RCA jacks included with the Octal 6 had the same color insulator insert: black. The plate on the chassis back that identifies and numbers the input and output jacks contains no information about channels. In short, there is no way to determine which connectors are right channel, and which are left, except through experimentation, looking it up in the manual, or opening the chassis. While this was the norm for kits and products made 40 years ago, it no longer is. RCA jacks with differentiated colors (standard red and white) would fix the inconvenience without adding complication to the assembly procedure.

A design using exposed tubes provides two aesthetic purposes: visible tubes are a reminder to all that the piece of equipment does indeed use vacuum tubes; and, often the design has a retro look that harkens back to the time when vacuum tubes were state-of-the-art technology. However, a look at old tube equipment reveals that most preamp tubes were shielded in one way or another. Many were double-shielded; they were covered by tube shields and placed inside a closed chassis to prevent amplification of unwanted vibrations. Having all six tubes exposed is a design compromise that improves the visual aesthetics but could reduce sound quality. Despite the potential for significant vibration problems, this preamp exhibited only minor problems when placed too close to the loudspeakers. Isolation of the Octal 6 using a heavy base and, perhaps, isolation products such as Isopods or Bright Star Audio's Isonodes will help. In addition, location of the Octal 6 away from vibration sources should take care of the most significant aspects of this problem.

Conclusions

Forty years ago, people who loved music but had little money purchased kits from Dynaco, Heath or other, smaller companies. Those with more money could choose more exclusive products, such as those made by McIntosh or Marantz. Today, those with an excess of disposable income can still buy a McIntosh or Marantz, or a host of products from other companies that are even more exclusive and expensive. In fact, due to high demand, both Mac and Marantz have reissued some of their classic tube equipment. However, as in the past, their products are quite expensive. Although Heath and Dynaco are gone or irrevocably changed, companies like Mapletree Audio Design are here to carry on in their place and make products that help fill the void.

The Mapletree Audio Design, Octal 6 kit provides the audio enthusiast with a high quality, well-designed and executed product that is great fun to assemble and is a worthwhile addition to any audio system where common sense and love of music are more important than impressing the neighbors. The Octal 6 has very good sonic performance in a visually attractive package. Due to its multiple topologies, it can use one circuit to help smooth out the gritty, rough edges of budget CD-players, while still able, with the click of a knob, to allow better quality sources to sound as detailed and musical as they should. In any mode, the sonic characteristics of the Octal 6 do not hinder music, but give a natural and pleasant presentation. Just as importantly, the Octal 6 is a modern equivalent of audio products from the past that based their success on high quality and great value. The MAD Octal 6 preamplifier is a fun and musically engaging product that will satisfy almost any music lover, and it is priced low enough to please nearly everyone. That is what value is all about.

Manufacturer's Comments

First of all, I wish to thank Richard for the time and effort spent in building and reviewing the MAD Octal 6. Such reviews are a great service to the audiophile consumer. I am pleased that the Octal 6 provided a rewarding kit building experience. I certainly relied on my experiences years ago with Heath and Dynakits when designing the physical assembly procedures and instruction manual. Richard's listening impressions reflect the design intent to a large degree. The choice of carbon film resistors was taken to achieve a fuller, warmer sonic signature than is often obtained with (cheaper) generic metal film types. Both the review and customers' feedback seem to indicate that this objective has been met. It is also interesting that for Richard (and other listeners), the sonic differences in the two active topologies are in the first instance, subtle. Nevertheless, listeners soon develop a preference, which seems about equally split between the two. With the Octal 6, you do have a choice.

Perhaps a word about "tube rolling" would be helpful. The 12SX7GT is considered by many to be the top 12SN7GT type ever made and can be directly substituted. Also, there are metal versions of the 12J5GT and 6R7G which can be tried. Also, if at some point in the future the user wishes to use 6 V versions of the 12J5 and 12SN7, heater rewiring is quite straightforward. For now, the 12 V versions are a much better deal, as the price for NOS 6SN7GTs is out of sight.

Two modifications have been made to the Octal 6 since the review sample was shipped, which have significant impact on the performance. First, an Alps "Black Beauty" center-detent balance control is now included. This special control (which is no longer manufactured) provides zero attenuation at mid-position. This gives the Octal 6 about an 8 dB increase in gain in all modes, which is particularly welcome in the passive mode. The precision and reliability of the control are also welcome. Secondly, the position of the volume and balance controls in the circuit has been reversed to lower the output resistance of the passive path. This can have a significant impact on high frequency response when driving capacitive cable runs to the power amplifier. This is a seldom mentioned problem with passive control "preamplifiers".

Lastly, I am not happy with the fact that Richard heard low level hum when using the Octal 6. The samples I have built as assembled units certainly had no audible hum close to the speaker when used with a power amp with a gain of around 10 and speakers with 92 dB sensitivity. Higher gain power amps or more sensitive speakers will of course effectively amplify any output noise from any preamp to audible levels. With the dc heater supply and the use of shielded cable on all the inputs, every effort was taken to make the Octal 6 a very quiet preamp. If a customer is unsatisfied with any aspect of the Octal 6 kit performance (relative to the published specs), I am happy to have it shipped back for a tuneup at no cost to the customer except shipping. On the same subject, assembled units carry a 3-year warrantee on parts and labour, exclusive of tubes (90 days) and shipping.

Lloyd Peppard, Mapletree Audio Design

Many thanks to Dr. Lloyd Peppard of Mapletree Audio Design
for providing background information and the unit used in this review.