The sonic success of the Ultra-Linear Transistor [ ULT ] concept was a wonderful advance in audiophile hardware, and once the designing was said and done, it was only natural that this concept should be expanded to cover other similar amplifiers. After the superior ULT Output transistors and circuit fine-tuning, the vintage Dynaco, rebirthed as the Stereo 120.ULT breaks new ground as a millenium standard, exceeding the sonics of the expensive mega-buck amplifiers, you know who they are, and you also know how much they cost !!!
On the other hand, similar vintage amps from competing manuafacturers offer similar opportunities for rebuild and upgrade along similar lines. Each amplifier has its advantages and disadvantages within the original topology and physical design. The 1971 Harman-Kardon Citation 12, was, in its day, considered to be a mid-priced unit of exceptional design and reliability. Although not nearly as many units of this set were manufactured, as the Dynaco Stereo 120, they were well made and well deserve attention to bring them into the millenium, sonically.
The basic price of the Citation 12 was about double that of the Dynaco, back in the 70's, and it weighed in at almost double the mass too. Not surprising, that this amp is still popular today, despite dated sonics, and reliability, but that makes it an excellent choice for a modern ULT based rebuild. It came in the basic spartan metal, with exposed heat sinks along its front edge, and also in a wood enclosure ( The Citation 12 Deluxe ) with a brushed aluminum front panel, but electrically their circuits are identical, either one can be rebuilt.
Unfortunately, its just over one-square-foot size, tends to hides its hefty 30lb. weight, which one suspects will tend to preclude stuffing it into a backpack to be carried over to audiophile friends ( to show-up the latest mega-buck three-man-lift power amps from Cello, or Krell ). Still, the positive attributes of the Citation 12 do include: dual power transformers and completely separate power supplies, minimalist design topology, a dual compartment steel chassis, and a combination of point to point wiring for the power supply under the chassis, and mostly printed circuit board wiring for the amplifier circuit section on a single plug-in circuit board.
Built like most of the amplifiers of its day, the Citation 12 lacks a regulated power supply, going instead, with conventional plus and minus supplies, with 12,000 uF. of capacitance in each channel. This may seem like it is about 8 times the capacitance as in the stock Dynaco 120, but when closely scrutinized, that capacitance is actually in two legs of +/- 44 Volts each, so it really is only about 4 times as large. Still it was a rather huge amount of capacitance in its day, in those four big blue electrolytic caps.
The Citation 12 amplifier's shortcomings are few, so they are an excellent candidate for a ULT rebuild. Some problems that have surfaced over time include the four most commonly known failure modes. These include: a problem in which the proprietary six-lead dual input transistors would develop loud, random, spike pulse noises, which would drive the amplifier to full output, or to blown outputs, often taking out the attached speakers' tweeters.
These input transistors are replaced in the ULT rebuild with the best of the modern day super-low-noise transistors for much improved performance. Also replaced, the associated resistors for proper biasing. The next most commonly seen failure mode: blown bias transitors, which immediately blows at least one of the driver transistors along with the output transistors. This of course is the typically seen "blown amplifier channel" along with either a blown primary fuse, or a blown rectifier bridge ( and a blown primary fuse ). Since this is a low Class A/B bias circuit, Harman Kardon should have been more careful designing the biasing circuit in the original design, of course, my rebuild fixes all of these faults.
Third most often seen: the output stages would blow for unknown reasons, despite the thermal breaker protection, and speaker line series breaker protection, taking out the attached speakers in the process. Lastly, in the earlier sets, the two main power supply bridge rectifiers may have been somewhat marginal, and they tended to short, often these parts can be seen to have developed cracks in the upper glass surface from excessive heat and thermal cycling, after they short-circuit. In later production sets, H/K used higher rated 6A bridge rectifiers, which may have helped prevent some of those types of failures.
In 1970, when the Citation 12 was designed, the primitive early output transistors of the day, were still not much better, or far removed from the slow, homotaxial JEDIC part numbered 2N3055. This despite using house-numbered and selected parameter devices with at best, slightly better charactoristics. I can't stress this more, the technology of the late 1960's/early 1970's was only so-so, and long-term, the transistors were still underpowered for the job. Also the type of devices used 1970, was pretty much the same as every other vintage silicon transistor amplifier made in that era, and sonically anything made in the 70's or 80's was definately much worse than is now possible thirty years later with 5th Generation ULT devices.
In the event of a short, or an unusually complex load, the lack of an output protection circuit, a current cutback design, current limiting, series output capacitor, or a fast fuse protecting the speaker, the loss of an amplifier channel, often meant loss of the speaker too. The set did incorporate circuit breakers, one of which was a special type of glass-encased self-resetting circuit breaker, which was present in the speaker output line. This device was of questionable long-term reliability, and took some "long" time to go into "break" mode. So how well it protects, is questionable. That type of glass enclosed breaker part is no longer made, and it is no longer available. However, I do think it was a neat and somewhat interesting idea. One question that comes to mind, is the actual functional time-constant, and how much the device cycled back and forth when problematic or failure conditions existed in the amp. So that is the question. The self-resetting breakers may have been better than having nothing present in the speaker line with the mostly inductive speaker crossovers of the day. Or, on the other hand, it may have been arcing-over, even causing spikes and pulses, and in fact precipitating blown output stages, guess its hard to tell which.
The Citation 12 was originally designed as a very wide bandwidth amplifier, extending from nearly "0" Hertz to nearly a megahertz. Now that is way beyond the normal audio spectrum, which does provide low phase shift within the audio range. Its a good idea, in theory, but may have been providing only a "small" sonic benefit. The problem would be that such a wide-bandwidth design, could have a very high frequency supersonic oscillation, one which occurs at a supersonic frequency, significantly above the audio spectrum. The original H/K Citation 12 design was only marginally stable. It was OK, but some loads with less than optimum bypassing caps could/would cause a very high frequency supersonic oscillation, and the output transistors would rapidly overheat.
While there is a self-resetting thermally activated circuit breaker present in each channel ( on one of the pair of output transistors ) these devices would not protect from the supersonic oscillation. These thermal breaker devices are known to make poor internal contact, and over time, to exhibit high internal resistances, often causing more amplifier problems than they ever tended to solve. I guess these techniques were better than nothing considering the ancient power transistors the designers had at their disposal. In the case of this supersonic oscillation, a rapid output transistor over-current in which the transistor dice gets superheated so fast, the transistor's case never has time to heat up before the dice cracks and explodes. So, case mounted or heat sink mounted thermal breakers are useless to catch this type of failure. Under normal operating conditions, and normal speaker loads, music never gets the output devices so hot that these breakers trip, but still, they can be useful under some abnormal conditions, like a partial short inside the speaker, or a pair of shorted speaker wires. So I would tend to leave these thermal breakers in place, if they are found to be working properly when tested with an ohm meter.
Like my Dynaco Level 3a Rebuild, the ULT Rebuild Kit for the Citation 12 Level 3 Rebuild includes replacement of the primitive output transistors with the 5th Generation, ULT Super-Matched-Pair of devices. Other minor parts changes to the Amplifier Circuit board, and to the power supply also contribute to the rebuild in terms of reliability and sonics. The input jacks are changed to precision machined gold plated input RCA's. New input capacitors, along with 25 Ampere power bridge rectifiers are placed underneath the chassis. Along with replacing the output devices, on the PC Board, all of the circuit board transistors, along with selected caps and resistors are replaced, very much like in the Level 3a Dynaco rebuild. The result is a very impressive sounding, and much more reliable amplifier, with double the output stage margin, lower noise and distortion, and of course, my ULT Output Devices for superior sonics. This amp has fewer wires to replace, so it takes less time to do this rebuild.
The end result, the Bender-Citation 12.ULT has much lower THD readings than the original, and a neutral, cleaner, more detailed sound ( much like a tube amplifier ) and the sound of my rebuilt Dynaco Stereo 120, at Level 3a. Some sonic variations are necessarily chosen, and the Citation 12.ULT Kits, will get the latest in low noise-design transistors, based upon all of the prior ULT sonic improvements that have come before in my Dynaco ULT Kits. The type of sound that the Bender-ULT amplifiers provide is a significant achievement, which makes them the sonic bargain of the century... In the areas of linearity, signal to noise ratio, high-end extension, soundstaging, and bass response, I design for the best in neutral and detailed sonics. This Citation 12.ULT Level 3 Rebuild Kit was long delayed, but Beta Kits were all shipped out at the end of May, 2003, so the long refinement process is completed.
Now that that is done, it is possible some minor improvements or suggestions of the Beta Testers will make for some small changes. Nothing yet to report, but I guess the kit builders won't all be done until sometime later this summer.
Due to some occassional requests for a "lower priced" kit, I'm considering making up a Citation Level 2 Kit, which would use the unmatched Level 2 ULT parts. I still have enough for maybe a dozen Citation 12 / Level 2 Rebuild Kits. These would be priced about $450. or $200. less than the current Level 3. Also a Citation Kit using older style transistors ( not ULT's ) could be priced at $200. If enough people email me with interest for either of these products I would consider making them up, if I have the time. The "email to link is just a few lines below...