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I'll bet that many of you like me, have had little time to devote to things audio.... Somehow I have managed to keep the flame burning in spite of the lack of time. This time I will share with you the discovery that many of the newer CD players with 4-8 times oversampling converters can be greatly enhanced by undoing rather than by doing; several simple modifications for the ARC M-100 monoblock power amplifier; problems with so called Mullard 6DJ8's; a cheap homebrew braided interconnect; the XLO interconnect, Mogami Neglex 2534 microphone cable, and the so called "Water Cable" from Purist Audio.
In my quest for better sound from the overdescribed Sony CDP-790 CD player, I recently decided to prepare the player for the inclusion of a tube analog output section. I removed the operational-amplifier previously installed (a 5532AN forced into class A operation) and decided at that point to try a passive network consisting of two cascaded low pass networks to achieve reasonable stopband attenuation somewhere above the audio band. The results are stunning - depth became more than just paper thin, detail and resolution improved greatly, placement of instruments within the sound stage is much more discernable, most of the digital edge has disappeared, and the bass extension and authority are also greatly improved. The network consists of the following components connected to the voltage output of the AD1860 DAC (or the I/V converter used with a conventional current output type DAC such as found in older Philips 14-16 bit CD players.): a 1K resistor in series with the voltage output, which is in turn connected to a series RC network consisting of a 220 ohm resistor and a 2200 pF polystyrene grounded at one end (a single order low pass shelving filter), then the node between the 1K and 220 ohm has another resistor (or a series of resistors) equalling 1K connected between it and the output, and a 1300 pF polystyrene to ground in parallel with the output. (a single order roll off) Please note that the stop band attenuation is not great and the filter is down only 18 dB or so at 200 KHz, if your CD player uses 4 times oversampling or less this attenuation will probably not be enough. See the included graph for an idea of the over all performance of this filter. Other similar approaches with air core inductors will yield more attenuation in the stop band and will provide somewhat flatter high freq. response below the cut-off. More to follow - I have a pair of AD847 op-amps I will use in place of the internal ones in the AD1860 for I/V conversion, and I will be using air core inductors for increased stopband attenuation. Who knows, maybe passive analog filtering will catch on in CD players used with the active pre-amplifiers required to drive the long interconnects found in some systems, or in those systems that are primarily based on LP's.....
I recently had the opportunity to convert a pair of ARC M-100's to triode operation, and amazingly the modification was really quite simple and consisted of unsoldering the 100 ohm screen resistors located at pin 4 of each 6550 output tube, and then soldering the free end of each resistor to pin 3 of the respective 6550. The bias resistors R50, R52, R54, R56 must be changed to approximately 220K in order to bias the output stage properly, and it may take several iterations to find just the right value.... In addition I replaced the 0.01 uF shunt coupling capacitors with MIT film and foils. I am not sure that the difference due to the capacitors was all that significant. I also replaced all of the wire wound power resistors in the signal path with paralleled RN-70's of equivalent rating and resistance. The amplifiers seem tighter and considerably more extended at the extremes when driving my MG 1.4's, clearly the top end is much cleaner, and the bass better controlled. Current delivery into low Z loads is enhanced. (Lots of Kick/slam at the bottom end.) The sound was also warmer, more detailed, lacking the slight stridency or edginess that characterises this amplifier when operating in tetrode mode. The apparent sound stage depth increased considerably and the timbre of musical instruments seems much more natural. This modification is NOT recommended for the faint hearted as the pc boards are easily damaged by poor soldering technique, and frankly - some areas of the board are cramped... Just try replacing those bias resistors on the bias board mounted just behind the front panel, and you'll see what I mean.. The effort involved WAS more than repaid by the end results - just be careful to allow the power supply to fully discharge before removing the cover, and if you don't know the proper precautions to use around high voltages don't remove the covers! I cannot be held responsible for any mis-haps (including accidental electrocution) that may occur!!
I am pained to have to report that some Mullard brand ECC88 / 6DJ8's recently purchased by a friend of mine from a major vendor have proved to be 100 % unreliable. I don't recommend any current product sold by ANY vendors for this reason. Real NOS bought from certified private sellers and in the original Mullard boxes have proved to be just fine. Insist that your seller guarantee the origin of the tubes you are buying particularly if you are paying a premium for what is being represented as the real thing....
I recently had the opportunity to audition several popular audio cables including the new XLO interconnect, the so called "Water Cable" and several cheap or home brew cables. I'll start by telling you that I think the XLO is about the best interconnect I have ever heard, unfortunately this is marred by the worst connector I have ever seen.... XLO Electric's dealer in Boston claims that this connector is the most transparent RCA plug available, but it appears to be a gold plated version of a connector that my employer purchases from Mogami as part of a complete cable assembly purchased primarily for its low price. Has any one else had problems with its reliability ? The XLO cable exhibited very good low bass extension, smooth, clean extended treble response, lots of depth and ambience, and a warm, open mid range. It had the best balance and detail of any cable I have used including the StraightWire Maestro... The music seemed cleaner and more palpable with a degree of presence only rivalled by the Maestro. It has to be one of the most transparent interconnects I have yet encountered, and it seems to be compatible with virtually anything..... The Water Cable was very good, but really wasn't in the same league with the XLO in my system. It was somewhat less extended in the extremes, and exhibited somewhat restricted dynamics in comparison to the XLO. Detail was good, with plenty of air around instruments recorded in large acoustical spaces. The fluid inside the outer tube looks rather like old sludgy motor oil, but infact contains a special fungus in suspension in water. One of the conductors looks to be a conventional irradiated PVC insulated AWM conductor probably of copper, the other is a small white plastic insulated conductor of mysterious composition. Both conductors are contained within a clear plastic hose and are partially immersed in the fungus water solution. Interesting.... BUT it does sound quite good. It also costs about twice the amount of a comparable quantity of XLO cable. I recently started looking for a better cheap cable than the Radio Shack Mega Cable I had been using. Andrea kindly agreed to braid four conductors in a flat weave for me, and the end result was quite interesting. The sound was cleaner but somewhat less open, was more detailed and extended at the extremes than the Mega Cable, but lacked its ambience and soundstaging ability. The approach looks promising, but better quality cable is required as I was only using standard 20 gauge teflon insulated silver plated copper wire, and I think this polluted the results. I thought that I would settle on Mogami Neglex 2534 microphone wire as the cable of best compromise, as it has to some extent many of the better attributes of all the above cables. It does not have the ambience recovery or high frequency extension of the XLO, but it does get the timbre right and has really excellent low frequency extension. Detail is not as good either, but it is acceptable and the mid range is warm, moderately well defined, leaning slightly towards the romantic. Unfortunately the dynamics seem to be overly compressed and the resolution overall does not compare well even with the Mega Cable I have been using. ( I would swear at times that it even sounded a little fuzzy? ) At barely $1.00 per foot it may be a good deal in systems that are excessively bright and transistory sounding, however in systems with a more neutral presentation it softens the impact of musical transients and blurs details. I don't get the impression that its impulse response is particularly good. I will continue to use the Mega Cable for the time being as the interconnect between my power amplifier and pre-amplifier and Neglex between my CD player and pre- amplifier as this seems to be the best overall compromise. This cable was featured in TAA many years ago and I believe it is still available from Old Colony....
Sadly, I have to inform you that due to circumstances almost beyond my control Kennedy Audio has temporarily suspended operations, except for book sales and this column. It is my hope that time and money will permit some limited business activity by mid 1992.
Thankyou again for all your letters and encouragement. Please keep them coming. For those of you who are interested in vintage audio components I have written a little book entitled: "A Guide to Vintage Audio Equipment for the Hobbyist and Audiophile" This book covers a range of audio components with which I have had significant experience or exposure as well as providing basic definitions for a lot of tube amplifier related jargon. Book cost is $14.95, ($19.95 2/98) shipping and handling is $5.00.
I am considering publishing an article on modifying the Dynaco Stereo 70 based on a circuit I designed a few years ago. That is if sufficient interest warrants yet another article on this subject.....
I have moved in order to be closer to my job and also to get away from a city full of people who covet my car EVEN MORE than I do...
My new address will be:
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©1998 By Kevin R. Kennedy