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| IONOVAC SPEAKERS The August issue with Daniel Schoo's discussion of the Ionovac tweeter ("Care and Maintenance of Dukane Ionovac Tweeters," p. 50) came at an opportune time, as I had two Duk 10 "add-on tweeter sections" in for re pair. Here are my experiences with those units and some additional information accumulated along the way. Although the cabinets were in very good condition, the insides were very dirty due to extended use, and there was what looked like cigarette ashes in side the throat of the driver. I disassembled everything and cleaned all the sub assemblies and covers with liquid detergent and water. The hardest cleaning was getting at the accumulated grunge, which was way down inside the horn just in front of the grille. I scraped this grunge carefully away and used nail polish remover to clean the inside of the throat as best I could. Fortunately, the electrode looked al most new, and the quartz cell was very clean. The connecting cable between the power supply and the horn assemblies were shot and needed to be re placed (the significant degradation of the insulation is probably due to the ozone generated by the plasma). I was able to clean and re-solder one 6DQ6 plate cap, but discarded the other, making a replacement plate cap from a paper clip shaped to fit by winding it around an appropriate size drill bit. After replacing the power cable and reassembling the pieces, I used a Vari ac to power up each unit. During this process I discovered one bad electrolytic section in each tweeter. I suggest that you check all four sections with an ohmmeter, even though it takes time to unsolder one terminal. My tipoff to possibly bad capacitors was that 5A fuses had been installed instead of the correct 1A size. The bad electrolytics had partial shorts, which caused the power supply to draw enough current to blow the "normal" 1A fuse. I also noticed that the 100 ohm 5W resistor in the power supply runs very hot and was right up against one electrolytic, burning a hole in its cardboard cover and hardening up the insulation of any wires in the vicinity. Repositioning the wires and lifting up the resistor will, I hope, eliminate further unwanted heating. Once the input voltage was up near 100V, the electrode began to glow and the power supply voltage on the first unit was dead on when the input AC was 115V. The first unit functioned normally after applying audio to its input terminals, but the second unit had no glow and ended up having a modulation transformer with an open secondary. I could not locate either PA transformer mentioned in the article but was able to find another workable substitute--a Merit A-3013. The 16k ohm secondary is obtained by connecting to terminals "com" and "5." One point mentioned in a couple of places in the Ionovac literature that came with the units but not mentioned in Mr. Schoo's article is that the low frequency content of the audio supplied to the tweeters should be reduced in order not to overdrive and harm them. The literature recommends a two pole (12dB per octave), 3500Hz high-pass filter. In the back of these particular Duk 10 units was a DuKane network with the designation 9A770, along with an "L" pad volume control. Neither of these components were connected and no additional information was sup plied. I determined the correct wiring for the "L" pad and was able to take apart the potted 9A770 network and determine its components. Let me try and describe this network. Inside was a 3.7µF capacitor in series with a 0.6mH inductor (choke) in series with a second 3.7µF capacitor in series with another 0.6mH choke (all four components are in series). There are six screw terminals on the cover of the net work. Terminal #6 is at the start of the series chain; terminal #3 is between the first capacitor and choke; terminals #2 and #5 are paralleled together and connected between the first choke and the second capacitor; terminal #4 is connected between the second capacitor and second choke; and terminal #1 is connected to the end of the last (second) choke. I can't deduce the full intent of this network, but came up with a two pole, 3500Hz high pass filter by connecting the Ionovac in parallel with one inductor (terminal #3 plus, terminal #2 minus) and connecting the "plus" side of the audio signal to the capacitor (terminal #6) and the minus side to terminal #5 (connected internally to 2). Finally, my thanks and a question for Mr. Schoo. Does he have electrodes available? Charles King Clinton, Conn. Daniel Schoo responds: The story that Mr. King relates about his restoration of a pair of DuKane Ionovac speakers is a familiar one. Many of the problems he discovered and solved are to be expected for a piece of equipment that old. His success at getting them going again is commendable. Dirt and accumulations in the horn and cell are typical because of the high temperatures and the tendency of air convection to draw dust into the horn, where it is heated and burned. Mr. King's observation that "the electrode looked almost new and the quartz cell was very clean" suggests that it had been replaced and not operated for very long. Ordinarily a dark orange to brown crust forms on the tip of the electrode and the inner surface of the cell after several hundred hours of operation. When that happens the plasma often takes on an orange cast or bright orange glow. I usually clean the electrode at about 200-hour intervals. Cleaning the electrode and cell will re store the plasma color to normal after a short restarting break-in time. During the break-in you will often see some initial bright flashes and orange glow until things settle down. I am not aware of DuKane ever recommending cleaning the electrode or cell during its operating life. Their solution to problems was to replace the cell with a new one. Other problems such as cracked insulation and burned plate caps are common. The cable DuKane used was a common rubber covered wire susceptible to heat, age, and the ozone that Mr. King points out. Every one of the units I have seen had an overheated plate cap. I suggest replacing every one whether it is still serviceable or not. They were a poor design and prone to failure. His use of a paper clip to manufacture a replacement is ingenious. I would use a more conventional approach and buy a good ceramic shell or spring wire type clip that might be more reliable in the long run. Electrolytic capacitor problems are always to be expected in anything more than a few years old. As is standard practice with antique radio restoration, even an old electrolytic that seems in good shape should be replaced to minimize the possibility of catastrophic failure later. The cardboard shell capacitors that DuKane used were not known for long life, although many of them continue to function even now. Before powering up a supply, I always check the electrolytic filter capacitors and/or replace them in equipment of this age. Failure of the modulation transformer is too often seen. I found the specifications on the Merit A-3013 transformer that Mr. King used as a replacement. It is a typical 70.7 constant line to voice coil transformer used in multiple speaker sound reinforcement systems. The A 3013 has approximately the same turns ratio as the original, but it is not exact. The A-3013 has a 16k ohm primary, which is the same as the original DuKane. It has two secondary windings of 3.5 and 7 ohm. The DuKane transformer used a single 8 ohm winding. While the Merit transformer will work satisfactorily, the speaker sensitivity and rated maximum input voltage will be changed from the original specifications. The lower impedance primary winding implies a higher primary to secondary turns ratio. This will result in higher sensitivity and a lower maximum input voltage limit. Because of this difference, using this speaker in a pair with another that has an original modulation transformer will cause unbalance in the volume levels from one speaker to the other. You should either replace the trans former in the other speaker to match or, preferably, find another transformer that has the exact same turns ratio as the original. It seems that fewer and fewer sources are available for this kind of transformer. Line to voice coil transformers are still common, but the selection is becoming very limited in types with the required 70V line and .312W taps. The Stancor A-8105 is still listed on their web page. Finding a stocking distributor is another issue. I completely agree with the statement that Mr. King made about limiting low frequency energy into the Ionovac. The scope of my article was mainly on maintenance and repair and assumed that the speaker was already part of a system, so I made no mention of this. A little more emphasis on crossover use and protection is appropriate. While the speaker itself has greatly de creasing output at frequencies below the 3500Hz recommended rollover, the electronics can and does respond. The modulation transformer is capable of response to below 20Hz and will couple audio at low frequency into the oscillator circuit. Although the horn unit will not reproduce audio at this frequency, it will cause modulation of the plasma and can quite easily drive it into cutoff causing serious distortion. I would like to stress that it is very easy to overdrive an Ionovac, so you should take great care not to exceed the maximum rated input voltage at any frequency to avoid distortion and damage. Overdriving promotes modulation transformer failure and arcing of internal components. It is essential for you to use a high pass filter or crossover to limit the high levels of the low frequency content in audio program material applied to the Ionovac. Mr. King's request for electrodes is not the first I've had. I have had electrodes made in small quantities for testing. I have done extensive research on electrode material, burning different alloys for thousands of hours and observing the results. I touched on some of this research in the article. As a result I have found materials with a very good service life. In response to several requests I have decided to have a limited quantity of electrodes manufactured. As of late September they have not been delivered, but I expect to have them soon. When they are delivered I will perform an inspection and life tests to con firm that they meet specifications. After testing I will be happy to make them available for sale, subject to supplies on hand. The quartz cells are difficult and expensive to make. I have no plans to have any quartz cells made because of the high cost. On the bright side, you can clean a quartz cell so that it remains in service through several electrode changes. SACD/DVD AUDIO BIAS As a fellow audio hobbyist, I enjoy Gary Galo's writings, reviews and comments, and I enjoy the mod space he works in. For example, I enjoyed his tips and advice for using Canare and DH Labs audio and data cabling and have built up sets for such use. [So I hope you'll take my comments as constructive and not negative. As we all know, such issues of tone don't always transfer especially successfully in the written medium.] After reading Gary's review of the Onkyo universal disc player (12/03, p. 52), I'm left wondering whether there isn't an editorial bias for SACD and against DVD-Audio. The author's comments about a dearth of high-quality DVD-Audio recordings seem inexplicable when original, high-resolution recordings from AIX Records and others are widely available and commonly cited as good sounding. He also laments that players with combined SACD and DVD-Audio capabilities are relatively rare when in fact many such universal players exist and have for quite a while. They have been somewhat scarce in the high-end space, but Pioneer has had three or four models ranging in price from below $200 to thousands of dollars. Denon, JVC, and others make universal players and have for quite a while now also. Still, I enjoyed hearing about the use of linear power supplies--which some what surprised me--and the use of specific AD and crystal chips in the Onkyo. Notes about the digital output formats supported by the player and its value as a transport are of particular interest to me, though it was not clear whether his comments applied as a CD or DVD or SACD transport. Like Gary, I have a Pioneer DVD player with 96 × 24 output connected to an Assemblage D2D-1 up-sampler and upgraded DAC-3.0 for which I'd be interested in a new universal trans port. I was therefore pleased to learn that the Onkyo sends relatively full resolution DVD-Audio outputs to the S/PDIF jacks, since I would want to connect and use these components as he did. I especially valued and empathized with his comments on the widely vari able quality of the purportedly high-resolution recordings available on SACD and DVD-Audio. (The same could be said of recordings in general.) I fully agree that for these new media to be explored meaningfully, you need good new recordings. Poor transfers of old analog production masters and relatively low resolution digital releases are sadly disappointing. Though some high-resolution digital releases of older analog recordings such as those from Classic Records like the VOX Rachmaninoff can offer good sound, and even newer analog tape masters can have excellent sound, I still find that high-quality analog tape has a discernible sound. While that sound can be pleasingly even, it does impose a slight sonic character, the very act of which is philosophically anti-hi-fi. Original, higher-resolution digital recordings have the potential to pro duce a new level of sound quality, so I hope we all can keep an open mind and not deprecate either of the new formats before they've been given a fair chance. Based on what I've heard so far, particularly from 96 × 24 PCM recordings on DVD-V, they offer good hope for improving the fidelity of recorded sound. Jeff Chan Los Altos, Calif. Gary Galo responds: I can assure Jeff Chan that there is no editorial bias against DVD-Audio, in favor of SACD. On the contrary, my initial "bias" was in favor of the DVD-Audio format, due to its PCM compatibility, and the resulting ease of using outboard D/A converters with the existing S/PDIF interface. But, my opinion has changed due to the dearth of material using the full resolution of the format. The only way DVD-Audio can compete sonically with SACD is if 192kHz sampling rates are employed. 96kHz sampling rates offer less than half of the bandwidth of SACD, and associated phase shift problems due to the steeper filtering that must be used. After receiving Mr. Chan's letter, I checked the AIX website. I found that all of their DVD-Audio releases have stereo pro grams at 96kHz?the only 192kHz recordings they have issued are five tracks on their DVD-Audio Demonstration and Test Disc (which I have ordered, along with the 96/24 Stravinsky and Ravel disc--I may report on them in a future issue). DVD-Audio will always be at a disadvantage in surround format, since 96kHz is the highest sampling rate available for multi channel (i.e., greater than two) programs. SACD has the same resolution in stereo and surround. Mr. Chan is correct that there are now a number of inexpensive players that will handle both formats. I agree that DVD-Audio should be given a fair chance. But, it is up to the major record companies to give DVD-Audio a fair shake. If they continue issuing DVD-Audio recordings at sampling rates of 44.1 or 48kHz, the for mat will never get the chance it deserves. REVIEWERS -- SACDs, PLEASE! I just completed a two-year project, a 220W stereo tube amp. It has too much transparency for CDs--that is, their slight but masking grain and massed-strings congestion are ruthlessly exposed. Not so with an SACD that was recorded directly in DSD--these sound excellent. Lest you doubt, consider an article a few years ago in Stereophile: they com pared the SACD and CD layers of a hybrid disc, listening and measuring. Sampling the same short segment of complex orchestral music, spectral analysis showed that from 10-20kHz, the CD layer had about 8dB less energy than the SACD. But the CD layer sounded brighter! (And hard.) How could this be? High-frequency IM and compression, of course. The top octave with its complex detail is compressed, hence the lower level; but it's also intermodulated into spurious frequencies, hence the "sandpapering" into a brighter, hard sound. (Listen to a CD string section after hearing one live!) This may be masked by the high order distortion of solid-state amps, but not by most any good tube amp. In any case, I strongly recommend reviewing all components with good SACDs (or DVD-As). Otherwise, with CDs, a component judged as "hard" or "bright" sounding may simply be revealing more truth! This is nothing new--The Absolute Sound has been saying this about 16/44 CDs ("Perfect Sound Forever") since their infamous inception. Dennis Colin Gilmanton I.W., N.H. POWER TRANSISTOR SUBS In response to Bill Wallace's Help Wanted letter in aX 12/03 (p. 63) in search of a 2SK134 or 2SJ49 power tran sistor, I suggest he look at and specifically at http://www.profusionplc.com/hitachi.htm. Profusion has a wide range of substitutes. I have used a couple and found no problems at all. A note should be placed at checking bias after replacing. The safest way is to power the amp up with a Variac (that is, variable AC) while monitoring the standing current, eventually rebiasing the amp to come to the original or desired value. I note that they sound best at 600-800mA, but the power supply/heatsinks should be able to cope with these as well. Guido Tent Guido@tentlabs.com SOFTWARE FOR THE HOME As a long-time subscriber---from back in the early Audio Amateur days---I would like to contribute a lead to a free software program that could be of interest to many readers as an aid to audio analysis, especially in home projects or by someone just starting to delve deeper into the hobby (avocation?). You may choose to post the URL or do a capsule product report in an upcoming issue. The product is called "Audacity," which is a wave editor/sound analysis program that may be just fine for some of the lighter or casual inquiries into the sound produced by home stereo equipment. The URL is. Please keep up the great work. Audio Amateur, Glass Audio, Speaker Builder and now audioXpress have filled a very real need in the audio hobbyist community. Dennis Bayzer Dbayzer@earthlink.net PREAMP SERIES Regarding the Adcom GFP-565 mod (aX 11/03, 12/03, 1/04, and 2/04), are the preamp outputs capable of driving a 600 ohm pair of headphones? Thanks. I've enjoyed your work over the years. Les Winter Joanleswinter@aol.com Gary Galo responds: The short answer to Mr. Winter's question is "yes." The AD811 buffer used in the line stage described in Part 3 can easily drive 600 ohm, and can actually drive headphones with impedances as low as 100 ohm. For headphones with impedances in the 40-50 ohm range, the series output resistors R239 and R240 can be lowered to 49.9 ohm. With any other impedances, make sure that the sum of the series output resistor and the head phone impedance is at least 90-100 ohm. Very low impedance headphones will be impractical for this circuit since the voltage drop across the series resistor will be high in com parison to that across the headphone, resulting in low volume levels. In the article I cited from the 2002 Analog Devices' apps book, Walt Jung suggests a 4µH air-core inductor in parallel with R239 and R240 to compensate for excessive voltage drop across the series resistor, but I still think that 40 -ohm headphones are a practical limit for a circuit such as this. Very low-Z headphones are best driven by a small power amplifier. SERVO SUB I wanted to express my interest in the article "A Servo Dual Voice Coil Subwoofer" by Daniel L. Ferguson (11/03, p. 18). I would like to see a project that carries his preliminary findings to a final hobbyist level product. Some ideas that I have that might be good for a future article are: 1. 15 ” woofer downward firing in a small cabinet. 2. Integrated "plate amplifier" from a commercial source. 3. Servo electronic module for insertion between preamp (or home theater receiver) and sub amp. 4. Circuit board design for easy DIY assembly-Old Colony could offer the board. Packaging is left to the hobbyist. I have built and enjoyed transmission line subs for years. However, the size of the enclosure makes the integration of a multi-sub system into a typical room difficult. Now that plate sub amps are low cost, have reasonably good sound, and offer high power, the Servo Sub makes sense. A Servo Sub would also be a great project for me to build for my children. Their bedrooms do not have enough space for a transmission line sub. Again, the compact size of a Servo Sub would allow all to enjoy tight, well-de fined bass. Sincerely, J.L. Ritch Ritch_joe@hotmail.com Daniel L. Ferguson responds: I will attempt to respond to each of your suggestions in the order you gave them. 1. I am in the planning stages now for a project article incorporating a high-excursion 12” woofer. It looks to be a good compromise between size and output. The problem with 15” drivers is that few of them have a low enough Qts. This is made worse by driving only one voice coil, as that essentially doubles the driver's Q. At this point I have found maybe one 15” candidate, and I'm guessing it will require a 3ft 3 box--larger than I prefer. 2. I thought about the possibilities of using a plate amplifier but have some concerns. Successfully using a plate amplifier for this project could be a matter of luck: a. First, I don't think you can use the built-in electronic subwoofer filter, as the crossover characteristics may not be compatible. If you build my subwoofer filter and bypass the built-in filter, your chance of success is increased. b. The amplifier must have flat frequency response down to 20Hz (or less) and minimal phase shift. I have no idea how accurate these low-cost amplifiers are. I'm not saying it can't be done or shouldn't be attempted. At this point, I don't know which plate amplifier has these attributes. 3. My circuit can, in fact, be inserted be tween the preamp output and subwoofer amplifier. However, it includes the electronic crossover, which must be ahead of the servo circuit in the signal chain and is an essential part of the servo. So if the subwoofer amplifier has an electronic crossover built in, it must be bypassed. On the other hand, I have always found it less problematic to use the speaker level out puts to the main speakers as the signal source for any subwoofer. 4. I tend to do all my circuit boards on a project board from Radio Shack, so there are no plans at present to design a custom circuit board. I will provide details of the board layout in any future article(s). Regarding servo subwoofers and reduced size, I am finding that there are limits on how small you can make a servo enclosure. When the air spring becomes too stiff, the driver response can become asymmetrical. Using only one of the voice coils exacerbates this as the motor strength is weakened. It will be interesting to see how the 12” driver I chose will perform in smaller and smaller box sizes. Thanks for your well-thought-out suggestions. Perhaps we can put them to use in the future. I've read the great article in your November issue ("A Servo Dual Voice Coil Subwoofer") and was puzzled as to what configuration of closed box the author (Mr. Daniel L. Ferguson) used on this project. I'd like to try to build one myself, but I need to know how the passive voice coil is sitting in the box--behind it or merely outside of the box like on his setup velocity bracket. Thanks a lot and keep up the good sound! Pascal Sauvageau Gravy Audio Montréal, Canada Daniel L. Ferguson responds: Mr. Sauvageau raises an interesting question. I didn't show the closed test box in my article (it was too ugly) because I assumed that the term "closed box" was a sufficient description. To set the record straight, I used only one woofer in the final experiments, and the magnet was inside the box. It never occurred to me to install the woofer with the magnet outside the box, which would have made some of the hook-ups easier. In my opinion, it shouldn't make any difference which way the driver is facing. AMP AUTHOR It was a pleasant surprise to see Mr. Cordell's recent letter exchange with Douglas Self after many years of apparent disappearance into the audio woodwork ("Xpress Mail," 8/03, p. 65). I've long been fascinated by his amplifier design with the error-correcting MOSFET output stage that appeared in the JAES and an International Rectifier applications note. Here's to hoping that you can induce him into updating the design as a DIY article for audioXpress! Brian Lenharth Tucson, Ariz. SUPERTEX FETS I was reading David Davenport's line stage article ("Odyssey of a Line Stage, Parts 1 and 2, aX 2/03 and 3/03) and noticed the depletion mode MOSFETs. He lists K & K Audio as a source; however they only seem to have these available as part of a kit! It would be nice to be given "real world" suppliers for the listed devices! I was aware--in the back of my mind--of the fact that Supertex made these things, which are basically high-voltage JFETs. A visit to the website did not help with sourcing the parts, but a call to the company did. I was told that Arrow, New Horizon, and All American are distributors for the parts, but the salesman did not have the slightest idea who had what, or if they had any at all! He did say that there were 4000 of the Dn2540N5s? TO-220 (the only useful one for CSs) sit ting in the warehouse in Hong Kong if a distributor ran out. Supertex has a note: "not recommended for new designs" for all of the TO-220 devices--not a good thing, as these are single source parts! However, when asked about the implications of this, the salesman said that Supertex will, allegedly, notify everyone who has ever purchased these devices, whether they are to become obsolete--certainly a comforting thought to those of us who have called to re-order a part and were informed that it was eliminated . . . several years ago. A visit to Arrow's website showed that they have hundreds of the DN2540s, LND150s, and so on. Forget about the surface-mount parts! Arrow, at least, wants you to buy 2k of them, as they are on tape and reel. If you order online, there is no minimum order, and you can avoid a $5 handling fee. A thought: what is the 500V LND150 good for with its 1mA Idss? Would it be possible to make a cascode, CS loaded source follower buffer--basically an active probe for measuring high impedance, high voltage (±600V) nodes in tube equipment, using four devices? Ever see what happens to the bias volt age/current when you hang a meter or scope probe on the output tube grids when you have a 220k grid resistor? This gets into "tweak" territory, where I have seen opinions by various authors in your magazine and others. First of all, tubes: Mr. Perkins wrote an article years ago about cleaning tube pins and how this would improve the sound. I have noticed that "new" tubes are coming through with bright, plated pins. It would be nice to have an assessment of what improvements are attributable to this. Also, from my experience, I would judge that the biggest difference in the sound of various tubes is their microphonic behavior. How do tube shields affect sound? I was exchanging tubes in a BF Bassman Head I refurbished for a client--trying to get the hiss/noise level down--and found that a more microphonic tube actually started feeding back when the steel-spring-loaded shield was installed! How does the shield affect the "sound" of the tube as an amplifier? Amp guru Ken Fischer, in his "Trainwreck Pages" (the uncensored version is contained in Gerald Weber's first book, Tube Amp Talk for the Guitarist and Tech, avail able from Old Colony Sound Lab), con tends that the electron beam, within the tube, can "see" the grounded potential through the plate as possibly added capacitance, and this will adversely affect the sound. Will adding the P.E.A.R.L. or IERC tube coolers, which are non-magnetic, affect a tube's performance? Ken also explained--in a personal correspondence--that EL84s, at least, will put out appreciably more power if they are really cooking and their cathodes are really hot. How about circuit board materials? Andy Nehan and Mike Van Evers have both suggested Teflon circuit boards, writing that fiberglass is a rotten insulator--especially at tube voltages, I would imagine--and is hydroscopic. I have dealt with "Fender disease," where leakage, due to moisture, in their carbon-loaded fiberboard can easily add 0.75V DC on the first two preamp tube grids. It would be nice to see some sort of comparison--at least electrical--between commonly used construction materials, including the phenolic terminal strips of old. John Nickerson Old Harbor Company Chatham, Mass. David Davenport responds: Thank you for reading my article and taking the time to send a letter, which covers a wide variety of topics. I am not sure I am qualified to address all of them, so I will limit my response to your question and comments on the article. You are correct about the importance of being able to source parts, so I was concerned about your not being able to order the DN2540 from my recommended source. I called Kevin Carter at K&K Audio and asked him about this. He said that although the MOSFET was part of a CCS kit, as you mentioned, he does in fact sell the MOSFET alone, and has sold a good number of them. Kevin is not in the general electronics parts business, so he doesn't list them on his website. As to the TO-220 package becoming obsolete; unfortunately the DIY market does not drive the semiconductor industry, so we must take what the telecommunications industry says it wants. I suspect that the TO-220 will still be around for a while, and then perhaps a NOS MOSFET cottage industry will surface. HORN SPEAKER In the January '04 issue Bill Fitz maurice presented a very interesting article on the construction of his DR250a horn speaker ("The DR250 Horn," p. 20). In his closing statement he says he built a unit without the ducts and using piezo tweeters. I am wondering how the performance of this version compared with the original ducted version. I am more concerned with the low-end response as opposed to the HF. Do the ducts have a dramatic impact in the LF response below the 50Hz point? I would appreciate any comments the author might have on this. Thank you and keep the speaker building articles coming. I will attempt to build this DR250a as soon as I finish the classic ALTEC A7 (828 cabinets). Can't wait to find out whether the DR250a can produce home theater thunder on the same scale as the A7. Gerald Hynes APPLIED COMMUNICATIONS Winnipeg, Manitoba Bill Fitzmaurice responds: The sealed rear chamber version of the DR250a is stronger than the vented version from 100 to 150Hz, but weaker below 100Hz. How you build yours depends on whether you plan on using a sub with it. If you don't plan on using a sub, and wish for maximum bass extension, tune it to 50Hz. If you prefer to cross over to a sub at 80Hz or so, tune it to 80Hz, and if you choose to go to a sub at 100Hz, omit the ducts entirely. As to how the DR250a compares to a stock A7 with the original or equivalent drivers, I think you'll be very surprised at what the 250a can do. Just remember that the trade-off of getting so much power and bandwidth from such a small box lies in the lack of flat response. So for stereo or HT, use at the very least a 4 band parametric EQ, though I'd recommend a 31 band digital EQ for best results. HELP WANTED Reflecting on tube experimentation encouraged in audioXpress articles, I was wondering whether any readers or regular contributors to the magazine have experimented with double beam tetrode type 832A in a triode mode SE, parallel SE, or P-P application--found in the WW2 VHF Bendix transmitter type BC-625? Also having difficulty locating source of a schematic for the RCA amplifier type MI-4288-JY using two pairs of 6L6s (metal) in parallel P-P. Many thanks in anticipation. Ken Domminney 11 Sycamore Close Eastbourne, BN 22 OSJ UK England Tel/Fax: +44 (0) 1323 500174 Readers with information on this topic are encouraged to respond directly to the letter writer at the address provided. --Eds -- Also see: SHOWCASE--A Bulgarian Builds a Spencer Amp |