hollowstate

Just thought I would add a note or two about the Octal 6A SE. I built the prototype as the request of a customer who wanted me to use some "premium" passive components. He was not a technical person and had no specific components in mind so I thought the best route would be to choose parts which were likely to push the sonics in different directions for the two active circuit topologoes of the Octal 6. This makes more sense than trying to predict the effect of replacing stock components in a design with so-called premium parts--you may get an effect, but is it better or just different? So I tried to push the "classic" topology (common-cathode + cathode-follower)further toward a full-bodied, gutsy character by using carbon film resistors and paper/oil caps. There are lots of choices but without much hard experience, I chose Riken 1W resistors and Jensen oil/paper caps (actually, due to a supplier backorder, one set of Jensens and one pair of 1 uF/400 V caps salvaged from a Tektronix scope--probably similar to Sprague Vitamin Q's). For the SRPP topology, I aimed to enhance the detail and analytical capabilities of this circuit by going with metal film resistors (I chose Caddocks) and film and foil caps (I chose Hoveland Musicaps). Again, there are lots of other choices and I have no experience with comparing these kinds of parts. As a skeptical engineer, I was astonished that the differences in the two topologies was no longer subtle. The CF circuit was indeed "fatter" sounding--even the bass sounded more like that you might expect from a SE power amp. The SRPP stage was lighter and more airy. I prefered it to the CF circuit in my casual auditioning--I wanted to get the unit it shipped to the customer as quickly as possible. The bottom line is that the use of permium parts to produce two sonic flavors in a single preamp is a realistic goal and has nothing really to do with higher quality. My feeling is that one might achieve similar results with less costly components of similar construction. I have a bunch of Russian 2uF oil/paper caps and boxes of old Philips 1W carbon film resistors that could be used in the CF circuit and generic metal films (e.g. Beyschlag-Centralab) and Sprague 716 polypropylene/foil caps for the SRPP. If I only had the time to experiment . . . One last note--one of the benefits of the Octal 6 SE is that with the extra cost, I felt inclined to do a "industrial/military" wiring layout with cabled wires, right-angle bends, etc., shown off by a plexiglass bottom plate for the preamp. So if your friends are not impressed by the sound, you can show them the innards! I welcome comments/suggestions. Regards, Lloyd

As a manufacturer and engineer, I have to agree with Mark's analysis. The circuit design is the most important element in making good music. If the design isn't right, you will know about it right away or perhaps after a long listening session ("my ears are killing me!"). If you are enjoying what you hear and hate to turn it off, it is a successful design, regardless of how little it costs. Now if you change components, tubes, etc. it may sound different, usually in subtle ways. However, it isn't really accurate to say it sounds "better", just different. You may like it better in the short or even long term but, again, it isn't related to what you spent on the new parts. I think the term "upgrade" is probably the wrong description of these modifications. If you design something that you plan to manufacture and sell, even on a small scale, the engineering challenge is to produce the best product you can at the cost target you have decided is reasonable. The MAD componenets all have parts selected to work reliably within the circuit design requirements. Reliability includes long life so meaningful parts quality is not a waste of money--hence the Alps pot substitution available on most preamps. Lastly, if customers wish to susbstitute stock items with specialty parts, and it brings them excitement and satisfaction (and they can afford it), I would say go ahead. Just remember though that it is no longer really a Mapletree Audio design, but your own creation; something different. Regards, Lloyd

Hi Mike, Glad to get some questions about the Octal 6A preamp. I have purposely taylored the sonic presentation of each of the two active circuit topologies of the Octal 6 by using carbon films in the classic common-cathode/cathode-follower (CCCF) circuit and metal films in the SRPP circuit. Both use a single Solen polypropylene capacitor coupling the output signal in each channel. Since my aim was to offer two (three, including the passive option) different sonic choices, depending on the listener's choice of program material, the associated componenets, especially speakers, and mood, it might be interesting to go further in that direction, using carbon composition (e.g. Allen Bradley) resistors and paper/oil caps in the CCCF circuit and high-definition film resistors/capacitors in the SRPP circuit (you all have your experiences/opinions on what these might be). The only upgrade I normally offer with the Octal 6 is the Alps volume control as it offers a long-term reliability advantage in addition to any subjective sonic improvement. The new channel gain controls are plastic element types and are not in series with the signal at normal full operating position--same situation as with the Alps Black Beauty balance control I had been using. So if you would like to specify what passive components you want in each circuit, I will be glad to oblige with the increased cost equal only to the diffence in component costs. This can add up of course (e.g. the Beyschlag-Centralab metal film resistors I use are $0.12 each in quantity). I have obtained stocks of 12SN7GTs from a number of sources. AES always has a supply--they are increasing in cost but still a bargain compared to 6SN7GTs. Regards, Lloyd

Thanks for the good input Dean. My experience in designing/building DHT/SET amplifiers was not as positive as many of you, probably due to the lack of appropriate speaker systems. I was also plagued by low level hum that persisted as long as I stuck to ac filament excitation. I cannot tolerate audible hum, especially in a component offered for sale. But as there are quite a few popular SET designs out there, I felt that I couldn't do any better given the general simplicity of the circuitry and limited options for doing something a bit different. Hence the 12A6-based class A push pull. It is based on the Mullard/Leak topology using a pentode for the input stage. You are correct in surmising that there is considerable global feedback as you can actually see from the frequency response/loop gain plots--amd it is responsible for the low output impedance and low noise. I know many of you prefer the sonics of zero feedback amplifiers. So the Stealth 5 is not everyone's amp but those who have tried it have been positive in their response. Thanks again for your input. Cheers, Lloyd

Thank you for the welcome to the Klipsch forums. I hope that I can learn something about the special needs of owners of Klipsch speakers and at the same time let you know what is available from Mapletree Audio Design (MAD) that is of interest. As an introduction, and to answer kjohnsonhp's questions, I will provide a short description of the MAD products of interest to members of this forum. In many ways, my design philosophy is rooted in the era that produced the classsic components and kits. I think that the main improvements over these designs lie in the availablility of higher quality passive components (resistors, capacitors, controls, etc.), and perhaps the deletion of excessive tone controls and other filter/equalization circuitry. Also, since tube audio is not a mass market business these days, the financial bottom line is not the only factor governing engineering design decisions. Lastly, I favor new old stock octal-based tubes over NOS and currently manufactured miniatures in terms of overall sonic performance, probably influenced by their inherent low distortion. Power Amplifiers: The newest addition to the MAD lineup is the Stealth 5 monobloc. It is a class A push-pull design using 12A6 output tubes producing 5 Watts. In terms of power output, it is well suited for use with efficient speaker systems. It also has a very high damping factor, which is an advantage in driving single-speaker systems which often present a widely varying load impedance across the audio spectrum. The high sensitivity and inclusion of a volume control in the Stealth 5 makes operation without a line preamp possible if desired. Preamps: The MAD "flagship" component is the Octal 6 line preamp which includes three switchable circuit topologies--passive, classic common-cathode/cathode-follower, and shunt regulated push-pull(SRPP). The two active topologies offer slightly different sonic characteristics which gives the user increased flexibility in matching to different source material, powoer amps, and of course, speakers. It is available in kit or assembled form. The latest MAD preamps come in a modular package which includes a phono stage, line stage, combination phono/line unit, plus the choice of two separate power supplies. The phono preamp circuitry employs a combination of passive and active equalization and with the separate power supply, offers very low noise performance. Specialty components: The MAD Eye 2 is a stereo signal monitor/power indicator using magic eye cathode ray tubes that can be used with any system components. The very popular Ear+ headphone amplifiers are designed to drive a wide range of headphone types and impedances with very wide frequency response and low noise. I invite you to view more details of all the MAD components on the MAD web site hollowstate.netfirms.com. You can download the user and kit assembly manuals plus read what owners have said about the MAD units. There are also links to reviews of MAD products. Lastly, I am glad to answer all questions (there is no such thing as a dumb question!) wether on this forum or by private email. I look forward to hearing from you. Lloyd