KISS 101
INTRODUCTION
by Andre Jute
This is a project to design an ultra-fi tube amp on the net. The project is aimed at the rawest newcomers to amps as well as the technically proficient who are interested in how the ultrafidelista think. Anyone can pitch in but keep it simple, keep it on topic, and leave the personalities off so the threads do not degenerate into another flame war.
The amp will be a two-stage resistance capacitance (RC) coupled 300B. The intention is not to build a cheap amp. I shall be designing and building my copy with parts I have to hand, which were selected for sonic glee rather than by price. Those who read audiophile newsgroups may consider my mix of outrageously expensive and good value parts bizarre. I've written about classical music since I was a teenager, I'm a constant concert goer, the rest of my audio chain is best quality gear, I have good test and measuring gear when I want to supplement my ears, I have access to world-class professional musicians to play live for me as a test against their disks, and I have designed and built quite a few amps, so when I say a component is chosen for sonic reasons, I don't mean because it is cheap or sounds like a boombox. I might add that I was trained as a psychologist and economist (both of which are generally statisticians of a more elegant turn of mind than mere technicians) and when I worked in advertising spent hundreds of millions of my clients' money on research, including the sort of placebo research that pretentious audiophiles from the wrong side of the tracks deify ABX. In short, I back my taste with teeth.
The amp will be simple enough to be adapted to parts you have on hand or can afford to order. The idea for those who are absolutely new to the hobby is to swap in parts until you are happy with the sound and then to start changing the design of the amp until you're happier still. It is not my amp, it is yours. Do what you please as long as you take the usual safety precautions.
Note the warning at the bottom of every page. THE VOLTAGES IN THIS AMP WILL KILL YOU. GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
GENERAL TOPOLOGY
The official name of this amp is the Real McCoy Type 39 Mk VI Amplifier >The Tradition< because it is the 39th hi-fi design I drew and the sixth major topological iteration or significantly variant design group based on the original in the ten or twelve years since. Its short name, 'The Tradition' is obvious as it will be a very traditional design. I have built this amp many times in different versions, so I know where the design process will arrive. But we shall take it stepwise all the same, so that you can design something else with the calculated steps. I'm going to shorthand both the product we design and the process The KISS Amp. KISS stands for Keep It Simple, Stupid.
Where we shall arrive is at a linestage integrated stereo amplifier with on each channel a 417A driver resistance capacitance (CR) coupled to a 300B power tube which is output transformer (OPT TRX) coupled to the loudspeaker. The signal section will be sensitive enough to take its signal directly from most CDs without a preamp. The output power will be enough to drive sensitive speakers.
Along the way to arriving at the ulrafi version we shall design an ultra-silent amplifier which I shall refer to as the "standard good" amplifier. This is not a put-down by faint praise. It is in fact the highest level of industry standard, against which all other single-ended amps with ultrafi pretentions are measured.
The power supply in the ultrafi version will be tube rectified, choke input. The standard good version will be silicon rectified but can later be upgraded to tube rectification.
Whether either of these amps is high-level hi-fi or ultra-fi depends on the quality of the components selected, and on the layout and construction. Both will have zero negative feedback and will operate strictly in class A1, never drawing grid current or approaching cutoff under any condition of operation. It will, if you build it with my recommended components and values, be utterly silent, with a pleasing harmonic distribution.
GETTING STARTED: WHY WE START DESIGNING AN AMP AT THE SPEAKER
The problem with designing a good audio component is that it requires thought about matters we are inclined to take for granted. Ask yourself, What is an audiophile, and what is his relationship to his amplifier?
An audiophile is a man who listens to music. The music comes on some sort of a disk and is played on some sort of a speaker.
The source is usually the most easily changed component. It is often the one on which the audiophile has the least choice and over which he exercises the least control. In the case of the CD, it is also the one on which for almost any amount of money over a not very high minimum the return will be disappointing. For the purposes of The KISS Amp we shall assume the music arrives at the amplifier from either a pre-amp or a CD player capable of putting out 2Vrms. Most CD players can, and most preamps put out much more voltage.
The most important element of an entire audio chain is the loudspeakers. It is the loudspeaker which interacts directly with the listener. If the loudspeakers are poor, the finest source and amp in the world will not make the music sound good. We are about to design an expensive amp. My best advice is that your speakers should cost at least as much as your amp and possibly more. With the exception of some novelties, every single pair of my speakers cost at least twice what my most expensive amp cost to build. Even my commercial amps are all cheaper than my speakers. The purpose of DIY audio is to let you build hi-fi of caviar quality on a hamburger budget. The amp we will be designing and building in a shop will cost you between twelve and fifty thousand American dollars; you can easily build a version of it that will come very near for under a thousand dollars.
The thoughtful audiophile therefore chooses his speakers first, then his sources and only then does he build the link, his amp. That is the answer to my question, a real audiophile has no relationship to his amp unless he is the sort of person who tells people what his Rolex cost or has a nodding dog in his car to distinguish it from all the other family sedans. He relates to his speakers. Practically, you need to know how many watts your speakers require to drive them before you can decide anything whatsoever about the amp.
It is generally accepted that the best loudspeakers use point-source, full-range drivers rather than multiple drivers. Coaxial cones are point-sources and the Quad electrostatics mimic a point source extremely effectively.
The speakers we shall design for are horns with 8in drivers of about 100dB. Specifically, we are designing for Lowther PM6A fitted to Fidelio type bicor cabinets. You can buy drivers and cabinet-wood and build them yourself for a huge saving off the ten grand plus dollar price of a pair delivered at your door. I built mine. The amp will also drive horns with similar Fostex and other drivers, and of course smaller horns and Voigt pipes. I also publish a high-sensitivity loudspeaker design called The Impresario (because I created it for a music promoter to use in his office) which uses a universally available guitar fullrange coaxial driver and so can be very inexpensively built. (Yeah, you're absolutely right. There is no such thing as a free lunch. It will turn out to be a big speaker even if economical and easy to build.)
Finally, by simply selecting a different output transformer (OPT TRX) or wiring up the one I recommend differently, the amplifier will be suitable to drive speakers of a sensitivity of 93-96dB.
Single 300B are not suitable to driving speakers under 90db sensitivity though I have used a pair in parallel single-ended (PSE) to drive Quad ESL-63 which are around 85dB sensitive with surprising results.
THE VOLTAGES IN THIS AMP WILL KILL YOU.
GET EXPERIENCED SUPERVISION IF IT IS YOUR FIRST TUBE AMP
All text and illustration is Copyright © Andre Jute 2004
and may not be reproduced except in the thread KISS xxx on rec.audio.tubes
