Articles for Audiophiles by Steve Deckert
AUDIO PAPER #002
A SINGLE ENDED TUBE AMP FOR THE MASSES
Part II - The Design
If your not an amplifier builder this may become laborious reading after a while, but initially you should understand that the audiophile's fascination with sound stage and imaging (the motivating factor for all tweaks) is well supported in this design. I have found that the determining factors that map your systems sound stage and control imaging are mostly if not completely governed by impedance matching. Finding the exact right setting on your pre-amp volume control for best sound could be an example. When you break-up a pre-amp and amplifier into their respective gain stages and begin to play around with the input and output impedance of each you start to see why no two combinations of amplifier/pre amp will ever sound the same. This is the reason for the large concern about finding "good matches" in equipment and why some good gear sounds bad. To all "miserable" audiophiles that have found out the hard way that money alone can't solve the problem, I would say in fact that this is the problem.
This ability to tweak is what I feel it will take to win the hearts of the serious audiophile and what it will take to be compatible with the expectations of the solid state conditioned masses. This is not because it has millions of bells and whistles (and it doesn't) but because the operator will be able to "discover" the best sound by changing the configurations of the amplifier. It makes sure that whoever buys it is sheltered from the alternative which is taking your chances with odds of getting just the right impedance match between all of your components.
The schematic in figure 1 is what I have prototyped and am currently listening to. The final version will be very similar to this. As you can see the amplifier has 3 gain stages, or optionally two, depending on where you select to have the input go to. In the two stage (direct) mode, the single passes through one resistor and one capacitor the tubes and the output transformer. I'm sure you'll find merit in this "less is more" approach when you listen to it. The additional stage making this optionally an integrated amplifier for all practical purposes gives additional impedance balancing and more gain so that you can listen to it at louder volumes.
The tubes I am using are a 12AU7 and 12AX7 for the first two stages into a pair of 6BQ5's (EL84's) for the finals. I chose the single EL84 because it is a scaled down version of the popular EL34 but in my opinion has a better sound. The single EL84 uses less current which is making this amplifier a cost effective reality to build. I think it has better sound than larger tubes because the plates are small and tightly packaged around the screen grid meaning there is less distance for the flow of electrons between the two. The other tubes are perfect for this design because they are the most popular of their kind and the easiest to find. This means the guy that buys it can play musical tubes by trying several different brands without spending any real money.
The biggest feature will be the switch for changing the amplifier operation from Pentode to Triode. If you need more output, and like you music to sound a little analytical with better specs you can run it in pentode. If you want to here a slightly warmer more pure musical sound with less power switch it to Triode. Obviously there are several combinations you can get from combining the features of this amplifier each with its own sonic signature. You surely will be able to find one that you love.
The outputs on this amplifier will run most any impedance of loudspeaker, 6 to 8 ohms being ideal (at least with the current iron I plan to use). The outputs can bridged by strapping the positives of both channels together if you happen to have two of these and wanted to run them as mono amplifiers, one for each channel.