Steve Deckert
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CSP2 Designer’s Notes
The CSP2 is one of those things that comes from constant R&D. It started as a project one night to see if I could come up with a headphone amp that could replace the MLB. For a dedicated headphone amp, the MLB was at a price point that kept people who were not into headphones from trying it. I wanted to do some less expensive circuit designs and see what was possible. You see, I’ve heard magic in headphones - it requires a lot more then a cheep pair of headphones and a headphone jack. I know my customer base, and what they like. I know if they heard this magic I’m talking about they would embrace it. By doing so they would expand their audio experience in many wonderful ways. It’s a fact that it would improve the sound of their two speaker system because it’s such an excellent tool for hearing what the music is suppose to sound like.
I know why there are not more audiophiles into headphones. I was one of them - and being the type who is really taken by good sound stage depth I never particularly cared for that in your head imaging of headphones. Headphone enthusiasts will often remind you that “good” headphones are typically not like that and while partially correct, most fail to realize that it is in fact the headphone amplifier that makes the most difference.
In designing our first headphone amp, the MLB, I wanted something that was really top drawer - something that made those who heard it suddenly take headphone listening seriously. To do that I choose the more expensive approach of using top quality output transformers to drive the headphones. After listening to several candidates I choose Sowter Transformers from England. A lot of effort and work went into the design of the MLB and it turned out great. It demonstrated that headphones don’t have to have that in your head sound.
So back to the bench… I’m basically thinking that by eliminating the expensive Sowter transformers we could get the price way down, and that basically leaves OTL. I choose not to go with OTL designs when I did the MLB simply because I only knew of two ways to accomplish it, and both had little appeal to me. The first way is to run several tubes in parallel to get the output impedance down low enough to drive headphones. With every tube you parallel, you loose transparency. I just can’t get into the sound, too spoiled. The second way is to use a super large value capacitor in place of the output transformer. This second method would ideally hit the goal of reducing cost, but I have never considered it seriously.
This is one of those defects of too much thinking. I can say that now because I built one thinking it was going to be inferior in almost every way. For years I would never waist my time trying it. I listened to a couple OTL headphone amps and concluded that the extra large cap just couldn’t compete. I was wrong. In the one I built, I was completely shocked at first listen. Even fresh of the bench it was so pleasing to listen to that it made the MLB sound almost hard. The ambience was so prominent that it sounded as if I were at the venue, not wearing headphones. The bass was unbelievably low and organic. Music that draws your ears to it rather then music that penetrates or sometimes violates. I was amazed that it sounded so much better. Had I tried this from day one, things would have been different.
Still fascinated that this sounds good, I wanted to hear it in the context of a preamp driving our amplifiers in my reference room so I added some line level outputs to the circuit and gave it a listen. After about 20 minutes I realized this may be as good as the CSP preamp so I warmed it up and A/B’d the two. It was better then the CSP. Slightly more defined and focused / a bit more powerful sounding but otherwise almost identical. This was another surprise.
And that’s how it happens. Serious now, it changes from a project to consuming all of the R&D time and my full attention for awhile.
This comes at the same time as our new Anniversary platform which trades the black 10 x 6 x 2 inch steel chassis for a 2mm thick T6 Aluminum plate that has been stretched to 14 x 6.5 and mounted into a solid walnut base. The higher mass chassis are less effected by resonances and offer an extreme high quality appearance and other advantages.
The final prototype when viewed as a replacement for the CSP has several advantages over it besides sounding slightly better. The CSP circuit consisted of a gain stage with low plate voltages driving a single cathode follower with a trim pot between the two stages for calibration. Due to the lower then normal voltages that in part made it sound so good, the input stage had to be adjusted to match different input voltages. It was well adept at offering lots of gain to lower input signals of 2 volts or less, but had to be adjusted internally with the trim pots for voltages higher then 2 volts.
The CSP2 circuit consists of a gain stage with higher plate voltages driving a SRPP stage similar to the original MLB design. The stages are direct coupled. The SRPP stage drives the output capacitors. The line level is taken directly off the outputs via a 100K precision trim pot. The output impedance is about 10 times lower then the CSP so using line level outputs has no audible effect on the sound of the headphones.
The output level is adjustable and the trim pots are accessible from the outside, one per channel. With a 2 volt input, the line level output can be adjusted smoothly between 0 and 36 volts with virtually no distortion on the scope. Factory setting will be a 4 volt output giving it plenty of authority with our amps. High power amplifiers and or amplifiers with input sensitivities of 1 volt or less will benefit from adjusting the voltage to around 1 volt. Lower power tube gear that sometimes requires 5 volts or more to drive it because it lacks enough gain stages, will be best served by settings between 5 and 8 volts. This insures complete synergy and compatibility with virtually all amplifiers. Having adjustable gain allows the serious audiophiles to run the volume control wherever they feel it sounds best while maintaining the desired level of output at all times. Independent adjustment on each channel also allows for channel balancing.
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