A U D I O... P A P E R
Zen Triode Preamp model CSP2 designer's notes
2006 by Steve Deckert
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, Iíve
heard magic in headphones - it requires a lot more than 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 Iím talking about they would embrace it. By doing
so they would expand their audio experience in many wonderful ways. Itís a fact
that it would improve the sound of their two speaker system because itís 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 ďgoodĒ 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 donít have to have that in your head sound.
So back to the benchÖ Iím 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 canít 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 waste my time trying it. I listened to a couple OTL headphone amps
and concluded that the extra large cap just couldnít 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 ambiance 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 than 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/Bíd the
two. It was better than the CSP. Slightly more defined and focused / a bit more
powerful sounding but otherwise almost identical. This was another surprise.
And thatís 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 than 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 than 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 than 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.
Having the line level output variable means that any noise in the system will
be reduced rather than amplified. For example, the CSP2 has a noise and hum
level of 0.4 mv with the volume turned all the way up, and the trim pots all the
way up at the full 36 volts of output. That alone is by far the cleanest thing
Iíve ever built, but since the average user will have the output level adjusted
to around 4 volts. That means whatever residual noise floor there is will be
dropped by around 8 times. When I set the units up for a 4 volt output the level
drops to the limit of my meters at 0.1 mv Part of this comes from the decision
to implement a DC power supply for the tube heaters. These improvements will
make the preamp far more forgiving when it comes to tube quality and should
invite more NOS tube rolling with better results.
The general layout and configuration has a few improvements also. There are
still two inputs and a selector switch, but the locations have been changed
resulting in half the wire being needed to interconnect the input jacks to the
first gain stage. Now a simple non-shielded run of silver/ Teflon wire can
interconnect the two. The output besides having the obvious headphone jack, also
has a 3rd mono jack that can be used to drive a subwoofer or mono
center channel etc.
As with all the products built on this platform, we are using our more
expensive world voltage compatible transformers. These are significantly larger
in physical size to accommodate the extra voltage windings and meat CE ratings.
These can be wired to any voltage requirement.
One of the biggest secrets behind the original CSP was the fact that the
power supply is so large that it has no variance in output just like a regulated
supply, but does it naturally and without a lot of heat. The CSP circuit draws
around 30 ma and the power supply can deliver 150 ma. Power supply size and
quality are always the first place corners are cut in the world of mass produced
hi-fi gear. This is why we do the exact opposite. A lifetime warranty is just a
pleasant bonus made possible by this type of thinking.
In the end we have a product that looks 10 times better than both the MLB and
CSP that it replaces and sounds better than both for less than the price of the
original MLB. Occasional production glitches from delayed English transformers
are also eliminated.
If someone who was new to the whole tube world wanted to hear it at itís
absolute best, they could get the CSP2 and a decent pair of headphones without
breaking the bank. They could then experiment with one or two CD players to get
a feel for how much improvement in sources is possible and in the process would
indeed be able to hear the magic I so often go on about. Unlike if someone buys
just a preamp or amplifier and introduces it into their system, this way is a
And as I mentioned, it can be the worlds best tool for evaluating cables,
sources and becoming familiar with how recordings actually sound. Armed with
this as a reference you can EASILY hear huge differences between sources and
cables whereas in your listening room those same differences are often unheard
due to other things handicapping the system including and especially the room
and speaker locations. It allows you to hear frequency response that is actually
flat without the peaks and holes. You never hear the music get sharp, dry or
offensive. The bass is completely flat all the way down to around 5 Hz. Bass
with no holes or peaks down to any frequency is impossible in any room and in
even the best rooms true 20 cycle notes are mostly fantasy. Like I said, it will
let you hear what it was intended to sound like - and that reference alone will
cause you to improve your system by your knowing exactly what you want it to
sound like. This is so much easier than being blind with no true reference and
just trying to make something sound good. It ultimately means you make far fewer
mistakes when purchasing gear and spend far less money as a result.
Input level Controls added in the final 2010 revision
in 2010 the final revision for this amp came with the model number
CSP2+. This somewhat minor revision did two things: Improved
the internal layout, reducing the amount of wire used and added a pair
of input level controls to
compliment the existing output level controls. While completely
unnecessary for line level use, there is an improvement for headphone
use. Some high impedance, high efficiency headphones simply get
too loud, too fast. This makes it harder to adjust the volume
control in fine increments. It also puts the master volume control
at the very beginning of its range before it gets too loud, again a
less than ideal result. By adding input level controls, it is now
possible to drop the gain for the headphones in the same way the output
level controls can drop the gain on the line level outputs. So, if
you were getting loud at one or two clicks of the master volume with
your headphones on, you can now use the input level controls (one per
channel) to drop the source level - effectively multiplying your master
volume control's range by up to ten times. Instead of two clicks
to get loud, you can adjust it to take 20 clicks to get loud - giving
you 18 clicks of adjustment into lower volumes that you didn't have
before. It was the perfect upgrade to this well loved
CSP2+ OTL Headphone amplifier/preamplifier