The last three years has been a wonderful experience of designing and
fine-tuning the vintage Dynaco Stereo 120 into a modern amplifier meeting
or exceeding the sonics of the best and most expensive amplifieers on the
market, regardless of price. Additional testing and comparisons to many
amplifiers costing several times the Bender-120.ULT's price, has made me
wish that I could tailor the sound to match that of vacuum tube electronics.
While absolutely mimicking "tube sound" has proven not to be possible,
as transistors and tubes are just two different species; there are some
sonic alterations which are possible. So I wanted to share these possible
sonic variations with the ULT Kit builders, who will now be able to make
these minor sonic improvements and / or choices in the type of sound that
they want. The Bender-ULT amplifiers are still the sonic bargain of the
century... its just that you can improve on it in the areas of linearity,
signal to noise ratio, high end extension, soundstaging, and bass response,
if you so choose.
Please keep it in mind that, these new options produce some very slight
graduations in these parameters, which under some circumstances, may not
be obvious, or even evident with many recordings, or when using associated
equipment, which limits the functionality withing a given parameter. For
example: increasing the unweighted signal to noise of the power amplifier
from say, 105 dB to 115 dB, while a significant achievement, and and quite
measurable on an expensive lab instrument, that improvement may have almost
no sonic effect, if the signal to noise ratio of the stages driving the
amplifier ( especially the preceeding phono stages or Tape Deck ) only
achieves a SN/R on the order of 55 dB. or 65 dB.
However, using a 24/96 or 24/192 CD / DVD player with appropriate source
material and a wider signal to noise ratio, or even qwell recorded CD's
having a wide dynamic range, will likely show improved effects in system
SN/R. Compared to many other amplifiers the Level 3a ULT is already quite
close to perfection. Still, in areas of linearity, speed, soundstaging,
and, to a lessor degree, the bass response and extension tend to be very
speaker / amplifier interactive. That is, the amplifier and the speaker
interact, in effect, making minor improvements in the amplifier appear
as improvements in the speaker. This is what I am trying to achieve.
The Level 3a - Option 1 is: The Chassis / Amplifier Upgrade
This was formerly called: "The March, 2000 Revision" which optimizes the
overall amplifier's Signal to Noise Ratio. Better connections and audio
cables, requires a higher mechnically secure machined input jack, so the
new input jacks helps bring the audio signal into the amplifier with less
degredation feeding the input stage. Any losses introduced at this level,
are simply gone, so the Super Matched Pair of Input transistors is replaced
in the input stage. While these transistors were fine, I felt that they
limited the final signal to noise ratio of the amplifier, with their small,
but noticeable levels of shot noise. So their performance did put
me on an extended search for something better, for a bit of improvement.
Using a specialty super-low-noise transistor for Q1, improves performance,
as does replacing the related biasing resistors associated with Q1,
optimize the operation of the new transistor. In the second voltage gain
stage a new Q2 is a high Ft device that improves bandwidth and distortion.
Then new resistors in the Driver Stage, and Output Stage completes the
Option 1 Upgrade.
The Level 3A - Option 2 is: Amplifier / Power Supply Upgrade
Upgrade Option 2 minimizes amplifier distortion in the second voltage gain
stage, using a new Q2 found to be superior for the overall distortion of the
amplifier is used. The transistor formerly used in this position, from
Option 1 is then moved to the complementary current driver stage along with
its complement. These are fast transistors, and this keeps the overall
open loop distortion ( before applied feedback ) lower for a better sonic
signature. The results of the Q1 / Q2 / Q3 / Q4 changes along with some of
their associated resistors increases the gain of the amplifier slightly,
by around 4 dB. This means less loop feedback, which one would suppose,
would be instrumental in causing the distortion figures to rise, in which
case a 0.1% level would rise to around 0.25%, all other things being equal.
However, the choice of superior transistors chosen, most specifically for
Q2, actually results in a decrease in distortion, more than offsetting
any increase that would be seen from the reduction in loop feedback. A
case where fine tuning of the circuit reaps performance benefits, without
actual changes in circuit complexity or circuit topology, clearly showing
that transistors have improved over time, where small signal devices are
now catching up with the ULT Output Transistor's Technology.
In the regulated power supply the capacitance reseevoir is split, using
dual large reservoir capacitors, each one much less than half the physical
size, but half the electrical size of the prior C12 for each channel.
Each new C12 cap is fed by a power diode, which isolates the power supply
lines from audio signals which would cross-couple back to the opposing
amplifier channel. The improvement: the channel to channel separation,
and in the sonic soundstaging. Lastly, use of high voltage, precision
[ 2% tolerance ] Film Caps in the Output Zobel Network enhances the
amplifier output stability under large voltage swing and high power
transfer output conditions; isolating the output stage from some
non-linearities when the stage enters and recovers from clipping.
The Level 3A - Option 3 is: A Power Supply Upgrade
Use of 8 Ampere high voltage Hexfred Ultrafast or 8 Ampere UHR Soft
Recovery Power Diodes, provides some superfast sonic benefits. Price
wise, they are pretty expensivee, subjectively, they improve the
bass region in ways that seem to defy explanations. Either of these
Ultrafast or Soft Recovery type diodes seem to work about the same.
Perhaps it is a difference in the diode turn-off time, which causes
the residual ripple spectrum of the power supply harmonics to be
decreased, in the lowest frequency range. Anyway, I suspect that
intermodulation distortions are reduced. I can't measure it, I can't
prove it, but I like the effect, as it appears to be a positive
effect, with no sonic downside. As a final complement to the prior
Option 1 and Option 2 distortion reductions / bandwidth increases,
it leaves me almost speechless.
I encourage my Beta Testers, especially those whose amplifiers are
Level 2, to Upgrade to Level 3, for the Level 3's clarity of the
sonic details, and also because that allows you to also experience
these new Upgrade Options. I'm sure you will be happy with these
incremental, but very measurable improvements. If not, parts of
the Option 1 are reversible. If the original input transistors were
carefully removed, they could be reinstalled, if that prior sonic
signature is preferred... But, I kind of doubt that anyone would
do that, it's not likely that it will be necessary. The improvements
in Signal to Noise Ratio, Channel to Channel Separation, and
Soundstaging will most likely be considered not only technical
improvements, but sonic improvements and preferable to the prior
- Steven L. Bender
This page is under construction
( Like software, the job is never done!....)
Last Update - 04-01-2001 / 6:00 PM.