(An Andrea Ciuffoli's
design, a 711's construction)
This is the
version of the amplifier constructed by Andrea Ciuffoli with 300B
fitted in instead of 2A3.
Introduction
I classify this project as
one of the best audio amplifiers, in terms of sound quality, that have
ever been made.
This is a TOP REFERENCE
amplifier that has been designed with no compromise in mind. It is a VERY
expensive project intended for the most advanced builder. You definitely
cannot reproduce the quality of this amplifier with low cost components.
The main results match these items:
top sonic performances
two stages only from DAC chip to the loudspeakers
power about 7-10watt rms with 8ohm nominal load
very low distortion about 1% at the max power
good damping factor with an output resistance near 1.6ohm
very good low frequency response about 5Hz (-3db)
Although this is an Andrea Ciuffoli's design, in this page I present my
approach, with some minor modifications that can improve the amplifier to my
oppinion.
Components
All the audio transformers used in this project are Lundahl
products:
Lundahl is one of the world´s
leading manufacturer of audio transformers used in many professional studio
recording and radio stations so they have a very big experience and all these
products use a custom audio C-core.
The total price of the transformers
cost me about 1000 Euros.
As far as concern the input stage I have tested many combination starting from parallel
6SN7 to the 6H30 Russian incredible tube but with the 5842 Raytheon the sonic result is
much more higher. But note, the 5842 has a much shorter life (about 1000
hours compared to other 10000 hours of other tubes)
The other advantage using the 5842 is the
high voltage gain that allows to connect
the amplifier directly to the DAC chip so we will have only two tubes on
the signal path. In fact I am proposing in this same page also a digital input.
The tube diode rectifier GZ34 should be the Mullard version. Some DIY don't
like the sound of the GZ34 because consider this to much hard near solid
state diodes but this is not true because depend by the power supply design.
Also, 5R4 can be used. The appearance of the military 5R4 is nice and
military types of this tube are long lasting.
About passive components types, I don't leave many choices:
Holco or Caddock resistances on cathode for their excellent non-inductive and
precision characteristics, ELNA Cerafine capacitors on cathode
and power supply.
The ELNA Cerafine capacitors contain super fine ceramic particles which,
through chemical reaction, improve the discharging speed between the anode
and electrolyte with very low distortion.
Any other electrolytic or polypropylene capacitor give very lower sonic
performances introducing some alteration. Only the Blackgate WKZ could
give better sound, but I have not tested it and I know of some problems
during startup phase.
Ciuffoli's quote about wire
was: About wire forget the teflon! use only stranded tinned copper wire
with pvc isulator and for loudspeakers I am testing the elevator cables
(paralleled wire with a good section). A very good quality stranded tinned copper wire could be buy directly
from E-Z-HOOK that carries an extensive
line
of fine stranded and extra flexible wire.
Well I have an objection on
this. At least for the signal path, use silver plated or 99.9% pure silver
wire, with teflon tube insulation. These wires are just excellent, and
this can be practically proven from their use to critical performance
radio coils. I do not believe that silver plated is worst than 99.9% pure
silver wire. In fact you do not gain too much from using 99.9% pure silver
wire, as the most signal is carried out the thin external layer of the
cable. But the advantage of using 99.9% pure silver wire is that you
further ensure that it's external layer will be the same as the inside,
99.9% pure silver material. I am saying this because some sellers sell bad
quality silver plated wires that get some corrosion after some years. The
insulation is mandatory as it prevents from accidentally touching cables.
But it also has another advantage. It further prevents dust and humidity
from reaching the surface of the cable, which will result in corrosion
after some years. Also I do not agree with Ciuffoli about teflon. Teflon
has the lowest relative dielectric constant (about 2) after the air (about
1) and it also has one of the lowest tangent losses (about 0.0002). It has
also one of the best maximum temperature characteristics (about 250
degrees Celsius). Additionally, it is a highly non-chemical reactive
material, it does not react with silver at all and it can be easily
cleaned. So I think there is not better material than teflon.
Teflon is the choice for tube
sockets too. This material is found to have the best sonic properties,
since it gives very low acoustic coupling of the tubes to the chassis.
This greatly reduces the microphonic effects, better than any other
material can do. The disadvantage is that teflon sockets are not easy to
find and they are very expensive.
For the I/O, superior
quality plugs have been chosen. Audio Note AN-STR "MEISHU" silver plated
speaker terminals and CMC brand pure silver RCA female connectors with
teflon insulation.
The mains switch is a
military one, pulled out from an old aircraft surplus device. A better
switch couldn't be chosen. These are super long lasting and solid. The
switch is double bank and it has three positions. Position SW1 (a and b
bank on), position OFF (a and b bank off) and position SW2 (a and b bank
on). By using this, I can choose manually the time for the protection of
the anodes of the tubes. First switch to position SW1 to preheat the tubes
for as long as you like. I always like things to be made manually so I can
control them better. Then switch to position SW2 quickly (quick enough not
to turn off the filaments) to apply the high voltage. It is a technique
that I apply to all my constructions that require manually preheating and
high voltage applying with just a single switch and no semiconductors.
Ciuffoli suggests not to
use a mains filter because he does not like it's sound. Well, I disagree
about this. A clean mains means clean power to the tubes which results
always in cleaner sound. Much filtering is done in the PSU and many money
is spent for a clean and stable PSU. So Why not having a cleaner mains
too? Mains filters are bandpass filters that do not allow frequencies
below or above 50-60Hz to pass into the power transformer. There is an
even better solution (although much more expensive) that also keeps the
frequency and the voltage of the mains stable. Power generators at power
stations rotate at 50-60Hz but they are not stable neither provide a
stable voltage. There are devices that can be connected in a way similar
to the connection of a UPS at a computer and they keep the frequency of
the mains stable. They also prevent from mains surges and they also keep
the voltage of the mains stable. They provide more than one plugs, to plug
in many devices. My cousin has tried such a device in his cheap Chinese
EL34 push pull amplifier and he was surprised to actually listen to the
great difference in the sound. Thus, I would not use a mains filter inside
the amplifier, but I would definitely use one of these devices.
The main schematic
The LL1660/10mA
in the 1:2.25 configuration have a right gap for 20mA.
If this amplifier will be used with a tube preampl. use a 100K 1/4W
Caddock resistance instead the stepper attenuator.
If this amplifier is used after a CDplayer use the DACT 100K +100K stepper
attenuator.
Power supply
The 0.56 ohm resistors could be adjusted to have correct 5V on filaments
of tubes and a value range from 0.47 ohm to 0.68 ohm should be good to
test and match any rectifier.
I have used 2 x 1Kohm MP816 Caddock resistors for the automatic bias
of the 2A3 but a single normal 470ohm 25W could be used but sonic performance
could be compromised.
Also this resistors could be adjusted to have the perfect bias current
120mA.
Digital Input stage
This project give a first class sonic performance but this can be easily
compromised by a bad input signal so I suggest you to don't use a preamplifier.
Instead, take the output directly from a DAC, unless a phono stage must be used.
Using one of Ciuffoli's top DAC projects, all the DIYer's will reach the same
sonic result.
The DAC normally included into commercial CD player (even these of significant
price) have a rather low performance because the common usage of high NFB
operational amplifiers "destroy" the sound given compression effects and
poor soundstage stability.
Sonic results
The sonic performances are the best of all my projects!
This is a very reference amplifier and I suggest it to everyone if
the power output is enough.
Layout views
click on the images to get real scale
Layout size
Attention:
some 100uF+100uF 500V ELNA Cerafine capacitors are higher so you should
use an higher chassis or you can use 100uF+100uF 350V!
