The "Nemo" line
level preamp and power supply for line and phono stages :-
The "Nemo" preamp
that needs a better
name, and the "Bunnerong" power supply.
( There is a coal fired power station in NSW at Bunnerong, and a friend
said that the preamp power supply
looks like that power station...something about all those large value
electrolytics, with
potted power transformers at the rear.)
The Nemo is in its
3rd
regeneration; it should be close to heaven because its been born again
3 times.
It started with a circuit that was just a simple common cathode gain
stage with 1/2 a 6CG7, resistance loaded,
with cathode follower direct coupled and a cathode follower output.
Conrad Johnson used such a simple good sounding circuit way back in
about 1955....
A DACT attenuator was placed before the gain tube.
The gain was a little too high at 26dB.
Next I tried a µ-follower instead of gain and CF, and with a
shelving HF cut network and switch
after the gain attenuator. Both these two options sounded well. NOS
Siemans 6CG7 tended to sound better than Russian 6H30 which we tried.
The 3rd and present form is described in the schematic and notes below.
It sounds quite excellent with Sieman's NOS 6CG7circuit, and gain has
been pegged right back
to only about 12dB, by means of the shunt FB which also allows the
shelving network to be in the
FB loop.There are less HF losses in the signal path and balance is more
accurate.
Controls are from
left to right, source
select, HF shelf levels, gain, mains on/off, phono amp on/off.
The line stage for
dec 2005....
The line stage amp
only has one gain
triode and one output cathode follower, so a pair of 6CG7 are used for
both channels. V1 anode has its dc supplied through the MJE350 constant
current source which was selected to keep the ac load on the tube high,
and THD low.
The first and input stage of the amp is around V1 which has a local
shunt FB network formed
with R1 & R5. But also R6,7,8 and C3,4,5 can be switched to give a
shelved HF response
above 2 kHz so that nasty harsh recordings can be tamed a little if
required.
The input signal
is selected via a DACT
switch and fed to V1 grid. There is 12dB of gain in the stage
with the shunt FB so THD is low, and Rout is also low, and so the
100k DACT 24 position switched attenuator is an easy load for the gain
bloc.
V2 is a simple cathode follower output buffer with fixed bias.
There is some shunt regulation of the B+ voltage with a zener string.
When the phono amp is separately switched on or off, there are no
strange LF noises
or cone wobbles in speakers due to B+ rail variations.
I do not believe
that Auricaps or many other brands of coupling capacitors sound any
better
after having recently trialed the above line stage where Wimas were in
one channel and Auricaps were in the other.
A customer friend and I used the same mono sound source through each
channel in turn with me trying to trick my friend when I asked him to
say which channel was better. After about 6 changes of with 2 different
recordings, my friend
could not pick any change or state any preference which was better than
chance, ie, he liked the
Wimas just as much as the Auricaps. I certainly could not hear any
difference let alone a "better" sound with Auricaps.
However, my friend is proceeding to have me replace all the Wimas in
his preamps and power amps with Auricaps. I will always consider that
my customers are always right, and work as directed, but I don't myself
think I am missing out on better
sound because I have not used Auricaps in my own system.
I do not think
many of the myths about special parts have much validity. The circuit
topology and design
and careful tube choice are far more important to the sonic signature.
The main reason to use better quality parts
is reliablity and tolerance quality. For example a cheap Taiwan made
dual gain potentiometer may have 15% different
levels at the -20dB gain setting but otherwise work perfectly for 3
years. Then it begins to make noises.
hence Ii won't use less than an Alps Black pot which is 26dB more
expensive but they will last 40 years
with good matching on their tracks.
The above Nemo has a DACT switched attenuator which is still working
well after many years, and the level match
is even better than an Alps pot. I doubt there is any difference in the
sound of either.
Tube choice for
all the above preamps does have an effect on the sound. This is true
especially with the Rocket which has no global FB for RIAA eq it is
more prone to trouble from noisy tubes and microphony.
Unless tubes are properly tested for noise and microphony, not just
tested in a tube tester, then the sonic outcome will be sub-optimal. I
find the least noisy and least microphonic tubes give the best
performance in terms of detail,
dynamics, conveyed natural warmth, musicality, tonal balance, sound
stage, imaging and air.
Tubes that are old and tired, and possibly not so NOS as you may think
they were when you bought them
will perhaps sound less than optimal. It does no harm to have a few
different tubes to try.
NOS Mullards are supposed to have a "polite sound" less perceived bass
and treble while NOS Seimans
are supposed to give the best all round dynamics.
I find the NOS tubes made here in Australia 40 years ago are very good.
As everyone can see there isn't much room under the chassis with
Auricaps.
The messy tape with writing on it is just so I know where things are
when I have to service the amp and normally the
bottom cover plate completely covers over such mess.
Everything is hand wired with three dimensional circuitry. The wires
from input sockets to source switch
are very fine teflon insulated solid silver plated wires which are
fragile but supposed to be good sounding.
The ""Bunnerong""
power supply schematic for
line level and
phono stages, dec 2005.
This power supply
is a stand alone unit
about 250mm long x 220m wide x 170mm high. It used to have a tube
rectifier and tube regulator for a line stage only when I sold it to a
client who ordered it with those features.
But that client sold it to another client of mine who contracted me to
do the above phono amp stage.
I suggested we increase the power output capability and upgrade the
supply by
removing tubes and using
all solid state active devices.
That made a lot more room for some decent sized capacitors and so that
the power output could be
doubled to suit both line stage amp and phono stage amp, and all
without getting hot like it used to.
I most certainly
do not believe tubed power supplies improve the sound one bit.
T1 is an Navy
potted transformer used
for its HT winding only. T2 is a generic 100VA tranny bought from Jaycar
electronics parts in Oz and it runs cool and quietly enough after I
potted it.
The large value power supply caps used for the dc heater supply are
what I have aquired from miscellaneous
sales here and there but they are very well over rated for the job.
C2, C3, C4 and R1, R2 form a CRCRC filter for the B+. Further RC
filtering is done within the chassis for the line stage and phono stage
which has a solid state follower regulator, so there is NO NOISE
from the rails or heaters in this amplifier set up.
The dc heater supply does have a shunt regulator to steady the heater
voltage applied to the line stage.
This was prudent because the switching on and off of thre phono amp
meant that the heater voltage
varied too much to the line stage unless it was regulated. This simple
regulator allows the removal of the
line stage umbilical cable and the regulator transistor, 2N3055 will
pass all the heater power,
about 12.6V x 0.6 amps, and without getting too hot. Zener diodes alone
are not as effective as the use
of the zener and trimmer diodes shown that are working with the power
transistor.
It is a rugged and simple regulator.
The heater supply to the phono stage has been made adjustable, see R14
to R18 because there may be a time when someone wants to change a tube
type to something with more or less heater current.