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.