LINE LEVEL PREAMP 2003.
Fig 1. Amp on bench,
Fig 2. Rear amp on bench,
Fig 3. Schematic of 1 channel of amp,
Fig 4. Underside of chassis.
Full explanations are included.
This preamp was custom built in about 2003 for a customer who
something with tubes to see if it made his music any better. He
Pardon the morning break mug of coffee and apple, but they indicate the
size of the amp and its ultra simple remote control inside a block
of re-cycled mahogany.
Remote control is only for volume up or down only. Sensible motor speed
is used to control an Alps pot, 27mm square body. Manual volume control
is possible. The aluminium case work is hand made, machine engraved,
powder blasted to an even matt finish then anodized. A blue LED indicates
on/off, and 5 sources are selectable, and there is a pair of record outlets.
The infrared "sender" electronics and battery is within the small
The IR receiver in the amp drives a motor to turn the pot.
The inside of chassis volume is packed with PSU and point to point wiring.
The rear panel has mains input, fuse and each channel has 7 RCA sockets
5 inputs, 1 output, and 1 record output.
Here is one channel of the line level amp. No balance control was
because the customer agreed there was no need for one.
Gain is set for less than 2 times because the only source the
was CD, or other fairly high input sources.
There are Constant Current Sources anode or cathode loads to
to 12AU7 so that Ia = about 4.0mAdc for idle current for each 1/2
The CCS use means the dc carrying load element to the tube acts as
with R > 2Mohms, so that there is no ac loading by a resistor
B+ to anode
or from B- to cathode which would have values of about 30k, and
AC coupled loads such as the Volume pot of 50k after V1 anode,
resistance of 120k, the total load = 16k.
The result of using the CCS dc supply transistors as passive high
dc elements much reduces the distortion by approximately -10dB,
use reduces THD by a factor = 1/3, a welcome amount which probably
translates to a much larger subjectively perceived betterment in
The THD is somewhat determined by the level of signal before the
Let us assume the CD maximum level is 1.4Vrms, then maximum V1
signal would be approximately
16Vrms, and THD = 2% if there
was zero shunt
NFB, ie, R3 and R8 were omitted.
The use of the volume control at the 12 o'clock position would
the output cathode follower would slightly reduce this to 1.4Vrms,
create far to much volume level in most power amps which need only
to cause clipping.
Therefore the use of a shunt NFB network using R3 and R8 is needed
overall gain at maximum volume setting = 1.7 times. With maximum
= 1.4Vrms, Output = 1.7 x 1.4 Vrms = 2.4Vrms, and THD < 0.1%,
the NFB is reducing the THD by factor = 1/4, and much less signal
exists at the
The average level from a CD player may be only 0.2Vrms, and the
THD is approximately proportional to output voltage, so it will be
THD under normal operating conditions = < 0.03%, and quite
The maximum average signal
levels ever wanted by most people
and the low gain of the preamp provides this easily.
This suits people with normal 50 watt power amps which require a
1Vrms of input for the maximum 50 watts at clipping.
The sound from this type of circuit has the natural warmth of the
detail and dynamics, a sense of smoothness and spaciousness and
This type of preamp is the very essence of what captures the soul
the music and
warms hearts of anyone listening. It will seem to smooth out
may otherwise accompany the efforts of some of the more raucous
Celine Dion; it will make an old Italian violin or cello have you
piano will never sound so good, massed strings will sizzle with
the searing harshness of solid state. Brass will have sparkle
During the years I've made such preamps, people have found that
solid state preamp with a tube pre-amp such as the one above has
for the sound even if they have a solid state power amp.
One fellow had a listen to this type of preamp and asked if I'd
well known brand of solid state preamp as a trade in for a preamp
model and I could only politely refuse him because how would I
sell an SS
preamp after anyone with good ears had a listen to a well done
After many years the 2003 preamp above is still working perfectly
has tried a few different 12AU7 from the suppliers around the
brands can have a slightly different timbral quality, or voice.
The measurements would all be nearly equal. Why some tubes sound
never been fully explained.
One may find many fine wines from different wineries to be all
and enjoyable, and nobody worries whether they have the same
So it is with many tubes from different makers. I supplied the
twin triodes from my collection of about 40 12AU7 at the time so
a pair of spares.
I rigorously test tubes for low noise and low microphony and try
carefully match them for brand and gain. Usually with 12AU7 the
dB between samples and there is no need for a balance control.
The 12AU7 is an old fashioned choice of tube because these days
like to use 6922 / 6DJ8 / ECC88 or the Russian 6H30. I usually
12AU7, or 6CG7,
or 6SN7 and I don't think more modern type numbers such as 6922
sing as well
as the best of the NOS 12AU7 where they test well for noise and
Some more recently made Russian tubes and selected NOS do not have low noise
and low microphony. Usually I find the tubes that pass the quality
for low noise
invariably sound glorious, probably because the tube is as perfect
manufacturing as possible, with tight fitting electrodes and a
vacuum and a
well made cathode.
These aspects of a tube cannot be judged by the appearance or just
with a tube tester. The electronic testing I perform reveals the
a tube beyond the capability of a "tube tester" which only tells
tube works or not, ie, if it has the rated correct emission.
The other fabulous triodes such as the 6SN7 and 6CG7 have slightly
than the 12AU7 but they will work almost identically as the 12AU7.
the 12AU7 needs a 9 pin mini socket, and uses 12.6V at 0.15A
pins 4 to 5
for heating but 6CG7 needs 6.3V at 0.6V. 6SN7 needs an octal base,
are not interchangeable. 12AT7 or 12AX7 may be used instead of
alterations to cathode R and CCS current setting R. All have been
tried, and the
12AU7 or 6CG7 are probably best. Usually, if 6CG7 is used, then
may be used without any circuit alteration.
I don't make a lot of preamps, and I doubt I will attempt to make
sized preamp such as the one pictured which delighted the owner
actually know exactly what he would be getting when he ordered it.
But when he did get it the appearance matched his expectations and
well with the rest of his gear. I now prefer to make a line stage
preamp slightly larger.
In October 2011, I have begun to make a line level preamp using
triodes which are single unit triodes only 35mm long, dia = 8mm,
size is quite
a lot smaller than a normal 9 pin mini tube.
These tubes do not use a plug in socket and have leads which
to a circuit board, thus allowing considerable saving in circuit
may be clamped to a chassis with copper or alumimium U straps to
The sub-mini tubes were originally produced to suit wearable
with battery supplies but later were used wherever compact size
especially in military gear for aircraft where reliability was
Their production has long ceased, but stocks remain. Their
ceased because of the change to solid state in about 1960 for 99%
applications. I'm planning to use a similar circuit for the next
as in the 2003 model, but instead of using MJE340 for CCS, it is
and effective to use a sub mini triode. So a total of 8 individual
tubes will be
used and they will fit easily to a chassis about 350mm long x 150
x 55mm high
with a PSU box encasing PT and diodes at one end and heatsink and
at the other, and with plenty of room for the remote volume
stepper motor to drive a DACT switched attenuator.
It is fairly crammed within chassis area but "compact circuit style" does not
spoil the sound.
Critical signal cabling is short and input and output cabling is
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