Line level preamp from  2003.
This page include:-
Fig 1,2 & 3, pictures of line level preamp supplied in 2003.
Fig 4 is the schematic for the input and output stages.
Full explanations are included.
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Fig 1


This preamp was custom built in about 2003 for a guy who wanted to try something with tubes in it
to see if it made his music any better. He was delighted. Pardon the mug of coffee and apple but it was morning tea time
when I took the photo and you can seen the compact size.
It features remote control, 12AU7 twin triodes for the gain and buffer stages, on switch, gain control
and source select switch.
The remote control has only two buttons, one for volume up, one for volume down.
The infrared sender electronics and battery is within the small wood block. The IR reciever in the amp drives an Alps pot
for volume control.
The hand crafted anodized aluminium cases for the body and top mounted power transformer case were made possible by a guy I knew at the time who had a small business anodizing specialist parts for many people including Halcro amplifiers.
He has since closed down and such other local crafts people are hard to locate.
The the use of anodized aluminium is not an easy process because the slightest blemish in the metal can easily be seen.
I now no longer try to use such a finish.
Fig 2

There are plenty of  inputs, plus a rec output at the rear. The cables used to no5 input above are some hand made
cables using two 4.5mm wide copper foils between clear adhesive tape. I don't believe they improved the
warm detailed and accurate sound from this preamp.
Fig 3

It is fairly crammed within this amp but a compact circuit style does not spoil the sound.
Critical signal cabling is short and input and output cabling is well shielded.
Fig 4


Here is one channel of the line level amp. No balance control was included because the client said he had no need for one.
Gain is set for less than 2 times because the only source the owner has is CD or other silver disc formats.
There are constant current dc supplies so that 4.8mA is the working idle current for the 12AU7.
This means that there is no ac loading by a resistor from B+ to anode or from B- to cathode which
would have values of 25k. The signal loading on the gain tube V1 = 50k of the volume pot
and the 120k nfb resistor so a total of about 40k. the unbypassed R6 of 2.7 k gives about
5dB of negative current FB. The loading on the V2 cathode follower output buffer = whatever the outside world load may be.
The result of using the CCS dc supply transistors as passive high impedances reduces the distortion by a factor of
approximately 10dB. The THD is determined by the level of signal before the volume pot.
Assuming the cd max level is 1Vrms, then the V1 would have 1.8Vrms output.
The maximum output is approx 40Vrms when thd would be  about 3% of mainly 2H.
At 1.8Vrms output THD will be 0.1% approx.
The maximum average signal levels ever wanted by most people is 0.25vrms. This suits people with normal 50 watt power amps which require a maximum 1Vrms of input for the maximum 50 watts at clipping.
Therefore with the average level with gain control at the -12dB position, about 2pm, there would be 1Vrms at the
output of V1 and thd will be less than 0.05%.
The sound from this type of circuit has the natural warmth of the music, excelent detail and dynamics, a sense of smoothness and spaciousness and precise imaging. This type od preamp is the very essense of what captures the soul
of the music and warms hearts of anyone listening.  It will  seem to smooth out the harshness that may otherwise
accompany the efforts of some of the more raucous singers like Celine Dion, it will make an old italian violin
or cello have you in tears, and piano will never sound so good, massed strings will sizzle with warmth without
the searing sound of solid state. Brass will have sparkle without edge.
Several times during the last 4 years people have found that replacing their solid state preamp
with a tube pre such as the one above has done wonders 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 accept a well known brand of solid state preamp as a trade in for a preamp like this and I could only politely say no because how would I ever sell it after anyone with ears
had a listen to a tubed preamp?

The preamp above is still working perfectly and the owner has tried a few different 12AU7 from the suppliers
around the world since different brands can have a slightly different timbral quality, or voice. The measurements would all be nearly equal and why some tubes sound different has not been fully explained. But like many fine wines, they are
very drinkable, and enjoyable, and nobody worries whether they have the same chemical makeup.
I supplied the best 4 selected 12AU7 twin triodes from my collection of about 40 12AU7 at the time so my customer has a pair of spares.
I rigourously test tubes for low noise and low microphony and try where possible to carefully
match them for brand and gain. Usually with 12AU7 the gain is within a dB between samples and there is little need for a
balance control at all. The 12AU7 is an old fashioned choice of tube because these days the premier makers like to use
6922 / 6DJ8 / ECC88 or the russian 6H30. But I don't think such modern made tubes can sing as well as the
best of the NOS 12AU7 where they test well for noise and microphony.
Some recently made tubes and selected NOS do not have low noise and low microphony. Usually I find the tubes that pass the
quality tests as I perform invariably sound glorious, probably because the tube is as perfect a sample of manufacturing as possible, with tight fitting electrodes and a hard vacuum and a well made cathode. These aspects of a tube cannot be judged by the appearance or just by testing with a tube tester. The electronic testing I perform reveals the real condition of a tube beyond the cabability of a "tube tester" which only tells anyone if the tube works or not.

There are other fabulous triodes which can be used such as the 6SN7 and 6CG7 which have slightly more gain than the 12AU7 and which are almost identical electronically.

I don't make a lot of preamps, and I doubt I will attempt to make a similar small sized preamp such as the one pictured
which delighted the owner who didn't actually know exactly what he would be getting when he ordered it.
But when he did get it the appearance matched his expectations and it blended in well with the rest of his gear.
I now prefer to make a line stage preamp slightly larger.

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