The above preamp picture was taken in 2000.
It  was initially built in 1994 as my first hi-fi DIY project after a 30 year period where I was not involved in handcrafting any electronic equipment except several pairs of speakers and was the first major audio project I undertook.

The preamp chassis is brass and aluminum composite with a mild steel box to enclose the power supply,
and both the HT and heater transformers are potted to reduce stray magnetic and electrostatic fields.

After a couple of years it became necessary to re-build this preamp with convenient features to facilitate
A-B testing of other equipment also developed during the last few years, so two pairs of 12AU7 output buffers  and two dual gain controls were added to give two pairs of L and R outputs so two pairs of power amps could be connected for the A-B evaluations. This allows me to keep the levels for two different power amps
exactly the same while using a switch from the speakers to either power amp outputs.

The original power supply was dual mono but did not need to be, and just the one supply with a separate filament transformer is now used. Passive CLCRCRC filtering was used for the B+, and a regulated DC supply was added for all heaters.
There is more than enough individual RCRC filtering of the B+ shown in the power supply schematic
to ensure each channel is working just as separately as they would if they were each completely
mono bloc constructionn.

The schematic of the above 10 Tube preamp was firmed up by April 2000........

A few people have emailed me over the last 4 years to tell me they built this preamp, or to get additional information about the power supply. I did not provide exact power supply details because I felt anyone building such a preamp should at least be able to design and build a power supply of their own, and I did have some generic power supply designs in my power supply pages.
Power supplies do NOT affect the sound quality if built to a reasonable level of quality.
There is now a suitable power supply schematic further down this page, see the details for the 2004 10 tube preamp power supply.

In about 1996 I was very favorably influenced by some of the ideas promoted by Allen Wright who wrote a provocative book, The Preamp Cookbook in 1988 to make a little money and to spread the gospel about preamp construction with tubes.

Allen has a lot of information about preamps et all at his website at http://www.vacuumstate.com

I began to think seriously about how to make a suitable low noise preamp for a moving coil cartridge that I thought I should try because many people had told me that generally MC were better than MM.

Allen began in 1988 to use a high transconductance j-fet such as the 2SK147 in cascode with a triode tube to give a gain block of up to about 800x at quite low thd and with about 20dB less noise than when using a well chosen and engineered 12AX7. Allen's Four Valve Preamp of 1988 was a landmark design where he combined the best aspects of a j-fet with tubes. The j-fets such as 2SK369, 2SK147 have low noise, high gm, and are very suitable for cascode use with tubes. The j-fet only has to amplify the tiny input signal about 20 times, and the distortion is low even without any NFB since the signal is so low.
Triodes then can take over amplifying the signal to a higher level, and since their dynamic range is 20 times that of a j-fet the distortion is still low at signal levels below 10Vrms, so loop NFB need not be used for RIAA eq or for any other reason.

Although my original 10 tube preamp was an excellent performer with a Shure V15 MM cartridge
with high output, I eventually bought myself a Denon 103D moving coil and replaced the Shure.
The Denon has only about 1/16 of the Shure output, and the 12AX7 is just too noisy no matter how carefully one selects the brand of tube. I found  that even a paralleled 6DJ8 was too noisy even though calculations
and theory suggested this should not be so. So I decided to use Allen's basic idea of  cascode phono input stage.

Since 2004 my prototype preamp has the following general topology
and to allow for low output moving coil connection:-

Each channel,
Phono input, 2SK369 j-fet driving into cathode of SE 6EJ7 frame grid pentode wired as a triode
used in a 'cascode' circuit.
RIAA passive filter,
12AT7  µ follower in second phono gain stage,
12AU7 µ follower line level gain stage which is fully bypassable,
12AU7 µ follower tone control stage with Baxandal feedback circuit,
giving gain = approx 0.94 with +/- 9 dB of max cut and boost at 100Hz and 10 kHz.
The tone stage is fully bypassable, ( and is seldom used except when testing speakers and sources to gain a very approximate idea of response problems ).
Balance is controlled by carbon track pots immediately prior to the output buffer stages.
Gain is controlled by carbon track pots placed immediately before the output cathode follower buffers
which are 2AU7 with transistor constant current sinks.

Most of the time the line stage and tone controls are not used, with the input signal
applied directly to the gain controls, and the following buffers, so that long cables to
power amps across the room may be used, which saves having a remote volume control.

For each channel, this 10 tube preamp has one MC input, which can be used for MM input if the low gain circuit setting is chosen by removing a bypass capacitor in the j-fet source circuit. 
There are 4 line level inputs, and one record output. 
There are two volume controls per channel with separately buffered outputs.

The 10 tube preamp of 2004 above has undegone some changes from the picture right at the top of the page
taken in 2001.
The switches and some RCA sockets originally mounted in the top plane of the chassis in 2001
have all been moved to either the rear panel or front panel, and their quality is improved since I used some
high quality NOS rotary wafer switches instead of the rather awful taiwan made el-cheapos I began with.
The holes left behind have been covered over with well glued kitchen bench top plastic laminate
with a blue pattern, although it looks grey in the picture; photos never do tell the full story.

The schematics of the 10 Tube Preamp 2004 are in five separate sheets which follow :-

As you might be able to tell if you have been to Allen Wrights' website, or if you have read his Preamp Cookbook, that some of Allen's ideas have not been included.
I don't always agree with all the experts in this field of human endeavour!  For example, I DO NOT think there is any sonic degradation to the sound caused by electrolytic or plastic coupling capacitors as long as their quality is sufficiently good, and the value of capacitance is well chosen. So I have even used a small sized 6.8 uF non polarized electrolytic plus polypropylene 2uF to couple the low impedance MC cart to the input fet gate, Q1. In this location the j-fet gate is biased at +1.4V through R1, 47k, to allow variable phono stage gain by adjusting a link in the j-fet source circuit to allow C5/C6 to be connected by the link to the junction of R2 & R3 when high gain is desired. 
 
If the C coupling from the low impedance MC cart is not a high enough value, then low frequency noise generated in the 47k biasing R1 will not be shunted by the low MC impedance, but appear at the j-fet input gate and will be amplified by the large amount of gain this amplifier has at low F since it is a phono amp with RIAA de-emphasis curve filtering, which basically allows all the LF to pass through but attenuates F above 50Hz.

Allen's original 4VP didn't have variable phono gain, and the whole preamp with its cascode line stage was considered by some people to have too much gain overall, so that if you used a high output cartridge, sensitive power amp, and sensitive speakers, the system was almost unusable because it would roar at you when the volume control was set to a very low level
I like to see variable phono gain and to use a fully bypassable line level gain stage.

I also very much like the use of large electrolytic filter caps for rails of  phono stages to also stabilize the rail to prevent LF ripple being amplified by the high gain at LF. Hence the C4 = 470uF. There is  also C16, a 2uF polyester cap with very short leads between the B+ at between V1 and V2A anode supplies and 0V, to make sure the bypassing is effective to high RF.
I found that the circuit as shown tended to oscillate at about 100MHz if the circuit board area for the j-fet plus V1 was larger than 50mm x 50mm in total area, and if leads to capacitors and track lengths were too long. Phono stages like this one need to be designed as if they were RF circuits; small is beautiful in this case.
Bypass capacitors C1, C5,6,7,8,9 are very important.  I also supplied the dc to the heaters to V1 via RF chokes and bypassed the heater wiring well with C to prevent parasitic oscillations.

Tubes such as high transconductance frame grid pentodes like the 6EJ7 make fabulous triodes for this purpose, but whatever you do, don't try to use them as pentodes driven by a j-fet as shown. The extremely high resultant gain WILL be impossibly unstable at some high RF and the tube will be weirdly microphonic, and you'd think you had the bells of St Mary's Cathedral connected to your amp.

V2A and V2B form a µ follower, or bootstrapped follower as it used to be called when invented in about 1943. This type of gain stage is my favorite because of the clear sound, the nice measurements and simplicity. The efficiency is good because there is no wasted current in separate anode resistances to supply Ia dc or in cathode resistors to sink Ik dc in a separate cathode follower.
The set up as shown with a 12AT7 gives a very healthy gain of 36x even with the V2B cathode R21 unbypassed. The load seen by the V2B anode is approximately the open loop gain of the top tube x R20, plus R19 in parallel. Effectively, the RL of V2B = 166kohms, which is over 10xRa for the 12AT7, so THD is extremely low, and the much maligned 12AT7 can sing as sweetly as any tube can.

I first tried this amp set for low gain to suit the MM ShureV15. Then I changed to MC Denon 103D
with the gain set high for MC. After only 3 bars of Mozart I realized how much better MC could be.
I have retired the ShureV15.

In 2005, I developed a superior cascode MC amp circuit which is described in the page on Preamps 2
listed at the index page.
 

This is the line gain stage and tone control stages. I like the 12AU7 as a preamp tube,
and it really sings in µ follower mode. There is some local shunt NFB to control and reduce
the gain to a sensible level since even a low µ triode like 12AU7 has too much for most line level gain stages. The balance control is incorporated in the shunt NFB path.
Both balance and line gain is fully deletable.
The tone control stage has a Baxandal NFB network with mild amounts of boost/cut.
I lent this amp to a customer for a fortnight and he used it for a week with tone controls included, and he needed to be told that it could be deleted; he could not tell when the tone control stage was included in the signal path or deleted out of the circuit.
 

The above pairs of output cathode follower buffers have transistor CCS  to reduce any ac loading effect
and maximize open loop gain of the 12AU7, and hence minimize the thd, and allow the load powered by
the cathode follower to be lower than would otherwise be used.
 

Note that there is NO B+ regulation. The anode current consumed by each channel = 20mA
approximately, so about 40mA max for both channels, and at about +300V potential, which is only 12 watts. This low amount of B+ power is easily filtered by passive C, L and R components. Using  active tube regulation or active solid state regulation tends to add a lot to complexity and I have found such things create unwanted heat and tend to be unreliable. I sure DO NOT believe in silly notions that tube rectifiers sound better!
This amount of B+ power is similar to that required by an old AM radio, and in fact the type of power
transformer found in many an old AM radio will provide the B+ power needed by this preamplifier.
However, the 25VA radio transformer will probably not have the necessary heater windings to
enable 12.6Vdc x 1.8Adc to be generated, so anyone building this amp should use a single transformer specially wound, or have a radio transformer for the B+ and an auxiliary transformer for the heater voltage with a 19V x 2A winding, ie, with a 40VA rating. Most of the power drawn by the amplifier is filament heater power. Hammond Engineering of Canada also supply a range of suitable power transformers.
My 10 tube preamp has the power supply within a steel box where there are two power transformers
instead of the single unit shown on the schematic; the B+ is from a potted Navy spec transformer and the low voltage transformer is from Jaycar, an Australian parts supplier, and it is a general purpose transformer that has several taps and allows up to 2A at 30Vrms output. I potted this LV transformer in a steel can filled with dry sand to keep it quiet and reduce stray magnetic fields.

The combined effect of the potting for both transformers and mild steel sheet box for the power supply does reduce the stray magnetic fields just enough to allow the very magnetically sensitive phono input circuitry to be placed at only 450mm away from the power supply in the same chassis.
It is always better to use a remote power supply and umbilical cable, but in my case i got away with the PS being close to the phono stage because of the two layers of magnetic shielding. I used brass and aluminum for the main parts of the chassis which looks nice, but plain thick mild steel is actually better, especially if other devices such as CD players in plastic cases are located near the phono amp, say on the shelf below the preamp.
I have seen even expensive CD players radiate stray magnetic fields that migrate into phono
amps like this and cause hum.

There is about 0.2Vrms of 100Hz hum at C4, and this is reduced by a factor of 0.001 by L1 and C5,
and then again by the R2/C7 R3/C6 filters before each channel by a factor of 0.015,
and then the additional RC filters at the top right hand side further reduce hum to quite negligible levels.
The time constants of all the R & C filtering give good B+ rail stability that is immune to mains level fluctuations.
In fact, the amp can be turned off, then back on again after 2 seconds repeatedly and there are no audible
changes to the sound, or slow wobbles in speaker cones since so much stored energy is contained in the electrolytic capacitors.
Regulation of the B+ was not needed because even in phono the LF noise was utterly mimimal, and since below 3Hz the total amp response reduces steeply due to so many RC couplings between stages.

However, Q5, Q5 form a regulator circuit for the totally dc heater supply which is biased at about +56Vdc
so that the dc voltage difference between heaters and cathodes does not exceed the ratings of 90Vdc
and arcing or current leakage from cathode to heater circuits is prevented.
To achieve good ripple rejection in the 1.8 amp dc heater supply, the easiest way is to use a regulator
rather than have an ungainly large choke and huge capacitors.
But please feel free though to use as much L and C and or R as you can afford.....

The overall performance...
The bandwidth is 3Hz to 100kHz and the preamp will drive any known solid state or tube power amp inputs.
The output impedance of the cathode followers will allow the use of 10 metre long interconnect cables
and direct connection of the 'record out'  select switch pole to a recording device such as a sound card,
so the phono preamp stage can be directly connected to the sound card if required.

For those interested in having a preamp built, tube choices will be from my very limited NOS stocks of tubes after full testing for noise and microphony. Excellent sounding NOS Siemans tubes will be happily supplied providing you can afford them, and find a supplier. NOS Siemans produced what i think give the
best subjective sound quality in terms of dynamics, detail, bass/treble balance, vocals, musicality, bloom and warmth.
The subjective quality of tube amplifiers differ with tube choice. All the preamps built have at least 5Hz to 100kHz bandwidth, and typically measure 0.02% thd at 1Vrms output. Usually we only need about 0.1Vrms to drive power amps, and thd/imd from the SET pure class A circuitry is proportional to output voltage, so at low output voltage levels the thd/imd is so low it is quite inaudible.
The measurements do not indicate a correlation to the marvelous sound quality heard,
so small triodes have a happily mysterious character within, because tubes that measure similarly
and at thd/imd levels <0.02% can have quite different sonic signatures. For example, in a recent
test with 3 other friends present, we all agreed that in a line stage being tested for the afternoon,
The order of preference for the 6CG7 tubes tried in a line stage placed Siemans NOS first,
Australian Miniwatt NOS second by a nose, NOS Mullard third by a chest, and recently made russian
EH6CG7 a length behind, and struggling.
But for difficult female vocalists, ( my respects to Celine Dion ), the Mullards were the most
beneficial to the sound, although nothing will save your ears from Kylie Minogue, except the "off" switch.
We don't know why the EH6CG7 sounded so rough; perhaps the batch was made on friday
when the staff are impatient to be off to their weekend lodge in the forest.....

As with all my amps, I only use point to point wiring where possible and hand wired boards with
hand laid copper wire tracks. I use mainly Wima polypropylene capacitors for the signal path couplings and 1% x 1 watt Welwyn metal film resistors, and other well regarded parts.

I do NOT believe that Auricaps or many other brands of coupling capacitors sound any better
after having recently trialed a 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. 

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 I 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.

Should you have any further special requirements, please feel free to ask me.

For more information on preamps I have built and sold recently for discerning clients,
please go to the page, Preamps 2.  

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