JE Labs phono and line preamp



Phono stage

For severaI years I have been hacking the Dyna PAS chassis with various cascoded and cascaded phono circuits all employing negative feedback RIAA EQ in search for a circuit that will satisfy my musical requirements. My research ended when I discovered this unglamorous 2-stage cascaded passive RIAA EQ phono preamp circuit at the back page of an RCA tube manual. 



The most important lesson I learned in this excercise is negative feedback chokes the sound! I did not alter any component values except for using Ni-mH batteries for cathode bias which I thought improved detail and definition. I strongly advice against the use of a cathode follower and make sure the circuit drives a minimum load of 220K as originally specified otherwise you will lose bandwidth at both frequency extremes. I use an Alps 250K but have also tried the cheap Alpha 250K dual log taper carbon pot sold by AES. It does not have the tracking precision of Alps but at normal volume settings the channels are balanced enough.



I have been using this phono circuit since the early 90s starting with a PAS chassis with Telefunken ECC83/12AX7. Through the years I developed fondness for octals and nowadays I almost exclusively use 6SL7s/5691s which further improved the bandwidth and depth due to its slightly greater transconductance, vis-a-vis, the 12AX7. 


JE Labs phono stage adapted from the RCA circuit
There are people who swear by using high transconductance tubes like WE417A or 6DJ8. To hear what this was all about, I bread boarded a famous circuit using these tubes but did not find the sound as appealing, must have been just a matter of taste...I find the superior linearity of true audio tubes like a 12AX7 or even better yet 6SL7 octals play a lot more LPs.

MC transformer



For MC cartridge use a transformer step up device like this Tamura TKS-83 or mic transformer  to boost the signal to MM level. I am not a fan of active MC stages and rather not go there...

76>6SN7 cathode follower line stage

Another very simple topology with a single gain stage direct coupled to a cathode follower. Originally I was just using a 6SN7 dual triode for both stages but was inspired by the Berman article in SP 13 to try a type 76 as the gain stage. I use a lower B+ supply and run the tube at lower plate dissipation and also prefer the sound with the 2.2K cathode bias bypassed with a 100uf/16V cap [in this case I can justify using Black Gate]. 


At this signal level the direct coupled 6SN7 cathode follower is sonically beneficial and the low output impedance results in less interaction with long and/or capacitive interconnect cables. [Cable geeks take note: many times the "difference in cable sound" you hear is more a function of load/capacitance especially if your preamp does not have low output Z].


Transformer coupled line stage using Tango NP216N


I briefly flirted with this design as a linestage and in spite of the fact that all my SETUP friends liked the sound [whose ears I respect], I eventually went back to the above linestage. The Tango NP216N is a wonderful piece of iron although I have to admit that I needed greater bandwidth than it has to offer. However this is a circuit that offers a solid 600 ohm [as used in studios] output impedance and can even be wired in balanced mode.
At some point I may try this topology again at the output of a tube microphone preamp project. Please do not take my seeming lack of enthusiasm as a dismissal of this topology. I included this schematic because it may well be appealing to other ears and/or applications.


Joel's preamp
12SL7 phono + 12SN7/Tango NP216N line stage + external PS

Nonong's preamp
5691 phono + 76>Tango NP216N line + external PS

Power Supply


During my Dynaco hacking days, I experimented with regulated HV using solid state devices which was 'in tune' to the high end sensibilities of the time. I also chanced upon a Heath tube regulated [6L6GC] Lab PS and hooked this to my preamp, there was a slight improvement towards my goal but my ears longed for a more natural sound.

When I acquired my first copy of the Radiotron 3rd edition, I regressed to 1930s technology. This is the PS circuit I've been using for the past couple of years for deluxe versions of my preamp. A full choke input followed by another choke/capacitor stage in order to get a very well filtered B+ supply.




I use a bridge rectified and highly filtered DC filament supply for the heaters without having to use silicon regulators. If you pay close attention to the schematic you will notice that there is a voltage divider network [120K series connected to a 47K and 68uf/160V combo shunted to ground] at the output of the B+. This network is mounted on the preamp chassis and keeps the filament floated above ground to further minimize hum and noise and also to keep the heater to cathode voltage on the 6SN7 cathode follower below maximum limit.

Note: the numbers in the schematic pertain to the terminals of a 4-prong connector/umbilical cord to connect the preamp to the power supply.

If your power transformer has a 5V, 3A rectifier winding, you can experiment with rectifiers. My favorites are the 5V4G and 5Y3, but have also used GZ34/5AR4 and 5U4. If you know how to use a DVM, you will note that different rectifiers will produce higher or lower B+ voltage which alter the operating point of the circuit. In general, the different voltage is what changes the sound not the rectifier itself.


Prototype

Built on a PAS chassis
12AX7 phono + 6SN7 line + external PS

Octal prototype
6SL7 phono + 6SN7 line + external PS

5691 phono + 76/6SN7GTB line
Gen Rad PS

under the hood

Variations on a Theme


5691 phono + 6SN7GTB line
External PS


6SL7 phono + 6SN7 line
External PS

Simple version 


6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS


6SL7 phono + 6SN7 (single gain) line stage
no cathode follower +
simple CLC PS


76>6SN7 line stage + CLC PS, ca. 2006
6SL7 phono + 6SN7 (single gain) line stage +
PS in one chassis

My last project in the USA - a single chassis preamp in 2007 for my brother's system. With careful attention to wiring to avoid ground loops, it is just as quiet as the 2-chassis version.

76>6SN7 line stage preamp
with remote
volume control
JE Labs goes high tech - built by Joel Villanueva for this system and featured in the November 2011 Hi-Fi Show. It is a minimal line stage preamp with remote volume control on one chassis from Harana Audio.

Have fun and happy listening!

Comments

  1. could one be able to replace the phono cathode part with resitor and cap bypass instead of using batteries that would have to be replaced constantly, if so what would the best values be?

    ReplyDelete
    Replies
    1. 2.2k 1/2-1W RESISTOR bypassed with A 100uf @ 16V cap can replace the Ni-Cad/Ni-mH.

      Delete
  2. Hello JE,

    How are you? I am gathering parts to build the 76 linestage with the 6sn7 cathode follower. I have a couple question regarding the power supply. At the power supply junction which has 290V B+, how did it went down to 280v going to the preamp circuit? do i need a dropping resistor to adjust the voltage to 280v from 290v? and also what bridge rectifier should i use for the the dc heaters. IF I decide to use AC will it still hum even if I lift the heaters?

    Thank you

    ReplyDelete
    Replies
    1. The difference between 290V and 280V is negligible (less than 10%). For the filaments use at least a 5A bridge rectifier. I never tried AC on filaments and speculate it will be noisier.

      Hope this helps!

      Delete
  3. JE - I have build your 45 amp and the 76 pre-amp and they work beautifully together. Both pre-stages are very quiet.
    I have tried to hook the pre up to several ss amps only to be knocked over by feedback from the speakers. Is this common.
    Best,
    Comer

    ReplyDelete
    Replies
    1. The 76 tube can manifest its noise with amplifiers more sensitive than the Simple 45/2A3 circuit (~ .8V-1V to full power). Ex: SS amp (or another tube amp like my 300Bdx) with less than .5V sensitivity. I have a friend using a JEL preamp to biamp - JEL SE2A3 for the midrange and high frequency horns and the woofers driven by a generic T-Amp - with no problems.

      Delete
  4. I have build your 76 pre and now I want to bulit the phono stage following your idea with 6sl7/6sn7 but I have not the the new modified schematic, could please to provide me this one ?

    AK

    ReplyDelete
    Replies
    1. Ak,

      There are no modifications to the phono stage and all the schematics are above.

      Happy listening!

      JE

      Delete
  5. Some years after of the initial build and thousands of listening hours I decided to upgrade the pre with new materials and components.However I am wondering why did you use two 6SN7 instead of the two halves.

    ReplyDelete
    Replies
    1. The layout looks more symmetrical with 2 x 6SN7s not to mention that 20 years ago they were cheaper. I wired the sockets so that periodically I can interchange tubes between channels so both triode halves are used.

      Delete

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