Wednesday, June 20, 2012

JE Labs phono and line preamp


!!!WARNING!!! 

The voltages found in this circuit can be lethal, build at your own risk!!!

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 GTA or GTB cathode follower is sonically beneficial due to its low output impedance. Thus, it can drive longer interconnect cables without any detriment to the frequency extremes. [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!

38 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
  6. I was thinking of trying to build your phono and line stage on a spare pas chassis. I'm thinking I would have to use the version of the line stage using only the 6sn7. Is it possible to post this version of the line stage schematic for general interest? Thanks

    ReplyDelete
    Replies
    1. Sure, as long as you give credit or link back to my page!

      Your project reminded me of my Dyna PAS hacking days. Before building the octal preamp, my PAS had an RCA ohono circuit on the original PC board and used a perf board to mount a pair of 6SN7s for the line stage. I mounted a 6V filament transformer to the back panel. The original PAS transformer was only providing B+. I've also done it with 12AX7s in the phono and 12AU7s in the line stage.

      Good luck with your project and happy tinkering and listening!

      JE

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    2. Thanks so much for your reply, but I think I wasn't clear in what I was asking. I was wondering if you could post a schematic for the 6sn7 line stage so I could build it. Figuring it out on my own is beyond my primitive understanding of electronics. I've read your old site and blog for many years and listen to a JE Labs FF preamp that someone built for me and enjoy it very much. I'm just now figuring out how to build something that's not a kit. I am a photographer as well, so maybe there is some knowledge I could pass to you as well. Thanks again,

      alan

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    3. What led to the confusion is the fact that the 6SN7 line stage schematic is exactly like the 76/6SN7 above. If you want less gain, use a 22K/1W plate load resistor (instead of the 33k) on the first stage. I've used both and the choice depends on the gain structure of your system.

      Hope that helps!

      JE

      Delete
  7. Hi Joseph,

    I built many of your preamps over the years with much success. It’s a great design and I thank you for sharing it with other hobbyists.

    I am thinking of building the phono section on a single enclosure. What care should I take to ensure that I don’t end up with a humming/noisy unit? Also, can I build it without the 6SN7 buffer stage/volume control after the phono stage? I am trying to mate the phono amp directly with an integrated 2A3 SET stereo amp.

    Thanks in advance for your feedback. Again, I really admire your impeccable workmanship. It certainly had a big influence the way I build amps.

    Best regards,

    Min @ Tube Audio Lab

    ReplyDelete
    Replies

    1. Hi Min,

      Nice to hear from you again!

      I used the RCA phono stage circuit (with loctal tubes) in this SE2A3 mono integrated amp -http://jelabs.blogspot.com/2016/09/je2a3-amp.html

      I was careful with the placement of components and paid close attention to wiring to avoid ground loops and other potential source of noise and hum.

      This integrated amp is just as quiet as a preamp/amp combination.

      Hope this helps!

      JE

      Delete
  8. Hi Joseph,
    I have been using your phono stage design for several years now and still love it (I modified it to use 7F7 tubes.) I would like to use it with a passive preamp and wonder if you can suggest a simple cathode follower circuit to lower the output impedance.

    ReplyDelete
    Replies
    1. The cathode follower section of this RCA circuit sounded best to my ears when I experimented with CFs many years ago -http://diyaudioprojects.com/Schematics/12AU7-Cathode-Follower-Tube-Preamplifier.htm - I've used it with 12AU7s and 6SN7s so a 7N7 will work just as well.

      Connect the .22uf cap from the second stage of the RCA phono to the grid of the cathode follower and you are all set.

      However, I still maintain that the RCA phono sounds best w/o CF. But with a passive preamp you need this for full frequency response.

      Good luck and Happy listening!

      JE

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  9. Hi Joseph,
    ...just build the line stage 76-6sn7 on your schematic and Im verry happy with the sound.The only change I made is,that I use 22Kohm cathode resistor on second stage(6sn7) becouse I have this at home and not 24Kohm like in your schematic.Plese tell me if th sound will be worse becouse of that,and if I must use the 24Kohm resistor like in your schematic??Plese for answer and nice regards from Slovenia,Rado

    ReplyDelete
    Replies
    1. 22k should work just as well. No need to worry!

      JE

      Delete
  10. Hi Joseph,
    ....thanks verry much for the answer.Just one more question.My good friend just tell me he have one pair of old RCA 37 triode at home.I know that the 76 triode and 37 triode are not 100% totaly the same,but is there a chance that I can put this 37 triode now in line stage instead of 76 triode??
    Regards,Rado

    ReplyDelete
    Replies
    1. The 37 will "plug 'n play" with a little loss of gain. You decide which sounds better to your ears. There's no right or wrong here. Your musical satisfaction is paramount!

      JE

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  11. hello Joseph. Thanks for your help in the past. I am considering your above linestage and am curious if you have an opinion on using the 2C22 tube instead of one half of the 6SN7? As much for interest as any real sonic reason.
    Thanks in advance
    Dave

    ReplyDelete
    Replies

    1. Hi Dave,

      Sonically the 2C22 is a good alternative to the 76. Technically the 2C22 is even more linear but to my ears I put them on the same level. The 76 has the edge in terms of air and refinement while the 2C22 is more robust. That said, I have much better luck finding quiet samples of the 76. IME, a bad 2C22 is exponentially noisier and more microphonic than the worst 76. Maybe you'll be luckier than me.

      Hope this helps!

      JE

      Delete
    2. Thanks for the information.
      I was thinking of using the 2C22 as the follower stage in PLACE of the 6SN7 not instead of the 76. Does that alter your comments?
      Also, in you power supply Dx, what kind of caps would you recommend for the 50uF ones? Obbligato has 47uF PSU film/oil caps for reasonable prices
      https://www.partsconnexion.com/OBLIG-77685.html
      or would you search out old PIO ones on the auction sites?
      Similar question regarding the 10K uF caps in the filament supply. Are any good quality long-life electrolytic cap recommended over others?
      Thanks in advance
      Dave

      Delete
    3. In my experience and opinion, the extra linearity offered by the 76 or 2C22 will only be more apparent in the voltage gain position. No amplification takes place in the cathode follower circuit. All it does is lower output impedance.

      I've used Cerafines and LCR. Since those are no longer available, I use decent quality $10-$15 50uf+50uf @ 500V generic caps available from Angela Instruments or Parts Express. I also like axial Sprague Atoms. Beyond that, I'm not very particular as long as the required capacitance and voltage ratings are observed.

      However in the audio circuit, I only use carbon composition or carbon film resistors + paper in oil caps. Nowadays, I use Vitamin Qs or Russian K40/42Ys in my projects.

      I used to recommend Jensen copper foil in oil which were slightly more detailed and transparent. Unfortunately I had a 100% failure rate with them. I rather listen to music than repair my equipment.

      Anyway, I think I covered most of this in the Shop Talk section - http://jelabsarch.blogspot.com/2012/06/je-labs-shop-talk.html

      Hope this helps!


      JE

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  12. hi Joseph

    I have built your 300B Deluxe monoblocks. I use a separate pre amp all the time but I think gain is to high. my DAC volume doesnt even go 8 oclock. i only placed 100k in replacement of the pot. how coul i lower the gain or sensitivity of the amp yo accompodate a separate pre amp. i have high sensisivity speaker. 98db. appreciate your help.

    ReplyDelete
    Replies
    1. The most expedient solution, if not the most technically elegant, would be to install a 100k input level pot on the JEL SE300Bdx amps Beyond that, lowering the gain of the amp requires a total redesign.

      Unfortunately DACs are designed in the context of mainstream audio components NOT for the DIY/SET/High-Efficiency speaker crowd.

      Stay safe!

      JE

      Delete
  13. Hello once again JE. I'm still working on the 76 preamp with 2C22 followers. An additional couple of questions . I was interested in using a pair of VR glow tubes to regulate the B+ and would probably need a higher voltage transformer (Hammond 273BX comes to mind). What is your opinion of this plan or would this deviate too far from your design to allow comment? Thanks in advance

    ReplyDelete
    Replies
    1. I can't comment on VR tubes because I've never used them.

      The last time I used a tube regulated power supply was in the early 90s. It was a Heathkit Lab PS. You'll see it in this picture - https://3.bp.blogspot.com/-8cZ_ZqYNWaE/Uv7iv4pByMI/AAAAAAAADUY/22PakAHEwtQ/s1600/basement-system.jpg - it's above the turntable w/o dustcover, right of center.

      I eventually ditched it because my ears preferred passive regulation using chokes. But that's just me. I'm not discouraging you.

      Build your 76/2C22 preamp project with VR regulators and if it sounds good to you, enjoy! If not, then the search continues.

      That's what makes DIY more fun and educational.

      Delete
    2. Thanks for the encouragement!

      Delete
  14. Hi there. What is the gain of the line stage? I need 15÷20 dB for bridging a .5V phono stage with my 45 SE. Thanks!

    ReplyDelete
    Replies
    1. On the bench, I measured 1 Vrms input = 8 Vrms output, so gain = 8

      JE

      Delete
  15. Hello, I am currently building your design and I would like to know what the output impedance of the line stage is? If it is suitably low, would the line stage be appropriate to drive a load of approximately 300 ohm, such as a pair of headphones?

    ReplyDelete
    Replies
    1. I never measured the output Z but guesstimate it to be around 400 Ohms. I wouldn't use a 6SN7 cathode follower to drive headphones.

      Try this instead - https://jelabs.blogspot.com/2021/03/parallel-single-ended-6sn7-headphone-amp.html - I've used this amp to drive dynamic and planar headphones from 30 ohms > 600 ohms, ex: ATH-2, SR80, Fostex T10, T20, T50RP, Yamaha YH-1, AKG K240-600 ohm, Senn HD6XX etc.

      Delete
  16. Thank you so much for your schematics and advice. I' planning to build both phono and line in one chassis. A couple of newbie questions:

    1. In the phono pre, how many batteries is it advisable to use for stereo? 4 (one battery per triode per channel) or 2 (one battery for both channels)? Or even 1 battery for all triodes? Is an AA sufficient?

    2. Also, in the phono pre, what are the ratings for the .01µF and the 3300pf caps?

    Thanks.

    ReplyDelete
    Replies
    1. I strongly advise NOT TO build the circuit because your questions reflect an inadequate ability to understand and interpret a schematic diagram which involves dealing with potentially LETHAL HIGH VOLTAGES!

      No offense intended, I'm just trying to protect you!

      JE

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