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Thursday, June 27, 2013

The Octal Phono Preamplifier, Part 3 : Power Supply


The schematic of the signal section of the Octal phono stage was already shown in part 1. What is still missing is the power supply. In this post I will show two possible variants. The first one is quite simple without any chokes and only a single power transformer.

This PSU uses a hybrid full wave rectifier with a 6BY5 and two UF4007 silicon diodes. The smoothing of the B+ voltage is done through a couple RC filter segments using electrolytic caps. I used my smallest power transformer which only has a single 6.3V heater winding. So this had to be used for the rectifier and signal tubes. The rectifier is hooked up directly to the heater winding, while the heater voltages for the signal tubes are rectified by a bridge of Schottky diodes. A small dropping resistor brings the voltage to the desired 6.3V. Smoothing is done by a bank of 4 10.000uF electrolytic caps. Not the most elegant solution. Better would be to have separate heater windings. But it works like this. I wanted to use the smallest power transformer which I have so that the whole phone stage fits into a single wooden chassis as used with my 6AH4 line stage.

All the resistors in this circuit should be at least 5W or higher rated. The 100K/10K voltage divider at the end of the B+ filter chain serves as bleeder resistor and provides a positive bias voltage to elevate the heater potential.

The supply was used like this in the first prototype build and works nicely without hum, even with the power transformer in the same chassis.

The first user of this phonostage however asked if he can have a chassis style which has the tubes visibly exposed on the top. Something in the style of the recently built D3a phono stage. With only the tubes on the top of the chassis, this would be a bit too empty. So it would need something else to fill the surface. This would be possible by bringing the power transformer on the top as well and by adding some chokes. So I designed another power supply which is a bit more elaborate:

Separate transformers for the B+ and heater voltages of the signal tubes. Both supplies with choke input filter. 

The assembly of this version will be covered in part 4. Stay tuned.

Best regards



  1. Hi Thomas
    Realy great audio engineering :-)
    i humbly learn from every pages.
    I am happy finder your blog is make me
    sleepless at night
    From Western Electric & Sakuma San
    inspiration next step
    to 21 century innovations and with new Elrog DHT's ?
    German excellent audiophile tradition in action
    Yes this up the party time
    Thank You mr Thomas Mayer
    Best regards from France

  2. Hi Thomas,

    I'm very interested when see your blog about audio. Please inform me about :
    1. Which one of your best product Preamplifier using 10Y Tube? and what is the speacialities from any preamplifier?

    2. Which one of your best product Preampower using 211 tube?

    Best Regards & Thank you,
    Teddy (indonesia)

  3. Hi Teddy,

    my best line preamplifier is using the 10Y or 801A

    I have 211 amps with different drivers. The best uses 211 as driver

    You can contact me by email for further discussion: thomas - at - vinylsavor - dot - de

    Best regards


  4. Hallo Thomas,
    würde dein Netzteil aus dem Octal Pre MKI hier Sinn machen?
    Das mit der LCLC Siebung und dann pro Kanal nochmal LC.
    Grüße Hannes Schrotter

    1. Diese phono war als low cost Variante gedacht, warum Du dann ausgerechnet ins Netzteil mehr Aufwand stecken willst erschliesst sich mir nicht. Aber das schöne an DIY ist ja, dass Du das selbst ausprobieren kannst nach eigenem Gusto

  5. Ok, da wäre es wohl sinnvoller gleich den Octal pre MK1 oder MK2 zu bauen.
    Grüsse Hannes Schrotter