The Modular Preamplifier, Part 4: Phono Stage PSUs

Hi!

After the introduction of the 6BY5 tube in the previous post, I will write about a power supply that uses it in this article. As mentioned in the 6BY5 tube of the month post, two of them will be very well usable in a bridge rectifier configuration for preamplifier supplies. I'm using them this way in the power supplies for the phono gain stages of the modular preamplifier.

In this modular preamplifier, the phonostage is split up into two separate gain stages (input and output stage), separate LCR RIAA module and MC transformer. Each gain stage has it's own separate power supply for maximum isolation and flexibility. The gain stages will use the E55L which was presented in a Tube of the Month article in last November. They will be run at around 150V B+ and about 35mA. Taking some voltage drop into account in the output teransformer primary and local LC decoupling which will be in the gain section, a voltage of approximately 175V is needed.

The schematic shows the high voltage part. The two 6BY5s are wired as a bridge, both heaters fed from the same winding and referenced to ground. About 300V AC are needed from the transformer to get the 175V out with the voltage drop in the chokes and rectifier. Final voltage will be determined once the PSU is tested under load. The transformer allows adjustment of the voltage.

Heaters will be supplied by DC, rectified with Schottky diodes and powered by a separate transformer. A choke input filter is used for low switching noise. About 12Vs AC are needed to get 6,3V out with the losses in the rectifiers and filter. Again the final voltage will have to be trimmed when tested under load.

The 100mA B+ transformer and low voltage transformer for DC filaments are used from my own range of custom wound power transformers. Details about these transformers can be found here.

All elements of the modular preamp will use the same chassis style as shown in the posts about passive line stage and MC transformer. The circuit is divided into sub modules which are premounted on metal plates:

All these plates are assembled in sandwich fashion:

The complete assembly slides into the wooden enclosure: There are holes on the top and bottom of the enclosure to allow airflow around the tubes.

A rotary on/off switch was chosen so that the same knobs can be used as on the passive line stage

Usually the glow of the tubes are good enough for me to work as on/off indicator. Since they are mounted inside, a LED was added to the front:

The PSUs for phono input and phono output stages are identical. Assembly of the gain stages themselves will be shown in the next post. Stay tuned!

Best regards

Thomas