Regulated Power Supply

“Loekie”

Draft:: Ideas for a power supply

Last modified: 20-Apr-2003

For those of you that want to get Loekie up and running fast with lots of fun I recommend building Granny. Although Granny has been developed for Tweety, by using the same connectors in Loekie as for Tweety it is perfectly possible to mix'n'match power supplies between Tweety and Loekie. Below you therefore find the shcematics of Granny, it works, proven track record.

I therefore recommend to everybody wanting quick results (no patience) with Loekie to build the power supply according to the above schematics. The vertical dotted line indicates the boundary between the two housings of the power supply and the amp. Of-course you can build everything in one box but I do recommend using two separate boxes as in my experience the noise floor is lower when the power supply of a phono amp is seprate from the amp itself. Inside the amp I used for the 350V/22uF caps Black Gate types, the two 22uF caps for the heaters are simple Philips/BC types of 25 Volts or so.

Alternative Supplies?

My intentions are to learn by doing, and therefore I need to improve on myself in every project. Therefore I will design another power supply for Loekie in the same box as I use for Granny. And like I indicated before I will keep the rating such that Tweety and Loekie can mix and match with the two power supplies. It will also allow me to make a 1-to-1 conparison between regulated power supply and a "simple"one and hopefully they extra investments of this page do pay-off. In any case, at this moment I cananot use both amps at the same time and that's a pity.

Schematics

As shown on the pictures of Tweety and Loekie the small aluminum boxes used for the housing do not allow much room for experiments. For some time now I'm thinking about using a tube rectifier too but that's probably not realistic especially when you do not want to make use of electrolytic capacitors (everything else is much bigger). Even with Granny I found it difficult to find the best layout for using a Choke next to the power transformer. A tube rectifier with more than 1 choke i certainly not gonnan work in these boxes.

In het buizenboek van zur Linde kwam ik een interessante schakeling tegen (pagina 202): een stabilisatieschakeling op basis van 1 enkele ECC81 buis. Grappig genoeg wordt voor de gelijkrichting wel gebruik gemaakt van diodes (1N4007) maar volgt er een redelijk simpele stabilisatieregeling op basis van een buisje. Ik neem mezelf dus even voor om de ruimte in de kast nog even extra efficient te gaan benutten, want die buis met die paar extra componenten moet toch passen (naah).

Dit is het schema waar ik de laatste dagen eens goed naar gekeken heb. Niets is vast totdat ik het gebouwd heb, maar het is in ieder geval een begin...

Ik heb de schakeling van zur Linde uitgebreid met een extra RC en LC filter. Dit omdat de door zur Linde aangehaalde rimpelspanningen volgens de simulator bij lange na niet haalbaar waren zonder een extra stukje filtering. De schakeling is niet in staat om zeer veel stroom te leveren, alleen al omdat de stabilisatiebuis in serie met de belasting is geschakeld. Maar tweety verbruikt ongeveer 1.3 mA per kanaal dus dat moet prima te leveren zijn door deze schakeling. Als ik heel eerlijk ben denk ik dat stabilisatie eigenlijk onzin is, want met 300uF aan capaciteit in de voeding kunnen schommelingen in de spanningvoorziening ook goed worden opgevangen, Tweety en Loekie verbruiken zo goed als niets.

Het aardige is dat de schakeling regelbaar is over een vrij groot bereik: van 100-275 Volt. Helaas was volgens de computer simulatie het gedrag in de buurt van de uiterste regelwaarde (275 Volt) niet geheel ideaal. Daarom ga ik uit van een hogere voedingsspanning (mede om de spanningsval over de eerste RC filter sectie te compenseren). Als ik uitga van een secundaire spanning van 300V~, dan hou ik voldoende spanning over om evt. de stabilisatieregeling naar boven uit te breiden tot 300 Volt. Da's mooi, want dan kan ik dezelfde transformator gebruiken als voor de Tweety (en dezelfde smoorspoel).

Wel wil ik nog even goed kijken of er in geval van overbelasting niets onprettigs gaat gebeuren ...

Alternative?

There was an alternative design that I decided to build. It is found in an article of Peter van Willenswaard, the book of zur Linde and also Triode Dick decribes a ripplykiller based on the same basic principle where a tube is used as a shunt (parallel load to the amplifier).

I decided to build this variant as the previous one would involve more components and it would mean that the tube is in the B+ supply line. Some design decisions:

• I will use Audyn Cap 33uF capacitors in the last stage of the power supply
• The umbilical cord is shielded and contains only 6 small wires. I will use two wires for B+ and ground and 4 wires (2 by 2) for the heater supply.
• I will use a 20H choke (made by AE-Europe)
• Resistors are all Beyschlag 1W/1% types.

Above you find a picture of the power supply in a finished state. In the upper left corner one can see the tube.

And ...

Immediately after finishing this power supply I noticed some things I did not like about this setup:

• The heater supply has to feed 4 instead of 3 tubes. And since I do not cool the 78S12 voltage regulator for 3 tubes (no problem), I had some problems with the heater supply overheating and therefore switching in protection mode (=off). As a result, the power regulation of the 4th tube did not work and the voltage of the B+ was rising rapidly to over 330V. This therefore is a problem: If at some point in time the regulation circuitry failes I risk blowing up my amplifier...
• I checked the sound quality of both power supplies by listening and comparing Tweety and Loekie with both Granny and the new power supply (all combinations) and my conclusion was that the new power supply in fact did not improve the noise levels more than Granny already did. In fact, I liked it less. OK I have been tweaking the circuit for a while searching for the best setup but the conclusion was the same: An excellent power supply but not better than Granny.

Back to the old way ...

I therefore decided to go back to the old proven unregulated power supply. I removed the tube and all it's components and replaced it by another resistor/capacitor filter. The capacitor was another big 33uF Audyn capacitor (which is a better slution than just another Electrolytic capacitor).

Here is a picture of the finished powersupply ...

© Maarten&Annemarie, August 2002, 2003, 2006