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the 845 Single Ended Amplifier

by Andrea Ciuffoli - 2001
contact me for any problem or question: webmaster @

This is a expert only project because the high power supply voltage, about 900V, it can kill!
Keep always one hand, left hand is better, close to your back.

25 Apr. 2007 - NEWS:

1 jan 2002 - NEWS:


This project born to integrate in a single component all the experiences made on the previous projects.
Here no compromize has been accepted in any stage.

The main results and targets should match these items:

Here the photo of stand at the
Hi-End Milano show (17-18 feb. 2001)

The following photos are refered to the prevous test version with only 2 x 100uF capacitors.

Here the output characteristic of the tube 845 with the specific operation point (click to enlarge)

You can calculate the output power with:

600Vp / sqrt(11500ohm / 8ohm) = 600V / 38 (turn ratio) = 15.8Vp => 11.2Vrms => 15.6W on 8ohm in pure class A1 operation

but the driver stage implemented in this design with a 300B is able to control the 845 in class A2 operation to get a sure power of 20W


All the audio transformers used in this project are Lundahl products and the distributor list is available here.

Primary Windings:-
 0-115V @ 50/60Hz
 0-115V @ 50/60Hz
 For series/parallel operation

Secondary Windings:-
 0-1000-1050-1100V @ 0.4A rms
 One tap used at a time
 0-380V  @ 0.2A rms 
 0-380V  @ 0.2A rms 
 0-200V  @ 0.2A rms

Primary Windings:-
 0-115V @ 50/60Hz
 0-115V @ 50/60Hz
 For series/parallel operation

Secondary Windings:-
 6 x 0-6.3V @ 3A rms
 2 x 0-10V  @ 4A rms
 1 x 0-6.3V @ 4A rms
 2 x 0-5V    @ 4A rms

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so He have a very big experince and all these products use a custom audio C-core.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer. A good review of these components can be found on the article published in Hi-Fi World.

The output tubes used are the Golden Dragon 845.

The driver tubes used are the Golden Dragon 4-300B that I consider better than normal 300B The input stage is the very good Golden Dragon 6SL7. The vacuum diodes used are the 6D22S Svetlana to use with the PC509 connector.

About passive components types,  I don't leave many choices:  Caddock resistances on cathode, ELNA Cerafine capacitors on cathode.
In all my previous projects I have used in the power supply the ELNA Cerafine capacitors with a very high sonic performances and compact size.
If we skip any economic aspect and no size problem the first components is a paper in oil capacitor.
In comparation test the paper in oil are best of the ELNA Cerafine or other electrolityc type.
Searching a paper in oil capacitor is very important buy new product as the Jensen and not an old stock.

About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator and for loudspeakers the same.

A very good quality stranded tinned copper wire could be buy directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

The main schematic


Power supply

Before to insert the 845 tubes in the sockets switch on the amplifier without the fuse and set the trimmers to get about -180V from the grids of the 845 and the ground.
After switch off, insert the fuse, insert the 845 and switch on the ampl.

Digital Input stage

This project give a first class sonic performance but this can be easily compromised by a  bad input signal so I suggest you to don't use preampl. and use one of my DAC projects.
Using one of my top DAC projects all the DIYer's will reach the same sonic result.
The DAC normally included into commercial CDPlayer (even these of significant price) have a rather low performance because the common usage of high NFB operational amplifiers "destroy" the sound given compression effects and poor soundstage stability.

Layout views

click on the images to get real scale

external size 56cm x 42cm


 Bias board

click on the images to get real scale