New 845 amplifier - version 2
by Andrea Ciuffoli - starting in the September 2007

BEWARE! This is an expert-only project because the high power supply voltage, about 900V. Be careful, it can kill you!
Keep always one hand (better if it is the left hand) behind your back.

INTRODUCTION

This project is born to be simple and secure but with high electrical performances and great sonic results.
The sound quality has been obtain using the best components on the market and a good circuit.
All the test on first version confirm a very sonic high performances and I consider it like one of my best amplifiers.
The use of normal diode bridge instead of common vacuum tube diodes give a many advantages and the sound is always good.
This project use only common vacuum tube in current production because I don't like old stock items difficult to find and with short life time.

These are the characteristics of this design:

• no feedback
• short signal path
• single ended design
• full class A1 operation
• output power about 20w
• very good frequency band
• cheap chassie easy to build
• few points at 900v to reduce risks
• no SRPP (totem pole) and uFollower
• no voltage duplicator on power supply
• short assembly time (about 5 evening)
• no cathode follower with local feedback
• only one cathode bypass (on the first stage)
• simple bias point set using multi turn trimmers
• soft turn-on to increase vacuum tube filaments life
• only one resistor in the signal path (on the first stage)
• only 3 stage design (the min. for an integrated 845 amp)
• no noise using indirect heating driver and all dc filaments
• simple power supply design, but with hi-end characteristics
• very low magnetic flux using a R-Core power supply transformer
• max energy transfer in the driver stage using an interstage transformer
• max voltage swing on driver stage obtain with the interstage transformer
• only one capacitor on the signal path (Jensen copper film paper in oil)
• high sensibility, so to use this amp. with any source also without a preamp.
• low output impedance (near 1.5 ohm) better than 90% of common vacuum tube amps
• some transaction to class A2 are allowed using a good driver stage and solid bias circuit
• ultra low distortion obtained with many measurements and tests so to get the best bias points

Very cheap and good chassie created with these componentsç

1. 2 marine plywood 19 mm 38 x 38 cm
2. aluminium 3mm 37.5 x 37.5 cm
3. aluminium 3 mm 37.5 x 23 cm
4. stainless steel studding M12

• photo 1 - chassie starting phase

• photo 2 - chassie starting phase

• photo 3 - chassie starting phase

• photo 4 - start of chassie

• photo 5 - power supply section

• photo 6 - power supply section

• photo 7 - power supply section

• photo 8 - power supply section

• photo 9 - power supply section

• photo 10 - pcb filament power supply

• photo 11 - aluminium work

• photo 12 - vacum tube layout

• photo 13 - output transformers

• photo 14 - output transformers & output transformers

• photo 15 - another vacuum tube layout

• photo 16 - bias circuit pcb

• photo 17 - input and driver layout

• photo 20 - front view

• photo 22 - back view

• photo soft turn-on circuit

• photo bias power supply

• photo bias circuit

• photo DACT and Q-audio cable

COMPONENTS

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

to add input cables and DACT stepper attenuator cost

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so it has great experience and all these products use a custom audio C-core.

About passive components types,  I don't leave many choices:  Caddock resistances on the signal path, Sanyo OS-CON capacitors on cathode and Jensen AudioGrade electrolytc capacitors on power supply.

You can find many components of this project at Audiokit online shop.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer.

The Custom R-core has been custom design with 2 secondaries 6.3V 5A necessary only if you want to use the SV572-3 instead of the 845.

About wire forget the teflon ! use only stranded tinned copper wire with pvc isulator.

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.

I have used the model 9506 with WVDC (Working Voltage Direct Current ) of 5000V for all the main power supply connections, the model 9504 for the signal and the model 9510 for the output loudspeaker connections.

All the wires near the tube socket have been cover by Sterling tubes in order to avoid that the high temperature will destroy them.

As visible in the photos to fix some components I have used some good PTFE Insulated Twist Eye Terminals by Grennan Audio.

After my big experience with the cables published on "Cable test" I can assert that the quality of cables is very important.

So like internal input cable I have used a good Q-Audio Tao

For the R-core power supply transformer contact:

SCHEMATIC

MEASUREMENTS

Here are some plots generated with my Clio system by Audiomatica.

With a driver stage of this type (107Vrms with only 0.18% THD) I expected a great performace.

The distortion spectrum is perfect driving with this amp. a 5 ohm load until 11-17W and the output impedance is less than 1.5 ohm.

So the result is a perfect amp. and I am sure there are very few 845 amp. around with this performances.

DRIVER

OUTPUT R CHANNEL

OUTPUT L CHANNEL

MORE OUTPUT