the new 300BXLS Single Ended Amplifier

by Andrea Ciuffoli - 2004

Introduction

The main results and targets should match these items:

simple project with only 4 iron
middle power (min 9W on 8ohm) to drive medium sensibility loudspeakers (about 89db)
optimized in 2 months to get a perfect distortion decay with true measurement with Clio system
top sonic performances
3 stages only from DAC to the loudspeakers without pre-amp.
very low distortion
good damping factor with an output resistance near 1.5ohm
very good low frequency response about 10Hz (-3db)
no anodic high voltage so suggested also for beginner
soft start power supply
compact size

Components

The KR300BXLS is an ultra-linear, low frequency, high power triode tube capable of producing 15-25 watts of pure class-A power. The KR300BXLS may be used in 300B amplifiers, however you will still obtain only 8 watts. Therefore we recommend the KR300B for these applications. However, if your amplifier has adjustable bias, the KR300BXLS will provide more power. Increasing the plate voltage and current will result in higher power than is obtainable with other 300Bs. Consult your equipment manufacturer before increasing the power of your amplifier.

The KR300BXLS features a new ribbon filament construction and the KR Electronics exclusive patent design with 32 cathodes.

As with all KR Electronics vacuum tubes, the KR300BXLS possesses a vacuum of 10-8 Torr, a maximum grid current of 2.0 microampere and absolutely constant emission over the life of the tube.

All KR Electronics vacuum tubes are warranted for a period of 2 years.

Information from:

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

2 x LL1623/120mA + PCB_B Lundahl output transformers (price about 148$ each)
1x LL1669A Lundahl power supply transformer
1x LL1638/4H 400mA Lundahl power supply inductances (price about 65$ each)
4 x 534 50ohm Spectrol precision wire wound multi-turn potentiometers
2 x 64W 20Kohm Spectrol multiturn cermet trimmer

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.

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

The input and driver stage is the dual triode 6GL7 or 6EM7 with high-mu unit and low-mu unit..

About passive components types, here I have used normal metallic strate resistor connected in opposite phase to search a compensation of their inductance and ELNA Cerafine capacitors on cathode and power supply.

The interstage capacitors used are the very good Auricap by Audience.

About wire; forget the teflon ! Use only stranded tinned copper wire with pvc insulator, and for loudspeakers use the same.

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

Follows some other datasheets about the parts to build the anodic switch-on delay.

These components could be buy from RS Components

Never switch-on the anodic power supply without this delay circuit !

The Design

Before inserting the 300BXLS tubes in the sockets switch on the amplifier without the fuse and set the trimmers to get about -90V from the grids of the 300BXLS and the ground.

The dot near the resistors show the direction of the components

The Result by Clio system

The distortion spectrum driving with this amp. an 8ohm load show a value about 2.5%.

and here the distortion on second stage