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the my new reference 300B Amplifier
max performance to drive high efficiency loudspeakers
started 16 february 2011


This new reference design born like an upgrade of my 4300B SE and It is the best choice to drive hi-end full range loudspeakers with high efficiency like
Lowther, Fostex and Fullrange-Speakers.
In this project the budget will be not considered so it will use only the best components on the market and it is very near to a dual mono design.
The basic version is very simple and with very few parts but it is possible make some changes to increase the output power and eliminate all the electrolytics on the signal path (fixed biad and led bias).
This amplifier is superior to any other 300B design because it have a very low distortion, an extended frequency response and an incredible high damping factor.
These characteristics are obtain by the use of an ultra linear driver stage and a high output transformer impedance (5600 ohm instead of normal 2500-3500ohm).
Someone to reduce the distortion of driver stage use strange driver with dynamic current source but in this case the sonic performances are compromised.

UPDATE 03/09/2021:  Jeff Wagner have reduced the noise to zero using a pair of the Tent Labs 2.5A filaments power supply, using this solution you can eliminate trimmers relative to the 300B filaments.  If you continue to use automatic bias the 1Kohm and 220uF 63V are necessary.



This amplifier use direct heating tubes with AC filaments so at the startup some 50-60Hz hum can be ear.
The two trimmers Spectrol mod. 534 should be set for the lower hum and any position is good at the startup.
I don't suggest a DC filaments power supply if you use a unique power supply transformer but you can try it.

It is possible to increase the output power from 6W to 8W using fixed bias, in this case change the LL1648 with LL1669A to get an extra 100V secondary.
For the fixed bias a dedicated power supply for the negative grid bias (~ -60V) will necessary and you must increase the value of the  0.39ohm 3w on 300B filaments because the LL1669A have only 6.3V secondaries.

It is possible use LED bias in the driver stage, I have tested green led with good result and the same frequency response, only a little bit less voltage gain.

If you use fixed bias for the 300B and led bias in the driver you can exclude all electrolytic capacitors in the signal path.


All the audio transformers used in this project are Lundahl products.

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 input stage tube is the  D3a in triode connection.

The output stage tube is the Golden Dragon 4300B. The vacuum diodes used are the GZ34 Mullard.

About passive components types,  this project use the best combinations:

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.


Follows the first measurements on the basic version (no fixed bias and no led bias).




For the 10Kohm Caddock I have used this A1882 heatsink.