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Thursday, November 3, 2016

The Differential Power Amplifier - Part 3


Here some photos of the finished push pull 300B monos and some initial sound impressions.

All tests on the raw amps have been completed and they got their final housings.

One of the amp chassis in it's beautiful walnut case.

The back side with the connections.

The power supply:

Power supply back side:

One channel:

The stereo combo consisting of 4 chassis:

Another view

Close ups to the tubes of one channel:

Top view of all 4 chassis:

And of course the big question, how do they sound, especially compared to the single ended amps?

They exhibit quite a different character. While single ended amps can tend to be euphonic and more on the forgiving side. These are extremely clean sounding and well controlled. Very direct, but never loose the musicality or flow.

And of course lots of power with excellent bass control. Are they better than single ended amps? No, just different. The choice is a matter of taste. If you want a sound more on the romantic side, the single ended amps are the choice. If you like clean and well controlled sound (probably closer to the truth). These are the ones to go for. Especially the lack of capacitors in the signal path is showing with a very un coloured and high resolution reproduction.

With the ample power and low output impedance they can drive a wide range of speakers.

The sound of these grows more and more on me and I will give another update after living with them for some time. An ultimate version of these amps with all silver transformers and plate chokes is already firmly planned! Which probably says more about their sound quality than any description...

Stay tuned for more impressions of the differential sound!

Best regards



  1. Are these the most powerful amps you have built?

  2. No, the SE 211 amps have similar power and I have built amps with 30W per channel in the past

  3. Hi !
    seems to be nice tubes. (300b)
    Where are they actually buid ? (what city ?)
    thanks !

    1. Hi! Those tubes are made by my own company. They are manufactured in Hagenow. click on the label Elrog on the left column to find everything about them

  4. Hi Thomas,

    Impressive and inspirational as always...I have read your comment about the 801A in Tube of the months..

    "Hi Josh!

    I did build PP amps, but not with the 801A. With its high plate resistance the 801A is not really usable for Class A PP

    Best regards


    I guess you meant as the power tube, not the driver, correct ?

    I am still impressed that you are able to make the voltage swing work for a 300B with only one tube which has a gain of 8...I would never thought about this, but you demonstrated it is possible ! Wow. I will have to try the step-up with irons my self now...the combination of 801a and Elrod 300B must be breathtaking.

  5. Hi!
    Yes I meant as output tube. Interstage transformers with the required primary impedance are available. This amp has not the highest input sensitivity, obviously. But still well manageable .