Distortion in a Luxman SQ38F

[MattiaM]

I recently acquired a Rohde & Schwarz audio analyser and decided to start running all the amplifiers I have through it. One of these amplifiers is a Luxman SQ38F that I recently restored, and that sounds amazing.

I will start by saying that valve amps are not something that I am very familiar with, as I have pretty much only worked on solid state stuff.
One of the things that really surprised me is the behaviour of THD versus frequency: see attached graph at the rated 30W output power.

I assume that the rating for the Luxman at <1% at 30W is at the often standard 1kHz, in which case the THD of mine is much lower. However, is a curve like this a normal behaviour in valve amplifiers?

 

[zackthedog]

Yes, it can be. Much depends on the output transformer, but for a good discussion of this, see Dave Gillespie's thread about restoring the Heathkit W2. Overall it's an excellent introduction to feedback amps and their behavior:

Regilding the Gilded Lily: Heath's W-2M

INTRODUCTION Immediately following WW II, there was very little in the way of truly high fidelity amplification equipment available to audio enthusiasts. But then came D.T.N. Williamson's Landmark design that introduced the Williamson amplifier to the world in the May, 1947 edition of Wireless...

www.audiokarma.org

 

[MattiaM]

Thanks a bunch! It looks like that thread is going to be a very good read!