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The Most Powerful Advanced Amp Parameter...


Fractal Audio Systems
is Transformer Match. I've written about this before but it's time for a refresher.

When people try different tube brands or rebias their amp to use a different type of tube they make all kinds of hyperbolic claims about those tubes but it isn't really the tube that made the difference. Well it is but it's not because the tube is doing something special. It's simply because the tube has a different transconductance (gain).

Amp designers choose an OT turns ratio such that the amp is "matched" to the load. However "matched" is a nebulous term since tube gains vary, speaker impedance is variable and bias point is adjustable. Therefore there is no absolute turns ratio that ensures perfect matching. Matching implies that the swing at the power tube grids just pushes the plates to the rails. If the output transformer is undermatched, the grids will clip before the plates hit the rails and vice-versa. Designers also select the turns ratio based on personal preference. Some designers prefer undermatched OT since this gives a more "open" sound, while others prefer overmatched since this gives more touch response. For example, a Trainwreck is highly overmatched.

For a given OT, if the tubes have higher gain than originally then this effectively overmatches the OT and vice-versa.

Now this matters most for non-MV amps that get their distortion from the power amp, i.e. old Marshall, Fender, etc.

So... if you are going to experiment with any advanced parameter, start with Transformer Match. A little bit in either direction can make a big difference. Note that the Transformer Match parameter is relative to the internal value.


Self-Admitted Software Thief
What is the best way to set this when using a tube poweramp with PA sims off?

There are many parameters that remain active for obvious reasons when the power amp sims are off, I wish there was more clear direction from fractal on achieving desired results when using the axe as a preamp only.

I grabbed a woofer tester and test my speakers to set the speaker tab parameters.

Any way to test my power amp input for useful information?


And deeper into the rabbit hole I go. Thank you Cliff for sharing such valuable insights. These can keep me entertained for quite some time.


Fractal Fanatic
Thank you very much for this. Not only for the heads up, but for the detailed explanation!


....while others prefer overmatched since this gives more touch response...
I use this ALOT. When you're going for that 'old amp' dynamic response, I get this to where it feels right relative to the volume I'm at, add a little 'squish', and enjoy the love.

Lol! The funny part was lastnight I was just messing with this parameter and thinking to myself over it if this is what really does this and that... And now the question has been answered! Thanks Cliff!


Fractal Fanatic
Went to a 'tube guru' for my Princeton a few years ago. he came out with several different 'sets' he'd hand-picked and we listened to 'em all. I was AMAZED that my amp could go from a really horrible (IMHO) compressed-to-hell sound to a clear, open sound with just a set of tubes (I settled on the most 'open' of the 5 sets).

Now I finally understand a bit more of how and why the tubes were able to so drastically shape my amp's character.

Thanks Cliff.
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