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Re: Good sound from FETs-driving-tubes powestage?

"Better than tubes-driving-tubes? That's news to me."

Glad I could be the first to break it to you. It shocked me too, believe me. However, I started reading about how difficult it is to really get the driver of the output stage right and where a couple of guys claimed that this was the primary source of distortion in many tube amps. This high voltage MOSFET really controls the output tube such that there is NO sloppiness in the bass (also great output trannies) and yet the mids and highs sound like pure SET. The secret I think is that there is no feedback and the fets are running single ended class A with no feedback, which should produce mostly low order products like a good tube stage (actually according to Boyk and Sussman's model it should produce only 2nd and 4th harmonics due to the quadratic nature of the mosfet).

I know the Tenor OTLs. Fantastic sound and you can really hear how the regulation tightens up the sound compared to some other OTLs. I know this because I had a pair of Silvaweld OTL Tube Reference amps using a similar tube complement to the Tenors. The sound was crystal clear and extremely dynamic. Truly a beautiful sounding amp; however, they were not as well regulated and as a result the bass was somewhat sloppy. The mids and highs were nearly perfect (when biased correctly...too low bias and they began to sound a bit cold). I only sold them though due to the unbearable heat they generated (500 watts per monoblock all the time). The room went up at least 5 degrees C in a couple of hours!!

However, the KR hybrid is just as dynamic and clean sounding as the Tenors with even better bass. Hard to believe but at least to my ears true. Here is a link to the smaller brother of the amp I own. You will see that it has pretty nice measurements for a tube amp in general (let alone a SET).

http://www.vi-fi.nl/assets/s2dmain.html?http://www.vi-fi.nl/xtrartikelen/testen/100000982d090451e/index.html

The one I have gets about 10 more watts. Notice the linearity of the output vs. frequency (no sharp rise in the bass due to tranny saturation big key to good bass performance). Notice also the -3db point is 80Khz...without negative feedback! Distortion at 1 watt has only to the 5th harmonic at about -105db. At 10 watts some higher harmonics but all below -110 db. Even a decent damping factor (always above 5).

The Tenor at 10 watts looks like this:

http://www.soundstagemagazine.com/measurements/tenoraudio_75wi/

Not too bad either but there are more higher order harmonics and there is more power supply noise in the distortion spectrum. Also, the progression is not as smooth probably for two reasons, 1) it is essentially a pushpull amp and 2) it uses negative feedback. Also running in Class AB it will perhaps have a bit more mismatch in the zero crossing (a SET has no discontinuity there). Finally, it has a more complicated circuit which means more active stages with each adding some distortion.

Honestly much of this is made up for by not having the output transformer. OTLs really do sound amazingly transparent and the KR is first tube (output) amp that I have heard that really measures up in this regard. I have found that the OTLs though that are using negative feedback (this means Graaf, Tenor, Silvaweld, and some Atmasphere) to lower the output impedance to a reasonable level do sound a bit less rich harmonically than the best zero feedback tube amps. Not so cold as most solid state and a bit different but not perfect either. Call OTLs my second favorite amps :), with "normal" hybrids my third favorite (I still love the tone of my Sphinx Project 14 hybrid...tubes in Mosfets out). I have heard one, from AcousticPlan, that is very close to sounding like a good OTL.


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  • Re: Good sound from FETs-driving-tubes powestage? - morricab 17:38:21 03/09/07 (0)


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