tube hybrids question.

[azira]

I've seen a couple of tube-SS hybrid amps floating around the forum. Probably the most popular is the VBIGC (valve buffered inverting gain clone). Generally, if a tube is used in a hybrid, it's in a preamp stage, why is that? This paper on Dr Leach's website suggests that the noticable differences between a tube amp and a SS amp is predominatly in the output stage. I wonder if a designer was looking for a "tube" sound, they would do best to build a SS preamp/VAS followed by a tube buffer.

http://users.ece.gatech.edu/~mleach/...ubeVsTrans.pdf

In some ways, this is just my interpretation of the results of this study so I'm interested in hearing other oppinions... hopefully with some good supporting material to look at.
--
Danny

[SY]

The hybrids of the sort you describe are generally designed because of an esthetic, a philosophy, a perceived market need, or sociology, rather than for pure technical reasons. There have been amps done the opposite way, too (the oldest one I can remember is the Audionics BA150).

Then there are people like me who mix solid-state and tubes within the same stage, but at least I've got some technical justification.


The choice all depends on what your design goal is- define that tightly and the appropriate technological implementation with generally be obvious.

[Pjotr]

Hi Danny,

Putting a CF in front of a GC appears to me a kind of fashion. It certainly adds a kind of “tubie” flavour to the sound, probably masking the “cleanness” of the SS amp. It’s just a matter of taste IMHO and/or for the reasons SY mentioned.

Putting a tube in charge as a substitute for the input/VAS combo is a different matter, can second your observations about that. Tubes are simply very good voltage amplifiers, having large input voltage compliance and low distortion.

Cheers

[azira]

Originally posted by Pjotr
Hi Danny,

Putting a CF in front of a GC appears to me a kind of fashion. It certainly adds a kind of “tubie” flavour to the sound, probably masking the “cleanness” of the SS amp. It’s just a matter of taste IMHO and/or for the reasons SY mentioned.

Putting a tube in charge as a substitute for the input/VAS combo is a different matter, can second your observations about that. Tubes are simply very good voltage amplifiers, having large input voltage compliance and low distortion.

Cheers

Thanks Pjotr, insightful comments, agree about it being more like a fashion. I have a question though..

You're suggesting that a cathode follower circuit would be a dominant factor in creating a "tube" sound regardless of loading. A speakers inpedance being much lower and more variable than a the cold constant of a opamp shouldn't have an effect, nor transformer coupling.

Therefore a CF buffered SS amplifier should have a similar effect... wouldn't that be the case with the tube-preamp + SS power amp that was used in the study? I believe there were no subjects who could reliably detect the difference between a SS pre or tube pre.
--
Danny