I did not make my mind about it but I have some “concerns”. Using SS amps and hearing different SS amps I do not like their bass and the notion of a powerful enough DSET to be able to drive ULF in A1 was born. Sounds logical. However, recently I employed a vacuum tube crossover before my SS amp and it looks like addressed all my frustrations in SS sound. Listening the very same SS amp with bass colored by tube crossover I do not experience the typical sense of overly “pressed” bass and hard “room clipping”, quiche in opposite, the room sound very soft with bass being almost like a butterfly sits on your shoulder – very different from the sound of the same SS amp.

I certainly do not propose this as a universal solution for better bass but since I did get in my estimation a better bass then I would like to think what have I’ve done.

I did not test my tube crossover but I am sure if did then I would find it quite distorted. A sequence of 4 cathode followers with 12AX7 – it got to be brutal. If I use the same crossover as a line-level buffer (with no filters) then I am sure that I would not find it transparent enough. Furthermore I would be most likely bitching about bass. However, in case of multiamping and use the crossover only as low pass filter for ULF it look like the distortions that the crossovers create become virtues.

So, I wonder, is it anything in my finding about this way to use ULF worth to be capitalize in more global scale or it is just my erroneous perception have found a satisfaction in my private case?   I do not have answer to it. But what I would like to do is to propose to someone who use powerful SS amps to drive bass and who have own dissatisfaction with some aspect of its quality to try to color that bass with passing it across a “weak tube” buffer. It is possible that harmonic distortions that small signal tubes do have very positive impact to whatever they impact.

I know that it sounds like foolishness but I do have two identical SS amps with two identical filters and after much contemplation decided to abandon the elegant and very clean amp with passive line filter and to employ very non-graceful solution with 4 active tubes staged before my SS amps. If somebody would do it I would very much criticize this approach. However, in my defense I do have my well recognized reasons.  What I am doing not is sharing my reasons with you and proposing to confirm it with “second opinion”.

I do not feel that my employment of active filter is right thing to do system design-wise; even it gives subjectively better sound, closer to where I would like it to be. I would like to know if it possible to make it “properly”. The proper implementation in mind would be the use of powerful SS amp to drive ULF that will have the distortion pattern (or whatever is responsible for sonic subjective improvement while I use tube crossover) similar to what my current tube crossover + SS amp gives. My task is very limited – I do not need a full range SS amp. What I might be OK with is DSS a dedicated to bass only SS PP amp. It is highly possible that all that SS amps need to drive infra-bass is to have a few harmonic shaping stages in a signal pass. I did not know if anybody look into this but I do not mind if somebody do and let me know about the findings.

Paul S wrote:

While you are merely being empirical, there are actually plenty of devices that aim to "benefit" the end sound of SS gain by using tubes to "condition" the signal from DACs, etc.  Also, there are plenty of commercial widgets out there that say they will bring the glorious "Tube Sound" to a given system.  I have to say, I regard this stuff as basically soft-headed, and results I have heard literally speak for themselves. Who wants to "listen to tubes"? OTOH, the injection of 2nd order harmonics is a well-known "remedy" for the typicall hi-fi "Upward Trend", not to mention the fact that LF is really a different animal in terms of getting it to "work" with the rest of a system in a given listening area, and I'm all for reverse engineering according to what "works", sonically, no matter what the experts say.

I was thinking myself what I was attracted in my ULF with tube crossover was just a simple “tube sound”. They I realizes that there is something more to it. To relive my retaliation you need to ask yourself: what would constitute “tube sound under” 25Hz? When I realized that there is nothing that I know about “tube sound” in ULF region then I stopped to look at the problem as “tube sound” and begin to look at it as sound in the tubes.

What I find interesting is that there is nothing in that sound of my tube ULF that I like for it specific “tubbiness”. It is rather that sound ULF is better integrated with the sound of my playback and the ULF has less some negative elements of sound that I explicitly noted when use SS amp/ BTW, I did not feel that my installation demonstrate “tube sound”…

That all bring me to think that if my little tube crossover did such a pleasant job with my ULF them how great would it be if the ULF would be coming from a powerful SET that would not need any 4 stages of active crossovering….

Guys, I am not kidding, something is VERY right in the sound of that tuber crossover combined with my SS amp. Even the ULF in this configuration has some properties that I would like to have improved but the from more holistic perspective the integration with the rest of my playback is beyond reproach, much better  then I was able to get from pure SS amp. In fact it reach the desirable level of seamlessness that I had in my old room with all tube gear driving the entire system and including bass driven by my 6C33C DSET.

As in anything that I am trying to expose at my site there is some interesting educational aspect. I always was pitching that in case multi-aping all channels must be driven by amplification with the same harmonics structure and the most important with the same pattern of changing harmonics with volume. I spent a LOT of efforts to accomplish it in my old room but try to do it if you need 150W of SET power for ULF? It looks however that that tube crossover made the tandem of crossover and SS power amp pushed my integration to more proper integration route. I do not think that I got where I would like to be but the direction of overriding the stupid harmonics of SS amps with tubes I find is very perspective.

So, how many people out there run tube powered playback with powerful SS amp at the bottom? Many-many of them. How many people have bass sound differently than the rest of the system? Practically all of them. Do you feel that it is an opportunity for a right commercial product to be introduced to address a problem? With proper explanation of the subject and proper demonstration of the success I think it might be VERY successful product and the most important that it will have absolutely no competition.

Take a look what I propose. I powerful 100W-200W SS class A/B output stage with PS that has a few voltages switches. Thai would allow loading output stage properly against load of multiple impedances. Of course the adjustable idle current – this along with voltage would assure the perfect symmetric clipping at desirable power level against ANY load. A tube based gain stage and tube based driver stage. A low pass adjustable crossover between the stages, perhaps 12dB per octave in open loop and 12dB in local feedback.  A harmonic correction circuit with exposed controls to user. Now pretend that it all come in one package, properly calibrate and voiced, would it be wonderful?

If some of manufactures would like to undertake this task then I will be willing to contrite my efforts to help with evaluation of prototypes and offer some sonic recommendations.  I think it might be a very good and useful product.