Your journey with lower-watt tube amps -- Can a kit be good enough?

@atmasphere 

I think it was Einstein who pointed out that a thing can be too simple to do its job well.  

Everything should be made as simple as possible, but not simpler. 

-Albert Einstein 

@mclinnguy Thanks!

So what is actually simpler? Many say SETs are. But they have problems that are very difficult to solve like the elliptical load line I've mentioned several times. I think they are just too simple.

In a conventional push pull tube amp the problem of core saturation in the output transformer is solved (so no elliptical load lines). This allows the the transformer to have much higher inductance at low frequencies and generally lower distortion at any frequency.

If the PP output circuit is biased class A1, it induces no more noise in the power supply than an SET. If the circuit is fully differential then the zero feedback distortion signature can be just as pleasing if not more so then that of an SET.

What I'm getting at is there is a thing I like to call 'elegance'; a simple solution to a difficult problem.

The original Ultra Linear concept was elegant. It was so popular that many sought to get around the patent by simply moving the UL tap to a slightly different spot, sacrificing some linearity to avoid paying a patent royalty. This was so prevalent that most UL transformers made today still do not conform to the UL patent, despite it being long expired. That's why some people like UL designs and others don't- they are not all equal. But the original idea worked quite well and displayed that quality of elegance. 

In case those reading these words aren't familiar with the concept, UL operation allows a pentode power tube, either single-ended or push-pull, to have the linearity of a triode combined with 90% of the power of a pentode while being much easier to drive than a power triode. Anyone doubting this probably should read the original patent issued to Acrosound in the 1950s (David Hafler, one of the inventors, left Acrosound for Dynaco; knew how to break the patent and did; the UL patent was never licensed to Dynaco). 

Again, in a nutshell: They shine when not presented with bass or asked to make much power.  I've already explained why they sound good when they do so.

@atmasphere Ralph, I'd like to test that with the implementation of my SETS (I've got the low power part covered with high efficiency speakers and modest listening levels). So I figure I need to split the audio signal before it gets to the SET and create one for the SET and one for my powered subwoofers. The first being treated with a high pass filter and the latter a low pass filter. It so happens I own a MiniDSP 2x4HD which I think can do the job. The only limitation is that I need to then use the internal Dac of the of the MiniDSP and it is not my preferred Dac. Are there alternative approaches that won't cost too much? My music server is a Mac mini running Roon. Roon can do DSP but as far as I know I can only treat one signal stream at a time and there is no way to split the original signal in two and get two outputs from the computer. Any ideas would be appreciated.

Is there possibly a way to split the output from a Dac and then apply the high and low bandpass filters using some combinations of resistors and capacitors as done in a speaker crossover?

@bruce19  You could install a coupling capacitor between the source (perhaps a preamp) and the SET, with a value calculated to have a -3dB point at the desired crossover frequency.

Frequency in Hz = 1,000,000/Capacitance in uF times Resistance on Ohm time 2Pi  (Usually when this formula is used the one million is a 1, but the capacitance is in Farads, which is too large to be easy to use. So I do the formula this way so I get practical audio values)

So if you wanted 60Hz as the crossover frequency and the input impedance of your amp is 100KOhms, then the capacitor would be about 0.027uf (that yields a 59Hz -3dB point).

This cap can be tiny since it does not need to handle much Voltage. So you might be able to install it inside an RCA connector of the interconnect cable.

@bruce19 

If you already own the 2x4, there's no harm experimenting with an active crossover setup like you described, which sounds like it would work fine. If you're pleased with the proof of concept, then you can consider the DAC question. Just my 2¢! 🙂