Pushing to the limits

 

 

 

 

 

 

 

We’re constantly obsessed to get the most out of our lives. Not a product of the capitalist world we live in, but a fact of our human nature. Its evolution.

When it comes to sonic power, unfortunately we are not too distance from this thought. We want more Watts. Yes, pure power. My generation back in the 80s got misled by the audio product marketing and their unrealistic metrics (e.g. PMPO)  to fudge the real power of a solid state amplifier.

When I built many years ago my first 10W push-pull amplifier I discovered a new music sound in valve amplifiers like many of you out there. In fact, things changed when I got my single-ended 45 amplifier up and running. With its mere 1.5W it is outstanding in terms of sound. However, I regret not having an extra Watt or two. Playing rock or even free-jazz records is not the best amp for it.

I’ve been exploring the push-pull route, but single ended is deep in my heart. That is why I’m building a 4-65a SE in class A2. Perhaps its 10W will be the combination of single ended sweetness and power. I don’t know yet, but I will soon find out.

The sound of the 6C4C in single ended is remarkable. My mind is constantly thinking of options to compare and contrast 6C4C and 4P1L in PSE. What I found is I didn’t like the most the 4P1L to 4P1L amps I heard in the past.  A bit too bright perhaps given it’s H3 levels? Perhaps, but what I did found I liked was the 4P1L to 6C4C combination.

That is why I continue looking at this Russian babe 🙂

Again, looking at my LL1623/60mA which can provide an effective anode impedance of about 6KΩ (5K6:8 mode) we can see that the optimised operating point for A1 operation is about 300V/50mA:

This bias point will deliver about 3W @ 3.4% when driven at 126Vpp approximately.

I will reuse in this design a previously presented 4P1L driver (in filament bias) but with an LL1671/20mA in 1:1 IT mode:

The bias circuit is similar as previous designs with the MOSFET source follower. So let’s see at the performance of this design:

THD is about 3.4% when 3W is delivered into 8 Ω load. The H3 component is down to 0.4%. Above 3W, the grid current impacts the driver performance and harmonic profile. Looking at the  grid and anode current you can see how the curves are flatten out given the poor performance of the driver in A2.

I think this amplifier should perform really well. The combination of 4P1L, filament driver and IT in a coupling cap-less design is very promising.

An additional Watt will beat the 45? That is yet to be proved…