300b lovers

One variant we did not try was a center-tapped inductor to load the 6SN7 plates, and direct coupling between the input and driver tubes. That requires a high B+ voltage for the driver, but that’s no problem when the input + driver have their own power supply that’s fully isolated from the output section power supply.

It’s a good question if this is better or worse than a special interstage transformer. The center-tapped inductor shares many of the design challenges of an interstage transformer without necessarily having any advantages. This falls into the category of "build it and see". A minor challenge with a 450 to 500 volt regulated B+ supply, but nothing impossible.

Another similar option, instead of a center-tapped inductor, are paired current sources and direct coupling between input and driver tubes. This has an ugly disadvantage that a small DC imbalance in the input tube turns into a 5 to 10 volt offset for the driver tubes, which is intolerable because the driver section is then grossly imbalanced with different operating points for each driver tube.

To prevent this, it would require something like an opto-isolated DC servo circuit to balance the plate voltages of the input tube. Doable but kind of nasty, adding a lot of pointless complexity that adds nothing directly to the sound ... basically, another point of failure. The alternative would be manual adjustments (with limited range) on each current source and a meter so the user could hand-adjust DC balance.

Alternatively, the center-tapped inductor, because it has a moderate DCR of a few hundred ohms, limits the amount of plate-to-plate DC imbalance to a volt or so. However ... small as that is, that’s more than an interstage transformer, where the output going to the driver grids is always perfectly balanced, with zero DC offset from grid-to-grid.

What the interstage transformer does is offload circuit complexity to the ingenuity of the transformer designer. The circuit schematic looks simple, but what’s going on inside that transformer is very complex, requiring sophisticated modeling tools to fully understand.

I am still puzzled why capacitors measure so well, yet are so audibly colored. And why on Earth do they require hundreds of hours of active operation, not just polarized but signal going through them, to finally stabilize sonically? What’s going on inside them? My only guess is a (very) slow electrochemical process that subtly alters the dielectric properties of the plastic film.

By the way, there’s a very intelligent discussion of the latest John Curl/Parasound amplifier design here:

Parasound Discussion

Note they are talking about fully isolated input+driver sections with 117 volt rails. That might sound insanely high for a transistor amp, but overshoots in input sections of transistor amps are a big deal, and these are very high-powered amps with serious voltages appearing in the feedback loop.

By the way, cascoding the input section is how you get both voltage resistance and a hundredfold reduction in Miller capacitance. Since Miller capacitance in transistors is grossly nonlinear, this is a very good idea. Tubes exhibit Miller capacitance too, but it is an order of magnitude lower, and it is stable and predictable instead of being nonlinear. There are cascode tube circuits as well, but they offer no improvement in linearity (unlike transistors), and are mostly seen in phono preamps and FM tuner input sections. In the tube universe, pentodes behave similarly to a pair of cascoded triodes, and are more commonly used when a cascode is called for.

Certainly a great benefit in a phono preamp where you have to overcome the wall of noise in the very first stage, and yes, cascodes are more linear than pentodes. The EF86 mike-preamp pentode so popular in the Fifties and Sixties is now very expensive and hard to get, so cascodes make more sense today.

Don’t need all that gain in a power amp in the absence of feedback, but if we ever needed feedback, yes, that’s a good way to get it.

I completely agree about the benign nature of low-order distortion. Most of all, it reduces nasty high-order IM distortion which is objectionable and obviously electronic-sounding. I also agree about the heavy 2nd-order distortion of SETs, which limits their effective dynamic range. Clever design techniques can mask the 2nd-order distortion (like coupling caps with complementary sonics) but the IM distortion remains, and is audible with symphonic, choral, and heavy rock music. (Music with a sparse spectra, like jazz quartets or chamber music, doesn't expose IM distortion, but music with a dense spectra turns IM distortion into a wall of noise that goes up and down with the music.)

Since transistors are notorious for high-order distortion, cascoded differential sections are about the only way to tame the things, while also getting rid of nonlinear Miller capacitance. The classic John Curl topology.

Yes, Don and I are working on a medium-price (by high-end standards) amplifier, most likely a stereo integrated amp. Not much to say about it now, since it's mostly conceptual at this point. If people are looking for value, I think Spatial may still be making Don's previous Kootenai amp, which is a superb amp.

Thanks for the update, Don.

Yoder, there was a lot of progress sonically after the first prototypes. What we have now is actually simpler in some ways. We went for a pair of spacious monoblock chassis mostly to simplify assembly, and to improve cooling a bit.