300b lovers

Wow, is that the first Atma-Sphere with an output transformer?

Sounds like the 300B amp is well on track, and I imagine the next version will have current sources in the appropriate positions. I’m curious what your experience with a high-power current source in the output stage turns out to be. It didn’t work for us, but it did work for Allan Wright back when he visited me in the late Nineties.

Wow, is that the first Atma-Sphere with an output transformer?

No, nor the first to be shown in public or sold. We make a small 5Watt/channel integrated amplifier that is suitable for desktop, bedroom, headphones or a main system if your speakers have the efficiency.

That one uses EL95s which are a little brother to the 6AQ5, running class A1 ultralinear. The tube is very easy to drive so the voltage amplifier is a differential amplifier using a 12AT7 with a constant current source. The amp can sit on a single sheet of notebook paper with room left over but its built with good parts throughout and an overbuilt power supply. The chassis has its corners welded and ground so it can be polished and chromed. We've been having fun with a variety of finishes and color schemes; blue with black (the transformers be the contrast finish), blue with chrome, black with chrome, chrome with black, grey with red and so on. They are entirely handwired point to point. 

It easily keeps up with SETs of the same power; it has a greater amount of usable power owing to greater linearity.  It also has wider bandwidth both on the bottom end and the top end (goes out to 100KHz).

This amp is one of the projects we did which aptly demonstrate why SETs are impractical and obsolete.

That's not to say we're not also having fun with SETs. We have a 300b SET project as well. It uses some techniques we use in out OTLs to minimize distortion driving the power tube.

Sounds like the 300B amp is well on track, and I imagine the next version will have current sources in the appropriate positions. I’m curious what your experience with a high-power current source in the output stage turns out to be. It didn’t work for us, but it did work for Allan Wright back when he visited me in the late Nineties.

Constant Current Sources are the key to getting differential amplifiers to perform. A PP output section is often wired as a differential amplifier so a CCS can work really well if output section is class A.

Yes, it’s difficult to get people to understand that Class A push-pull is quite a different animal than the much more common Class AB push-pull. I understand the reasons for Class AB ... it’s much more efficient, but Class A working is not the same, since device switching and the saturation region are avoided.

It’s also kind of funny seeing the solid-state fraternity tap-dance around various sliding-bias schemes and the marketing attempts to call them "Class A". Well no, they’re not, since the clever sliding-bias system is always just a little behind the musical content. And genuine (thermal) Class A transistor amps require stupendously large heat-sinks or fan cooling.

I remember all the marketing about "cool-running" transistor amps in the late Sixties. Yes, they ran cool if the Class A region was 1 watt or less, and the rest of the operating envelope was Class B. The A/B switching region was especially offensive with the quasi-complementary amps of the first and second generation. The third generation, in the late Seventies, finally had access to decent quality complementary devices, although dog-slow by modern standards.

Class A working is actually easier with vacuum tubes, since you don’t need a monster heat sink. Solid-state reliability drops pretty fast above 80 deg C, so heat sinks and efficient thermal design are mandatory. By contrast, vacuum tubes operate quite happily at very high temperatures (short of red-plating). If you can accept lower power, it’s easily within reach by altering bias points and the primary impedance of the output transformer.

I agree with you about the merits of single-ended vs Class A push-pull (or balanced). The only indisputable advantage of SE operation is avoiding the zero-crossing region in the output transformer, but this comes at a massive cost in core size and the requirement for a large air gap, which in turn erodes bandwidth. Skillful  output transformer design can work around the problems of the zero-crossing region ... this is largely a solved problem.

I have done every permutation and combination of ways to build this 300 PP circuit.  I originally started with high quality CCS and then went to LC coupling, which sounded more natural and musical to my ear, and then finally went to full transformer coupling.  As was stated above, this was impossible with off the shelf iron from even a very well know major European manufacturer.  Those ITs rang like a bell at 15K and up.  It took several iterations of custom design work in collaboration with Dave Geren at Cinemag to get it right.  Dave's easily make it down below 20 Hz and out past 30 KHz with no oscillation and no loading network needed.  We all have our taste, and all versions of the amp were wonderful, but there is magic in the final version that simply wasn't there with CCS or LC coupling.  A lack of solid state coloration and wonderful tonality and presentation of instruments to my ear.  The piano sounds like a piano, not a nice rendition of one.

We are in production after two years of prototyping.  Sounds like @atmasphere is early on their journey with a PP 300b.  Certainly far enough along to demonstrate a prototype at a show, much as we did in Seattle last summer.  It is undoubtedly a different topology than ours and I wish them well.  I am sure they will get their version to where they want it to be as we have with ours.  They will sound different of course due to the preferences of the designers.

Sounds like @atmasphere is early on their journey with a PP 300b.  Certainly far enough along to demonstrate a prototype at a show, much as we did in Seattle last summer. 

We started prototyping with DHTs seriously about 20 years ago. Back in the 1990s we built a 300b-based OTL which we showed at CES. 

The trick with the CCS is doing a good one. A lot of the circuits that you see on the web leaver performance on the table. We've had CCS circuits in our amps for the last 35 years.

'A lack of solid state coloration' is matter of avoiding the distortion signature that is endemic in so many solid state amps. This has to do with proper design of the feedback loop and a whole lotta loop gain in an amplifier design. The 'solid state' sound is just bad feedback application and is part of why feedback has garnered a bad rep in high end audio. 

Feedback has been known to give tube amps a 'solid state' sound too; this is because the feedback signal has been distorted prior to mixing with the audio signal its supposed to correct. Its like having an incorrect map to guide you through town. Norman Crowhurst wrote about this problem nearly 70 years ago but did not suggest a solution, despite it being rather simple. Peter Baxandall 'rediscovered' the same problem in solid state amps about 20 years later but he suggested more feedback, which doesn't solve the problem.

Its only been in the last few years where we've seen self-oscillating class D amps where this problem has been solved pretty consistently. You can do it in class A/AB solid state amps too if you're willing to work out the 3rd or 4th order feedback loops that need to be involved. Most designers just use a resistor which won't cut it...