300b lovers

Ralph, I see you introduced a PP 300B amplifier at one of the recent hifi shows. How do you think it compares to your Class D amplifier, in subjective terms?

The prototypes at the show sound very similar to our class D amps; the big difference is the bass- the prototypes roll off at a higher frequency, and owing to no feedback and the OPT design don't throttle back their power when dealing with higher impedances, such as seen in the CALs at the show, which are bass reflex. So there was a bit of extra energy in the double impedance peak of the speaker. Its fun, but not correct.

Essentially that OPT loads the tubes at their power peak rather than a bit higher where the amp would have had a better chance at being more like a Voltage source. If the next iteration of the OPTs had been available, the bass would have been a lot more like the OTLs but ultimately lacking the extension that up until now has been something that OTLs do better than any other kind of tube amplifier.

The class D has the same relaxed mids and highs but is a little more transparent- its easier to hear what's going on in the rear of the sound stage if you have a 2-mic recording.

I think we can do better with the 300bs by using some of the tech we use in our OTLs. So the next PP version of the amp will be quite a bit different- with LF bandwidth similar to our OTLs with no sacrifice in high frequency bandwidth.

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.