300b lovers


I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations.  I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is.  Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.  

Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence.  They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music.  They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding.  Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a  unique circuitry to mate with his 300b monos via XLR connections.  Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it. 

I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system.  300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm.  Of course, 300b amps favor efficient speakers so carefully component matching is paramount.

Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public.  Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC.  I will be there to hear what I expect to be an outstanding sonic presentation.  

To allay any questions about the cost of Don's 300b mono, I do not have an answer. 

 

 

whitestix

Hi @atmasphere ,

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

 

@charles1dad My point was addressing a comment made earlier by Lynn about overshoot in amps employing feedback; simply that if you do it right its not a problem. The OP mentioned using Spatial Audio Triode Masters who were a dealer of ours and have used our OTLs and class D on their speakers. It didn’t seem that off topic, especially if we discuss the issues of signal coupling, operating points and the use or lack of use of feedback.

As I understand it, you are particularly enamored of SETs; perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing.

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

@alexberger Ringing contains higher ordered harmonics which can be heard as brightness and harshness. You also get lower orders which contribute to richness. Both are colorations and will obscure low level detail.

@atmasphere 

perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing

Different pathways to audio nirvana is something I’ve acknowledged long ago. It’s an undeniable individual journey with numerous successful outcomes. What I have found to be most pleasing and satisfying for me certainly may not be the choice for another.

I don’t believe my comments above contradict this perspective. I was merely comparing two earnest efforts to build amplifiers that are vastly different in concept, design and implementation.

Charles

As a minor diversion, I should describe the "Golden Age" amplifiers I keep referring to. This aren’t just the amplifiers made in the Fifties and Sixties; it describes the majority of PP tube amps made since then, including today.

There were only a few basic Golden Age circuits, or topologies, as we like to call them. (Topologies omit circuit values, but are easily worked out once you know the tubes.) The first was the Williamson of 1948, but it had the drawback of marginal stability. Still, it dominated the US market until 1955 or so, when the much simpler Dynaco variant came in. (The Dynaco topology simply omits the driver stage of the Williamson and uses the phase splitter to drive the output tubes. More distortion but more stable.)

The Mullard became the prototype of many tube amps as the better-performing alternative to the Dynaco circuit, and is still widely used today. Let’s walk through it.

There’s a high-gain input tube, typically either a 12AX7 or a pentode like an EF86. This is direct-coupled to one half of a differential stage, with the other grid AC-coupled through a cap to ground. Because the grid of the diff stage is at 150 volts or so, the cathode is a little bit higher, maybe 155 volts. This requires a large value resistor that goes all the way to ground, so the diff stage is frequently called a "long-tailed pair". A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

The diff pair are a pretty good phase splitter, and unlike the split-load inverter of the Dynaco circuit, audio-frequency balance is not too sensitive to load. It also has more drive capability than the split-load inverter, and unlike the split-load inverter, it has some gain, too. So a win all around.

And we’re not talking about a lot of parts here: 3 triode sections, and the output pair. A Dynaco is even simpler, with 2 triode sections, and the output pair. The only coupling caps with either circuit are between the grids of the output pair and the preceding circuit, so not really complex, and simple enough that a stereo chassis, running off a single B+ supply, is quite practical.

The point of the high gain (in the input section) is to give feedback something to work with. Feedback requires "excess gain" to work its magic; you need 20 dB of excess gain to get 20 dB of feedback, which will reduce overall distortion tenfold. In a pentode or ultralinear connected amplifier, the output impedance is way too high to use with most speakers. The feedback really comes in handy here: 20 dB of feedback reduces output impedance tenfold.

What limits applicability of feedback is loss of stability if too much is used (I’m not going to get into Nyquist Stability Criteria here, nor phase margin, settling time, etc.) In other words, if we slap in another gain stage and try for 40 dB of feedback, it will just oscillate. At full power. And take out a tweeter before damaging itself and letting the smoke out.

A more clever approach is wrapping local feedback around the most distorted stages, like the output section, and then add overall global feedback on top of that. This was done in the McIntosh, Citation II, and a few other amplifiers. This really gets the distortion numbers down, but clipping can get ugly, and settling time from transients can be an issue. Multiple feedback amplifiers can be quite sensitive to operating conditions. It’s more often seen in modern transistor amps as "two-pole compensation", and is not trivial to design.

Note: To puzzle out a schematic, by convention, signal flow is left to right, just like you’re reading this. To see what a tube is doing, look what the grid (the dotted line) is connected to. Often, there will be a coupling cap, typically 0.1uF. If it is much smaller than that, like 30 mmF or 30 pF, it is bypassing RF or has something to do with stability. Larger caps are cathode bypasses or power supply. The plates (the flat-topped dingus) is the output of the tube and typically heads to the right side of the schematic.

You usually have to stare at a phase splitter quite a while before the function becomes obvious. One side is quite simple, coming directly from the input tube, but the other side can be pretty weird. A diff stage can be puzzling, because the DC connection is a high-value resistor going to the other grid, and the AC connection just goes to ground through a 0.1uF cap. The "other half" is actually driven from its cathode, not the grid.

What gives away a split-load inverter, or "concertina" stage, are the equal cathode and plate resistors. This is a dead giveaway you are looking at an inverter, since no other tube stage uses equal resistors ... for one thing, it’s kind of useless for anything else, since gain is a bit less than unity.

I leave the "floating paraphase" as an exercise for the reader. I kind of like them, actually, because current drive for the power tubes is pretty good, although balance is only so-so.

A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

Usually in a differential amplifier, the plate resistors are matched if both halves are driven. In the case circuit of the above description, only one side is driven. So if a CCS is not used, the plate resistors can't be matched; one side must have a slightly higher plate resistance to compensate for the mu (gain) of the tube of the un-driven side, so as to get equal outputs from each half.

A good CCS eliminates this problem (which is nice since in the real world you can't count on the mu of each section to be equal or matching that of the tube specs on paper). A good CCS is both inexpensive and reliable, allowing the tube to be removed from the circuit while active (hot plugged) without damage.

Further benefit can be had from placing a CCS in the output tube cathode circuit, if you control the output tube(s) bias using fixed grid bias. The cathodes are tied together and the CCS feeds them; thus improving the differential effect of the output section, which reduces distortion and makes it slightly easier to drive due to increased gain.

Of course, if you have the input tube be a differential amplifier too, it can accept a balanced or single-ended input and can have excellent performance if a CCS is used for this stage as well. But its not a good idea to direct couple both plates to the succeeding driver stage; its OK to do one but the other should be capacitively coupled so as to prevent DC offsets of the first stage of gain from causing distortion in the driver.

The original BAT VK-60 of the 1990s used a differential input direct coupled to a differential driver; to deal with the DC offsets a potentiometer in the cathode circuit of the input tube allowed the plate voltages to be equalized. I found this approach to be problematic (we had tried that back in the early 1980s; one obvious problem is that it requires the user to make this fairly critical adjustment...) and often causes more problems than it solves.

So in the quest to keep the number of coupling capacitors down but retain easy operation, we started using a differential cascode voltage amplifier. The advantage of this was that all the gain of the amplifier was in a single gain stage, consisting of three dual-section triode tubes, one for the input differential amplifier, one for the top of the cascode, being plate loads for the bottom tube sections, and finally a 2-stage Constant Current Source for the circuit, tied to a B- supply of equal potential to the B+ supply. The CCS prevented changes in the AC line voltage from affecting performance of the voltage amplifier from 107VAC to 126VAC the difference was only 17 parts per million. So you couldn't see any performance change on an oscilloscope over that range!

So that allowed for enough gain, low distortion (once the correct operating point was set up), and only one pair of matched coupling caps (of a small value, in our case only 0.1uf, further minimizing the sonic impact of the coupling caps). They drive a pair of cathode followers which are direct coupled to the output tubes. So the power tubes obtain their bias voltage from the driver; therefore the bias and DC Offset controls are in the grid circuit of the driver tube. This allows for instantaneous overload recovery and rock solid bias control of multiple high-capacitance triode grids, with low frequency response to 1 or 2Hz no problem at all.

If you use a coupling cap in the critical area of the grids of the output tubes, it must be large so as to get good bass response since the grid bias network must be of relatively low impedance to properly control the power tubes. This means that the driver tube has a difficult load to drive and the large coupling cap can cause blocking distortion and slow the overload recovery. While this really isn't much of a problem driving pentodes, using this topology to drive triodes is a bad idea IMO/IME.

I've been describing how our OTLs work but obviously this would work well with a 300b too. We've managed to get our OTLs to 0.5% THD which is pretty low distortion for a zero feedback circuit! SETs by contrast tend to be about 10% THD at clipping which might be only 7 Watts. Since the OTLs tend to be much higher power capacity, the tendency is, for any power level the SET might have, the OTL has distortion that might be 2 orders of magnitude lower or more at the same power level. This is why they tend to be so much more transparent than SETs.  I've no reason to think this cannot be applied to a 300b circuit with similar results; SETs have the distortion they do out of the topology rather than the power tube that is used. So a pair of 300bs could be used to much greater advantage!

For those that might want to see more about how our OTLs work (and how this might be a topology for a 300b amplifier), there is a DIYaudio.com thread from several years ago that has a schematic and discussion. A lot of this would work very nicely with a 300b; for example the Circlotron output can be transformer coupled of course and have all the advantages (such as zero DC saturation of the output transformer) it offers.