I want one
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.
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@gladmo Spatial Audio Lab will be making the amps and preamp. All parts are ordered and I hope to be going to Salt Lake City in early November to train them on the builds: https://www.spatialaudiolab.com/news/september-2023-update If you have interest, then contact them.
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I’ve been following … no … have been massively influenced by Lynn’s Karna amplifier and the reasons behind the topology. My first effort is on Lynn’s site as the PP813 Grand Aurora so I’ve been at this for about 20 years. It has now morphed into all DHT with finals of GM70 so not part of the discussion in a 300B thread. These types of amplifier are not easy to get right. Even though the signal path is ridiculously simple, it is still challenging and the supporting power supplies are anything but easy. Finally after all this time, I’ve succeeded in getting to the sound I was after. Probably as I’m not in the position to commission a super IT and maybe as my current one is not so super, I’ve ended up DC coupling the 1st to the 2nd stage in a similar way to Kevin Carter’s method; choke loaded 1st stage. Don mentions in the thread DC coupling was not an option for a commercial product, but I’m left wondering, was this method evaluated in the many permutations of coupling configurations for the front end? I’ve also got a line stage, which again, is heavily influenced by Lynn’s designs; the Raven but using 12B4; initially used as a headphone amplifier. The system is better for it being in the chain than out despite having no gain. Many thanks to Lynn for providing oodles of thought-provoking posts over the years to fuel the ideas leading to the success I’ve had.
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The problem of DC coupling vacuum tubes in a balanced circuit is maintaining DC balance ... during warm-up, in steady-state operation over hours, and as the pair age over the life of the amplifier. A small DC imbalance error in the first stage becomes very large in the second stage, resulting in a massive current imbalance in the second stage. This can be servoed out by a housekeeping circuit, using a bit of analog logic, but if that ten-cent opamp fails, it takes out the entire amplifier. I have seen that happen while I was sitting in the listening room of the editor of the magazine I write for, Positive Feedback. A cheapo servo circuit in the preamp took out the entire power amplifier and the bass driver. $50,000 worth of damage in a few seconds. I don’t care how it sounds, that’s just bad design. DC coupling without a servo basically doesn’t work. Small drifts become big ones over time, and the circuit will have to be manually re-balanced by the user whenever tubes are replaced, which will happen many times over the life of the amplifier. I think the horror of transformers has been taken much too far. There’s a reason they have been used so widely in studios for the last eighty years. They are problem solvers. Output transformers take the pint-size currents of output tubes and multiply them 28 times, or more. Input transformers reject common-mode noise and RFI, presenting a clean, quiet signal to the input grids. Interstage transformers sends the power of both driver plates, summed together, to whichever grid needs it the most (grids take turns going into Class A2). Looking at the driver section, I don’t see the appeal of a cathode follower drive circuit. Adding an additional stage is not exactly direct coupling, and it requires another regulated power supply with oddball voltages for both plus and minus. It’s not a simplification, it’s considerable added complexity, and for what gain? There’s no improvement in slew rate, which is controlled by the current available to drive the Miller capacitance of the power tube grid. The Blackbird has 32 mA of current from each side of the driver, far more than the usual 8 mA of many other amplifiers. The driver can even enter Class AB for a half-second or so, so 32 mA is not the upper limit for grid drive. The least necessary transformer is between the input tube and the driver tube. The driver grid is relatively easy to drive, and no clipping is seen in that part of the amplifier. The biggest annoyance is the slowly drifting DC imbalance of the input tubes. In an AC coupled circuit, it doesn’t matter ... it’s only a few volts out of 150 or more. In a DC circuit, though, it controls the bias of the driver tubes, which is a big deal. You really don’t want one tube at be at 50% power while the other is at 90% power, and you don’t to burden the user with meter and knob twiddling on a regular basis. Most of all, you never want to give the user the power to destroy their own amplifier with a thoughtless knob twist. The 6SN7 DC balance will drift ... not by much, but by a few volts. I do not want it controlling the 6V6 bias point, and most of all, I do not want a solid-state servo circuit to control the 6V6 bias point. That circuit will fail sooner or later. One option that was considered was a center-tapped inductor for the 6SN7 plates, with direct coupling to the 6V6 grids. But the performance of the inductor, against expectation, was actually worse than the dedicated transformer, and the transformer completely eliminates DC imbalance at the 6V6 grids. Like all transformers, DC is not getting through. Don and I tried all the more complex options that would give supposedly better operation. They were worse. Removing current-source coloration is non-trivial and difficult in a very transparent amplifier. Bipolar transistors and MOSFETs are audible, even as plate loads. All of the coupling caps were colored, some much worse than others, but they were all colored sounding. (Once you hear capacitor coloration, you cannot unhear it. Just ask Don.) That was a source of great disappointment. The high-value load inductors had their own set of issues, mostly excess stray capacitance that could not be removed. I did not expect the transformer to win, honestly. Don and I tried everything else, and the more complex options were always a step downward. In a zero-feedback circuit, you hear every single part. It’s a dumb truism in audio, but simpler usually does sound better. Not that I’m a fan of 2-stage amplifiers or full-range drivers ... there’s such a thing as too simple. Every designer has to find the balance point between simplicity and complexity. |
No doubt a painstaking balancing act to conquer. Charles |
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