An oldie but a goodie.
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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The tricky bit using regulators is bandwidth. The output of any regulator has a certain impedance. What you are looking for is a linear impedance curve across the entire audio band. The tube regulators often had a problem with this; the output impedance rises when the regulator meets its bandwidth limit. At this frequency the regulator has to be bypassed with a capacitor that keeps the output impedance as linear as possible. It won’t be perfect- usually you wind up with a step in the output impedance at the crossover point. Too much capacitance can cause the regulator to run hotter and with tubes, you can have reliability problems that might occur when the capacitance is charged during warmup. Solid state regulators often have much wider bandwidth so don’t usually need so much bypassing, but they can have stability problems so are often bypassed with a small capacitance at the output to prevent it oscillating. Adding additional capacitance to the output of the regulator will do little to improve sound quality and if enough capacitance is added, will increase the heat of the regulator and could threaten its reliability. If the regulator is properly bypassed and operated well within its limits then they will tend to be neutral. To help the regulator along, its a good idea to do as much as you can to minimize noise at the input of the regulator. For example a PI network is helpful to reduce the amplitude of the sawtooth waveform at the regulator’s input; this will reduce the work the regulator has to do, which can reduce its operating temperature. |
@lynn_olson - what are your thoughts on using a balanced input stage to drive a single ended 300B? My front end hardware is all balanced and I like using balanced connections. I’m thinking an LTP with a CCS on the cathodes and a push/pull interstage on the anodes. Output of the IT driving a single 300B with fixed bias connected to the other side of the IT secondary. I haven’t decided which tubes to use for the input stage, but was thinking about triode strapped D3As. I have a nice pair of Monolith Magnetics SX-11 output transformers that I want to use. |
@jaytor You’ll want as much gain out of the voltage amplifier as you can get, since the more gain also yields better Common Mode Rejection Ratio. But you’ll need a good Constant Current Source to optimize the gain stage. The D3A, triode strapped, will give you a good mu value to work with, although you could get that with a single 12AT7 and be in the same ballpark. A 12AT7 will allow for plenty of bandwidth. If you use a driver transformer as a plate load, the issue you’ll be up against is imbalance of plate current between the tube sections. The better your CCS the less you’ll have this problem, and matching is a very good idea. The more current that isn’t cancelled in the magnetic core you can also expect greater distortion. So you can see getting the plates to have equal plate voltage is important. To this end, using a balance pot in the cathode circuit to balance the plate currents, while effective, has the effect of also reducing the differential effect, increasing distortion and reducing gain and bandwidth. So matched tube sections and a really effective CCS are paramount. One thing to consider is its really impossible to get a perfect center tap in a transformer. Its always going to be a little off. Since its often difficult to get really well matched tube sections (they should be matched on a curve tracer for best results), I prefer to use RC coupling to a cathode follower, which is in turn direct coupled to the grid of the power tube. The power tube would thus obtain bias from the driver tube. If the driver tube gets weak, the power tube will conduct less also, preventing damage. I explained this topology earlier in this thread. If you are considering using a differential amplifier as the input voltage amplifier, you really should consider a bi-polar power supply of equal B+ and B-. This will improve the differential effect and the effectiveness of the CCS. If you have such a power supply then you have a good way of setting up that cathode follower I mentioned. If you go this route, care must be taken to make sure the plate of the driver tube is well bypassed so even at full output there is no noise, no artifact caused by the signal on that plate. This will really help the amp assume greater authority. |
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