It seems there is some demand for a Schiit all tube phono preamp. As most of you know, we already offer the Mani as our first phono entry. It offers variable gain settings, and two different input impedance settings. There are a variety of cartridge options with gains that can vary significantly. This makes the Mani suitable for a variety of those cartridges, either moving coil or moving magnet. Such are a benefit of solid state design – an ease of phono gain design variability.
Tubes have real world constraints which impact costs and features. The first is an unsuitability for all but the highest output moving coil cartridges. In 1978, I introduced the first ever what was referred to at the time “pre-pre amp” made with tubes. It used Western Electric 417A tubes which were just adequate for moving coil cartridges in a noise sense. The problem was microphonics, which were such that a mosquito beating its wings within 50 or so feet were intolerable. The tubes were chosen as a result of evaluating dozens of tube types by manufactures in the USA, Europe, Japan, and Russia. This eventually led me to the conclusion that middle to lower output moving coil cartridges and tubes are mutually exclusive.
The next issue is gain – this is due to my philosophy of no feedback vacuum tube design. No feedback dictates the use of low distortion tubes at a fixed operating point (bias). The gain ends at a level for moving magnet (common, ubiquitous) cartridges (the majority of my favorites). The only way to provide variable gain is to add another gain stage and then passively step it down. As a lifelong minimalist, the only better thing than an extra very good gain stage is no extra gain stage at all.
The next constraint is no tube rolling outside of the 6DJ8/6922/7308 family. This is for several reasons. First, the no feedback operating point limits the tube installed. Outside of the designed-in tube family, the varying family of curves will cause gain to vary, and distortion to increase. Because a phono preamp starts with signals in the 1 millivolt or even lower, it requires the filament supply to be regulated, to minimize hum. If the current increases significantly over the regulated spec, the hum and heat will do the same. Finally, the RIAA equalized output of the preamp will vary significantly. That is because the source impedance of the tube is a component of the RIAA equalization in a passive design – which every preamp I have ever built features. One design parameter that is very important is the deviation from the required RIAA equalization. The majority of the competition at any price point has variations of 1-3 db being average. The Mani is within 0.2 db either way. The majority of feedback RIAA eq tube designs tend to have significant bumps up at the bottom end, which could give rise to “tubby bass” reputations of tube designs.
Finally we get to cost. The highest quality resistors for the tube preamp are 3-7X the cost for a Mani. Film and foil polypropylene caps at tube voltages are not cheap, and the 20 times power design is much more expensive, not to mention the regulation. The metal for the enclosure will also be at least 6x. The net result is that such a proposed design will be 4 to 5 times the price of a Mani.
Given the above info, what say you all?
Just a personal taste thing.
