There shall be 2 flavors, 1 is for those with tube power amps, the other is for those with solid-state amps that desires more tube warmth and smoothness but also requires higher driving strength for the SS amp input stage.
Flavor 1 - two 5687 + two 45 + one 5U4G, 3 outputs preamp
Flavor 2 - two 5687 + 0ne 6X5, 2 outputs preamp
The 1st stage is a mu-stage, having a 5687 (other triodes are possible as well, like 6C45, 5842/WE417, 3A167M, etc) triode loaded with another triode that is configured as constant current source. The upper triode will act as a high impedance load for the lower triode. Such topology is much superior to simple resistor loaded or choke loaded types, with the cons of needing higher B+ and an additional tube in the circuit.
The 2nd stage is just a choke loaded gain stage with a direct-heated-triode like 45 or 2A3. Other choices could be 71A, 6B4G, 31, 01A, 801A and even the 300B! 45 is chosen as it fits nicey in to this application due to the sound character that is considered rather transparent and linear among the direct-heated-triodes. The current requirement to bias the 45 is just nice as well and not too taxing on the power design as very low ripple is needed to be used as preamps.
Some may ask why not just mu-stage the 45 as well? Of course you can! I'm not doing it as mu-staging the 45 will increase the power supply B+ voltage to over 500Vdc and there aren't many 500Vdc+ capacitors that sound that good compared to their normal 450Vdc/500Vdc brothers.
If you build the complete version, you shall get 3 outputs, 1 low and 2 medium output impedance outputs. The low output impedance channel can be used to drive subs as that is the worst sounding among the 3. The 2 medium output channels can be used to drive tube or SS power amps with input impedance of 25k or more, the higher the merrier. If your power amps just happen to have very low input impedance, then use the low output impedance channel, it is not as good as the other 2 but it is still a very good preamp output.
The frequency response of the preamp will be 5Hz - 100kHz @ 3dB. It also depends on the wiring methods and components used. One can copy the design but the results will differ depending on the skills of the builder. None is created equal. Don't blame if your version does not sound good. :)
The next article we shall talk more about ULTIMA in details.
