Asymptot Σ  |  DualPath Valve Microphone/DI Preamplifier

The philosophy of the Asymptot Σ is to combine a classic valve sound with and a 'one-knob' set and forget style, that let you stay focus on the performance and not on settings and metering. Despite the fact that this preamplifier use tubes as active amplification devices, you will not find the sound of the Asymptot Σ in any other commercial products on the market today. Why ? Because we decide to use a particular input tube (12SN7) which is not famous for beeing used in microphone preamplifiers... This med-µ tube give us some very interesting design possibilities, and here is what came out !

What is the DualPath ?
The Asymptot Σ give you the opportunity to choose between two differents configurations (Phat and Cool), which affects all aspects of the preamplifier : sound, headroom, distortion, gain, etc...
- Phat Mode : In this mode, the preamplifier exhibit more harmonic distortion and less headroom than in Cool Mode but got more gain and give a sound that seems larger with a sweet but colorfull high-end.
- Cool Mode : In this mode, there's some subtle differences, mainly the sound is more natural, flat and it's almost IMPOSSIBLE to overload the preamp due to a huge headroom, at the cost of less gain. Note that the DI Input is only active when the preamp is in Cool Mode, there's no DI/Hi-Z in Phat Mode (but you can choose the opposite).

As you can see the front panel is kept very simple and there's no need to explain most of the controls. The only unusal switch you can find on this preamplifier is the 'High Voltage' one. This switch let you put the full High Voltage on the circuit or keep it to a small amount (around 100v) to preserve tubes life. I decided to not switch from zero to full High Voltage for two reasons. The first reason is to avoid the grid current to flow through the input transformers and magnetise them when there's no High Voltage on the heated 12SN7. The second reason is that the tube will become stable faster if there's a smaller difference between the Safe 'Low Voltage' Mode and the Normal use mode.

The signal path is also kept to the minimum :
Input Transformer > Input Tube > Capacitor > Potentiometer > Output Tube > Capacitor... and that's all.
Yes there's no Output Transformer and the signal is kept unbalanced. Why ? Because Output Transformers are not really necessary with this kind of preamplifier for an Home-Studio with short wires between equipments, so why add a component in the signal path which is not really necessary ? Yes it can introduce a pleasant 'transformer sound', but a good Output transformer is very expensive and 99% of the time you will like a more neutral sound better. I like to 'tune' the color of a gear where there's NO possibility to avoid coloration, so we choose the flavor we prefer as a compromise in this places.

No PCB !
No i'm not a PCB hater... i like to use them where they make sense. And precisely in the Asymptot Σ using a PCB would be a bad idea. This design is really 'simple' and it's a better use of time here to take it to solder components, than to design, etch & drill a PCB. You can build something really strong that will last for decades with a precise and intelligent hard wiring. It's easier to test too, and easier to troubleshoot and adjust, So forget any PCB for this project and just follow the guidelines to design it in point-to-point the right way.