I am finally finished (at least for now) with my new preamp build. I started this project almost three years ago, and have built five different circuits for the tube line stage - the first two being John Broskie's Aikido designs, and the last three being my own designs.
This build is a fully balanced (differential) design. I'm using relay-based input selection and a balanced relay-switched stepped-attenuator. These are implemented using AMB.org delta 1 and 2 boards, but the control is done with an Arduino processor with my own software so I could use a TFT display and custom IR remote control. The code also controls power sequencing and an output trigger for my power amps.
The stepped attenuator is implemented with Vishay Z-foil series resistors and Audio Note silver tantalum shunt resistors.
There are actually two separate line stages - one is opamp based using AMB.org alpha 24 (A24) boards to drive my woofer towers, and the other is my own differential tube design. The A24 boards are powered by a Neurochrome Preamp power supply which provides a very low noise +/- supply.
The tube circuit is powered by independent custom Toroidy transformers for each channel, fed with a Neurochrome Smart Soft Start. The transformer outputs are filtered with a CRCRC circuit which then feeds a shunt regulator based on DIYAudio contributor Salas's SSHV2 circuit and implemented with my own custom PCBs. The shunt regulator voltage is 200V and is set up to supply ~28mA for the tube circuit.
The tube line stage that I finally settled on uses a pair of 6SN7 tubes, each with their sections paralleled, in a differential circuit. The cathodes are loaded with independent cascoded BJT current sources, with a resistor tying the cathodes together. This allows me to optimize the DC balance and reduce the gain a bit (to 13db) to better match the rest of my system. The anodes are loaded with a Lundahl LL1660 push-pull output transformer with a Nanocrystalline core in a 2.25+2.25:1 configuration. This circuit uses no global feedback.
Is that Attenuator in the second picture a really high end attenuator? It is the same that is used in a preamp I'm looking at purchasing.
Brandon - I was reading through this thread and realized that I never answered your question from earlier this year - sorry. My previous preamp uses a Khozmo relay-switched attenuator. This is a nicely design circuit and I would consider it fairly high-end. It doesn't use the most exotic resistors in the signal path, but uses nice sounding ones. Much nicer than the typical Alps conductive plastic potentiometer used in many commercial preamps. I used an Audio Note silver tantalum as the final shunt resistor.
