Single Channel 601 (high end!!)
These are single channel preamps based on the Ampex 601. They have UTC HA-100x input transformers with 5-way input impedance switches, dual (selectable) output transformers, 2 tube / 3 tube option, remote power supply with phantom power, DC tube heaters, Line / DI input, and rack ears. The audio capacitors are Teflon, polypropylene, and polystyrene. The pots are Alps and Bourns conductive plastic pots. I use precision metal film resistors, silver plated hookup wire, silver solder, and high quality Nichicon and Panasonic electrolytic capacitors.
The pad on this unit was an after thought and an experiment. It is placed AFTER the input transformer. With the HA-100x transformer, there is very little risk that one will overload the input transformer with a mic. Combine this fact with the complexity of a multi impedance mic pad, and putting the pad after the transformer seemed like a good idea at the time. Unfortunately this high impedance pad colors the sound, which the customer thought was a happy accident. It is a darker sound and the transformer is being driven harder. It isn't something I plan to include on future builds. If a pad is needed, I recommend an in-line pad compatible with one of the impedances available.
You can hear me flip the pad switch about halfway through the example:
Nils 601 color-pad
The Hi-Z INSTR / line input accepts instruments like electric guitars and basses as well as line level signals. It has input impedance of 250k Ohms. One can also use a matching transformer and go in through the mic input for more gain if you want to overdrive it. In the example below, the lead guitar was recorded this way. The bass and rhythm guitars were recorded via the INSTR input. The preamp was feeding my A/D converter and recorded into my DAW.
This example file is a real song. A customer sent me a "checking out my new preamp" song he recorded with his new 9er preamp. (a rebuilt 403) He recorded piano, voice, and bass. The bass was recorded with the DI input. I had asked him if he was happy with the quietness of the preamp. He answered my question in the lyric - hehehe. I loved the style and humor in this song and could not resist adding some tracks. I used these preamps during testing to record Lisa's vocals, acoustic guitar, drums, and sax.
Yes You Are
by Paul Christian
I've kept the gain structure and tube circuits close to the original Ampex 601. I believe that certain vintage devices are discovered and loved because they were happy accidents that sound good. "There are as many ways to design a tube preamp as there are designers."** A lot of people will operate certain pieces of gear wide open, like power amps, for most headroom. That will be excessive and noisy with the 601 preamp. The master gain control's nominal position is 50%. Even with a vintage passive ribbon, you will rarely need to raise the gain above 60% or so, but it is there if you need it. Lowering the gain allows for coloration, tube compression and all the way to fuzzy tube distortion. Opening the gain increases the headroom.