IntroductionThe EMF12PA is a high performance pre-amplifier. The design uses three ECC83's (12AX7's). The circuit is inspired by the famous Marantz 7C pre-amplifier, but there are few similarities between the Marantz 7C and the EMF12PA. The EMF12PA is quite simple to build and the original is built on a PCB, but it can be built using point-to-point wiring as well. A simple passive power supply will do the job for those of you who wants to build a replica, but the original pre-amplifier uses a slightly more sophisticated power supply in order to get the last drops of performance out of it. The EMF12PA uses large amounts of negative feedback, so this project might not suite all, especially those of you who thinks that negative feedback is a true tool of the devil. The circuitThe input stage is a high impedance voltage amplifier. It's pretty much a bread and butter setup. |
The input stage has a bypass capacitor placed in a rather strange place, but that is to prevent HF oscillation, since the circuit has a bandwidth that is basically DC to daylight. Second stage has the same gain as the first stage. Nothing out of the ordinary there either. The output stage consists of a cathode follower. This gives the EMF12PA sufficient drive capability so that long line-level cables or hard to drive power amplifiers can be used.
PerformanceValve complement: ECC83 (12AX7) All measurements were done with a Tektronix VM700T. |
