Posted on Feb 5, 2014

This is a clean preamp that I designed to use with my bass, a power amp, and a cab at a rehearsal space. (Rehearsal space being a fancy name for the singers basement. ) I am very proud of this preamp since, unlike the mu-preamp, I designed it from the ground up. Although there is nothing special about the design, at least it is all mine. The desig

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n started as an all transistor design using only MPSA18s. ( Original design ) The original design had three stages: input buffer, first gain stage, second gain stage. I did not design the input buffer, I got it from the Acoustic Control 360 Bass Preamp schematic. I breadboarded the original design and it worked. Then I realized that a FET could provide both the high input impedance and the first gain stage. So I switched to the current design. And got rid of the part I did not design in the process. You may notice that the every stage has paired transistors. The second transistor acts as a buffer. For the first stage, it provides a high impedance to the FET to maximize its gain. For the second stage, the buffer is required to provide a nice low output impedance for the power amp. The power amp I am using has a 10K input impedance. I measured the output impedance of the NPN gain stage without the buffer at about 3. 5K. With the buffer it is about 16 ohms. Much better. The first stage is designed for a gain of 2. 5 and an Ids of about 1mA. The second stage is designed for a gain of 4 and an Ic of 5mA. This provides a gain of 10 which is 20dB. I chose a 24V supply voltage because that is the largest wall wart style supply that is easy to get. The 24V supply I used produces close to 32V at the low current used by the preamp. I used an LM7824 to regulate the voltage to 24V.

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