Description
A few years ago, I decided to design my own 1U rack preamp. My goal was to design a simple solid state preamp that can get tones similar to Steve Harris and Geezer Buttler (Paranoid album). The eq and filtering sections can be modified however you see fit. I should also point out that this design has been tested and abused with the following gear: Fender Basses, QSC RMX1850HD Stereo Power Amplifier
, Fender BXR 400 (power amp), and Avatar B212 neo and non-neo cabinets. With some component tweaking, this design can be exactly what you are looking for.
Photos
Features and Specs
Front (left to right):
- Passive and Active input jacks
- Gain Knob
- Volume Knob
- Mute Switch
- Mid Cut Switch
- Low
- Low Mid
- High Mid
- High
- Power Switch
- Superbright blue LED
Rear (left to right) :
- Send/Tuner (buffered)
- Return
- Main-out Un-balanced (~22dB maximum output)
- Balanced TRS output with level control and ground lift
Schematic and PCB Files
FrankenP BP-1 v1.0.3 Schematic.pdf
BP 1.0.3 PCB Files.zip -> View Gerber .gbr files for free with Gerbv.
Operation
Input: Passive basses are connected to the preamp via J1. Active basses can be connected to the preamp via J2. The signal connected to J2 is attenuated by -6dB due to the voltage divider between R7 and R15. C10 helps to filter off unwanted RF from your signal. C15 provides input coupling. R39 is 1MOhm and could probably be omitted for an even higher input impedance. Opamp U1A is configured as a high impedance buffer to insure that your signal is not loaded in any way. Opamp U1B is used to set the preamp’s input gain.
Twin T Notch Filter: Switch J4 is used to cut the mids from the signal. The twin T filter adds punch to your sound by aggressively attenuating the mids around 500Hz. U2A is configured as a buffer to connect the passive filtering to the active eq section.
4 Band Eq: The eq circuit consists of 4 active bandpass filters. They are configured as follows:
Low control -> fc = 100Hz, A = 2, Q = 1.4
Low mid control -> fc = 500Hz, A = 1, Q = 1.4
High mid control -> fc = 2.5kHz, A = 1, Q = 1.4
High control -> fc = 5kHz, A = 2, Q = 1.4
The eq section is wired so that the center position of the knob provides a flat response. Turning the knob clockwise provides 12dB of gain. Turning counter-clockwise provides -12dB of attenuation.
Volume/Mute: The volume control is the master volume control for the preamp. J3 is used to mute the signal to both outputs. The signal still appears on the send jack so it is possible to quietly tune while the mute switch is engaged.
Send/Return: These jacks provide a patch point for unbalanced (mono) effects. It can also be used to provide a mono signal to a recording or sound system. A tuner can be plugged into the send jack.
Outputs: The preamp has two buffered outputs. One balanced and one unbalanced. J7 is an unbalanced (mono) output used to connect the preamp to a poweramp. U10 is a balanced TRS connection for connecting to a recording or sound system. R38 and R41 convert the signal to line level. R41 is used to adjust this level. U5 provides a transformer-less balanced output. Switch J8 can be used to safely lift the ground on the TRS output to eliminate ground loop hum.
Important Note on Ground Loops: All jacks are plastic and therefore isolated from the chassis. AC ground is connected to the preamp’s chassis; however this ground is not connected to signal ground within the preamp. These grounds are typically tied together inside your power amp. Connecting signal ground to earth ground in both devices will cause an undesired ground hum.
Graphs
The graphs below shows the preamp’s frequency response with different control settings.
Audio Samples
FrankenP BP-1 Sample.wav – Recorded with a Fender bass with a Duncan Quarter Pound Precision and Fender Noiseless Jazz Pickups.
Useful Links
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Design Links:
Items/Services I purchased:
I’d love to hear your thoughts. What would you change ? Which features would you add or subtract ?
. Instead of reusing the corroded crimp connectors, I soldered the wires directly to the speaker. I replaced the cheap knobs with Boss style knobs.
I bought from amazon is completely silent! 
