Hi everyone. For my latest Project Berocca video I put together a simple 2-band preamp as a project you can build yourself...
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DIY onboard bass preamp...
- Thread starter RobbieK
- Start date
Hi robbie...thanks for the great video and circuit...still got my spector pre sitting here ...Thanks again for your time and advice... could be a close cousin to the one you're replacing ...also just wondered ...would this new circuit work with EMG bass pickups as they are active and some pre amps i see won't work with active pickups....Thanks again
Yep this will work well with active pickups. For optimal performance, especially if you use a very low current (noisy) opamp, I'd lower the input impedance. In Project Berocca I've lowered the input impedance to 220K (by leaving out R1 and setting R2 to 220K) and also lifted the coupling cap (C1) to 470n. This is because the signal source is coming from the active blend control. But really, with a good opamp, the difference is negligible.
This was / is a criminally underrated thread post. Thanks so much for laying everything out for this Robbie! I am going to give building this a go. I have a couple of things I want to try with the circuit, but I'm quite experienced enough yet to know exactly how to alter the design to get the results I want. I am planning on starting a fresh thread to get some input on the things I want to try. Either way, thanks for all of this info!
A lump of putty to hold the components while soldering....brilliant!!!
I just built this as my first ever electronics project. It’s a great way to start learning. Installed it in my P Bass. Had to do a little router work. Love it , works great.
Be careful though or you may end up like me and falling down and endless rabbit hole. I’m now working on several pedal projects. My world is forever changed with blue tak and stripboard.
Be careful though or you may end up like me and falling down and endless rabbit hole. I’m now working on several pedal projects. My world is forever changed with blue tak and stripboard.
I built this and it has a nice transparent sound but is there anyway to increase the gain by adding a component somewhere?
Thanks mate, I'm glad you like the project. For more overall gain, you can add two resistors to the first opamp stage. One resistor (Ra) is a feedback resistor, between pins 1 and 2. The second resistor (Rb) has to connect pin 2 to the Vref.
Ra can stand on end in place of the original wire jumper, and if you make the board an extra row wider, you could easily add Rb to the left of the opamp, laying flat on the board.
The formula is very simple. The voltage gain is equal to (Ra + Rb) / Rb. So for a gain of 2 (6dB), you could use two resistors the same value, say 10k. (10+10) / 10 =2. For a gain of 1.4 (3dB), Ra could be 10k, and Rb would be 22k. For 9dB (gain of 2.8), Ra could be 18k, Rb would be 10k. For 12dB (gain of 4), Ra could be 30k, Rb would be 10k. Etc etc...
You don't get anything without consequences though. Since this first stage has quite a high input impedance, adding more than a few dB of gain will raise the noise floor, and if you boost the treble, you may hear some hiss. You could use a high end fancy low noise opamp chip of course, but these will have a higher current consumption, and you reduce your battery life.
If you lower the input impedance this will help as well. If you are using this with active pickups (like EMGs), it could be as low as 50k. Or if you are using it with say a single music man style pickup wired in parallel, and directly to the pre, I'd run the input z at 100k, and have a 10k or 25k volume pot wired on the output.
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Cool, but I'd recommend a fixed resistor in series with the trim pot if you are going to have it adjustable, with the trim pot as Rb.
Otherwise, when the pot's resistance is zero, the gain will shoot right up to whatever the opamp can handle. Also, all the adjustment will be in a small part of the sweep.
If you want to have adjustment, I'd recommend putting the pot in the feedback loop instead. With a fixed 10K resistor as Rb, and a 20k trimmer as Ra, the gain will go fairly smoothly from 0dB to around 10dB, and with 6dB in the centre.
FWIW, it's also usual practice to wire a pot in this situation with the wiper and clockwise lug joined. This way if the trimpot wore out or was faulty, the pre would be limited to that 10(ish)dB.
Otherwise, when the pot's resistance is zero, the gain will shoot right up to whatever the opamp can handle. Also, all the adjustment will be in a small part of the sweep.
If you want to have adjustment, I'd recommend putting the pot in the feedback loop instead. With a fixed 10K resistor as Rb, and a 20k trimmer as Ra, the gain will go fairly smoothly from 0dB to around 10dB, and with 6dB in the centre.
FWIW, it's also usual practice to wire a pot in this situation with the wiper and clockwise lug joined. This way if the trimpot wore out or was faulty, the pre would be limited to that 10(ish)dB.
Nice and interesting preamp which can be adjusted to the player needs. Built this and like how it sounds, but wanna try some other values as well.
I'm also comparing the schematics with some Baxandall passive tone controls, and it's not exactly the same.
Just for the reference, James tone control:
And Passive Baxandall tone control:
The treble part looks a little bit different in the Berocca preamp (kinda mixed from both circuits). Also, not sure what C6 does and can it be omitted (updated – looks like it needs to be here to stop possible oscillation). Just wondering if it is possible to reduce the amount of parts since I'm going to use this in a pretty tight space. Probably need to simulate this in LTSpice. Probably, voltage regulator could save some space comparing to voltage divider.
I'm also comparing the schematics with some Baxandall passive tone controls, and it's not exactly the same.
Just for the reference, James tone control:
And Passive Baxandall tone control:
The treble part looks a little bit different in the Berocca preamp (kinda mixed from both circuits). Also, not sure what C6 does and can it be omitted (updated – looks like it needs to be here to stop possible oscillation). Just wondering if it is possible to reduce the amount of parts since I'm going to use this in a pretty tight space. Probably need to simulate this in LTSpice. Probably, voltage regulator could save some space comparing to voltage divider.
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Can you please make a simple diagram for this situation? What pot should i use in the feedback loop? Thanks!
I have compared both schematics, and honestly I do not see significant difference in the frequency response, whether one uses a dual-cap topology or a single-cap topology, for either bass or the treble side of the filter. The nominal values of the caps need to be adjusted, obviously.
After building this preamp with centre of frequency at 3kHz and 90Hz (using the caps values in the .pdf) i notice that it has much more noise then any other preamp i build. I'm currently using a NE5532 IC. I'm recording it directly from the audio interface, it became worst in the amp. I used a 10k trimpot and a 10k fixed resistor since for me 6db are more than enough. Other than this hiss it works really well. How can i reduce this noise?
NE5532 is not a good choice in circuits needing a high input impedance, as you'll end up with a lot more noise that the 5532 is capable of when driven from a low impedance source. For high impedance applications without excess noise, you need to look for an opamp with low input bias current (in the picoamps to low single digit nanoamps). You have three choices for this: JFET input, CMOS, or superbeta. I settled on OPA2196 in my bass preamps, but it's surface mount only. In thru hole (PDIP8), I'd suggest TLE2022.
In general with op amps, you can have low noise, low power and low cost -- but you can only choose two
