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How to add a gain trimpot to this onboard bass preamp?
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difesa98 1 month ago
<span class="colour" style="color: rgb(51, 51, 51);">I'm currently trying to replicate this Yamaha BB5000A preamp but i wanted to add a gain trimpot since it was designed to be paired with active pickups. Active pickups have much more output and they are quite rare to be used in a bass. I was wondering if it's possible to add a gain trimpot to make it a "passive pickups friendly preamp". It's a 2 band preamp that uses 30k linear pots, since it's quite difficult to find this value, is it possible to change them with a more traditional 50k linear pots? It's a single layer PCB and this is my first time trying to trace. Once it's all done i will rearrange the geometric disposition of the components to make it smaller and double layer. I add a square pad which is the ground (to avoid soldering a cable direclty to the bass pot). B stands for "bass", T for "treble". I don't own this preamp, a friend of mine lend my his Yamaha.</span><br> <br> <span class="colour" style="color: rgb(51, 51, 51);">Here it is my project:</span> [https://oshwlab.com/difesa98/yamaha-bb5000a](https://oshwlab.com/difesa98/yamaha-bb5000a) (you can find all the value in the excel file)
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difesa98 1 month ago
@andyfierman [https://www.talkbass.com/threads/in-need-of-a-30k-pot-vintage-yamaha-bb5000a-active-2-band-preamp.1278945/#post-26471881](https://www.talkbass.com/threads/in-need-of-a-30k-pot-vintage-yamaha-bb5000a-active-2-band-preamp.1278945/#post-26471881)
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difesa98 1 month ago
@andyfierman First of all thank you for your patience! Breda & Co. is a fantasy name (it's the name of my city), i will also change the name of the project. just this morning i had a chat with this guy: [https://www.youtube.com/watch?v=xnGYsR1HVLo](https://www.youtube.com/watch?v=xnGYsR1HVLo)  [https://www.youtube.com/watch?v=-J8O16oM-tk](https://www.youtube.com/watch?v=-J8O16oM-tk) because he simply swapped the original preamp with a Glockenklang an Ibanez and an Artec and he claim not to have any issue in terms of too much gain in all cases. Probably the fact is that these active pickups have similar performance to normal passive pickup so any preamp should works. Probably the original preamp could work flawless with passive pickups. I'm interested in your analysis of the change of the frequency using B25K pots intead of the B30K (maybe could be an idea using a trimmer for an adjustable gain just to be sure it will be compatible with every pickups).
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andyfierman 1 month ago
@difesa98, Some points to note. 1. The front end the Aguilar pre-amp is a unity gain buffer. Therefore it does not increase the gain of the signal path. 2. Reducing the pots from 30k to 25k will increase the frequencies at which the treble and bass tone controls operate by a factor of 1.2 (30k/25k). They will also change the overall gain by around the same amount. The exact changes to the gain and frequency response are best analysed by simulating the preamp. Then you have to decide it they would be acceptable or if you need to alter some other component values to correct for the changes. You can do this in EasyEDA (or I can do this for you but I would charge a fee for that service). 3. It may be possible to adapt the first opamp stage to provide an adjustable gain. This is fairly simple to work out but again a simulation would help to verify the performance.  4. More information on the gain required to adapt for a passive pickup is required.  5. Your schematic includes a Title of Yamaha BB5000A and a Company name of Breda & Co. I hope you realise that you are publicly announcing that you have breached Yamaha's copyright (I assume that this is also why you reset your aguilar clone project to private).
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difesa98 1 month ago
@difesa98 After some digging i finally found a B25K with center detent!! hopefully this value should be a direct replacement to the original B30K
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difesa98 1 month ago
@difesa98 Here it is the first stage of the aguilar: after this part the circuit starts with C3 which is a 1uF standard capacitor and the Yamaha starts with C8, a 1uF electrolytic capacitor (this was the thing that make me think it could works). Maybe should i insert somewhere an anti-inversion diode just like this Aguilar? If yes, where exactly? thanks for any help! :)![Aguilar.png](//image.easyeda.com/pullimage/GqaXc9BFxzKSt8feJuv7XMuR5H2HBo8KQddxT2CQ.png)
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difesa98 1 month ago
Ok, i make the schematic out of that PCB. Probably it's not much clean but this is my first time :) I made a new PCB out of my schematic to verify that every net is just like the original and it seems to be all right. I have updated the photos in the project highlighting every components to make a trace control much easier. The original preamp does not have a pad for ground, il was grounded using an external cable to the back of the bass pot (the one that is PCB mounted) so i added a ground pad (the square one). For boosting the gain i thought that could be an idea using the first stage of the aguilar preamp clone that i have already build which include a j113 transistor, could it work? what about the potentiometer? i found that B25K it's not that hard to find instead of the original B30K but if it could be modified to accept B50K it would be significantly better because i can find these pot with a center click. <br>  ![New.png](//image.easyeda.com/pullimage/Z2VPoxHTsZ8nhqkpf1ti9PHgyUPunq17kqojagbh.png)
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difesa98 1 month ago
Oh sorry, i think there could be a simple solution like this one but for gain and not for volume: [https://www.youtube.com/watch?v=dyfFHO2XeKY](https://www.youtube.com/watch?v=dyfFHO2XeKY) I will try to make a schematic out of that pcb :)
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andyfierman 1 month ago
You can't seriously expect someone to advise on this without you having first put the effort into reverse engineering the PCB into a schematic. Please read and act upon:  [https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a](https://easyeda.com/forum/topic/How-to-ask-for-help-and-get-an-answer-71b17a40d15442349eaecbfae083e46a)<br> <br>
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