4 band onboard preamp development - First Project

[matmonster58]

A friend of mine has asked for a 4 band preamp for his bass. I've built a good number of pedals, but really haven't designed much myself. I'm taking on the project as a learning experience and will be documenting my progress here, mostly as just a log for myself but also to get feedback from the community. I am mostly looking to learn how to use Kicad and design pcbs.

Here is my preliminary design (pdf attached for better clarity). LTspice is what I use for school so it's what I'm using for my mockups


I designed the input stage after the EQD Arrows boost. It's not really a true buffer, but I like the sound of that circuit so I figured I'd put it at the beginning of my circuit. The EQ stage is taken from the preamp of a GK 800rb amp. I started with the EQ from that amp then tweaked components until I got a response I liked.

This is not a flat preamp, it has a bit of a 'preshape' that I think will sound nice. It also has a slight volume boost which I don't mind either. I could make a more neutral preamp, but I want to make something interesting that you can't buy off the shelf.

The eq bands result in +-10db at 30hz (very low but acting more as a low shelf) , 450hz, 1100hz, 10k+-ish (again more of a high shelf).

Since it is going to be an onbaord preamp, small size is a priority. Everything is going to be done SMD

If any one has feedback on things I could improve, components I could simplify or remove, or anything else, I'd love to heard it.

 

[Nostradoomus]

I don't have a lot to add but you may want to use a Quad op amp instead of 4 OP07s

 

[Passinwind]

Best practice is to include a series resistor just in front of the output cap, something in the 200R-1k ohm range is pretty standard. This stabilizes cable capacitance drive issues. You may want to go a bit higher on value on the voltage divider resistors too, I generally go with 220k for my battery powered onboard designs. I just recently tried OPA205 and OPA 2205 for a battery powered EQ pedal and highly recommend them. OPA145/OPA 2145 are fairly frugal on current and very good sounding as well. You might want to include 100nF bypass caps right at all your opamp power pins, X7R ceramics work well for that.

And then you might consider consolidating the Bass and Treble sections differently. I do a B/T module like that (i.e. standard cookbook opamp based Baxandall stack) and then separate single band midrange ones, or sometimes Bass and Low Mids in the first stage then High Mids and Treble in separate modules. You can see the different interactivities pretty readily in LTspice sims and even hear them if you run WAV files through your model.

Some of these ideas can be seen on my open source GitHib repository: https://github.com/Passinwind/PW3B-LPF/blob/main/V4 Schematics and BOMs.md . There's a link to my basic B/T or B/M board there, with Bass?Mids shown for that particular pedal implementation. One can download the Gerbers if they don't want to order a set through OSHpark, but in your case I'd much rather see you do your own thing.

 

[matmonster58]

I don't have a lot to add but you may want to use a Quad op amp instead of 4 OP07s

I'll be using a quad op amp , the op7s are just the generic op amp model in ltspice

 

[matmonster58]

Thank you so much @Passinwind ! I've spent many hours reading your preamp thread. Its definitely inspired me to try designing stuff myself.

I'm nearly through an Electrical Engineering degree. I've learned a lot of math and theory in the past couple years, but now I need to learn some more practical skills

 

[Passinwind]

Thank you so much @Passinwind ! I've spent many hours reading your preamp thread. Its definitely inspired me to try designing stuff myself.

I'm nearly through an Electrical Engineering degree. I've learned a lot of math and theory in the past couple years, but now I need to learn some more practical skills

I misspoke a bit last night, I've now revised the bit about the bass/treble section. It sometimes helps to have the treble section first so that it has less noise in front of it to get boosted, but as usual it comes to down to managing your trade sets. You can do noise analysis in LPspice but I've found actual measurements to be more instructive, since board layout and lead dress are also big factors.

Battery powered widgets are an interesting challenge and it's so nice to finally have more decent options for good sounding .low current opamps. My most recent battery powered EQ pedal is my new Swiss Army knife in the gig bag, I've been very happy with it so far.

 

[matmonster58]

Working on v1.1
I've implemented most of passinwinds recommendations and adjusted the frequency bands a little bit to get closer to what I want. I flattened out the eq "preshape" to be slightly flatter and the output is now closer to unity.

I kept the bass and treble section the same format as previously because I really like the shelfing response I'm getting from the treble control.

The bass control is a little lower than I'd like but it is fairly wide and interacts well with the low mids. I personally don't like much low end in my tone, but I'm not building this for me. I think my friend will enjoy the extra rumble


I've also reduced the amount of unique components. If components were similar in value and could be changed without much impact on performance, I made them the same. This will make the ordering and building process easier for me. There's just much less to keep track of now.