A DIY Rack Preamp

I build lots of bass widgets from scratch, but this one's based on the ESP preamp board from project 27, here: http://sound.westhost.com/projects-5.htm .

I also used his power supply board, project 05 here: http://sound.westhost.com/projects-4.htm . This has been replaced by project 05A.

The toroidal transformer was scrounged from an old Allen & Heath mixer. I used the best Clarostat and Bourns pots I could find, and a rack case from Sescom. The opamp is a Burr-Brown OPA2132. If you're looking to save money by rolling your own, forget it in this case. I probably have nearly $200 in the project at this point, maybe more. And it's a very simple design with no spiffy extra features. But if you built it into an existing case and have some usable parts kicking around, it could be reasonably cost effective.

One caution: the circuit boards are pretty unforgiving of rework, so you might want to buy two preamp boards and figure on throwing one away by the time you dial in all the values. I managed to do several mods without completely hosing the board, but I did have to do some trace repairs.

Some pictures, sans front panel graphics (which are in process):




How's it sound? I'm liking it as a replacement for the preamp in my AI Focus for fretless BG. I didn't intend it to be a recording preamp, but here's a noodley sample recorded straight to my soundcard, using my Crescent Moon fretless 4, with a Bart preamp and strung w/ T-I Jazz Flats. I'll shoot for a better recording eventually, this one is a bit noisier than it needs to be, mainly due to some issues with the onboard electronics in the bass.

Scary Bass Preamp Clip

Questions, comments, and general derision are welcome, as always.

 

I changed some of the coupling caps, going one standard size (e.g. 1uF to 2.2, etc) bigger, but I left the tone stack alone. It's an impedance-scaled Fender style stack BTW, and the values work out fine for my active fretless basses. You could throw the whole thing into the Tone Stack Calculator and tweak values pretty readily if you care to though. I also played with a few different bright switch options, and may still install a pot to allow fine adjustment. Finally, I doubled the size of the two gain setting resistors to reduce overall gain in each stage, since I don't need as much gain as was designed in as the default.

The power supply also runs power to one of my active basses through the XLR output on the front panel, plus I may want to add a few modules for EFX mixing, and I already had the A&H transformer kicking around. The ESP power supply is actually designed to run off a wall wart though, so your approach would work great if you don't mind using a wall wart. It's easy to breadboard regulator circuits, so you could save some money by doing that as well.

My last solid state DIY preamp was very similar functionally, but it looked really ratty. I have too many upscale gigs these days to mess with that, and I also felt like using all new components for once just for a change. I have a lot of junk parts in my shop, having done repair work for many years, and they make their way into a lot of my projects. My last preamp survived ten years worth of gigs, which is truly amazing given its lineage.

If you used Alpha pots from Mouser instead of the ones I chose, you'd save nearly 50 bucks right there. In a few years you may or may not hear any substantial difference, but in the short term it could very well be negligible. I just figured I'd use all metal film resistors, good grade caps (not audiophile-style ones though), sealed conductive plastic pots, and so on. The B-B opamp I used is maybe overkill too, you could buy an OPA2134 for a good bit less, or downgrade another notch to NE5532 or even the TL072 that the author used.

Hope this helps. I give the ESP design a thumbs-up so far.

 

There's an outfit in Seattle that is part of a German company. They do killer engraving work, but it would cost around $50 for what I need so far, including the panel stock itself. They offer free design software, and off you go. Here's the link: http://www.frontpanelexpress.com/

A local machinist has also offered to do one on spec that I might build more using my own design eventually, so I might let him take a shot at engraving one. I am also lucky enough to have several friends who do graphics design and/or silkscreening, so that's in the frame as well. A certain Talkbasser I'm related to has first shot though, we'll see what he comes up with.

I'm drooling over all the free space you have to work with in your rack box.

I figured I'd better leave lots of room. I have lots of scary plans for developing some of that real estate.

 

Heh, thanks for the SPAM Mal.

I have a standard Jensen DI transformer sitting here, guess maybe I'll throw it into the box. I'm also thinking about using a different model that would let me do a post EQ transformer-balanced output at line level though.

There's an Alembic F1X nearby that we can try out pretty easily, I'll work on snagging it for a test spin. I've played through it; it's a nice piece, as you'd expect. Mal just got an old blackface Bassman head, which definitely has that Fender mojo he loves so much as well. I had a great time playing through that amp, but we're both finding that it likes old school speakers better than the ones we're both using on gigs these days. My new pre was built specifically with the Schroeder 1210 in mind, and so far I'm pretty happy with the results. The pre works well with my Bart soapbars in passive mode too, which is a bonus I hadn't figured on.

Anyway, I thought I'd post a few test results for the preamp, done with Rightmark analysis software, which is freeware. I used an EMU 1820M sound card for the tests.The frequency response figures reflect the different tone control settings I used for the tests.

 

Necro bump, as I've recently re-done this one, and Rod Elliot has also released a cool updated version of his circuit board. This is a great first timer's DIY bass preamp build, IMHO. I ended up gigging regularly with the original version for a few years before moving on to the other builds you can find in the Amps FAQ. Recently I've added a new discrete FET front end, using a design another TB'er is working on for his own preamp design. Hopefully he'll post his own thread when that gets a little further along.



So here are revised test specs for the updated version:

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The new front end adds a bit more headroom and character to an already nice sounding circuit. Rod's new board looks like it offers some worthwhile improvements as well though.

 

Sure could Dave. The pictured power supply is way overkill and a wall wart could be used easily enough if one wants to avoid plumbing line voltage into their build. For more advanced builders rolling your own power supply is generally worthwhile though.

Originally I was contemplating adding one of my high pass filter/parametric EQ boards too. Feature creep, the DIY'ers' curse!

 

Rod added another opamp and restructured the last stage a bit, but IIRC you have to buy his board to see that schematic. I omitted the bipolar transistor in my build, but have since come to realize that buffering post-master volume is generally best practice.

As I alluded to in post #5 a zillion years ago, I changed the values for the coupling caps and the feedback ones on the noninverting inputs. I did reduce gain in both stages a bit, and I also messed with the brightness circuit a lot, going from a little edgy at all times to now a little more relaxed. I think I left the tone stack alone, but the next time I open this thing up I'll try to document everything I changed. I can run frequency response curve tests again too -- doesn't look like I ever posted them in this thread the first time. I can also throw the whole circuit into an LTspice model and run tone stack tweaks for you if that'd be helpful.