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DIY bass preamp/3 band EQ options
Old 30th May 2019
  #1
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DIY bass preamp/3 band EQ options

The time has come where I've decided to strip the electronics from my bass and replace them with something different. I've already settled on everything that will be going in however I can't decide what preamp/3 band EQ I should build. I've played through a friend's bass with a Darkglass tone capsule installed and to my ears it sounded fantastic - I like the EQ's frequencies (70hz, 500hz, 2.8khz) however it is out of my budget. There are a few DIY designs I have in mind but I can't find a calculator that calculates which capacitors to use for a specific frequency in order to achieve the same/similar as the Darkglass. Can anyone recommend me some DIY options/alternatives? Thanks!
Old 31st May 2019
  #2
Gear Addict
The easiest way to do this is to implement it in two stages, a standard Baxandall for the bass and treble controls and a separate stage for the mid control. It's just one extra op amp section, which costs peanuts, and you minimize interaction between the stages, which is always significant in a single stage circuit but more so in this case with such low treble frequency.

It would be nice to have frequency response plots for the Darkglass to see what slopes and Q they use, but assuming more or less standard values, the attached circuit will match its +/-12dB specs at each frequency. It uses 50k pots and easy to find R and C values. I didn't use any formulas, just tweaked it in LTSpice from simulations I already had. You will also need an input buffer and some extra stuff for 9V battery operation, I can add those if you're interested.
Attached Thumbnails
DIY bass preamp/3 band EQ options-maverick.png   DIY bass preamp/3 band EQ options-maverick-bt.png   DIY bass preamp/3 band EQ options-maverick-m.png  
Old 1st June 2019
  #3
I build them here (since 1975) and 2.8k shelf is more like a volume pot. It will interact with the lower band as the crossover frequency will overlap. I set mine at 70 low shelf, 650 hz bell mids and 10k hi shelf. These with the active panning option were used in some models of the Tobias bases. Input impedance is 5 meg ohms and noise is around -100 dbu.

Otherwise I use a two band sweep EQ version, that covers 40~2k lows, 1k to 20K hz highs. Dial-a-tone.
Old 1st June 2019
  #4
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Thanks very much for the replies.

Cabirio, I really appreciate the schematic, I will build it very soon (I've got all those parts on hand which is useful) and see how it sounds. I didn't think to use some form of frequency analyser to tweak/find component values but it makes sense and I will probably do something similar if I want to change the values. If you wouldn't mind, that would be very useful to add the additional stuff in - I might use a FET buffer for the input if not.

Edit: checked the Darkglass site and the 2.8khz adjustment is another bell filter similar to the low mid. The way they worded it makes me think there is in fact no LPF/shelf - just a low and two mid controls.
Old 2nd June 2019
  #5
Gear Addict
Quote:
Originally Posted by Maverick616 View Post
checked the Darkglass site and the 2.8khz adjustment is another bell filter similar to the low mid.
Yup, sounds like it. I've modified the circuit accordingly, also added the single supply stuff and input buffer. Let us know how it works out if you build it!

Edit: one thing I forgot to mention is that, for better headroom, if you're using a single 9V battery it would be a good idea to use a rail-to-rail output opamp at least in the last stage, e.g. you could use a TLC2272 or AD822 (both dual) for the last two stages and say an OPA134 for the input buffer. Alternatively if there's enough room in your bass you could go with 2x9V, the supply connections would be as in the schematic I posted before and at the input you would only need a 1M resistor to ground instead of the R-C-R arrangement shown here. In that case the OPA2134 would work just fine.

Edit 2: I've reworked the mid section to ease the load on the opamps at extreme settings. The response is pretty much the same as before.
Attached Thumbnails
DIY bass preamp/3 band EQ options-maverick3.png   DIY bass preamp/3 band EQ options-maverick-bt2.png  
Old 3rd June 2019
  #6
The OPA1641/42 is a better opamp choice, I use those here. 1.5 ma current draw, 5 nv noise and rail to rail outputs. Batteries will last pretty long.
Old 3rd June 2019
  #7
Gear Addict
Quote:
Originally Posted by Jim Williams View Post
The OPA1641/42 is a better opamp choice, I use those here. 1.5 ma current draw, 5 nv noise and rail to rail outputs. Batteries will last pretty long.
Definitely. If you prefer DIP though, another good option for the input buffer is the TLE2071, and if you want even longer battery life, a TLE2061 / TLC2262 combo will only draw 1.3mA total. Noise / distortion performance won't be the same but it may work for you. Note that the TLC22X2 are 16V max. supply, so single battery only.
Old 3rd June 2019
  #8
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You guys are awesome, thank you so much. I've ordered some stripboard so I should have it built pretty soon (after I've soldered a 144 pin QFP, that is). Would there be a way to adjust the bandwidth with a trimmer pot? I could easily add it to the schematic myself if I knew which components to switch out. This really isn't necessary but it would give me more options if I needed.

Once again, thanks!
Old 3rd June 2019
  #9
Gear Addict
Quote:
Originally Posted by Maverick616 View Post
Would there be a way to adjust the bandwidth with a trimmer pot?
The only circuit topology I know of where you can adjust Q without affecting frequency and/or cut/boost is the state-variable filter, but that's a much more complex circuit that uses 4 opamp sections... You could easily make the center frequency switchable (which IMHO would be more useful than adjustable Q and fixed frequency) by scaling both caps, e.g. 1n/10n would give you 1.1k, 4.7n/47n around 230, etc. Or, come to think of it, you could go with two dual opamps instead of one dual and one single and have an extra mid section at a frequency of your choice.
Old 4th June 2019
  #10
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Quote:
Originally Posted by Cabirio View Post
The only circuit topology I know of where you can adjust Q without affecting frequency and/or cut/boost is the state-variable filter, but that's a much more complex circuit that uses 4 opamp sections...
That's what I feared but thought I'd double check in case there's some secret to EQ design that I've missed. Just checking, but when you say scaling both caps, I assume you're referring to C5 and C2, or do you mean C5 and, for example, C7 etc?

I've just placed an order for the remaining parts I needed such as the Op Amps so I will report back hopefully next week with how it sounds. Currently I only have an active bass (well, that isn't in pieces) so I won't be able to give it a true listen.
Old 4th June 2019
  #11
Gear Addict
Quote:
Originally Posted by Maverick616 View Post
Just checking, but when you say scaling both caps, I assume you're referring to C5 and C2, or do you mean C5 and, for example, C7 etc?
Sorry I didn't make it clearer, I meant C1 and C2 to make the 500 section switchable. You have to scale both caps in a single section by the same amount to change the frequency but keep the max. boost/cut the same, as in the examples I gave: the frequency will scale inversely with the scaling of the caps, so if you halve the caps, the freq. will double, etc.
Old 4th June 2019
  #12
Gear Addict
I just thought of another cool thing you can do to make the circuit more flexible: if you simply switch C7 out (disconnect it from either side of the pot), you turn the 2.8k bell into a high shelf with +/-12dB at around 4k, see attached. Could be useful and it only takes an extra switch to do it.

Edit: if you do this, increase the feedback cap around the last stage from 10p to 33p and the output resistor from 100R to 470R to prevent oscillation due to cable capacitance.
Attached Thumbnails
DIY bass preamp/3 band EQ options-maverick-bt3.png  
Old 5th June 2019
  #13
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I think I'm going to need to route the cavity out to fit this all in . Thanks for the Hi-shelf idea, I've got all the parts adjusted for that. I've changed from a DIP14 OPA4134 to OPA134 & AD822. I managed to get lucky and find a seller selling 50k center detent linear pots for a good price but I only picked up 3 (which I regret; I could've added an additional stage in the future).
Old 10th June 2019
  #14
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Quick update: Preamp is soldered (minus controls) but I'm still waiting for my AD822's to arrive. The layout isn't perfect because I did it as I went along but I think it's pretty compact. I've decided to put this in an Ibanez SR500 which I am doing some work on so it might take another week before I get everything assembled. Once everything is assembled, I'll consider doing some recordings.
Attached Thumbnails
DIY bass preamp/3 band EQ options-preamp1.jpg  
Old 10th June 2019
  #15
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Great stuff, I love protoboard projects, currently listening to a headphone amp I made on it

A couple of things I didn't include in the schematic because I kind of took them for granted: another 220u cap from V+ to ground for general decoupling at the board and decoupling caps at the supply pins of the opamps, both the AD822 and OPA134 call for at least 10n (value not critical, up to 1u will work fine, X7R ceramics recommended). To keep them as close as possible to the opamps usually I put them underneath the board soldered directly to the pins.

Have you tried it with other opamps just to see? Any first impressions?
Old 10th June 2019
  #16
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How critical are the decoupling capacitors on the opamp supply rails? I've actually already got some capacitors under the opamps so I'm very tight on room. I assume I'm decoupling both rails to the center reference of the battery? I think I will get the potentiometers and switch soldered tomorrow so I can give it a test. The AD822 is expected to arrive tomorrow so I should hopefully be able to try it out!
Old 10th June 2019
  #17
Gear Addict
Quote:
Originally Posted by Maverick616 View Post
How critical are the decoupling capacitors on the opamp supply rails? I've actually already got some capacitors under the opamps so I'm very tight on room. I assume I'm decoupling both rails to the center reference of the battery?
No, it's just one between the two supply pins, and it's ok if you put it on the component side provided it's not too far from the pins.
Quote:
I think I will get the potentiometers and switch soldered tomorrow so I can give it a test. The AD822 is expected to arrive tomorrow so I should hopefully be able to try it out!
Great, looking forward to hear what you think!
Old 10th June 2019
  #18
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Thanks for clearing that up. One more thing: when you said that I should use the original schematic supply for 18v, I interpreted it as the center reference is ground and the positive and negative rails are the opamp supplies. Should I be using the center reference only as a bias for the unused opamp inputs like in the new schematic and the negative rail as ground?
Old 11th June 2019
  #19
Gear Addict
To clarify things, here's the complete schematic (except for the decoupling caps next to the opamps, which in this case would be from each rail to ground) if you use 2x9v batteries, I hadn't realized that's what you were going to use.
Attached Thumbnails
DIY bass preamp/3 band EQ options-maverick4.png  
Old 11th June 2019
  #20
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Thanks, that is how I had it wired but wanted to make sure I wasn't referencing the wrong schematic. The AD822 is here so I will test it very soon!
Old 4 weeks ago
  #21
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Whoops, I completely forgot to say how it sounds! I've only tried it with my guitar (bass is in pieces). The first thing I noticed is that it is incredibly clean (expected) with no hum or buzz. All the controls seem to work including the switch which is good news. This will hopefully sound great when in the bass. I will give another update once the bass is back together. Thanks!
Old 2 weeks ago
  #22
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Sorry for the long wait everyone - some family stuff came up so I had to put the project on hold for a week. I've only just got it back together and it sounds great when running it into my computer with an amp sim (apart from the fact I've got the treble and bass knobs back to front, whoops). It might be my wiring but there seems to be a lot of hum present with the preamp engaged. I'm going to look into it tomorrow then hopefully get some sound clips.
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