Building a pre-amplifier with 3 band equalizer for bass.

[space_astronaut]

Hello guys, I am a newcomer here. The last couple of days I was trying to understand how the 3 equalizer band works, however I've been struggling to understand the principle behind the scenes to calculate the bass, mid and treble frequencies. The formula to obtain the frequency is f = 2.Pi.C.R but the numbers don't match. I would like to build a bass pedal or, a more challenging project, a preamplifier considering this type of equalizer with a couple of 2SC5200/2SA1943.

The first diagram that I am trying to calculate the frequencies is:

The diagram developer told that the bass, mid and treble are, respectively, 28Hz, 10.2kHz and 18kHz.

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The second diagram that I am trying to figure out is more complicated because it uses two TL072, and this is the one that I am more interested in:

In the second case the bass, mid and treble frequencies are, respectively, 100Hz, 800Hz and 4.5kHz. In this particular diagram, which Resistors and Capacitors are responsible for the bass frequency and how could I change those values for 80Hz, 500Hz and 2kHz ? I would like to implement the second diagram to a bass pedal or pre-amplifier.

Thank you for your help.

 

[BuddytheReow]

The first diagram, at least from what I’m told, is more interactive in terms of the mid control. It can easily overpower the other two. The fix, I think, is to stick a resistor either before or after C9.

The second diagram separates the mid control entirely as it own stage to make all 3 controls independent.

Try fooling around TSC in the web in addition to breadboarding to tweak the values. You’ll need to search for an active baxandall or James tonestack. Chuck also wrote some boneyard articles about EQ which will help. They’re a little older so you’ll have to dig through a number of pages to get to them.

 

[thetimberowls]

Hello guys, I am a newcomer here. The last couple of days I was trying to understand how the 3 equalizer band works, however I've been struggling to understand the principle behind the scenes to calculate the bass, mid and treble frequencies. The formula to obtain the frequency is f = 2.Pi.C.R but the numbers don't match. I would like to build a bass pedal or, a more challenging project, a preamplifier considering this type of equalizer with a couple of 2SC5200/2SA1943.

The first diagram that I am trying to calculate the frequencies is:

The diagram developer told that the bass, mid and treble are, respectively, 28Hz, 10.2kHz and 18kHz.

----

The second diagram that I am trying to figure out is more complicated because it uses two TL072, and this is the one that I am more interested in:

In the second case the bass, mid and treble frequencies are, respectively, 100Hz, 800Hz and 4.5kHz. In this particular diagram, which Resistors and Capacitors are responsible for the bass frequency and how could I change those values for 80Hz, 500Hz and 2kHz ? I would like to implement the second diagram to a bass pedal or pre-amplifier.

Thank you for your help.

You can check out http://www.muzique.com/lab/gyrator.htm and play around with some values if you want to see how changing resistors and caps can affect frequencies (and the Q). You just have to study this and try and translate it to the schematic on the page. I'm not an engineer, so I'm just as lost.

Can I ask where you got the second diagram from? I'm wanting to build an active EQ, and those 3 frequencies you listed for it are exactly the ones I'm looking to play with.

 

[Tha’ToneSeaQr]

BUMP-I’m also wanting to build a bass preamp circuit with similar frequency bands.

*While I’m not able to give you the component values to modify the second schematic for those frequency values. I can say that 80, 500, 2k are (roughly) used in a couple of the most popular 3-band obp’s on market the past couple decades- The Aguilar Obp-3 & the non-vintage Bartolini NTMB-3
Because they’ve been so widely used (like Sadowsky’s 2-band) folks have put together schematics of these, with some digging you should be able to track one down and use it as a reference. I don’t know if it’s kosher to reference another forum here but check out the freestombox forum there’s a TON of commercial traced schematic's and original design bass circuits there.
For manipulating the different frequencies this photo from the Aguilar manual may help get you started~>

 

[matmonster58]

There are some good calculators for gyrator EQs online. This style of eq is used in the ppcb "6 band EQ" project. The easiest thing would be to copy this circuit, only use three bands, and change the values based on your calculator.

The eq in the first picture is a passive EQ like you would find in a tube amp. You can play around with different values in TSC. Check out the ppcb project "Box and All EQ"

There are schematics floating around for the darkglass tone capsule, shadowski preamp, music man 2 and three band preamps and several other. It'd probably be best to just copy one of those.

I have a thread where I am working on an onboard preamp based on a gk 800rb. Its a work in progress with no definite completion date. I'll continue to work on it when I have time.

Passinwinds open source preamp is the best resource for bass preamp design. That guy really knows what he's doing and there is tons of info here and on talkbass. The current iteration of the pre is pedal format but I think he's currently working on some on board stuff and there have been onboard versions in the past

 

[Passinwind]

Passinwinds open source preamp is the best resource for bass preamp design. That guy really knows what he's doing and there is tons of info here and on talkbass. The current iteration of the pre is pedal format but I think he's currently working on some on board stuff and there have been onboard versions in the past

Thanks for the shout out. There's an undocumented option for adding a "normal" treble band to the Bass/Mids board in the open source pedal design via a small daughtercard. I've done quite a few onboard preamps that way, but I also have an all in one three band job that looks like so:



I'm planning on putting this one up on Github eventually, with a board share and BOM, but I have NAMM builds going on right now and a lot of other irons in the fire, so I'm not sure when I'll get to it. I think I'm done with new preamp/EQ pedals for the moment, but I do have a lot of new ideas for onboard ones.

Edit: as far as figuring out component values for frequency response tweaking, LTspice or any similar modeling software has been my go-to for quite a few years, because changing one band tends to change other ones simultaneously, especially if they are all sandwiched between two opamp sections like the top one in the first post.