Boosting Frequencies?

[winslowj]

Howdy! I'm working on designing a pedal, and I was wondering if there are any schematics I could check out that boost signal below a frequency, while keeping the rest at unity gain. I saw somewhere that a non-inverting op-amp dealio with a capacitor between the output and negative pins would do that, but when I tried it out it seemed like it really was boosting the low-end but totally cutting the high-end.

I guess I could just split the signal and give one side a low-pass filter and a clean boost and then mix them back together, but I feel like there must be a simpler, less roundabout way of doing it.

[PRR]

Welcome.

> boost signal below a frequency, while keeping the rest at unity gain.

Generally, we cut the highs and then boost everything. Narrow boosts can be tricky.

[winslowj]

Cool! I'm kinda bummed about losing those crispy high harmonics from the clipping, but I might just do a Guvnor-type filter section, so the highs can still be dialed in.

[Danich_ivanov]

The simplest solution that comes to mind would be to take James tonestack/Baxandall eq, and keep only the part that boosts low frequencies. More elegant solution could be something with positive feedback, although it will likely be more complicated.

[jfrabat]

I am kind of a newb in this, but can't you use a wein bridge and boost it or cut it with that?  I did a semi-parametric EQ that uses 4 of them, but the Q is fairly narrow; you would need to adjust Q for what you are asking.

This is the schematic for the EQ I built (based on the PAiA 9303):

Another (similar) option is the ToneTweEQ (3 band, fixed frequency).

On either case, you can focus on the frequency range you want to hit.

[PRR]

> take James tonestack/Baxandall eq, and keep only the part that boosts low frequencies
> ...can't you use a wein bridge

At some point winslowj is going to have to draw-out the desired response shape. "Boost" to infinity? Or a long rising slope? Or a narrow ant-hill? Are the highs sloped-off or just flat?

For many audio EQ questions it pays to start with a Boss(?) GEQ7(?) grafic EQ pedal and dial-in a sound-shape, then try to find a simple compact equivalent.

I don't know when  crispy high harmonics came in?

[bartimaeus]

you won't lose the highs if you follow PRR's suggestion. if you cut the highs, then boost everything, the highs are boosted back to their original volume.

[idy]

Winslowj: have you tried going back to your first experiment (cap in the feedback loop) and reducing the cap until it didn't take out too many highs? You will find the simple filters like that have a slow slope, roughly (often) half the voltage per octave, and half the voltage is only a mild drop in volume... (1/10th usually quoted as 1/2 the volume) so we often use several stages isolated from one another. In your super simple booster you might:
1)Use a very small cap to ground at (or near) the input. Maybe less than 1n?
2) find a value in the feedback loop that helps but doesn't totally deaden the sound.
3) put another cap to ground around the output (where you have a resistor in the signal path.)
Now you could get the slope high enough that it doesn't suck everything out, but steep enough to make a difference.

[winslowj]

Cool! I don't know why it didn't really occur to me that the highs would be re-boosted, that's super groovy. I guess I'm just going to do a ton of tweaking on an op amp with a super small cap, like idy suggested, and then toss a little boost section after it! I guess maybe it's time to pull the trigger on a circuitlab membership, the frequency graph simulation on there is so handy.

[Vivek]

Could you please post a diagram of your desired frequency response ?

Do you need a hump ? or a shelf ?

Roughly how many dB ?

[Vivek]

Would a fixed tilt EQ do what you wish to achieve ?

[Vivek]

Another (similar) option is the ToneTweEQ (3 band, fixed frequency).

That is an active 3 band Baxandall

Recently I input into LTSPICE, the 3 band Baxandall from the NATIONAL SEMICONDUCTOR AUDIO HANDBOOK and also the ones in the PETE CORNISH CC-1 CRUNCH and the TALONS drives

I found from the simulations that the Mid control has very low Q and will act more like a level control since it is too broad. Also, most of the action comes at the ends of the travel of the Mid Pot.

Pete Cornish Crunch CC-1 use 3 band Baxandall in their Post-distortion section.
The mids have peak of 11.83 dB at 1410 Hz
3 db Bandwidth from 550 to 2982 Hz
Hence Q is 0.59 at full boost where Q is the highest.
at lower mid boost, the Q is even lower !!!!!
That is very low Q, it will almost act as a volume control rather than a mid, since it has such a broad effect

[Radical CJ]

A transistor gyrator in the feedback loop of an opamp might achieve what you want in terms of boosting a specific band. I've started playing with these and found they work well for my purposes without taking up to many components. Off the top of my head you could take a look at the schematic of the Metal Zone or DOD Grunge for examples (but there are probably other better examples).

However, I'm just a curious dude with a cache of electrical components, NOT an audio-electronics engineer, so there might be more elegant solutions. This website does all the eq calculations for you.

http://www.muzique.com/lab/gyrator.htm

[Vivek]

https://sound-au.com/articles/gyrator-filters.htm

[ElectricDruid]

+1 agree with Vivek - Rod Elliot's ESP pages are your friend.

I'd suggest Figure 5 from this page, a simple Bass+Treble Baxandall shelving EQ. Since you don't need the treble control, you can just leave that bit out. With his values you get +/-15dB for your bass end. You can play with the values to either change the amount of adjustment or the range where it starts to kick in.

https://sound-au.com/articles/eq.htm

[antonis]

+1 agree with Vivek - Rod Elliot's ESP pages are your friend.

+1 agree with you Tom for Rod Elliot's pages but NOT for gyrator posted by Vivek..
OP looks for a Bass boost and not for some kind of notch filter..

[Vivek]

Gyrators can do low pass shelves as well !

They can do peaking humps too

ie not limited to notch.

OP did not specify what kind of boost he needs at the bass end. A hump or a shelf ?

[Steben]

Hifi circuits 

[ElectricDruid]

Hifi circuits 

You prefer lo-fi?

[GibsonGM]

The Anderton Frequency Booster has bailed me out of more than 1 jam..might be worth looking at, being so simple...