How to reduce bass?

[suryabeep]

Hi everyone,
I built this circuit and it works properly but my only complaint is that there is too much bass. Can I reduce the bass by a) using a 25K pot instead of a 100K or b) use a smaller cap instead of C7- 1uf?
Thanks!

[Rob Strand]

As shown the circuit won't work.

1) It needs to have a load to the right of what you have shown.
    Another way is to put loads to the right of C6 and/or C7.
   (the load connects to  ground or vcc/2 depending on what follows.)

2) The way the pot is connected only works if panning.


Are you trying to vary the value of the parallel cap or pan between two different filters?

[suryabeep]

sorry, the schem shown is just part of the full circuit. The actual thing has a load, its an overdrive circuit.

[rainless]

"too much BASS?!?!?"

Son... there's no such ting... 

[diffeq]

Reducing 100n --> 10n and 1u --> 100n should do the trick. I don't think that changing pot value will have any effect at all. Let's wait what other members will say on the subject though.

p.s.: I'll agree with rainless

[antonis]

@suryabeep: Better post a scheme of what is before op-amp..
(it's more convenient to cut-off frequencies before amplification..)

I guess your circuit has some interesting clues, like input impedance.. 

[KimJongFun]

The capacitors and the resistor to ground form a high pass filter.
use this:
http://www.muzique.com/schem/filter.htm
To choose the correct values for both.

Example:
0.1uF and 100k = 15.9hz this is really low.
0.1uF and 10k = 159hz
1uF and 10k = 15.9hz
etc...
When you get the idea you can do:
0.047uf and 4.7k = 720hz
1uf and 4.7h = 33.9hz
or even bigger...

just examples, you choose any values you want.
If there is already a resistor next to the op amp, then you can chose it to form the high pass filter.

Here is a low shelf filter, more explanations on the picture itself:

(by the way 1meg = full reduction and 0ohm = none)
R1 and R2 set the maximum reduction when R1 is at 1meg (they form what is called a "voltage divider")

example for the same corner frequency (1026.67hz):

0.033u and 4.7k (green)
330p and 470k (red)

@suryabeep: Better post a scheme of what is before op-amp..
(it's more convenient to cut-off frequencies before amplification..)

Yeah, or simply not boosting them, I would choose the non inverting mode and simply put this variable high pass in the feedback loop... it would save one resistor.

[suryabeep]

Here's the rest of the scheme. I guess my query isn't so much 'reduce bass' as it is "how do I make it so that the bass pot has a larger range?". Currently it only makes an audible difference between 0-30% and after around 60% the pedal just sounds saturated.
edit: oh also I had an issue with gain knob range (it saturates very quickly in the rotation), but I think I can fix that by lowering R5 to about 10K or so. I guess it's really not that much of an issue, but I am a bit of a stickler in these matters.

[antonis]

IMHO, don't mess up with NFB loops tone modification..
(unless you're fond of complicated tone reactions..)

Just lower C1 from 22nF to 2.2nF and you'll result in a cut-off point (-3db) shift from about 7Hz to 70Hz..
(lower cap value for more bass reduce and vice-versa..)

You can try the same with Q1 4.7μF drain cap (is it C9 or C8..??) down to 220nF and omitting C1 from circuit..
(IMHO, two HPF in series with so much low freq cut-offs ( 3.4Hz & 7.2Hz ) is overkill..)

[Phoenix]

Surya, without a buffer following that bass control, it's not isolated from whatever is connected to the output, so the range and cutoff points (frequency) of the bass control will be entirely dependent on whatever it's connected to. If you connect to an amp with an input impedance of 1Meg, it will behave completely differently to if it's connected to a pedal with a 10k input impedance, and it will also vary with the setting of the level control. That's another argument for putting the bass control in the feedback loop of the second opamp.

[KimJongFun]

non inverting jfet buffer input, I don't the reason behind it but you can simply remove R12 (I believe, the resolution is quite poor) and connect the output to the source.

C9 and R11 form a high pass filter at.... 0.5hz
you can remove both since C1 and R14 form another high pass filter, R14 doesn't need to be 1Meg because of the buffered input. (option 2: removing the input buffer completly and chose a JFET op amp and keep R14 at 1Meg)
you can move R3 at the input, I believe you placed it here to prevent the op amp to oscillate.
You can add a capacitor from +9V to ground (low ESR) it's not only to remove the ripples from the power supply, but in general op amps will oscillate if they it's not present.
You MUST add a 0.1u ceramic from +9V to ground as close as possible from the op amp, for the same reason as above.

Then your bass control, you can either add an output buffer before the volume control or not, depending on what kind of bass control you choose.
For the low shelf filter I posted before, unfortunately you will have to add one except if you move it before ICB1.
The "panning" control, you don't need one (take the value of the volume pot... here 500k I believe in the RC filter calculator and you have the corner frequency).
Do the modifications of the picture in my previous message, except you don't have to add the resistor to ground, it's already here, it's your volume control.

EDIT:
Oh yes you do need a buffer (Phoenix is right), because amps usually have a resistor to ground at the input (or any pedals), so it will mess with the bass response, so yeah, you don't have any choice, you need a buffer.

EDIT 2:
In the feedback loop, you won't be able to use any resistor (or pots) because it will lower the boost, your only option is a switch or rotary switch.
Rotary switches to change the bass response are found in pedals like the Palisades Overdrive, Attack OD (protone) or the precision drive (horizon devices)... the bass response here is modified before the clipping stage, but you can do it after, it will only remove bass (before the clipping stage you can turn your overdrive into a fuzz... or gritty in the case of the precision/attack OD).

[suryabeep]

Thanks for the help! I've made some adjustments; here's my new circuit. would this work?
The JFET input buffer is just a tillman pre, so it's just there to sound a little better.

edit: welp I noticed something dumb, hold on while I fix it.
edit2: Fixed. how does it look now?

[Phoenix]

That's better, although I'd suggest you add a resistor in series with the bass control pot to set a MAXIMUM boost level, and make the range of the control more practical, else it will go to open loop gain at maximum and overload.
Also, no need to use such a large volume pot on the output, 10kA with a 1u coupling cap will keep output impedance at a maximum of 2k5 rather than 25k for a 100k pot. You should also include a build-out resistor on the output of IC2C, 100R is plenty, to isolate the output from capacitive loads and potential oscillation if the volume control is turned to max and it's connected to a capacitive load (poor quality cable or unusual pedal design). This would be in series with the final coupling cap C7 (currently 100n, suggested 1u), it can be between the opamp and cap, or between the cap and volume pot, it performs the same in either position, choose whichever is better for layout.

[suryabeep]

Cool, I've put in a 6.8K in series with the Bass control, and a 100R after I2c. Thank you so much for helping me!

[PRR]

how does it look now?

That "bass" control is really a mid/high *cut*. It seems to cut mid/highs to zero, which is odd.

When cut, you are left with 6.8K:680K of gain in the bass, a LOT.

Reduce the 680K. Even down to less than 100K.

Or find a better plan, or steal it more accurately.

[suryabeep]

@PRR - Thanks, good point. I'll reduce the 680K to 68K so it's less gain. Would the filter become a high-pass by putting the capacitor before the two resistors (25K 'bass' and the 6.8K series)?
Pardon me if this is all very basic stuff, I am but a youngling in the world of circuit design

[suryabeep]

Alright, so I built this up on a breadboard, and guess what? It doesn't work :|
I've got everything according to the scheme, except I subbed in a 500K for the 1M gain pot and 56K for 68K (R8), i figured 500K:1K is still good enough for gain. 
When I audio probe, I lose signal at pins 5 and 7 of IC2b. Weirder is, there aint no dirt at IC2a pin 1. none!
Voltages are:
1:5.97    8: 9.57
2:4.77    7: 1.37
3:4.33    6: 5.46
4:0V       5: 4.75
IC2c and d are both working fine, so the problem must be somewhere a or b.
Can anyone please help? I'm stumped, all my prior experience (not much) tells me that this should work 

[Phoenix]

Without putting too much thought into anything, you really want to be AC-coupling between IC2A and IC2B, and also R6, because of the massive amounts of gain that you have going on, to prevent any DC offset from slamming the signal into one-or-other rail permanently. That's probably why the output of IC2B is at such a low DC voltage - 1.37V - should be around 4.5V.

[suryabeep]

Phoenix, you are a true gentleman and a scholar. 100n coupling cap between IC2a and b solved the low voltage issue, so now there is sound. I wasn't entirely sure what you meant by ac coupling R6, so I just changed the scheme like so:

Now the gain and bass pots work, but there is a loud hum when I play through it using the neck pickup on my strat. Is this because of the lack of shielding and the single coil pups?
Also, the bass and gain controls are now highly interactive, which leads me to two questions: a) is this desirable? b) if not, how can I stop it from being so interactive?
Thanks as always!

[Phoenix]

Yes, your placement of C4 is exactly what I meant (else IC2A would be amplifying DC, not just AC), though I don't see the AC coupling between IC2A and IC2B, you probably just left it off that version of the schematic.

I would assume that the noise problem is due to the singlecoil/breadboard if the circuit otherwise works fine with humbuckers, so I wouldn't worry about it at this stage.

The interactivity of the gain and bass controls is exactly what to expect with your circuit arrangement, as they each rely on shared elements for their outcome. In fact, they're really just both gain controls, the bass control isn't really very frequency selective (it will shave some bass, but it will have a much bigger impact on all other frequencies).
If you want to make them independent, you'll really need to put them in seperate stages.
As to whether the interactivity is desirable or not, well, that's up to you of course!