High Pass Filter Mod need help.

[Migs 31]

Hi everyone,

I have been working on my mixing desk for a few months, modding,re-routing the sub-groups,added extra aux returns, changed pre-amp chips and mod the H.F. and L.F. on the mic-line channels.Where I'm stuck now is, trying to mod the HPF on the individual channels. Trying to replicate what I did with the HF and LF bands (putting caps in parallel,giving me an option of three Frq. with SPDT switches or ten Frq. with rotary switches) I dont seem to be able to change the darn corner Frequency on the HPF, or at least it's so subtle , that I can't hear it. I have identified Capacitor C11 as being the one responsable for the HPF corner frequency.Do I need to change something else to make that corner frequency shift either up or down? Is it possible to easely make it  a fully variable Frq. filter from lets say 30Hz to 300Hz? And the slope, does changing the slope from 6dB/Octave to let's say 12 or even 24 dB/octave have a role to play in this, maybe making the filter more "audible", more drastic? The original Frq. is 70Hz with a 6dB/octave, set by C11=22uF @ 25V.The values I have applied instead are, 2uf ,22uf and 47uf,  then 10uf, 22uf and 47uf combinations, hoping to get three different frequencies, with of course corresponding positives with + and neg with -. Find included part of schematic of a mic/line channel, top red circle being HPF, H.F. and L.F. section and lower circle being Mid Semi-parametric Eq section.I need help with this one folks, or pointing out at the right direction, other thread or document. I mean any help would be welcome.Thanks.

 

[abechap024]

HI.
I'm no expert but if you not hearing any change... there probably isn't any change going on. which means try a smaller capacitor until you hear something you like...and remember you can always series 2 caps to divide their value in half(ish)...great for maybe making the hp 3 different freqs without using a ton of caps...
sorry there are more technical people here that know their @#$ but maybe this will help?
Also the cut off frequency is a product of the circuits impedence and cap value...

 

[Harpo]

...I have identified Capacitor C11 as being the one responsable for the HPF corner frequency...

No, it is C9, C10, R19, R20 that set the cutoff frequency for your 2nd order hpf. Maybe google sallen-key filters.
C11 for blocking DC-offset with following 22K and maybe not shown other resistor(s) in parallel to ground is a hpf set below 1Hz.

 

[pstamler]

...I have identified Capacitor C11 as being the one responsable for the HPF corner frequency...

No, it is C9, C10, R19, R20 that set the cutoff frequency for your 2nd order hpf. Maybe google sallen-key filters.

What Harpo said. The cutoff frequency is actually about 80Hz, 2nd order (12dB/octave), with a Butterworth response.

Because it's 2nd order Butterworth, it stays pretty flat until just before the rolloff frequency, which means you'll only hear a difference on things that make noise below 80Hz. Using this filter to, say, compensate for proximity effect on an acoustic guitar will have very little audible effect.

Peace,
Paul

 

[PRR]

> Frq. is 70Hz with a 6dB/octave, set by C11=22uF

C11 22u against R21 22K is 0.3Hz. Maybe higher because there's other stuff || to R21, but still.... this is SUB-sonic.

As Harpo says--- look before that.

C9 100nF R19 18K is 88Hz.
C10 100nF R20 39K is 41Hz.

They are tightly joined, so neither together is the same as apart; still, this looks like a bass-cut. AND there's no unknown/variable loading after these RCs (like R21 has stuff going all over). That's gotta be the bass-cut, and of course is 12dB/8ve.

> fully variable Frq. filter from lets say 30Hz to 300Hz?

Freq will scale with (inverse) resistance.

Paul says its now at 80Hz, which is reasonable, anyway Paul knows his stuff.

For 300Hz you change both resistors down by 80/300; for 30Hz(?) you scale up by 80/30. So: 4.8K 9.6K to 48K 96K. If you can find a dual-gang 50K+100K pot you are golden. Such parts are rare. A triple-gang 100K+100K+100K can be jumpered; maybe there's a 5-gang 100K part for surround-sound?

Equal-resistor 30Hz-300Hz unity-gain 2-pole filter: Make C9 68n, C10 120n, use dual 100K 20% Audio taper pot with 10K fixed resistor.

> changing the slope from 6dB/Octave to let's say 12 or even 24 dB/octave

It IS 12dB/8ve. Changing to 6dB/8ve is easy: add 1Meg in series with R19 pot. Changing to steeper curves is not trivial: there's a cheat to get 18dB/8ve with a dual-pot over a limited range, but the shape turns from soft to bumpy as Freq is changed. 24dB/8ve really wants twice as many parts with four times the precision..... don't do this in the console. Just fudge the subsonics enough to get the sound on tape, then do any steeper filtering in the mix-down/ProTools. Digital tools let you find the lowest note in the passage and apply a 60dB/8ve sharp-corner filter; I did this to sort truck-rumble from pipe-organ.

 

[Migs 31]

Thank you very much gentlemen for correcting me and showing me the way( dunno but reminds me a Peter Frampton...)  humm humm, ermm, sorry... Yes, following your suggestions I looked into C9 and C10 with R19 and 20. Put back C11, tried the fully variable Frq. thing suggested by PRR. They had no Log Audio Taper pots at my local Maplin, so I had to do with a Linear one, double gang 100K with two 10 K resistors soldered to the middle legs, changed the two caps for 68nF and 100nF (closest I could find to 120nF). The blinking pot was too big,so I could'nt fit it inside the PCB so it's hanging out at the momment, I'll get a smaller better one soon.Wired the whole thing, tried some Bass, Kick drum and Acc.guitar and yes, seems to work.That in combination with the Ten switchable frequencies on H.F. and L.F. makes it a very flexible EQ, need less outboard gear. I don't have an oscilloscope and all that to check it precisely, but to my ear it seems to work, clearly.By the way, anybody has any idea why, when I switch beetween frequencies on HF and LF, the sound goes up in beetween??? But anyway, thanks again to you all, I have learned so much and hopefully this will help other people in the future.I go back to my other mods on the desk, I keep you posted.