Need help changing frequencies in Soundcraft EQ

[living sounds]

Hello,

I'm trying to figure out what and how to mod to get more options in the parametric EQ section of my Soundcraft 200B channel strips. See attachement for schematics.
Specifically, I'd first like to extend the low end of the low-mid EQ without changing the Q or lowering the highest frequency. A bigger cap for the 15n one (like 33n-100n) lowers the frequency, but also changes the Q somewhat and also lowers the maximum frequency. Is there a way to prevent this without swapping pots (I'd like to keep the very high quality pots in there)? I'd like it to go down to 50Hz or so.
Likewise, a higher maximum frequency in the high mids EQ would be great. A lower value of the 3.3n cap would do this, but it again affects the lower limit and Q. Up to 18khz would be great.

Is there a calculation tool online for this sort of thing?

Thanks!
Gregor

 

[tv]

It's a Wien bridge.
fc = 1/2 x PI x R x C

You will have to mod the values for the cap. pairs, in the same ratio as they are now in-circuit.

for LOW mid, change the caps.:
c21 = 33nF
c22 = 15nF

should (from top of my head !!!! check math) go down to cca. 68Hz, but the top freq. should be halved as well...

for HI mid, change the resistors:
r27 = r28 = 3k9

should (again from top of my head) go somewhere to cca. 18kHz, while the lower freq. should remain cca. 600Hz as it was.

------------------------------
IF you want to get lower in the LOW mid section BUT retain the sweep to 2.4kHz, you will have to mod the r31=r32=3k3 as well ( !!!! )
------------------------------


NOTE: this is from the top of my head, but should work. Wait for the confirmation or review of some other forum members.


***F-ed up: r31=r32 should be 3k3 (of standard values)

 

[Harpo]

as TV already said,
looks like bandpass centerfreq. = 1 / (2*PI()* sqrt((R27+VR3)*C19 * (R28+VR3)*C20)) for the 669.8 to 10519.83Hz range.
If you decrease both R27 and R28 to 3K9, range should change from about 670 to 18350 Hz with same Q.

Same for = 1/(2*PI()*sqrt((R31+VR5)*C21*(R32+VR5)*C22)) for the 147.55 to 2317.45Hz range.
Doubling C21 and C22 (just place same value cap in parallel) and decrease both R31 and R32 to 3K4 should change range from about 76.2 to 2300 Hz with same Q.

As range ratio changes from 15.7 to 26.6 for the Hi-Mid and from 15.7 to 30.4 for the Lo-Mid, you might try if you still feel comfortable with the pot behaviour.

Good luck

 

[living sounds]

I'll try that.

Thanks alot!

 

[tv]

The 33nF / 15nF combo is quite similar in ratio to the 15nF / 6,8nF original combo:

33/15 = 2,2
15/6.8 = 2,205882

IOW, this "standard values" substitution wouldn't hurt the Q-factor by much (if any - considering the component tolerances and potentiometer tracking errors).

How's that? (meaning that you could just reuse your existing 15nF caps. Pretty sweet, eh? Total cost 4 resistors + one cap per channel ... not bad.)

 

[living sounds]

The 33nF / 15nF combo is quite similar in ratio to the 15nF / 6,8nF original combo:

33/15 = 2,2
15/6.8 = 2,205882

IOW, this "standard values" substitution wouldn't hurt the Q-factor by much (if any - considering the component tolerances and potentiometer tracking errors).

How's that? (meaning that you could just reuse your existing 15nF caps. Pretty sweet, eh? Total cost 4 resistors + one cap per channel ... not bad.)

Yes, did it, works great! Although I used a better cap. This EQ sounds really good, especially with Mica caps for the high shelf and hi parametric EQ. And I haven't even tried a different op amp or biasing it into Class A.
Pot behaviour isn't a problem, I don't use the console live and the Q isn't that narrow anyway.
Now I'll add this mod to the recap of any channel.

Thanks again!
Gregor

 

[tv]

Oh, God forbid!

Now you are going to knock down kids on kvr how hardware just sounds better, am I right?

8)

 

[living sounds]

Oh, God forbid!

Now you are going to knock down kids on kvr how hardware just sounds better, am I right?

8)

Say what? When someone asks "how do I get this great sound" it just usually turns out to be hardware. ;-)

But I definitely prefer the EQs on this console even stock to all the plugin EQs out there. Except for mastering.

 

[romeojesus]

Hello,

I would like to lower the 150hz on the 150hz - 2.4khz low mid Eq on a soundcraft venue.
Looking at the venue eq schems they appear to be quite similar to the 200b.
I see two differences though:

- On the venue the sequence of 6k8 (r38) resistor and 15n cap  (c19) is inverted compared to the 200b.
- there is a direct connection between the 100k revlog dualgang pot´s upper wiper to the R38, C19 combo that
I cant see in the 200b schems.

Would changing the Venues caps.:
c19 to 33nF
c20 to 15nF

r36 and r38 to 3k3

as described in the above 200b mod also lead to the extended lowend in the  LoMid band of the venue?
(Regardless of the schem differences)

..bear with me... I know nothing about electronics

best

christian

 

[PRR]

> the sequence of 6k8 (r38) resistor and 15n cap  (c19) is inverted

Parts in series, no connection at center, may be in either sequence, no difference.

I have a suspicion that drawing has an error, but no matter.

If it works, but you want a different frequency, you don't need hard thinking. Leave the resistors alone. Double the caps to shift down an octave.