EQ "q" curves.

[j.hall]

i have a few sphere 3 band EQ's with fixed high and low shelfs

i have schematics to fully load them out for adjustable top and bottom.

but i don't have inductor values and from the way the boards look, i'll have a lot of trouble getting it all working.

so my question is this:

my low frequency point is 50hz. i like that point. but it's a shelf and when boosted it goes out to 200hz. that's BAD.

how can i change it from a shelf to a peack/dip filter?

 

[CJ]

just put an inductor in series with the shelving cap.
experiment until you get the right frequency as measuring inductance is far from an exact science.

 

[j.hall]

i need an inductor to change a shelf to a bell?

 

[SSLtech]

[quote author="j.hall"]my low frequency point is 50hz. i like that point. but it's a shelf and when boosted it goes out to 200hz. that's BAD.[/quote]
Can you be more specific?

I'm not sure if I understand you correctly, but I take you to mean that at full boost setting on the LF (50Hz shelving) the freq. response line doesn't return to flat until 200Hz, and that for 2 octaves from 200Hz to 50Hz it's a downward sloping line, straightening out at full boost below 50Hz...

If so then you're on entirely the wrong track. Yes, you can certainly make the filter into a bell instead of a shelf, but it should have essentially no effect whatsoever above the corner frequency of 50Hz. Remember the corner freqency of a shelf is equivalent to the center frequency of a bell at the same 'Q' (slope) setting, and that at full boost the equivalent bell shape will still not return to flat until 2 octaves above 50Hz (200Hz), the only difference being that it will return to flat an equivalent 2 octaves below 50Hz, i.e. 12.5Hz...

So either I misunderstand you, in which case can you clarify what you're asking, or you're on the wrong track entirely... a 50Hz bell that boosts only at 50Hz can be agreed upon to be largely unusable.

Keith

 

[nwsoundman]

I am thinking a change to the slope is what you may be looking for? How much boost are you talking? If you are boosting 12db then you have a 6db/octave slope. You can make that a 12db slope I would think. In a passive crossover you would make a 1st order filter a second order filter by adding a inductor in parallel with the load.

NWSM

 

[CJ]

I can design you a shelf filter for 50 hz.
Just tell me how many db per octave you want.
cj

 

[j.hall]

[quote author="SSLtech"][quote author="j.hall"]my low frequency point is 50hz. i like that point. but it's a shelf and when boosted it goes out to 200hz. that's BAD.[/quote]
Can you be more specific?

I'm not sure if I understand you correctly, but I take you to mean that at full boost setting on the LF (50Hz shelving) the freq. response line doesn't return to flat until 200Hz, and that for 2 octaves from 200Hz to 50Hz it's a downward sloping line, straightening out at full boost below 50Hz...
[/quote]

ok, you are following me to a point. and i'll clarify the rest as requested.

i like the 50Hz center. but i want to change the shelf to a bell.

now you mentioned a 2 octave bell.....i don't want that. i understand how Q works. i want 1/3 octave bell with the center frequency at 50Hz.

12db of boost on the shelf is indeed a 6dB per octave slope as it stands right now.

i want to make that a 1/3 octave bell centered at 50hz.

how do i do that?

 

[CJ]

what is your source and load resistance?
600 source and 10k load maybe?

 

[SSLtech]

[quote author="j.hall"]now you mentioned a 2 octave bell.....i don't want that. i understand how Q works. i want 1/3 octave bell with the center frequency at 50Hz.[/quote]

Since the bandwidth of a bell is usually defined as the width between the -3dB points, if you want a 1/3 octave bell, there would be 1/6th of an octave above the center freq. and 1/6th octave below the center frequency (to the two respective -3dB points). -Simplifying it a bit and assuming that the slope has settled into a reasonably steady section of the curve, if the signal drops by 3dB in a sixth of an octave, you'd be looking at a slope of six-times-three dB per octave... in other words 18dB/8ve. (in practice, owing to the flat section at the center frequency and the shallow slopes either side, the true response slope will in fact be rather steeper, more like 24dB/8ve, but let's go with 18 for now...)

This is a long way from the current slope of (assuming 12dB max boost and 2 octaves before the curve returns to flat) 6dB/8ve.

I have a few sphere EQs here and looking at the circuit I have to say you're probably not going to be able to do it. -that circuit is performing according to the nature of how that design operates. You have to used a much more resonant circuit in order to get a sharper slope. It's going to sound vastly different (but then.. you want a vastly different slope and thus a vastly different EQ, so nothing conflicting there).

I'd say don't waste your time changing the sphere. If you want it to work like a single band of a 27-band graphic EQ (for example) or even better, a single band of a parametric EQ, you're going to find it a lot simpler, cheaper and more serviceable to just build a seperate circuit to do just that, and leave the Sphere alone, not to mention available as an alternate LF EQ for the times when such a narrow bandwidth isn't what you want... (which is usually more often than not: The beauty of the Pultec for example, is that it's broad, gentle LF curves boost 'warmth' and low end without bringing attention to any single 'ringy' point.

If you're looking for a 'ringy' filter to add more 'note' to a kick drum (for example... one of the few occasions when it's more common to boost the LF at a narrow 'Q' setting in conventional EQ use) then sometimes it's even more helpful to have a parametric EQ, since the amount of 'ring' -as well as the musical note on which it is centered- can be adjusted.

The idea of adding a bell switch is a good and very useful one, and CJs suggestion will work, but if you want super-tight, narrow boost, then the spere is entirely the wrong topology to try and modify in my experience.

If you like the way the sphere sounds now, keep it as-is, and build a specialised tool for the specialised job. :grin:

Keith

 

[j.hall]

all right, that's what i needed to know.

which sphere EQ's do you have there?

the other thing i'm looking to do is finish these thing out.

i have a schematic and pcb layouts for a three band with rotary switches in all three position plus a fourth for HPF.

completeing the PCB's wouldn't be too hard, i've even found a small enough rotary switch to place in there since the thumbwheel can not be found.

i just need to locate the inductors (or more likely, something that will work)

any ideas?

 

[SSLtech]

[quote author="j.hall"]which sphere EQ's do you have there?[/quote]
We've got nine of the "vertical graphic EQs" (with the 10-way left-to-right 'paddle' switches for cut/boost) and sixteen of the three-bands like yours.

You might have to play around to get the right value, since the value can interact with the center frequency. There's too many variables for me to do it in my head and I like happy accidents. I'm lucky enough to have a sweep-response plotter here at work, so measuring the result takes 30 seconds... If it were me doing it for a customer I'd probably buy a few sample values, try each, plot the results and see which one looked closest.

CJ is better at the iron-and-copper math than I am, so he could probably tell you better!

Keith

 

[j.hall]

are you guys selling any of the 3 bands? my rack will hold 4 more.

anyway. is there any need for me to post the schematic?

i have a TIFF file of it and the front and rear PCB layouts. the scans are pretty rough but might be helpful.

i'd sure like some help on sourcing some inductors for these.