[sdiy] equalizer

[Seb Francis]

this is my rough understanding of this (I'm sure someone will correct me 
if I'm incorrect)...

dB/oct relates to the number of poles (stages) in the filter.  you get 
6dB per pole, so a 36 dB/oct filter is 6 pole which is quite a 
complicated circuit.

the Q comes from adding feedback to the filter

Seb


cheater cheater wrote:

>shall keep that in mind 8)
>what about filters of higher order?
>like 36 dB/oct?
>there's more to them than just Q - right?
>is there even a "standard" 36 dB/oct filter?
>
>cheers,
>D. 8)
>
>On 10/10/05, harrybissell <harrybissell at prodigy.net> wrote:
>  
>
>>.707 gives a smooth roll-off just like two cascaded RC sections...
>>1 gives a sharp corner
>>anything above that has a really obvious peak.
>>
>>I've found that if I want really smooth response, even a Q of 1 might be
>>too much...
>>
>>H^) harry
>>
>>cheater cheater wrote:
>>
>>    
>>
>>>I'll keep the book in mind,
>>>thanks!!
>>>
>>>How did you come up with "Q between 0.707 and 1"?
>>>
>>>Cheers,
>>>D 8)
>>>
>>>On 10/9/05, harrybissell <harrybissell at prodigy.net> wrote:
>>>      
>>>
>>>>cheater cheater wrote:
>>>>
>>>>        
>>>>
>>>>>Well, what I want is a narrow cutoff (like 24 dB/oct? no idea - hints?)
>>>>>Usual equalizers have 6 dB/oct - right? Or is it 3?
>>>>>          
>>>>>
>>>>Most are 12dB, some are 18dB.. I don't know of any common 24dB designs.
>>>>
>>>>        
>>>>
>>>>>So at 24 dB/oct:
>>>>>1. The BP would be real narrow and resonant and crappy and shtuff
>>>>>          
>>>>>
>>>>24dB could still have low, or high resonance.  Q of .707 to 1 would not have
>>>>a pronounced peak. BTW the BPF of the state variable is only 1/2 the cutoff
>>>>rate of the HPF and LPF.  The 'state variable' design in most commonly a
>>>>12dB slope. You 'can' co a 24dB but it gets complex
>>>>
>>>>        
>>>>
>>>>>2. The HP and LP would already give me the whole spectrum. So the BP
>>>>>would only add stuff - am I right? Or would HP+LP=everything-BP? :)
>>>>>
>>>>>And in other news:
>>>>>can anyone recommend a filter that:
>>>>>1. doesn't have a very resonant sound
>>>>>          
>>>>>
>>>>any one with a Q of .707 to 1...
>>>>
>>>>There are a number of standard filter designs. For memory I will fvck this up
>>>>and my listmates will jump my bones. They are Butterworth, Bessel, Chebychev.
>>>>One has better flat gain in the passband, one a better cutoff but small peak at the
>>>>corner, one has ripple in the passband but even sharper cutoff. They have different
>>>>shape of phase shift as well.  Decisions decisions. Most folk use the Butterworth...
>>>>
>>>>        
>>>>
>>>>>2. has a pretty narrow cut-off (24? 12? 18 dB/oct? which ones would you choose?)
>>>>>          
>>>>>
>>>>12dB
>>>>
>>>>        
>>>>
>>>>>3. has a pretty uncomplicated circuit (I'm going to have to have a lot
>>>>>of these in an equalizer it seems - and even more in a whole mixer -
>>>>>woe is me!)
>>>>>          
>>>>>
>>>>There are a lot of circuits. I'd start with one I like, then copy that first to get
>>>>experienced..
>>>>
>>>>4. (a plus but not needed) sweeps don't have the usual cheezy resonant whistle sound
>>>>
>>>>        
>>>>
>>>>>This goes for LP and HP. I'm counting on you synth freaks here!
>>>>>          
>>>>>
>>>>If you need to sweep the filters, for sure the higher cutoffs will need more
>>>>components. Usually a 12dB needs two tuning elements (dual pot, ota etc)...
>>>>24dB needs 4 etc.
>>>>
>>>>        
>>>>
>>>>>Regarding 3 - am I right to think that SMD components would make the
>>>>>[6x2x5=60 (!!!!) (4 bands)x(stereo)x(5 channels)] SIXTY filters easier
>>>>>to tune up? Don't they usually fluctuate less? In any case I guess
>>>>>it's a must considering space and such...
>>>>>I'm *so* buying metal film resistors... and 2% caps.... *cringes*
>>>>>          
>>>>>
>>>>and good caps are hard to find... especially in SMT
>>>>
>>>>        
>>>>
>>>>>Also: anyone got the Electrix EQKiller schematic?
>>>>>Or the Vestax DCR-1200 Pro?
>>>>>I'm very curious what kind of filters they're using, and what order they are.
>>>>>          
>>>>>
>>>>I'd recommend that you get a copy of the old National Semiconductor "Audio"
>>>>or "Audio/Radio Handbook"  (out of print but I think someone sells reprints). The
>>>>ICs shown are largely obsolete but the concepts are still quite valid, and it is
>>>>written
>>>>for the common man (well, common man skilled in the art :^) to understand. It shows a
>>>>graphic EQ and a 'room equalizing instrument' that are of interest.
>>>>
>>>>H^) harry
>>>>
>>>>        
>>>>
>>>>>Cheers!
>>>>>D.
>>>>>8)
>>>>>
>>>>>On 10/8/05, harrybissell <harrybissell at prodigy.net> wrote:
>>>>>          
>>>>>
>>>>>>Steven Cook wrote:
>>>>>>
>>>>>>            
>>>>>>
>>>>>>>A state-variable filter has three simultaneous outputs: lowpass, bandpass
>>>>>>>and highpass. I suspect that mixing all three outputs together would
>>>>>>>reconstruct the input signal with reasonable accuracy.
>>>>>>>
>>>>>>>              
>>>>>>>
>>>>>>True... but the corner frequencies cannot be (individually) controlled... and are
>>>>>>
>>>>>>really unlikely to be useful in an EQ application.
>>>>>>
>>>>>>H^) harry
>>>>>>
>>>>>>
>>>>>>            
>>>>>>
>>>>        
>>>>
>>    
>>
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