passive lowcut filter for mastering??

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matthias

Well-known member
Joined
Jun 4, 2004
Messages
768
Location
germany / frankfurt
hi,

I want to build a passive lowcut filter for mastering (stereo)...
to roll off unnecessary freqs...
each channel with a 12 pos switch to select the indivdual cut frequency...


calculating the frequencies is very easy using this great calculator: http://www.opamplabs.com/rfc.htm
this tool gives me the values for the resistor and capacitor that I connect in series to achieve a -6db lowcut filter.

so far.. so good...



1. do I need transformers for in and output??

2. can I use one capacitor for each filter and just vary the value of the resistor with the switch ??

3. should I use a certain resistor value and switch between 12 different caps??
if this is the way to go, is it ok to connect caps in parallel?? or wouldn't you suggest this...???


thank you
mat
 
If you look in the passive EQ meta there's a link to steffen's transformerless passive EQ. It's got opamp in/output buffers so you won't need transformers.
 
[quote author="matthias"]1. do I need transformers for in and output??[/quote]

Nope; if your equipment is balanced you can build a balanced filter:

hipass.gif


Note that when you use the calculator, you should use a figure for total R that's equal to the shunt resistor in the diagram paralleled with the load resistance. So if the shunt resistor is 1k and the load is 10k you should calculate based on 9.09k. If you don't know what the load resistance will be in any given situation...that's why buffers are in the world.

Also note that if you calculate C for a given frequency and load R using a standard formula for unbalanced filters, in a balanced filter you'd use 2C. So a frequency that required a 1µF cap in an unbalanced filter would use 2µF in a balanced one.

Formulas:

Unbalanced: C = 159000 / (F * Rtotal)

Balanced: C = 318000 / (F * Rtotal)

where C is in µF, F is in Hz and Rtotal is in ohms.

2. can I use one capacitor for each filter and just vary the value of the resistor with the switch ??

Yes, as long as the lowest resistance (remember to include the load) is still okay for the previous device to drive. I'd call 2k a minimum for prosumer gear. But instead...

3. should I use a certain resistor value and switch between 12 different caps??
if this is the way to go, is it ok to connect caps in parallel?? or wouldn't you suggest this...???

That's what I'd do, and yes, paralleling is fine. Hardwire the smallest cap(s) for the highest frequency, and switch caps in parallel for lower frequencies.

Peace,
Paul
 
Hi Matt,

I actually had the Pultec HLF3 filter schematic from a friendly person who sent it to me some months ago. But I had so much work to do at the time, that I completely forgot about it.

Well, here it is, now added to gyraf/resources under Equalizers:

http://www.gyraf.dk/schematics/schematics.html

Direct link:

http://www.gyraf.dk/schematics/Pultec_HLF3.pdf

Many thanks to the supplier!

Jakob E.
 
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