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Old 13th November 2012, 08:25 PM   #1
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Default Sharing a Single Inductor for Multiple Passive LR Filters?

Hi, I'm building a passive program equalizer and one of the main building blocks of the circuit is a first-order lowpass filter like the second diagram on this page:

http://www.play-hookey.com/ac_theory...s_filters.html

My plan is to use a stepped attenuator as a multiple-position switch to select the turnover frequency for a given instance of this filter. The idea is to switch the resistor value to achieve different turnover frequencies, but to use them all with a single inductor. (I've got a few very nice vintage inductors I'm trying to use.)

---edit: problem solved. Was looking at wrong circuit initially...---

Thanks for any thoughts or advice, rs

Last edited by ryansupak; 13th November 2012 at 08:51 PM.
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Old 13th November 2012, 08:45 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Its a high pass circuit not a low pass, a series capacitor is easier.

I can't see anything so crude being useful in modern EQ unit.

Nothing wrong with passive EQ, designed properly.

rgds, sreten.
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Last edited by sreten; 13th November 2012 at 08:49 PM.
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Old 13th November 2012, 08:55 PM   #3
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Thanks for the reply...I swapped the image out with the correct one.

In doing so, I also realized my problem isn't one, actually.

Such a broad filter is extremely useful as a "tone control" for treating entire songs of varying recording quality through on the fly. Some newer mastering EQ's and high-end DJ isolators are built around these kinds of filters. I'm actually replacing something very similar that finally broke...

rs
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Old 13th November 2012, 09:10 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

Its not a broad filter, its pretty useless as a form of EQ compared
to simple and much broader 1st order shelving passive circuits.

Inductors are hardly ever used in simple passive EQ circuits,
they tend to pick up hum, add distortion and are expensive.

rgds, sreten.
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There is nothing so practical as a really good theory - Ludwig Boltzmann
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Old 13th November 2012, 09:32 PM   #5
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I'll experiment with the filters I have planned and also with shelving filters to see which is the most useful in this case. Basically the EQ will be divided up into 3 "broad" bands, and each band can be killed completely. In practice the Low and High bands run close to 100% and the Mid runs at a varying degree of cut (it usually gets "played" like an instrument to some extent.) If all 3 band level knobs are turned to their minimum positions, no sound comes out at all.

As far as the noise issue, I am aware of it and am pretty sure these inductors (which I already have) won't be affected. They were originally designed for church pipe organ sound systems and the cases they're built in look literally bombproof.

Regarding the use of inductors, I've got a few EQ's, some inductor-based and some not, and the inductor ones do something to the sound I like, especially the bass. Not sure if this has to do with saturation or with something else, but I love it. I know of at least one high end studio EQ that touts "virtual inductor" technology on the bass band. (Speck ASC.)

Anyway...thanks again and I will surely look into a shelving filter instead of a low pass...

rs

Last edited by ryansupak; 13th November 2012 at 09:43 PM.
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