Pro Audio Design Forum

I need to add a HP filter, which is very useful on location recording.

The idea was to use a voltage follower to do that, but it seems to have some tricks, like what resistor you load it with at the input, or what chip to pick.

My idea is to put a switch and select between two caps going to the IC.

I saw something like that on a Satt mixer, but to my surprise they used a low value load resistor.

W.Jung is full of warnings about the voltage follower, and that's what got me curious. Because it's also very easy to assemble.

Maybe Walt will chime in with his concerns.

In general if you are going to add an op amp for a HPF you might as well make it 3 pole (-18dB/oct. You can put one underdamped active two pole in series with one real pole to make a 3 pole with less loss at the cut-off frequency. Butterworth alignment is -3 dB at tuning.

For non-inverting HPF I would look at Sallen & Key topology which may be called VCVS or something like that in filter nomographs.

JR

-18dB/oct is too steep a cut for location recording.

-6dB is more effective, as it can cut some of the low frequencies and give you the option to cut more later on. if you want to.

never mind...

JR

The first option, with the basic follower, is more or less what I was thinking about:

http://www.electronics-tutorials.ws/fil ... ter_6.html

Add a switch in the path, where the resistor is, and two capacitors to choose.

The question is what resistor value to use. Chip would be NJM2068.

Sallen-Key filter calculator.
http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm

How do I connect a simple toggle switch on a SK to select one combo or the other (flat and 100Hz)?

From what I've seen, selecting RC for 100Hz and 2Hz, all four values change.

The only way I imagine is using a dual IC with one filter on each. Then select one path or the other at V(in).

OK, let's try the simple again and please do comment about this option.

This is what Shure is using on some of their more reputable portable mixers, considered the industry standards.

The arrangement is not new, as I saw it on the latest mixer Stellavox released in the '80s, all discrete.

Same thing: flat with a large capacitor, HP adding a series capacitor + resistor to the chain, lowering the total cap value.

Any problems in adding that arrangement to a basic inverter? Values would be different, of course.

This is a cleaner drawing of what the filter would be like.

Now I will have to pick a frequency to cut at.

Opinions are welcome, particularly if you are familiar with recording dialog on location.