Voltage follower for high-pass

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Voltage follower for high-pass

Post by carlmart »

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.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Voltage follower for high-pass

Post by JR. »

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
Cancel the "cancel culture", do not support mob hatred.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

-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.
User avatar
JR.
Posts: 3700
Joined: Sat May 24, 2008 7:21 pm
Contact:

Re: Voltage follower for high-pass

Post by JR. »

never mind...

JR
Cancel the "cancel culture", do not support mob hatred.
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

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.
vintagedesign
Posts: 20
Joined: Sat Oct 17, 2009 8:21 am

Re: Voltage follower for high-pass

Post by vintagedesign »

Thomas TK Kristiansson
Sweden
Vintagedesign / TK Audio
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

How do I connect a simple toggle switch on a SK to select one combo or the other (flat and 100Hz)?
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

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).
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

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.
Attachments
Shure HP#1.jpg
Shure HP#1.jpg (24.53 KiB) Viewed 21925 times
carlmart
Posts: 169
Joined: Mon May 19, 2014 6:11 am

Re: Voltage follower for high-pass

Post by carlmart »

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.
Attachments
HP #1.jpg
HP #1.jpg (85.67 KiB) Viewed 21881 times
Post Reply