Turtlerock Forum

I want to make myself some handy doo-dads while I'm broke and bored. I'll be building a couple of simple pads as described in another thread into the connectors of some of short leads, along with a couple of phase reverse leads.

The other thing I want to make is some highpass filter leads. My desk has a highpass at 75hz, but I want to be able to go lower for kicks, floor toms etc. I've hunted the 'net for simple designs, but found only formulae in greek! I've gathered that a cap/resistor in series/parallel will do what I need, but buggered if I can figure out the required values for a 40 or 60 hz filter.

Has anyone made such a thing? What other handy cheap useful thingies have people made?

Hi Kurt

I suppose "cheap" is relative - the DI Joe active DI project is less than $100 is that's cheap, and its a great usable tool.

Re the HPF, I got this off the net a while back. Its written for compressor send/return sidechain loops, but you'll get the general idea.



Taking the HPF further, a 60Hz value would be about 120K for the resistor.

Let us know what you do.

ummm not quite as simple as that

don't forget the input impedance of what your little filter will be connected to. This will appear in parallel with the R in the examples from chrisp

120K is getting pretty high in value and would be swamped by most line in input impedances.

You could try a 1uF cap and a 20K pot in series with say, a 470R resistor. This ( depending on the input impedance of whatever you connected to ) would give you an adjustable range of around 16Hz to 340Hz. And because it's adjustable it allows you to deal with the shift in turnover frequency caused by the line in impedance.

Of course this is all done in an unbalanced circuit but can be adapted to a balanced circuit.

Also this gives you a 6dB per octave HP filter, this is pretty gentle, great is some applications, useless in others

happy filtering

Rob

Argh! Even simple stuff isn't simple enough for me.

So for dynamic mics with an impedance of 600ohm I would use 2 caps with a resistor across the signal leads?



Would I be right in assuming that a filter with a steeper rolloff would be much more complicated, probably an active circuit?

your drawing is correct for a balanced signal

here F( -3dB ) - 1 / ( 2*pi*R/2*C )

still get 6dB / octave, no way around this without active electronics

also it assumes a low output impedance from the source, putting it directly on a mic will screw around with the simple equation above. Also as i mentioned previously the input impedance affects things too and a mic pre input impedance is generally low-ish ie 1K to 3K

standby for the next issue of AT for an article about impedance :)

Rob

It's all too hard for my apparently electronics impeded brain to deal with :(

Had a brilliant idea for someone a lot smarter than me though. A barrel type xlr connector (M/F) with switchable high and low pass filter, phase reverse and pad that runs off phantom power. Damn that would be handy for live work.

rob wrote:standby for the next issue of AT for an article about impedance :)

man, couldn't come at a better time for me! Looking forward to it.