easy DIY passive filter

[PaRaNoId]

Is there a easy build for a very simple line-level (or mic level) passive high-pass filter? It's hard to find any designs catered to someone like me who only knows how to solder together XLR cables, etc. Ive read that all you need is one or two capacitors and a resistor. In fact I've read that over and over, but it doesn't tell an ignoramus like me how to piece one together. Any sites with super-simple designs like this? Or could it be as easy as connecting the hot pin to a cap (rating?) and then have that cap connected to the hot pin of the next cable? My ultimate goal was to piece together an 8 ch. XLR to XLR hi-pass filter set. Maybe 4 ch set at 100hz and 4 more set for 300hz....

[A27Hull]

The short answer is yes it can be done.

But someone more knowledgeable than me will have to help.

math wise, to figure a -6dB per octave high pass, use a little algebra on:

f = 1 / 2pi(XY)

X is an arbitrary resistance value

Y is an arbitrary capacitance value

f is the desired cutoff frequency (where the power attenuation acheives -3dB)

Using this formula, with 100 = f and 4.7uf = Y, I get a resistance of about 339Ω

Now, this is where I have to stop. I can't tell you the proper values appropriate here, nor can I tell how to implement it for a balanced line.

You may want steeper attenuation than -6dB per octave. At 50 Hz, if my thinking is correct, the attenuation will only be -6dB roughly. This will require more than a simple resistor/capacitor combination for passive implementation, maybe several back to back.

The real question IMO is whether or not it can be done practically, iow, can it be done for less that it would cost you to buy ready made items?

Good luck, and I hope to be schooled here as well.

[ulysses]

Yes, it really is that simple. A capacitor in series with the hot leg and another capacitor in series with the cold leg. The shield (pin 1) gets connected straight through.

And yes, it's as simple as calculating f=1/2πRC.

The complication is this: You get to choose C, and you most likely have some preference with regard to f, but the R factor will vary depending on what pieces of equipment you connect to it. If you use it to feed a microphone into a mike preamp with a typical 1300-ohm input impedance and you get a corner frequency of 250Hz, for example, and then you try to use it with a line-level source feeding a line-level input with a 25k input impedance, you'll discover the corner frequency is now down around 13Hz. You may not find that to be useful. Furthermore, using it with an input of unknown impedance means you won't know where the corner will be.

One solution might be to install a shunt resistor to force the R value in the equation into a particular ballpark. This resistor would be connected between pins 2 and 3 of the output XLR connector (after the capacitors). Problem is, in order for this resistor to effectively swamp wide variations in input impedances, it would have to be about one tenth the value of the lowest expected input impedance. In our example above, that would be about 130 ohms. Two big problems arise. First of all, you're loading the source very heavily, which may increase distortion in active circuits, change the frequency response of dynamic mikes, and combine with the source impedance to create a voltage divider that attenuates a bunch of the source signal. Secondly, the capacitance required to build a useful HPF working against such a low impedance would mean using physically large electrolytic capacitors. They wouldn't fit inside your XLR barrel connector, and their loose tolerances would cause an impedance mismatch between pins 2 and 3, which would destroy the common-mode rejection of the balanced circuit.

So the first thing we can do to save ourselves some headaches is to build two separate HPF gadgets. The first one will be specifically for use with mike preamps that have relatively low input impedances. The second will be built to work with the load impedances typically seen in line-level inputs and certain transformerless mike preamps.

Our "mike level" HPF might expect to see impedances ranging from 1000 to 10000 ohms. By using a shunt resistor of maybe 3000 ohms, we can be sure the impedance seen by the source will always be somewhere between 750 ohms and 2500 ohms. This means the corner frequency could vary over a factor of 3, but it's better than a kick in the pants. Going back to our 250Hz desired corner frequency and a 1300 typical load impedance (which will combine with our 3000 ohm shunt resistor to give 907 ohms), we find that we need 0.0000007 Farads of capacitance. That's 0.7µF (microfarads). But we actually have two capacitors in series in our circuit (one on pin 2, and one on pin 3). The signal current makes a round trip, passing through both of these capacitors. So we have to think about what happens when you combine two capacitors.

Capacitors are different from resistors. You put two resistors in series, and they add together and behave like one resistor whose value is the sum of the two. When you put two resistors in parallel, they sort of "divide". They behave like a single resistor whose value is the reciprocal of the sum of the reciprocals of the two resistor values. If they both happen to have the same value, we're in luck because this works out to being the single resistor value divided by two. (For seventeen identical resistors in parallel, it works out to the single resistor value divided by seventeen).
Capacitors are just the opposite. Put two capacitors in parallel, and they behave like a single capacitor whose value is the sum of the two. Put two capacitors in series, and they "divide" in the same way as parallel resistors. That is to say, the effective value of two capacitors in series is the reciprocal of the sum of the reciprocals of the two capacitors. Once again, if the two are of equal value this works out to the single-capacitor value divided by two. (For seventeen identical capacitors in series, it works out to the single capacitor value divided by seventeen).

Okay, so we have two capacitors in series and we want them to behave like a 0.7µF capacitor. Each capacitor will need to be 1.4µF. The nearest standard value is 1.5µF, and since we're using first-order approximations for the R value, we can settle for that here as well. Now we've got effectively 0.75µF working against a resistance that varies from 750 ohms to 2500 ohms, giving us a possible corner frequency ranging from 283Hz down to 85Hz. You'll have to look around and spend some money to find good, compact 1.5µF capacitors that are small enough to fit in a barrel connector, of high audio quality, and with a tight enough tolerance to preserve your common-mode rejection.

I'll let you do the arithmetic for our other example, in which we're expecting a line-level load impedance ranging between 10k and 50k ohms. I'll suggest a shunt resistor of 30000 ohms, which will mean the capacitors see an impedance ranging from 7500 ohms to 18750 ohms. Let's assume you're still looking for a 250Hz corner when used with your typical input impedance of 20000 ohms.

Here are the formulas you need:

f=1/2πRC

(Frequency equals the reciprocal of the product 2 time pi times resistance times capacitance)

Cp = 1/[(1/Ca)+(1/Cb)]

(The Capacitance Cp (formed by parallel capacitors Ca and Cb) is equal to the reciprocal of the sum of the reciprocals of capacitors Ca and Cb).

Let's see what you come up with for capacitor values! (Hint: They'll be a lot smaller than the ones in our mike-level HPF).

Two things I should mention about the above discussion. First of all, I have used the terms "impedance" and "resistance" almost interchangeably here. Impedance is a combination of resistance, capacitive reactance, and inductive reactance. We talk about the input impedance of a circuit because it may be a load resistance reflected through an inductance such as an input transformer. The effect is the same, as pertains to these calculations.

Also, I've completely ignored the source impedance of the microphone or circuit feeding the High-pass filter you're building. That's because it's usually rather small in relation to the load impedance, and disappears in a first-order approximation like we're doing here. If you wanted to consider source impedance, you would simply add it to the net load impedance (parallel combination of load impedance and shunt resistor) when calculating the corner frequency. Adding a typical 100-ohm source impedance to your 7500-ohm worst-case impedance will have a negligible impact on the corner frequency of your filter.

[Ethan Winer]

Originally Posted by PaRaNoId

Is there a easy build for a very simple line-level (or mic level) passive high-pass filter?

Here ya go:

Mike Pads and Other Small Gadgets

--Ethan

[A27Hull]

What would one do to change the order of the roll-off slope, add additional matched, other value caps? Say to reach -12dB passively?

Edit: Ethan's page suggests that higher order roll-offs leads to additional circuitry, and possibly more signal attenuation. Would the overall attenuation be so severe so as to negate the simplicity of passive design?

[ulysses]

Yeah, basically you would need additional filters, not just additional capacitors. Additional capacitors would simply combine with the first capacitors (as described above) to move the corner frequency of your 6dB/Octave passive filter. You could build multiple filter stages by isolating them with series resistors, and that's where your attenuation comes in. Or you could build multiple filters without attenuation by wrapping them around amplifier stages - and there goes your passive simplicity.
The good news is that the simple, gentle 6dB/octave slope of a passive filter is also the most benign filter in terms of in-band artifacts.

[tINY]

For a -12dB/octave passive filter, you get to play with inductors....

To fit those in a small box becomes a delicate balancing act of core size, saturation, and current density.

Or, like ulysses said, you could build a "tone stack" type network and make up the 10-30dB loss in your pre-amp.




-tINY

[A27Hull]

Thanks all!

[PaRaNoId]

Ok....for some reason i just can't put my head around all this. Right when i think i get it, a diagram shows up with something that confuses me.

Let's say i want a corner frequency of 100hz and i am doing the line level version expecting to see a 20k ohm console impedance....Is there a point to having a resistor in this config?

Also, my math skills are way gone. The reciprocal keeps tripping me up (pathetic, i know) With the above setup, what capacitors will i need to grab at radio shack?

[PaRaNoId]

just did the math using the equations on ethan's sheet; using a 20k console imput impedance, line source and desired 100hz corner. and got:

.0000017 ...am i way off here?

Does that mean i need two 1.7 (microFarads?) caps, one on hot and one on cold? Is there a specific orientation? By running pink noise into the filter i could later use an analzyer on my DAW to see a general idea of what's happening, correct?... I will be tinkering with the values by trial and error but just wanted to make sure my first numbers aren't waaaaay off. Thanks for all the help so far, you guys are the best!

[A27Hull]

What resistance value did you use? 20K?

oh, and scientific type calculators come in handy for reciprocal functions.


if you use no shunt, which I'm not familiar with the affects of not using one in passive filters, the capacitance for 20kΩ x 2pi x 100 Hz = 1/x comes to exactly 80 nf (nano)

to get 80 nf in parallel, one uses two .16uf (160nf) caps

Using a 1 megΩ shunt with a 20K input z gives equivalent 1/1meg + 1/20K =1/x

In this case x = Zeq of 19.6078 kΩ

You'll have to go way higher in the shunt resistor value to get the Zeq very close to the input impedance.

Now, using 19.6078K as R, puttinf that into orig formula, I get a capacitance of 81 nano farads...

Thats .081uf. or .162uf (162nf) per cap once doubled

[A27Hull]

Originally Posted by PaRaNoId

Does that mean i need two 1.7 (microFarads?) caps, one on hot and one on cold? Is there a specific orientation? By running pink noise into the filter i could later use an analzyer on my DAW to see a general idea of what's happening, correct?... I will be tinkering with the values by trial and error but just wanted to make sure my first numbers aren't waaaaay off. Thanks for all the help so far, you guys are the best!

If your math is right, yes. With no shunt, yes, exactly matched caps on both hot and cold. With a shunt, like this (edit the SR is can be anywhere along the line, and I put it in the middle, but the forum is shifting it way left..Ethan's sight shows something similar)

Hot --------C------------------------
l
l
SR
l
l
Cold -------C------------------------


Shield -------------------------------

Once you get it built you can test it with a signal generator and scope. Starting with a high frequency, slowly scale the generator down in frequency and watch the sine wave decrease in amplitude when it gets past the corner/cutoff freq. At that around 50 Hz, the voltage should have decreased t half of what say 200 Hz was.

I dunno if one can tell with pink noise, but surely if you are passing bass heavy line level signal through it, the bass will be less than usual. With a scope you could see exactly where (freq) it starts attenuating, and where its -6dB point is.

[A27Hull]

Also, try to match your caps if possible.

BTW, good luck with finding your caps at radio shack too..They really don't stock many different values and types. Your best best would be to order them. Perhaps more than a couple just in case. (they are probably cheap)

For matching, if you have an LCR meter or something, or access to one, try to find two that have the closest match.

Capacitors, like resistors have tolerances, because we live in an imperfect world. So, don't assume the caps will be their stated value. They are only guaranteed to be within so and so percent from the stated value, as per the tolerance rating.

[A27Hull]

Originally Posted by ulysses

I'll let you do the arithmetic for our other example, in which we're expecting a line-level load impedance ranging between 10k and 50k ohms. I'll suggest a shunt resistor of 30000 ohms, which will mean the capacitors see an impedance ranging from 7500 ohms to 18750 ohms. Let's assume you're still looking for a 250Hz corner when used with your typical input impedance of 20000 ohms.

12000Ω(2pi)(250Hz)=1/x

x = 53nf so,

106nf per cap.

Now to find the nearest value commonly produced...

[PaRaNoId]

OK... I'll try to make this even easier for my self, my math is awwwwfuuull (hopefully y'all aren't annoyed by me now)...

WHAT I DO GET---

freq(Hz?) = 1 / (2pi(R?)(C?)

1-pin#1 (ground/shield) goes through in this line-level, balanced circuit
2- these will be strictly operated at line level, going into a 20-30k ohms input
3- there will be identical ratings on capacitors on pins 2 and 3 (hot and cold)
each cap will be 2 times the value we get in the below equation for "C"
4- there should be a resistor of some type b/w pin 2 and 3

now..... this is what is completely killing me.....
what is "R" going to be? Is it expressed in ohms?
Is R the resistor value? Is R a value derived from eight other not-quite known factors, or is it simply 20,000 (ohms)? This is the element that i don't get. Ethan's layout looks good to me but it doesn't give me enough to find an R value considering the diagram shows an R1, R2, R22K, R and has unexplained numbers by the resistors.

What is C going to be expressed in? and what are my two caps going to be rated? and how to i say that and write it on paper I know i suck for asking someone else to do the math but im just trying to get a ball park estimate and not sure if im applying the info correctly.


Also....If price/time is no option, would certain brands/grades/types of caps and resistors improve/color the sound?

[A27Hull]

Originally Posted by PaRaNoId

freq(Hz?) = 1 / (2pi(R?)(C?)

yes, algebraically manipulated to 2pi(R)(freq)=1/C

or, C = 1/2pi(R)(f)

R in this case could be either or both resistance and input impedance. We sometimes use R and Z (impedance) interchangeably when the other factors of impedance are miniscule and relatively benign, such as in this case.

1-pin#1 (ground/shield) goes through in this line-level, balanced circuit
2- these will be strictly operated at line level, going into a 20-30k ohms input
3- there will be identical ratings on capacitors on pins 2 and 3 (hot and cold)
each cap will be 2 times the value we get in the below equation for "C"
4- there should be a resistor of some type b/w pin 2 and 3

1 yes
2 yes- but knowing the input impedance eases the calculation,
3 yes - as explained by Ulysses above, both are twice the calculated value, and both ratings are the same.
4 Yes and No. I've always thought of there being a "shunt" resistor across the lines, but thinking about it now, it would seem that the shunt resistor is really just a "load" of some sort. This means that we can forgo the extra resistor since the input impedance is the load.

Yes- if the input impedance is unkown, and you're trying to calculate R to make the total equivalent (extra resistor and the input impedance/resistance combined) the closest to the desired or specified input impedance.

Since there is no real easy way to determine exactly what the input impedance/resistance is, one uses a higher value resistor from 2 to 3 than the expected input impedance in order to make the equivalent resistance close enough to the desired/specified resistance/impedance.

No- if you don't care about trying to alter the input impedance, or you accept the specified impedance as your R value, then simply forgo the extra resistor and use the two caps of doubled value that we determine from the calculations.

now..... this is what is completely killing me.....
what is "R" going to be? Is it expressed in ohms?
Is R the resistor value? Is R a value derived from eight other not-quite known factors, or is it simply 20,000 (ohms)? This is the element that i don't get. Ethan's layout looks good to me but it doesn't give me enough to find an R value considering the diagram shows an R1, R2, R22K, R and has unexplained numbers by the resistors.

R is synonymous with both resistance, as when the load is purely resistive, but also with equivalent impedance, expressed in Z, but as for our case we're using Z as if it was a purely resistive load.

Impedance is really a combination of three things: resistance, inductance and capacitance. Since the input inductance and capacitance are relatively miniscule, we simply forget they are part of the equation and just use the input resistance value (20-30kΩ) and call it impedance.

Yes. R is the resistance value. Here we've been combining the guestimated R value with the input impedance by another formula, the formula for parallel resistance.

1/Rtotal = 1/R1 + 1/R2 ...+ 1/R (so and so)

it happens to be just like the capacitance formula used above..

1/Ctotal = 1/C1 + 1/C2 ...+ 1/C (so and so)

What is C going to be expressed in? and what are my two caps going to be rated? and how to i say that and write it on paper I know i suck for asking someone else to do the math but im just trying to get a ball park estimate and not sure if im applying the info correctly.

Capacitance (C) is expressed in some amount of Farads

For this case, your circuit will require caps in nano farad range. Thats 1x10^-9 or 0.000000(000).

As for voltage, line level at odBu is somewhere around .775 volts, so a 5v rating or higher should suffice.

Its alright to ask for help. I've learned a lot about the subject from thinking and calculating it myself with the help from those above.

Also....If price/time is no option, would certain brands/grades/types of caps and resistors improve/color the sound?

I can't offer suggestions as per the sound of a capacitor compared to another, but I can offer the offered values.

Looks like your stuck with two 150 nano (.15uf), capacitors or your choosing. Some type of film cap. Voltage ratings won't be a problem.

Here's a couple, but like I said, I can't speak on quality or characteristics.

WIMA

WIMA

[ulysses]

If you know the input impedance of the load that will be fed by this filter, then you don't need the shunt resistor.

Use a calculator that has a "1/x" button. That will save you lots of grief.

Your formula is: 1/(20000 * 100 * 2 * π)

Multiply out the denominator first (20k * 100 * 2 * π). Then hit the 1/x button on your calculator.

You should get 0.000000079577 Farads. Multiply that by a million to get microfarads. Approximately 0.080µF. Each capacitor will need to be double this value, or .16µF. The nearest common value is 0.15µF, so I would probably use that. If you want it to be exactly 100Hz, then you could round the capacitor value UP and then bring the R value DOWN by adding a shunt resistor. But this is a gradual filter, so I don't think you'll notice too much if the filter works at 106Hz instead of 100. So sticking with 0.15µF caps will be fine. If you're worried about it, 0.16µF caps can be found, or you can put a 0.01µF cap in parallel with each 0.15µF cap.

In this range of capacitor values, you're going to be looking at film dielectric. Polyester is the most common, but polypropylene performs a little better and should be fairly easy to find in this size. The voltage rating isn't too important, unless you expect to see some significant voltage (like if you were going to use it on a mike preamp input that had phantom power active - which would be pointless because the capacitors would prevent the phantom power from reaching the microphone. Polarity is not an issue for film capacitors, so it doesn't matter which way they face. What does matter is that the two capacitors be well matched. You can pay a little extra to get caps with a 5% or even sometimes 1% tolerance, which will give better noise rejection than caps with say a 10% tolerance. But another option would be to buy a handful of caps with 10% tolerance and measure them with a multimeter that measures capacitance. Pick two that give exactly the same reading on the meter. They don't have to be the two that are closest to the nominal value, they just need to be the two that are closest to one another. If you don't have a meter that measures capacitance, then don't worry about it. Just by the caps with the tightest tolerance you can find.

If you go to digikey.com and search for "capacitor" you'll find a bunch of things. Choose "poly film" and you'll get a parametric table that lets you narrow down the choices to help you pick something. You can choose a few different capacitance values (like 0.15, 0.16, and 0.18µF, for example); choose to only see parts they have in stock (they have thousands listed that are not stocked); choose only through-hole parts if you don't want to screw around with surface-mount caps). Choose parts sold in bulk or on cut tape so you don't have to buy a full reel (which is several thousand pieces).This will get you down to 128 choices and now it's time to make some decisions. If you don't have a capacitance-measuring multimeter, then select tolerances between 1% and 5%. If you know something about the space in which you have to fit these two caps, make some decisions that way (axial-lead caps may be easier to work with, but the selection is limited. Radial-lead caps may require a bit more space). Polypropylene caps will be physically larger and more expensive. If you're trying to fit this into a barrel connector, you may have to use axial-lead metallized polyester caps such as B32232A1154K. (Metallized film caps are generally more compact.) If space is not an issue for you (if you're building it in a little box or on a rack panel) then you could choose a 1% tolerance, polypropylene film capacitor such as ECQ-P1H164FZW.

[PaRaNoId]

awesome! I really appreciate the effort you guys have put into helping me. That is the reason i love this board so much! I will post when i finish to let you know how i did...-JON

[PaRaNoId]

will these do?


BFC238301164

[A27Hull]

yes, those are fine.

[PaRaNoId]

I did it! Yeah me!

I bought two polypropelyne .15 microfarad caps and soldered them b/w pins 2 and 3 with the ground going straight trough. On my DAW, i ran pink noise out and back in through my patch bay. Then using a freq analyzer (Voxengo SPAN) i measured the result- a very flat line. Then i patched into the cicuit i made (very crude TRS to TRS thing) and tested it again. My analyzer showed a very steady dip starting at about 250 hz and sounded relatively uncolored. So now, im guessing if i want to force the corner freq down, i should install a shunt resistor of a value of 20k or greater? Or is that because i should have used .16 micro farad caps? This is really fun!

[ulysses]

Originally Posted by PaRaNoId

I did it! Yeah me!

I bought two polypropelyne .15 microfarad caps and soldered them b/w pins 2 and 3 with the ground going straight trough. On my DAW, i ran pink noise out and back in through my patch bay. Then using a freq analyzer (Voxengo SPAN) i measured the result- a very flat line. Then i patched into the cicuit i made (very crude TRS to TRS thing) and tested it again. My analyzer showed a very steady dip starting at about 250 hz and sounded relatively uncolored. So now, im guessing if i want to force the corner freq down, i should install a shunt resistor of a value of 20k or greater? Or is that because i should have used .16 micro farad caps? This is really fun!

Glad you're learning and even gladder you're having a good time doing it.

Adding a shunt resistor (to the downstream side of the capacitors) will RAISE the corner frequency, not lower it. The C and the R are both in the denominator of our equation

f = 1/2πRC

which means that making R or C higher will make f lower, and vice versa. Your shunt resistor will be in parallel with the actual load impedance, and serve to lower the effective load impedance seen by the capacitors.

By the way, the reason this equation works is because you're finding the frequency at which the impedance of the capacitor is equal to the resistance of the load. Resistors, in theory, have the same impedance at every frequency. It's not dynamic or "reactive". Capacitors, in theory, have an impedance that continuously declines as frequency increases. This is called Capacitive Reactance because the impedance changes (reacts) based on the frequency in question. The 3rd component in this train of thought is an inductor, whose impedance (again, in theory) continuously increases with frequency, just the opposite of a capacitor. Its impedance is called Inductive Reactance.
So, if you have a series impedance and a shunt impedance, you get a voltage divider. If one or both of the impedances is reactive, it will be a voltage divider whose attenuation is frequency-dependent. In the case of your hi-pass filter, the series element decreases with frequency while the shunt is fixed. If you swapped them (series resistor with a shunt capacitor), you'd get a low-pass filter instead. Or, if you replaced the series capacitor with an inductor, you would also get a low-pass filter. These building blocks let you build many different kinds of filters.

[A27Hull]

I manipulated the formula to make R = 1/2pi(.075uf)(250)

Assuming Ceq = 75 nanofarads and 250 corner frequency.

That suggets an Zeq or Req of 8488.3Ω

Also, to define the cutoff frequency, we use the -3dB point in the downslope. So, if it starts sloping at 250, it may actually be near 100 Hz by the time the slope reaches -3dB. At first thought I guess it shouldn't be so high. However, it may be due to our gradual slope - because we are using first order filters (-6dB per octave.)

Best I can tell you is to experiment some more!

I can't tell you why it is so high. Someone with more experience will have to tell us.

[A27Hull]

Originally Posted by ulysses

which means that making R or C higher will make f lower, and vice versa. Your shunt resistor will be in parallel with the actual load impedance, and serve to lower the effective load impedance seen by the capacitors.

But if we didn't use a shunt in the first place, the input impedance or R in this case can't be raised. Is this right?

So, to force it lower, one would have to raise the cap value.

Using 8488Ω (not sure thats the correct Zeq) one would need approx .376uf on each leg

[ulysses]

Originally Posted by A27Hull

But if we didn't use a shunt in the first place, the input impedance or R in this case can't be raised. Is this right?
So, to force it lower, one would have to raise the cap value.

The other way to do it would be to install series resistors after the series capacitors. This would INCREASE the load impedance as seen by the capacitors and by the source. It would lower the corner frequency without increasing the size of the capacitors. The downside would be a deterioration of the signal-to-noise ratio (since it will also attenuate the in-band signal going into the load). So it's better to achieve this by increasing the size of the capacitors, until some other factor comes into play that makes that a worse idea.

[A27Hull]

Ah! Thank you!

[ripple_fx1]

Hello - does the math still work if the Rin of the mic pre is actually mostly (600-ohm stated xformer) Xl?
I've got a OpampLabs module I'm trying to hi-pass with a 200Hz corner.
There are small coils and caps for RF mitigation on the panel XLR.
Have a look and let me know if this is just an awful mess....
I could filter post module, but would prefer not to.
Cheers! JR

[GAS]

Hello, Just letting you know that I've had a 4 channel passive low cut filter in the works for a while and will be available really soon.

Check out recordingwithgas.com soon for more details.

[bren-hur]

Originally Posted by Ethan Winer

Here ya go:

Mike Pads and Other Small Gadgets

--Ethan

WOW! sick site... and very sick calculator tool... just made my life a hell of a lot easier, being not so great at the mathy type things...

[bren-hur]

Kind of an old thread, but thought i'd throw this out there anyway...

This has inspired me to put a Low cut filter in my preamp. I went through that wicked calculator tool and came up with the values of 6.9UF for a 100 cut and 2.3 for 300, this is for a 600ohm input impedance... I want to put it on a DP3T switch (on off on) and came up with this:


whereby with the switch in POS 1 (on) the 2.2 is in the circuit but bypassed giving no filtration...
In POS 2 (off) the 2.2 is in the circuit giving me (hopefully) a 300hz cut...
In POS 3 (on) the 2.2 and 4.7 are in, running parralell, giving me 6.9 and a 100hz cut...

Or so I think... I didnt put a shunt resistor in coz it was easier to put on a switch this way and I know what the input impedance is...

Does this look ok?? or is my theory flawed....