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diy hpf? Condenser Microphones
Old 4th September 2007
  #1
Gear Head
 
the yeti's Avatar
 

diy hpf?

is there a simple DIY high pass filter, passive preferably, plan? i would imagine it would be simple, either fixed at one point or on a switch for multiple points. i have a mercury m72s and love it on a lot of different sources but a hpf would be handy at times.
Old 4th September 2007
  #2
Lives for gear
 
ulysses's Avatar
A simple, 1st-order high-pass filter can be constructed with as little as a single capacitor. If you're trying to build one external to any other piece of equipment, you might consider building it into an XLR barrel connector. You would simply use capacitors instead of plain wire to connect the pin 2 input to the pin 2 output, and to connect the pin 3 input to the pin 3 output. Pins 1 would be connected with wire.

Pin 2 >----------) (----------> Pin 2

Pin 3 >----------) (----------> Pin 3

Pin 1 >-----------------------> Pin 1

In order to determine the correct value of capacitors, you need to know the load impedance of the piece of gear connected to the output of the filter, and you need to know the corner frequency you would like to have. (The source impedance of the gear feeding the filter factors in too, but in any modern, professional line-level gear, the source impedance will be low enough to be insignificant compared to the load impedance).

The two capacitors should both be of equal value, and should be very closely matched in order to preserve the low-frequency noise rejection ability of a balanced line.

In a balanced connection, the two capacitors effectively appear in series with one another, and so their net effective capacitance will be half of the individual capacitor's value. The formula is as follows:

C = 1/(2*π*R*ƒ)

Where
C = net effective capacitance, in Farads
π = 3.14159
R = the load impedance seen by the filter
ƒ = the desired corner frequency

For example, if you're feeding the filter from a low-impedance balanced source and into a good balanced load with an impedance of, say, 10k ohms, and you would like a 100Hz corner frequency for your HPF, then you need a capacitance of:

C = 1/(2*π*10000*100) = 0.000000159155 Farads, which is 0.159155µF (microFarads). In order to get this value from two caps in series, each cap needs to be twice this value, or 0.318µF. The closest readily-available value will be 0.33µF, which is a common value and fairly easy to find in good quality polyester or polypropylene packages which should fit into the barrel connector without much trouble.

But, what happens if you use this filter to connect to a different piece of gear? Suppose you use it to feed a high impedance (47k ohms), unbalanced input? Typically this would mean connecting only pin 2, and not pin 3, and using pin 1 for the ground connection. Now you have twice as much capacitance, and almost five times as much impedance. The corner frequency changes accordingly:

ƒ = 1/(2*π*47000*0.00000033) = 10.26Hz. That's not really going to work as a high pass filter. But this is what you get with a passive filter. It works great, so long as you can control how it gets connected. Making it more immune to loading is achieved one of two ways: Either you artificially load it with a resistor that swamps the variable load impedance it might see (which the source may not appreciate), or else you build in a buffer amplifier so that the load seen by the capacitor never changes. In most cases, the passive filter will work pretty well and they're cheap and easy enough that you can build a few different ones to accommodate different scenarios you might encounter. OR you could build a larger box with input and output connectors and a switch to allow you to change the capacitors, or change a resistive load, to alter the corner frequency.

Another scenario you might encounter would be if you wanted to use the same XLR barrel connector high-pass filter on a microphone line. The same principles apply, but the impedances are different. A typical dynamic microphone might have a source impedance of 150 ohms, and a typical mike preamp input might have a load impedance of 1200 ohms. In this case, the source is a significant enough portion of the load that it should be factored into the calculations. In the case of a series-connected capacitor such as our HPF, the source and load impedances appear in series. So our formula is:

ƒ = 1/(2*π*1350*0.000000165) = 714Hz. That's quite a bit higher than the 100Hz we get when we connect it to typical line-level gear. To get it back down to 100Hz, we would have to make each capacitor 7.14 times larger than our 0.33µF values. That's about 2.36µF each, which is close to the standard value of 2.2µF. These will be physically larger capacitors, which may mean a larger enclosure or a different style of capacitor - perhaps polyester instead of polypropylene. Another thing to consider is that you may want to protect these caps against accidental exposure to phantom power (you can't actually use them on a microphone that requires phantom power from the preamp, because the caps will block the phantom power from reaching the microphone). Choosing capacitors that are rated for at least 50V is a good idea (higher is okay).

Passive filters can also be made higher-order, for steeper low-frequency roll-off. The easiest way to do so would be to connect a large inductor from pin 2 to pin 3, after the capacitors. But large inductors are bulky and expensive, and open up a whole slew of other issues. You can also chain several capacitive filters together, with resistors between them. But a 1st-order filter is the most benign and should meet most needs most of the time. Start with that and see what you think.
Old 5th September 2007
  #3
Gear Guru
 
Ethan Winer's Avatar
 

Lightbulb

Quote:
Originally Posted by the yeti View Post
is there a simple DIY high pass filter, passive preferably, plan?
This is exactly what you're looking for:

Mike Pads and Other Small Gadgets

--Ethan
Old 6th September 2007
  #4
Lives for gear
 
paully's Avatar
 

Here's a great resource for DIY: LC Filter Design .

Best, Paul
Old 7th September 2007
  #5
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BradM's Avatar
Paul,

I tried to test out that website and I keep getting errors. Any tips to getting the thing to generate a filter? Have you used it successfully before?

thanks,
Brad
Old 7th September 2007
  #6
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paully's Avatar
 

Quote:
Originally Posted by Brad McGowan View Post
Paul,

I tried to test out that website and I keep getting errors. Any tips to getting the thing to generate a filter? Have you used it successfully before?

thanks,
Brad
>>>>>>>>>>>>>>>>
Yes. This thing used to work. I just tried it in various forms and also got nothing but errors . Sorry for the wrong turn. Their problem, not yours.

Best, Paul
Old 7th February 2013
  #7
Gear Nut
 
mogazi!'s Avatar
Hi,
sorry to resurrect this ancient post but would like to have a go at building a HPF myself and needed some guidance. As suggested by Ulysses I would make two versions; one for an unbalanced inputs and the other for balanced XLR inputs. But I'm finding the specification of my desk for the unbalanced part a bit confusing:

MIC/LINE INPUT - UNBALANCED
SOURCE IMPEDANCE.......10k ohms or less
INPUT IMPEDANCE........100k ohms


Which of these two values should I take into consideration when choosing the right capacitance condenser?
Cheers
Old 30th July 2014
  #8
Would it be possible to make one that does pass phantom power?
Old 30th July 2014
  #9
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Richard Crowley's Avatar
 

Quote:
Originally Posted by Cory.R.McLeod View Post
Would it be possible to make one that does pass phantom power?
Yes, but it would be considerably more complex and involve active circuits. So complex and fiddly that there probably aren't any examples of commercial products. If you have a phantom-powered mic, then we typically use the EQ controls in the mixer.
Old 4th March 2017
  #10
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travisbrown's Avatar
Quote:
Originally Posted by Richard Crowley View Post
Yes, but it would be considerably more complex and involve active circuits. So complex and fiddly that there probably aren't any examples of commercial products. If you have a phantom-powered mic, then we typically use the EQ controls in the mixer.
These pass phantom (in addition to being powered by it).
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Old 4th March 2017
  #11
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Richard Crowley's Avatar
 

Quote:
Originally Posted by travisbrown View Post
These pass phantom (in addition to being powered by it).
The AT8682 UniGate® was a "Voice-operated Switch with Adjustable Threshold" It is easy to simply switch a signal on/off without respect to levels or impedance.

The original question was about a filter. Analog filters are much more complex and very dependent on source and load impedances. Creating a stable "universal" filter that will pass phantom and work with unknown source and load impedances is nearly impossible.
Old 4th March 2017
  #12
Gear Maniac
 

AT8683 UniSteep

AT8683, which is hipass filter. Discontinued!

edit: as Richard said, they are active, a preamp for your preamp. They use nearly half of the channel's available phantom current, add a bit of noise, probably really useful in specific situations. Probably was not a big seller

Last edited by rfnoise; 4th March 2017 at 05:08 PM..
Old 4th March 2017
  #13
Lives for gear
 
travisbrown's Avatar
*nb. I saw after that rfnoise beat me to a response.*

Quote:
Originally Posted by Richard Crowley View Post
The AT8682 UniGate® was a "Voice-operated Switch with Adjustable Threshold" It is easy to simply switch a signal on/off without respect to levels or impedance.

The original question was about a filter. Analog filters are much more complex and very dependent on source and load impedances. Creating a stable "universal" filter that will pass phantom and work with unknown source and load impedances is nearly impossible.
Right. This is an 8683 UniSteep though, a fixed HPF. I think they passed at around 85hz. Audio Technica had other utility boxes in this line besides this and the gate: also a variable Limiter. UniLimit or something.

I'm not arguing whether it worked well or not. You had just said that you were aware of any commercial products that were active, passed phantom power. For better or worse, this did.

I believe this was meant specifically for between the microphone and preamp. Perhaps it was designed and functioned better for the narrower range of impedances of low-z mics, as opposed to the greater Z range of line level gear. You likely have more knowledge to assess that than me .
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