Inline high-pass audio filters

Chauffeur6 · #1 (**permalink**) 08-05-2009, 11:47 AM

Anyone have any experience with these, or something similar? I'm thinking of picking up a 200 or 300 Hz high pass to filter out some of the PL "hum" on a scanner audio output.

F-MODS In-line crossovers at Crutchfield.com

bassjunkie · #2 (**permalink**) 09-17-2009, 05:49 AM

Looks pretty good, do they require power?

Chauffeur6 · #3 (**permalink**) 09-17-2009, 11:04 AM

They're passive filters, not active. No power required.

kb2vxa · #4 (**permalink**) 09-17-2009, 05:38 PM

"Simply plug F-MODS crossovers in-line with the patch cords feeding your amplifier."

Critical information here, they cannot be used between the scanner speaker output and an external speaker amplified or not being both are low impedance devices. These are high impedance networks and must be used between the line (recorder) output and an external high impedance input power amplifier driving a speaker. KISS, keep it simple and use an external speaker having zero bass response, one strictly for mobile communications work such as the venerable Motorola Slim Line or equivalent.

Chauffeur6 · #5 (**permalink**) 09-17-2009, 06:07 PM

Who said anything about "speaker output"? I'm talking about line-level output between the unit and a PC for streaming/recording purposes. A 300Hz high-pass filter on said line would virtually eliminate PL hum (60Hz ground loop hum is already filtered out with a ground loop isolator/isolation transformer). I'm well aware of communication speakers whose frequency response starts at 300Hz, but again, this thread isn't about amplified signals. In fact, being a musician for nearly 30 years gives me some insight most don't have with regard to these applications dealing with audio signals.

dwh367 · #6 (**permalink**) 09-30-2009, 08:37 AM

Quote:

Originally Posted by Chauffeur6

Anyone have any experience with these, or something similar? I'm thinking of picking up a 200 or 300 Hz high pass to filter out some of the PL "hum" on a scanner audio output.

F-MODS In-line crossovers at Crutchfield.com

Just curious as to whether anyone has given this a try yet.

dwh367 · #7 (**permalink**) 10-04-2009, 05:03 PM

Since I guess nobody has given this a try yet, I am seriously considering it. However, the closest filter that Crutchfield has, that would filter out all of the PL tones, is a 500 Hz high pass. After thinking it over a little more in depth, wouldn't that also filter out some of the quick call II tones? They also have a 200 Hz high pass filter but there are still a few PL tones above that range or would the 200 Hz filter be better than nothing?

Chauffeur6 · #8 (**permalink**) 10-04-2009, 05:36 PM

Yeah 500 Hz is too high, 200 Hz is too low. The 300 Hz would be ideal. You can actually order them direct from Harrison Labs cheaper than Crutchfield has them.

Crossovers

Squad10 · #9 (**permalink**) 10-04-2009, 07:18 PM

Quote:

Originally Posted by Chauffeur6

Anyone have any experience with these, or something similar? I'm thinking of picking up a 200 or 300 Hz high pass to filter out some of the PL "hum" on a scanner audio output.

F-MODS In-line crossovers at Crutchfield.com

I don't have any f-mods experience. Have used the TS-64 to eliminate subaudible tone in scanners that I use audio accessories with, Since it is a CTCSS encoder-decoder, its highpass filter is designed for the correct frequency range. The TS-64 is pricey, but small enough to fit inside many handheld scanners. I've installed it right after the disc. output.

TS-64 Encoder-Decoder

Chauffeur6 · #10 (**permalink**) 10-04-2009, 07:53 PM

Another good option I didn't even think of, thanks Sq10.

kb2vxa · #11 (**permalink**) 10-05-2009, 02:04 AM

"I'm talking about line-level output between the unit and a PC for streaming/recording purposes."

Your purpose was never stated in your original post.
"Anyone have any experience with these, or something similar? I'm thinking of picking up a 200 or 300 Hz high pass to filter out some of the PL "hum" on a scanner audio output."
The words "audio output" make no distinction between line output and speaker output.

My response "These are high impedance networks and must be used between the line (recorder) output and an external high impedance input power amplifier..." pretty much covers it making your comments unnecessary.

"In fact, being a musician for nearly 30 years gives me some insight most don't have with regard to these applications dealing with audio signals."

That rather makes your question unnecessary now doesn't it? So why hassle this technician with 40+ years experience in audio applications among other things if you consider yourself on a par with me and my rather simple (KISS) style of answering questions? I see no difference between a power amp, sound card or similar hardware device having an unbalanced high impedance input where this application is concerned so why nit pick and try to make me out some kind of numpty when your question was answered properly?

OK, the bottom line here is the frequency range you want is 300Hz to 3KHz so the appropriate filters will eliminate what you don't want. Putting them back to back forms a bandpass filter so you won't have subsonics nor ultrasonics to contend with. Had you stated your question more to the point this is the answer you would have received from me at the start.

dwh367 · #12 (**permalink**) 10-05-2009, 04:04 AM

I hope I don't end up starting a war that will result in a thread lock, but I think I'm reading two different answers here. I am feeding the audio from the headphone jack of a Pro-2096 through a ground loop isolator and into the microphone jack of my computer which does not have a line in. I'm willing to pony up the money to see if it will work for the benefit of myself and others, but only if it has a half a chance of working. At this particular point, I'm not really sure if it will or not.

kb2vxa · #13 (**permalink**) 10-06-2009, 07:01 PM

Half a chance is better than no chance and IMO you have a good chance with the 200Hz high pass filter. I'd gamble the 30 bucks on it because while the highest PL tone is 254.1 few go that high. You seem to have your doubts about the 500Hz filter so the first choice is logical, the second questionable so you never really know until you try.

gmclam · #14 (**permalink**) 10-07-2009, 01:20 AM

If these filters operated as a "brick wall" then certainly 300 Hz is the value to get. I understand you want a filter which passes only frequencies above 300 Hz......

From the link you posted: "Simply plug F-MODS crossovers in-line with the patch cords feeding your amplifier. They filter out unwanted frequencies, with a 12 dB per octave slope. Low-pass models let frequencies below the cut-off point through and filter out the highs. High-pass models let frequencies above the cut-off point through and filter out the lows."

So my question is where is the '12dB' point? I think you may want the 500Hz filter which would have it's 12dB point one octave lower at 250 Hz. I believe the highest CT is 254.1 Hz so that would give you nearly 12dB for that tone and 24dB another octave down (125 Hz).

If the stated frequency is the 12dB point, then using the 300 Hz model means it starts to attenuate at 600 Hz.

dwh367 · #15 (**permalink**) 10-07-2009, 06:53 PM

I ordered the 300Hz High Pass filters today. I'll let everyone know how they worked when they come in and I get a chance to try them out.

911scanner · #16 (**permalink**) 10-18-2009, 01:05 PM

Sorry I'm late to this thread, but I have a lot to say (offer?)

I have used an F-Mod equivalent in the past. They work very well, so I'm sure you will have a good experience with them.

A high pass filter with a 12dB slope works similar to what was already explained, but let me add my $.02. At the next octave BELOW 300Hz, the output will be reduced by 12dB. It is an exponential drop, therefore at 2 octaves BELOW 300 Hz, there will be 24dB less sound, and so on. (It is BELOW 300Hz because it is a high-pass filter, which allows higher frequencies while CUTTING OUT the lower frequencies, thus the diminishment below 300Hz. From what I read, the OP & dhw367 seemed to understand this) The 300 Hz x-cover point is where the drop off of frequencies starts.

As for using a ground loop isolator dwh367, I just had to speak to that point. If you do indeed have noise in your set-up, then you will want to find out why and get rid of it. We used to have a saying in the +12v industry.... "Noise isolators are putting a band-aid on a broken leg". Meaning that while then can help or actually "eliminate" an offensive noise, it is better to find out the cause of the problem and fix it. If you have a ground loop problem, either make a better ground connection or find a better grounding point with less resistance (compared to a "zero ground" point, like the mounting bolt for the alternator). If you have an offensive power wire, then run your +12v feed away from that wire cluster. Not as easy, but it actually cures the problem vs. masking it.

Hope some of this helps. Feel free to contact me if you have any more questions about the F-Mods.

MM

gmclam · #17 (**permalink**) 10-18-2009, 05:02 PM

Quote:

Originally Posted by 911scanner

The 300 Hz x-cover point is where the drop off of frequencies starts.

Which is why I suggested that a 500 Hz filter would be closer to what is needed. The audio would start to be attenuated at 499 Hz and be 12dB attenuated at 250 Hz. Since the highest CT is 254.1, this would produce a better result than using a 300 Hz LP filter.