Building some passive filters - Page 1
Forum section: electronics / beginners

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 02, 2025, 08:28:42 am
content: I want to make some filters for a couple of things. The first being between my antenna and my preamp, and the second being between my mixer and my headphone amp. I only have parts to make RC filters right now. I was thinking about a first order 330~ KHz filter (1 ohm resistor and 0.47uF cap), but I have no idea how many watts the resistor should be rated for. Should I be using ceramic resistors? The line to the headphone amp is probably easier because I want to have the cutoff at somewhere between 500-1000Hz so that I'm only getting the difference from the mixer and not the sum.I'm having a hard time finding premade parts for this. I just don't know all the terms well enough I think. What I would really like to make ideally is a way to select different cutoff frequencies from the antenna by being able to switch between different caps or resistors...Any advice on this would be helpful. Thanks

author: ahsrabrifat
author_rank: Regular Contributor
timestamp: June 02, 2025, 03:23:08 pm
content: You have to use the formula P=I*I*R to calculate the wattage.

author: fourfathom
author_rank: Super Contributor
timestamp: June 02, 2025, 03:38:11 pm
content: Quote from: Amish_Fighter_Pilot on June 02, 2025, 08:28:42 amWhat is the output impedance of your antenna? If it's 1 Ohm then your series resistance will be 1+1 Ohm, giving you a 170 KHz low-pass. If it's 50 Ohm you get 50 + 1 Ohms, for a 6.6 KHz low-pass. The same applies for your mixer-output filter -- what is the mixer output impedance? The preamp input impedance also matters, unless it's much higher than your filter values.As for power, unless you are extremely close to a transmitter or other strong signal source the dissipation in your resistor will be in the milliwatt region at most. Virtually *any* resistor will be fine.

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 02, 2025, 04:15:00 pm
content: A 330 kHz, first order filter, between an audio mixer and a headphone amplifier.Hmmm. Hmmmm. Why? What do you expect to achieve?Are you having radio frequency interference, causing that you can hear some AM broadcasts on you headphones?If so, the National Semiconductor Audio Handbook has some excellent recommendations. Tell me if this is the case, to take a photo of the relevant information.

author: TimFox
author_rank: Super Contributor
timestamp: June 02, 2025, 04:23:29 pm
content: Many times, unwanted interference from strong AM broadcast is due to rectification by the transistors (including those in ICs) at the input of an audio amplifier, and cannot be removed by filters further downstream in the amplifier.Sometimes, an appropriate capacitor from base to emitter of the input transistor (in a discrete-transistor design) works better than one from the input to ground to minimize this rectification.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 02, 2025, 07:01:58 pm
content: Quote from: UnijunctionTransistor on June 02, 2025, 04:15:00 pmThe filter between the mixer and the headphone amp is 500Hz, not 330KHz. That filter is for the antenna preamp.

author: TimFox
author_rank: Super Contributor
timestamp: June 02, 2025, 07:08:37 pm
content: Quote from: Amish_Fighter_Pilot on June 02, 2025, 07:01:58 pmIf filtering audio, why not filter at 20 kHz or slightly higher?A high pass filter at 500 Hz removes everything below approximately C above middle-C on the musical scale; conversely for a low pass filter.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 02, 2025, 07:49:41 pm
content: Quote from: fourfathom on June 02, 2025, 03:38:11 pmThank you! This is exactly the sort of feedback I needed. I totally forgot to factor in the impedance of the system. I am still building the antenna, but it should be done in a day or two as I have time to work on it. I THINK it will be in the 150-200uH range with the wire I have available and the pattern I am following. It's a small spiderweb style antenna.My LNA is a Chinese model(LNA9037BAT) that CLAIMS to work from 10KHz-6GHz, and says 50ohm on it. The mixer is an AD831. The LO is just a function generator. What I am still less clear about is how much signal I am going to get from my transmitter. I'm temporarily using a function generator feeding an audio amp(TDA2050), but it is limited to about 80KHz maximum. I will be using a very small antenna relative to the wavelength(I estimate it to be 1/1,968th of a wavelength at the lowest frequency and 1/246th at the amp's limit). So the transmission efficiency is probably terrible. That's fine though since I'm transmitting and receiving in the same room. It will be rare that I even transmit over 3 meters from the receiving antenna. So very poor antennas on both Tx and Rx. How much signal I get at the LNA is still totally unclear to me.I'll have to measure the headphone amp's input impedance when I can get out to the shop. I suspect it's 50K Ohm if it's typical.My goal primarily is to remove as much signal above 450-500KHz(even 300KHz ceiling would probably be just fine). I have a bunch of ferrites too, but I don't know if they'd be of use in this project...edit - I forgot to add that I currently have no idea whatsoever what sort of transmitter antenna I want to use. Given I am transmitting over such extremely short distances would it actually be better to use a ferrite core antenna? What style should I even use? I want it to be very small. I think it might even be good if it was smaller than the Rx antennas.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 02, 2025, 07:56:09 pm
content: Quote from: TimFox on June 02, 2025, 07:08:37 pmI'm using a beat frequency; so I only need a certain window of frequencies around it. I only want the difference from the mixer and not the sum. Setting the low pass so low avoids problems when the sum is in audio ranges(which would manifest as high pitched squealing).

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 03, 2025, 02:11:53 am
content: Quote from: Amish_Fighter_Pilot on June 02, 2025, 07:01:58 pmCorrect!I had initially misread your post.

author: Terry Bites
author_rank: Super Contributor
timestamp: June 03, 2025, 04:10:34 pm
content: If this is to combat interfercence issues you need to examine and explain your system before using brute force filtering.You cant deisgn a passive filter without knowing the impedances in your system.Lets say you had 30ohm headphones and you want a 750Hz lowpass. Rule of thumb, the series resistor needs to be ten time smaller, ie 3 ohms. Then you would need a 6.8UF cap.Is this a balanced line system?An antenna connected to a preamp??If you use a lp filter how will any RF reach a reciever??What is the actual source of the interference?A cutoff of 500Hz to 1kHz is bang in the middle of your audio. Are you sure about that?You need to provide a much more accurate description to get sensible assistance here. Some photos maybe?

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 03, 2025, 11:30:27 pm
content: I will be able to put together some pictures when my LNAs show up. It does seem like I may need to consider other solutions.The best comparison I can find is the following system on YouTube. It's very similar to my setup other then the frequency range. Where he has the AM Distill bandstop filter: I need either a 400-500KHz low pass(it probably would be good to start the roll off lower than the target), or I need a bandpass filter that cuts off everything under 100Hz(to get rid of grid noise) and everything over ~400KHz.https://youtu.be/MV96ih1Tx38?si=2WykOJ44bxGuY4bW

author: radiolistener
author_rank: Super Contributor
timestamp: June 03, 2025, 11:38:18 pm
content: very strange decision to use RC filter, because it has pretty bad performance and loss. Much better is to use LC filter, for exampe 5-th or 7-th order Butterworth or Chebyshev

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 04, 2025, 07:09:56 am
content: Quote from: radiolistener on June 03, 2025, 11:38:18 pmIt makes perfect sense considering most of my electronic parts are salvaged. I don't have a lot of small inductors in my sorted bin, but I have a few computers that I need to scrap that have all sorts of fun stuff in the PSU.Thank you for the suggestions. I will attempt to figure those out. My experience with low pass filters is public address audio systems related so RC is used a lot. It seems like implementing such a method in my pursuit is impractical.

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2025, 07:52:22 am
content: Quote from: Amish_Fighter_Pilot on June 04, 2025, 07:09:56 amPassive RC filters can be acceptable when driven from a low impedance source and output into a high impedance load, which is the case for many audio circuits. When you have an impedance matched system (e.g. antennas) it's a terrible choice as the filters impedance naturally varies with frequency.

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 04, 2025, 07:56:10 am
content: That makes sense thank you. I'll try and draw something in LTSpice and see what I can figure out.

author: radiolistener
author_rank: Super Contributor
timestamp: June 04, 2025, 08:31:51 pm
content: Quote from: Amish_Fighter_Pilot on June 03, 2025, 11:30:27 pmHere is 5-th order Butterworth and Chebyshev LPF for Z=50 with 500 Hz cutoff, both calculated in Octave for two config - Pi and Tee, so you can select which one better fits your needs.Code:Z: 50.0 ΩFc: 400 kHzAp: 0.1 dB (Chebyshev)Butterworth LPF: Pi | TeeC1: 4.92 nF | L1: 12.3 uHL2: 32.2 uH | C2: 12.9 nFC3: 15.9 nF | L3: 39.8 uHL4: 32.2 uH | C4: 12.9 nFC5: 4.92 nF | L5: 12.3 uHChebyshev LPF: Pi | TeeC1: 9.13 nF | L1: 22.8 uHL2: 27.3 uH | C2: 10.9 nFC3: 15.7 nF | L3: 39.3 uHL4: 27.3 uH | C4: 10.9 nFC5: 9.13 nF | L5: 22.8 uH

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 06, 2025, 11:03:43 am
content: Thank you very much that is very helpful!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 08, 2025, 07:01:32 pm
content: I cant find any actual context in this thread. I'm guessing that this is some kind of LW reciever, but my mindreading skills only extend so far.Its worth going into detail because there are many aproaches to defeating unwanted signals. What kind of antenna are you using?Loop antennas can be easily tuned to reject out of band nuisance frequencies.Its going to be hard to implement the required LC filter for mixer image rejection from junk box parts because of the high capacitance and inductance values required.You might be able to slavage indutors from an old radio. The IF transformers within will have values in your ballpark.Ready made parts are the way to go though.This is one of the best online filter calculators I've seenmarkimicrowave.com/technical-resources/tools/lc-filter-design-tool/

author: Amish_Fighter_Pilot
author_rank: Regular Contributor
timestamp: June 09, 2025, 11:44:34 am
content: Quote from: Terry Bites on June 08, 2025, 07:01:32 pmI have a bunch of different home theater receivers in my storage unit and several of them need to be scrapped. I probably already have parts with the required values though. I'm still new to this though and I am trying to understand the virtues of the antenna I just built. I'll try to get pics up when I can. I will be replacing the steel with ferrite eventually.Anyway, the system can be a lot of things. I'm mostly using it for a sound sampling project, but I'm mostly just playing around. I'm really going to need several filters or an adjustable one for my low pass. I probably will also do a high pass to get rid of grid noise. I guess that makes it a bandpass technically right?

author: radiolistener
author_rank: Super Contributor
timestamp: June 09, 2025, 07:03:13 pm
content: calculating Chebyshev and especially Butterworth is pretty easy, here is Octave code that I wrote to calculate it for any order:Code:% Caluclate analog LC-filter elementsZ = 50; % in/out port impedanceN = 7; % Filter OrderAp = 0.1; % passband ripple in dBfc = 500; % cut-off in Hzwc = fc * 2*pi; % Hz to rad/sfprintf('Z: %sΩ\n', formatUnit(Z));fprintf('Fc: %sHz\n', formatUnit(fc));fprintf('Ap: %g dB (Chebyshev)\n', Ap);function s = formatUnit(x)% Format number with SI prefixprefixes = {'f', 'p', 'n', 'u', 'm', '', 'k', 'M', 'G', 'T'};exponents = -15:3:12;if x == 0s = '0.000 ';return;endexp = floor(log10(abs(x)) / 3) * 3;exp = max(min(exp, max(exponents)), min(exponents)); % clamp to supported rangeidx = (exp + 15) / 3 + 1; % index into prefix tableval = x / (10 ^ exp);if val < 1s = sprintf('%.3f %s', val, prefixes{idx});elseif val < 10s = sprintf('%.2f %s', val, prefixes{idx});elseif val < 100s = sprintf('%.1f %s', val, prefixes{idx});elses = sprintf('%.0f %s', val, prefixes{idx});endendfunction printElement(idx, g, Z, wc, ishpf)for i=idxCtxt = sprintf(' C%d: %9sF', i, formatUnit( g(i) / (Z * wc) ));Ltxt = sprintf(' L%d: %9sH', i, formatUnit( g(i) * Z / wc ));if mod(i, 2) == (~ishpf) % 1 for lpf, 0 for hpffprintf(' %-18s|%s\n', Ctxt, Ltxt);elsefprintf(' %-18s|%s\n', Ltxt, Ctxt);endendendfprintf('Butterworth LPF: Pi | Tee\n');g = 2*sin( (2*(1:N)-1)*pi/(2*N) );printElement(1:N, g, Z, wc, false);fprintf('Chebyshev LPF: Pi | Tee\n');epsilon = sqrt(10^(Ap/10) - 1); % ripple factorbeta = asinh(1 / epsilon) / N;gamma = sinh(beta);A = sin((2*(1:N) - 1)*pi / (2*N));B = gamma^2 + sin((1:N)*pi/N).^2;g = zeros(1, N);g(1) = 2*A(1)/gamma;for k = 2:Ng(k) = 4*A(k-1)*A(k)/(B(k-1)*g(k-1));endprintElement(1:N, g, Z, wc, false);As you can see calculating it is very easy by knowing pre-calculated g:3-th Butterworth g = [ 1.0, 2.0, 1.0 ]5-th Butterworth g = [ 0.618, 1.618, 2.0, 1.618, 0.618 ]7-th Butterworth g = [ 0.445, 1.247, 1.8019, 2.0, 1.8019, 1.247, 0.445 ]3-th Chebyshev g = [ 1.0316, 1.1474, 1.0316 ]5-th Chebyshev g = [ 1.1468, 1.3712, 1.975, 1.3712, 1.1468 ]7-th Chebyshev g = [ 1.1812, 1.4228, 2.0967, 1.5734, 2.0967, 1.4228, 1.1812 ]You can get L and C values by substitute 1, 2 and 1 as g(i):wc = fc * 2*pi;C = g(i) / (Z * wc);L = g(i) * Z / wc;For LPF Pi use C, L, C sequence. For LPF Tee use L, C, L sequence.For HPF use L instead of C and C instead of L.3-th Butterworth LPF Pi:C1 = 1 / (Z * wc);L2 = 2 * Z / wc;C3 = 1 / (Z * wc);5-th Butterworth LPF Pi:C1 = 0.618 / (Z * wc);L2 = 1.618 * Z / wc;C3 = 2 / (Z * wc);L4 = 1.618 * Z / wc;C5 = 0.618 / (Z * wc);