Crossover frequency of a 2.1 speaker system - Page 1
Forum section: electronics / beginners

author: poot36
author_rank: Frequent Contributor
timestamp: October 19, 2014, 10:21:23 pm
content: I have a somewhat broken midiland mli 450 2.1 speaker system that I am trying to modify for a lower crossover frequency for the sub channel. I have attached a picture of the reverse engineered schematic of the input section to both the sub amp as well as the left and right amp. The sub output is C4 and the left and right output is C7 and C8 (note these do not correspond to the component labels on the actual board). In my limited testing with my computer sound card and a scope I think that the -3dB roll off of this circuit is around 250 to 275 Hz. What would I have to change to get that frequency down to around 100 Hz? I am attempting to create a cheap sub crossover for a 10" 4 ohm car sub that is part of my computer and video game system. Unless there is a better way to get the low frequency's out of a standard stereoerio source. The sub amp chip is a TDA2007A wired in mono (bridged mode) and the left and right channel amp is a TEA2025B that has the left channel input fried as well as the end of the high volume part of the left channel volume pot probably due to a high voltage being applied to the audio input. I got this for free so I can not complain.

author: LightYear
author_rank: Newbie
timestamp: October 19, 2014, 10:58:51 pm
content: Assuming XFG1 and XFG2 are the channel inputs, there is just a fairly straight-forward two stage low pass RC filter leading to C4. The potentiometer confuses things a little, but yes the -3dB roll-off points of the two stages are about 226Hz and 159Hz. Re-draw the circuit one channel at a time in a ladder network circuit style, with all the grounds along the bottom rung, and it will come out fairly clearly.So to lower the cut-off, there's two options. You could add a third RC stage with a low cut-off, or you could modify one of the existing stages to lower its cut-off. For example, adding another 15nF cap in parallel with C2 would lower that stage from 226Hz to 113Hz. Of course, that's just a raw low pass filter determination - I make no assertions about the effect on sound quality!

author: poot36
author_rank: Frequent Contributor
timestamp: October 20, 2014, 12:00:41 am
content: XFG1 and ZFG2 are the left and right channel inputs. Would I be correct in assuming that R4 and C1 make up the low pass filter for one channel and R5 and C3 make up the second filter where C3 is shared between the left and right channels? I did notice that R5 and R6 are used to combine the left and right channels together, how much cross talk would this cause between the left and right channels that are tapped off just before them? I have looked at other 2.1 speaker system schematics and have noticed that most of them seem to use 10K resistors in this configuration for the sub channel so I assume it is not that bad of a thing to use. I also noticed that the left and right channels starting around 1 kHz would increase in amplitude and continue getting bigger until I reached my sound cards maximum frequency of around 22 kHz. Why is it doing this? I have also noticed that even through the sub channel trace on my scope almost goes away above 1 kHz I can still hear frequency's out of the sub up to around 10 kHz. Would changing the second low pass filter fix this? Also the left and right channels do not seem to reduce much in output even all the way down to 5 Hz! Is there a easy way to change this? One last question why does R9 and R10 have a cap in parallel with them?

author: LightYear
author_rank: Newbie
timestamp: October 21, 2014, 02:20:28 am
content: Quote from: poot36 on October 20, 2014, 12:00:41 amThat's my assumption. Either we're both right or both wrong!Quote from: poot36 on October 20, 2014, 12:00:41 amGood question. I don't know, but I can only imagine that in the current arrangement it is "acceptable" according to the manufacturer's criteria. How that changes as you change things is a more difficult question...QuoteYes, there's a high pass filter in there. I'd guess it is to cancel the inherent low pass behaviour of the speaker or other electronics in the signal chain.QuoteHigher frequency signals are only attenuated, not removed, so provided you've got the volume up enough you'll still be able to hear them. If that's not what is going on, then I imagine it's either an EMI issue, or the non-ideal behaviour of the capacitor.QuoteEasy? Maybe, I don't know. I think you might be in the realm of audio specialist design here. None of these electrical components are ideal, so getting good audio performance out of them is an art.QuoteHigh pass filter.

author: poot36
author_rank: Frequent Contributor
timestamp: October 21, 2014, 03:53:25 am
content: Thanks for all the great answers. I have been doing some more research in filter design and have come across a active filter that uses op amps. I have another 2.1 speaker system (Benwin model bw2000-3d-sw) that has a blown sub amp and has 4 op amps on the circuit board. It also has a 3D button on it and I am wondering if the op amps are being used for this function or the sub or both. Do you think this speaker system would be a better one to mod for my purposes (I am assuming that I could use the op amps to boost the bass frequency's unlike the passive filters in the other system)? This system does have a low pass filter made out of 2 47K resistors that one end of each is connected directly to the left and right outputs of the master volume control and the other ends are connected together going directly into the bass volume control. Interestingly the 2 470pf caps are connected to the left and right channels out of the master volume control. The output of the bass control goes through a 2.2uF electrolytic cap and then into one of the op amps and also through a resistor to other parts of the circuit not near the op amp at all.

author: LightYear
author_rank: Newbie
timestamp: October 27, 2014, 09:19:18 pm
content: Quote from: poot36 on October 21, 2014, 03:53:25 amWell of course - if your goal is amplification, then an amplifier is a better place to start than a passive filter. Only caution I'd make is that the more complicated the circuit, the harder it will be to make effective changes.

author: Fcsteve777
author_rank: Newbie
timestamp: October 28, 2014, 02:59:07 am
content: The active circuit may be the way to go. Also remember for passive crossovers you need to go off of an impedance chart for the particular driver you are working with and base the crossover calculation on the actual impedance @ the frequency you are interested.Ive recently gone thru this with some audio stuff that i have and bought a Mini DSP unit. it has some pretty powerful crossover functions and i'm going to use a master volume on it before it splits to my tube amp and powered subwoofer. Of course i had to go overboard and get the calibrated Mic and use their software to set all the crossover bands and slopes.

author: poot36
author_rank: Frequent Contributor
timestamp: October 29, 2014, 05:48:05 am
content: With the passive design it fed into a amp chip with 70 to 200 K? input impedance. I have decided to use the amp chip and some of the associated circuitry along with the active cross over circuitry that had a blown amp chip to get the result that I wanted. I am inputting the active circuits output signal into one of the passive circuitrys 10K? resistors before they join together. I am wondering why the active cross overs sub channel increases in amplitude the lower I go down in input frequency. This is very annoying because the amp chip can only handle 300mV p.p of input and when I drive the circuit with a standard rca line level output (1V p.p) it works until the frequency gets below around 200 or so Hz and then it can increase upto 2V p.p if the frequency goes low enough. What can I change in the active op amp based circuit to fix this? Interestingly the standard left and right single op amped channels do not have this behavior only the 2 op amps in series for the sub. The first op amp in the sub chain has a resistor across the positive and output terminals (the signal comes in on the positive side of both op amps) and the second op amp has the positive directly connected to the output. I will try to take a picture of the hand drawn schematic in a few days and hopefully someone will be able to figure out why as the frequency decreases the amplitude increases.

author: poot36
author_rank: Frequent Contributor
timestamp: October 30, 2014, 01:39:49 am
content: Ok I have attached the schematic of the active cross over system from the Benwin bw2000-3d-sw speaker system. Sorry for the semi bad quality. I am also curious as to how the 3D button is suppose to affect the left and right channels.

author: poot36
author_rank: Frequent Contributor
timestamp: October 30, 2014, 03:15:58 am
content: If I remember correctly around 200Hz the left and right channels did start reducing. The info that I can find on the web about this speaker model makes reference to 150Hz. Interestingly the left and right speakers were of the flat panel design not cone based! For what they were they did better then I would have thought they would. Two interesting links on this model:http://www.techspot.com/old-articles/reviews/hardware/bw2000_spk/bw2000-2.shtmlhttp://www.anandtech.com/show/312

author: poot36
author_rank: Frequent Contributor
timestamp: October 30, 2014, 06:24:31 am
content: The 10" sub is in a sealed enclosure that is attached to a ported enclosure (with a piece of pvc pipe in the port for tuning that I think is around 4" long) about 2 to 3 times the volume of the sealed enclosure. I built this enclosure based on the sub specs and what frequency (I think it was around 30Hz) I wanted to achieve a vary long time ago based on some books about subwoofer enclosures. I have no clue if it is correct or not. The sub is made by Pyramid has a orange fome surround and is rated for 150 Watts peak and 50 Watts RMS 4? coil. This would have been a good 10 or more years ago. I still have the warranty card and I think that it has some more complete specs on it if I can find it. I will measure the enclosure and try to find more specs on the sub.

author: poot36
author_rank: Frequent Contributor
timestamp: October 31, 2014, 04:14:24 am
content: Well here is the measurements of the sub 33" high 12 14/16" wide and 21" deep. The top sealed section is 10.5" high and the bottom ported section is 22.5" high. This is using 11/16 thick MDF on the sides and top and bottom. The front and back are probably the same thickness as the MDF but are plywood. I think that the board that is inside holding the sub is melamine (basically coated mdf) of the same thickness as the MDF. The port is 2 3/4" deep and 2 1/16" inside diameter. I found the warranty card stating the model of the sub is wc1020, it was purchased at London Drugs and the date of purchase was Dec 27 1999! The sub box was built some time after that date though (I think a few years). I think that these are the specs but I am not sure Vas (liter) 105,1 Fs (Hz) 31 Qts 0,492. Does this make sense for a sub box? I have found a pdf file with some more complete specs but it is in a different language. I have attached it to this post.

author: poot36
author_rank: Frequent Contributor
timestamp: November 02, 2014, 06:31:43 am
content: If I remember correctly it was a bandpass design with a cutoff frequency of around 1000Hz. I am not sure if this makes sense for a sub though. Maybe it was 100Hz?

author: poot36
author_rank: Frequent Contributor
timestamp: November 03, 2014, 06:01:08 am
content: That is not really a option/problem. Here is my current speaker setup. Wii U sends sound to 2 stereo speakers in my playstation 3D monitor over hdmi and sends analog audio to a 1970s kenwood stereo receiver that currently drives the sub from the right channel and two 4" or 5" Craig branded speaker + tweeter units from the headphone jack on the receiver to limit the volume to them also the audio comes out of the stereo speakers in the game pad. I do not want to connect the small speakers to the standard outputs on the receiver because that would result in it trying to drive an unequal or to low ohm load (the sub is 4 ohms and the speakers are as well). This setup works but I wind up with the sub been louder then the rest of the speakers and trying to output frequencys that it was not designed for. My current plan is to use the line level stereo output from the active cross over that limits the low frequencys and input that into the kenwood amp and then use the passive cross over amp for the sub channel. My only problem with that is the way the active cross overs sub section increases its output level as the frequency goes down. This causes the passive cross overs amp to clip due to the overvoltage (it has a 300mV max input level). If I can find a way to fix this problem with the sub circuit on the active cross over I believe that it would work well (I think).

author: poot36
author_rank: Frequent Contributor
timestamp: November 06, 2014, 04:31:13 am
content: Right now I am just using the Kenwood for the Craig (actually Curtis) speakers + sub. The Sony Playstation 3D monitor has its own speakers in it that are in use. Here is the flow chart Wii U -> hdmi -> 3D monitor, Wii U -> analog -> Kenwood -> Curtis L+R headphone jack + sub R speaker out on rear of amp, Wii U -> wireless -> gamepad speakers. There are 3 sets of stereo speakers in use with this setup. Here is what I want to achieve Wii U -> analog -> active crossover -> L+R out -> Kenwood, Wii U -> analog -> active crossover -> sub out -> passive crossover just used for the amp section -> sub. The problem that I have with this setup is the sub section of the active crossover increases in output amplitude as the input frequency goes down and ends up overloading the input to the passive crossovers amp section. How would I go about modifying the active crossover sub section to get a flat output amplitude regardless of the input frequency? I know that the L+R outputs on the same board do this although they are only one op amp in the signal path not two like the sub circuit. I have attached a picture of the setup sorry about the mess. The PS3 and Xbox 360 connect to the 3D monitor directly through hdmi and I use the headphone output on the monitor to listen to them but it is not very good because of all the hiss from the crappy amp in the monitor.

author: poot36
author_rank: Frequent Contributor
timestamp: November 07, 2014, 04:44:51 am
content: That may work but the active and passive crossovers have volume and bass volume controls on them so I think that I could find a way to get the volume problem solved. The model number on the Kenwood is ka-3500 and yes it has one volume dial. I do not really want to use the low quality headphone output on the 3d monitor if I can help it (that is the only audio out it has). I hope to someday place the Curtis speakers under the top section of the left and right sides on the desk so that they are off the floor (my bed is right behind all of this setup so I think that it would help prevent it from soaking up some of the sound).

author: LektroiD
author_rank: Frequent Contributor
timestamp: November 07, 2014, 08:54:33 am
content: Why would you need to alter the crossover frequency? This does not seem like a feasible move to me... The frequency the sub crosses over at should be the lowest frequency your satellite bass drivers can go to in the real world, otherwise you will be left with a frequency gap. The only time you would need to lower the crossover frequency of your is if you use larger satellites with a lower base frequency. Chances are, if that was bought as a system, it was designed to work with the satellites it came with, and the set frequency is matched for the whole system. Looking at the photo, the satellites don't look like they would be capable of going as down to 100Hz in the real world (not just on paper) - many speakers claim to have a bottom frequency of 60Hz, but in reality a 6" cone could not physically move enough air in the room to be heard from any distance, which is where the sub comes in.If it sounds wrong in your room, and you are getting boosted or dropped frequencies, then look at the modes of your room, and speaker placement (there are many online room mode calculators available online for free), you need to read up on room acoustics and look at speaker placement, rather than changing frequencies set at the factory which were matched for that system as a whole. Chances are it's standing waves and the resonant frequency of your room which is more the problem than the actual speakers, which could better be solved with a few bass traps in the corners and a reflection free zone (RFZ) opposite your satellites..I have set up a number of recording studios over the years, both commercially as well as repairing faults in existing setups. The amount of times I've gone into a home studio where the owner has spent thousands going through several speakers and getting the same results with boosted frequencies by placing them direct onto their computer desk causing early reflections from the desk itself. This isn't limited to recording environments, basically anywhere you have a speaker system.Read more about acoustics, and understand what might be causing your frequency issue here:http://www.indiana.edu/~emusic/acoustics/reverb.htm

author: poot36
author_rank: Frequent Contributor
timestamp: November 10, 2014, 06:41:09 am
content: I do not want to change the crossover frequency juts the output voltage. The problem is that the output voltage changes based on the input frequency. The lower the input frequency the higher the output voltage upto almost twice the input voltage. How would I change the circuit to keep the current crossover frequency but have the output voltage stay the same regardless of the input frequency? That is the only major problem I have right now with implementing this idea. I do not think that a proper sub crossover has its output voltage change with frequency. I think that this was done on these Benwin speakers because of the small (3" !) "sub" to try and make up for the undersized speaker.

author: poot36
author_rank: Frequent Contributor
timestamp: March 16, 2019, 04:57:14 am
content: Well I think I may have answered my own questions. I have changed R19 to 330K, R21, R22 to 100K, and C19 to 0.22uF and now the crossover frequency is around 68Hz and it does not appear to change in voltage any more through out its frequency range. I am mostly happy with it but I am wondering why it won't pass a square wave? It turns the square wave in to a sign wave on the output. I have upgraded the op-amp to a lm833 with a faster slue rate 5V/us vs 1V/us and it may have made the square wave to sine wave conversion issue more pronounced. What is going on here?