Electronics > Beginners
Crossover frequency of a 2.1 speaker system
poot36:
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.
LightYear:
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!
poot36:
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?
LightYear:
--- Quote from: poot36 on October 20, 2014, 12:00:41 am ---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?
--- End quote ---
That's my assumption. Either we're both right or both wrong!
--- Quote from: poot36 on October 20, 2014, 12:00:41 am ---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?
--- End quote ---
Good 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...
--- Quote ---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?
--- End quote ---
Yes, 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.
--- Quote ---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?
--- End quote ---
Higher 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.
--- Quote ---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?
--- End quote ---
Easy? 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.
--- Quote ---One last question why does R9 and R10 have a cap in parallel with them?
--- End quote ---
High pass filter.
poot36:
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.
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