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Old 26th July 2011, 08:22 PM   #11
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I've been looking at the website that I found and want to know what are the advantages/disadvantages of using more or less in the 'filter order'. Obviously I can see that the higher the order, the more op amps and components are required, but what about sound benefits from this.

I've been looking again at pacicblue's link to the 2.1 filter drawing and I have a few questions.

1. What are the crossover frequencies for the left/right and sub woofer channels (which components help us work this out)
2. Is the 5K pot used to adjust the gain of the sub channel.
3. I assume the sub woofer output is inverted.
4. There are 4 op amps listed in the drawing, is there any major difference between these?

Would there be any advantage of using a MFB type filter over the link that pacificblue posted.
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Last edited by portreathbeach; 26th July 2011 at 08:25 PM.
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Old 26th July 2011, 11:23 PM   #12
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Just found the website where bluepacific's link came from Subwoofer Filter Circuit

It says here that the crossover frequency is 200hz, this seems like the kind of thing I am after.

Would I simply connect my GainClone amp circuit to the outputs of this filter with the same 4.2uF cap and 22K resistor, or would this have to be changed. And also, where do I connect my 100k Log pot used for attenuating the amp. On my stereo amp, I have a dual pot connected at the input of my GainClones, would this be put at the front end of the active filter instead?

Thanks again
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Old 27th July 2011, 09:05 AM   #13
AndrewT is offline AndrewT  Scotland
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The main volume control comes before the gain stage that starts to split the signal into separate frequency bands.

You may need a sensitivity control before or after the low pass filter for the low bass channel. This channel will need to to be level matched with the two channel output. Once level matched that control is generally left unaltered until you change your speakers.
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Old 27th July 2011, 07:39 PM   #14
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Quote:
Originally Posted by portreathbeach View Post
2. Is the 5K pot used to adjust the gain of the sub channel.
Yes, that is the sensitivity control Andrew mentions in post #13.

Quote:
Originally Posted by portreathbeach View Post
3. I assume the sub woofer output is inverted.
No, because inverted x inverted = non-inverted. The mixer stage is inverted and the output gain stage, too.

Quote:
Originally Posted by portreathbeach View Post
4. There are 4 op amps listed in the drawing, is there any major difference between these?
They have different advantages. The LM833 and NE5532 were specifically designed for audio use, so may have a slight edge for the satellite section. But in the case of filters you usually prefer FET inputs over BJTs, so the TL072 or the LF3.. are good choices, too, maybe even better for the subwoofer section. What they have in common are wide availability and reasonable prices.

Quote:
Originally Posted by portreathbeach View Post
Would there be any advantage of using a MFB type filter over the link that pacificblue posted.
If you understand the circuit and know how to calculate it, it is possible to save one op amp channel, because you combine the mixer stage with the first filter stage. With dual op amps you have that channel anyhow and would then be left with one unused op amp channel. And you would only have one inverted stage, so the subwoofer output would be inverted.

Quote:
Originally Posted by portreathbeach View Post
It says here that the crossover frequency is 200hz, this seems like the kind of thing I am after.
Generally the subwoofer should cross at 100 Hz or lower. With only 3 inch satellites this will of course only work for low listening volume and you may indeed need such a high crossing for louder listening. It is also worth looking at the satellites' natural roll-off. If it is somewhere near the crossing frequency you may get better results with only a 12 dB high-pass or even none at all.

Quote:
Originally Posted by portreathbeach View Post
Would I simply connect my GainClone amp circuit to the outputs of this filter with the same 4.2uF cap and 22K resistor
Yes. You should only skip those components when you are 100 % sure that you will never use the amp without the cross-over anymore. A roll-off at 1,7 Hz is quite low for a three inch speaker, so you could use those caps for the subwoofer amp and replace them with smaller and less expensive ones for the satellites.
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Old 27th July 2011, 07:59 PM   #15
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Thanks pacificblue. I've been trying to find out why you can use more of less op amps to achieve 'the same' result. What I mean is, on the website I linked to in an earlier post (here it is again Active High-Pass Filter Design and Dimensioning), there is a 'Filter Order' option which obviously increases the number of op amps the higher the order you select. If you can get a highpass filter with only 1 or 2 op amps, why would you go for a filter order of 10 and use 5 op amps?

Sorry if these questions are silly, but I haven't had the time to read any books on filter design yet, I usually learn as I go, but I'm a little confused here.

If it isn't too much trouble, could you explain each part of the schematic and what each op amp is actually doing, because I have tried a few websites with filter designing guides etc. but cannot work out what the value of each component does in each part of the circuit.

Thanks in advance
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Old 28th July 2011, 09:03 AM   #16
AndrewT is offline AndrewT  Scotland
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Most opamp filters are 2pole (12dB/octave slope)
Cascade the filters (at the same frequency) and you add the slopes. 2active filters gives 4pole (24dB/octave slope)

Most normal Audio Band filters are in the range from 1pole to 4pole.

Look at unity gain Butterworth active filter.
Compare it to a Equal Component Value Butterworth active filter. This uses two extra resistors but in return it uses equal value components (for improved accuracy) and separates the Q adjustment from the Fr adjustment.
Finally look at the Multiple FeedBack Butterworth active filter. This uses one extra resistor and one extra capacitor, but now it separates gain from Q adjustment and is inverting.

Because all the filters are set to Butterworth and they are all 2pole they all have exactly the same effect in filtering the signal. There is a limit, usually quite far into the stopband where the slope degrades from the theoretical 2pole or 4pole with most filter types. It becomes quite complicated to attenuate this stopband error. For the time being ignore it and do further research later.

Cascade two Butterworth filters and the result is Q=0.5 and this is what Linkwitz Reilly uses for the L-R crossover. The Q of cascaded filters is the mutiplication (not addition) of the individual Q values).

All filters whether passive or active require the input and output conditions to meet strict limits. The usual is zero Rs and infinite Rload. These are fairly easy and cheap to achieve using buffers.
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Last edited by AndrewT; 28th July 2011 at 09:11 AM.
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Old 28th July 2011, 04:40 PM   #17
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Thanks again for the replies. I've just started reading about filters from this site Fundamentals of Active Filters

It seems a good starting point and isn't too hard to understand.

Andrew, you said previously that "The main volume control comes before the gain stage that starts to split the signal into separate frequency bands.", so are you saying that in the Subwoofer-filter-diagram that pacificblue pointed us too, that the pot should be after the first opamp on both left and right channels. What would the difference be if it were before the entire circuit?

I think I'll read up a bit more of active filters and try building a few. I found 10 x LM833 at work, so I have some op amps, must have some caps and resistors hanging around (probably not the right values though!)

Obviously the tolerance of the components is key in these circuits, what tolerance components do you suggest and what type of capacitors are best for filter circuits?
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Old 28th July 2011, 05:06 PM   #18
AndrewT is offline AndrewT  Scotland
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If one does not care about accuracy and takes what comes out then use 5% resistors and 10% capacitors.
If you do care, use 1% resistors and 5% capacitors.
If you are a stickler for slope and turn over and channel matching, you're probably in for 0.5% resistor matching and 1% capacitor matching.
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Old 28th July 2011, 05:26 PM   #19
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Thanks Andrew. I'll probably buy the 1% resistors and 5% caps and then test each one and use the closest ones.

What caps do you suggest? Ceramic, film, mica, tantalum? There's so many, but is there any better suited to active filters?
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Last edited by portreathbeach; 28th July 2011 at 05:50 PM.
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Old 28th July 2011, 07:50 PM   #20
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Most of the time the value will limit your choice. E. g. mica caps usually come in values below 1 nF. Bigger values are hard to get and expensive. Tantalum, like all electrolytics come in F values. The usual values in active crossovers will be covered by ceramics and film caps. Ceramics come in different qualities. The good NP0 or C0G only come in too small values and the others are no match for a film cap.
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