You will probably not like what any opamp xover does to the sound of your amp. Avoid it by installing a passive line level crossover (PLLXO) instead. I just finished removing an active high pass from in front of my amps, and installing PLLXO instead. The improvement is significant.
You can still use an active xover for the sub. You might not even need any additional xover on your mains if you can manage to integrate their intrinsic roll-off with an appropriate slope on the sub. If the speakers already drop off at 12db per octave, try a matching slope on the sub. You can always add another stage to achieve 4th order if necessary.
Don't expect to nail it perfectly on the first try, especially by using textbook calculations which do not account for phase differences and room effects. Using LR4 or any other configuration is no guarantee of success.
Peace,
Tom E
Thanks everyone, have learnt alot!
I have ordered the extra components needed to change the original circuit into a 2nd order butterworth low pass with cut off at 80Hz
I would be fine without the subsonic protection highpass but am unsure how to implement a simple summing NE5532 opamp with volume control using the components I have - nameley the 47K pot and all the spare resistors I have from the original circuit.
I understand that left and right signals need to join through equal resistors to form the summing junction and this then needs to connect through the pot for gain control which is drained to ground? This then goes on to feed the inverting input. Also a resistor is used to feed back from the output to the inverting input?
The non inverting input is then tied to ground, via a resistor?
Then simply build the butterworth as the next 'stage'?
Stu
I have ordered the extra components needed to change the original circuit into a 2nd order butterworth low pass with cut off at 80Hz
I would be fine without the subsonic protection highpass but am unsure how to implement a simple summing NE5532 opamp with volume control using the components I have - nameley the 47K pot and all the spare resistors I have from the original circuit.
I understand that left and right signals need to join through equal resistors to form the summing junction and this then needs to connect through the pot for gain control which is drained to ground? This then goes on to feed the inverting input. Also a resistor is used to feed back from the output to the inverting input?
The non inverting input is then tied to ground, via a resistor?
Then simply build the butterworth as the next 'stage'?
Stu
Just to be sure: is the design you have shown in the first post designed specifically for the subwoofer you want to use? If so, its peaking might be needed to correct for subwoofer roll-off.
Assuming that this is not the case, if you want to get rid of the high-pass and reuse the components, you could do this:
-Connect C11, C12, C15 and C14 in parallel, and put them between the wiper of the potmeter and the positive input of IC3A.
-Connect R8 and R2 in series and put them between the positive input of IC3A and ground
-Connect your Butterworth low-pass to the output of IC3A
-Put R4 and C13 between the output of the Butterworth low-pass and the amplifier that drives the subwoofer to block DC offset voltage
-Connect C13 the other way around, because the DC offset is bound to be negative due to the input bias current of IC3A
The first two modifications change the peaking second-order high-pass into a first-order high-pass with 2.82 Hz cut-off, which gives you only 0.0856 dB of roll-off at 20 Hz. That should be close enough to "no filtering". The capacitors between the potmeter's wiper and the op-amp ensure that there is practically no DC current flowing through the wiper. This is good for the long-term reliability, especially if you use a carbon track potmeter.
Assuming that this is not the case, if you want to get rid of the high-pass and reuse the components, you could do this:
-Connect C11, C12, C15 and C14 in parallel, and put them between the wiper of the potmeter and the positive input of IC3A.
-Connect R8 and R2 in series and put them between the positive input of IC3A and ground
-Connect your Butterworth low-pass to the output of IC3A
-Put R4 and C13 between the output of the Butterworth low-pass and the amplifier that drives the subwoofer to block DC offset voltage
-Connect C13 the other way around, because the DC offset is bound to be negative due to the input bias current of IC3A
The first two modifications change the peaking second-order high-pass into a first-order high-pass with 2.82 Hz cut-off, which gives you only 0.0856 dB of roll-off at 20 Hz. That should be close enough to "no filtering". The capacitors between the potmeter's wiper and the op-amp ensure that there is practically no DC current flowing through the wiper. This is good for the long-term reliability, especially if you use a carbon track potmeter.
Really appreciated Marcel - I have been googling but some of the finer details that you keep pointing out would take me a long time to learn!
I am in the process of creating a drawing....
And No, the original circuit is not specific to my Sub Woofer in any way, so I feel happier creating a better high pass
I'm presuming you asked for the caps to be in parallel which would be the sum of 4 caps at 47n each. This would mean 188n, I have 220n caps here and with a resistor to ground of 300K I would get a cut off at 2.41Hz
I am in the process of creating a drawing....
And No, the original circuit is not specific to my Sub Woofer in any way, so I feel happier creating a better high pass
I'm presuming you asked for the caps to be in parallel which would be the sum of 4 caps at 47n each. This would mean 188n, I have 220n caps here and with a resistor to ground of 300K I would get a cut off at 2.41Hz
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Thanks JCX but I may never have the brain capacity to understand the great linkwitz link you posted.
I think I'm getting to grips with a simple circuit, have attached a diagram of what I think Marcel was meaning.
I understand the initial two 10K resistors form the summing junction for the stereo input, the 47K pot then drains some of the signal to ground to form gain control.
Next, C1 and R1 form a simple highpass filter. If C1 is 220n and R1 is 300k then this cuts off frequencies below 2.41Hz and the signal is fully restored to its normal dB just above 20Hz
Then we have the opamp... this may be garbage but I have tried to calculate the transfer function using the calculations in my attached image.
It seems that if my DAC gives 2mV then I get 5.6mV at the output. This is assuming I have the pot at full gain so no resistance to ground
I presume that the resistor to ground that is part of the simple high pass filter will also affect the transfer function but I'm hoping the circuit will still function.
I will work on the low pass butterworth section next but would really appreciate some guidance from the experts as to whether the attached circuit will work
Stu
I think I'm getting to grips with a simple circuit, have attached a diagram of what I think Marcel was meaning.
I understand the initial two 10K resistors form the summing junction for the stereo input, the 47K pot then drains some of the signal to ground to form gain control.
Next, C1 and R1 form a simple highpass filter. If C1 is 220n and R1 is 300k then this cuts off frequencies below 2.41Hz and the signal is fully restored to its normal dB just above 20Hz
Then we have the opamp... this may be garbage but I have tried to calculate the transfer function using the calculations in my attached image.
It seems that if my DAC gives 2mV then I get 5.6mV at the output. This is assuming I have the pot at full gain so no resistance to ground
I presume that the resistor to ground that is part of the simple high pass filter will also affect the transfer function but I'm hoping the circuit will still function.
I will work on the low pass butterworth section next but would really appreciate some guidance from the experts as to whether the attached circuit will work
Stu
Attachments
R1 = 300k seems a bit high, I'd go for more like 100k. Shouldn't matter much down there and with a FET input opamp, but anyway.
You have your DAC output level wrong by 3 orders of magnitude - it'll probably be around 2 Volts for 0 dBFS.
(With rms levels depending on material and possibly output level control.)
You have your DAC output level wrong by 3 orders of magnitude - it'll probably be around 2 Volts for 0 dBFS.
(With rms levels depending on material and possibly output level control.)Your schematic is indeed what I meant, and using 220 nF instead of four times 47 nF in parallel will also work fine. Like sgrossklass already wrote, the output level of a DAC is typically 2 V RMS rather than 2 mV RMS when playing a maximum-volume sine wave.
I have the impression that your understanding of the potmeter is not correct. The potmeter is used as an adjustable voltage divider, not as an adjustable resistor. It always loads the summing junction with its 47 kohm resistance, which leads to a small reduction of the signal voltage at the node where the two 10 kohm resistors are tied to the potmeter. Depending on the wiper position (and neglecting the effect of the 300 kohm resistor), you send a fraction of this voltage to the op-amp.
One practical thing: the capacitors in the signal path should not be class-II (or class-III) ceramic capacitors as these are quite bad in many respects. For example, they distort much more than any other common capacitor type. Foil capacitors are much better, their distortion ranges from hard to measure (metallized MKT) to nearly impossible to measure (polystyrene, film-and-foil polypropylene).
I have the impression that your understanding of the potmeter is not correct. The potmeter is used as an adjustable voltage divider, not as an adjustable resistor. It always loads the summing junction with its 47 kohm resistance, which leads to a small reduction of the signal voltage at the node where the two 10 kohm resistors are tied to the potmeter. Depending on the wiper position (and neglecting the effect of the 300 kohm resistor), you send a fraction of this voltage to the op-amp.
One practical thing: the capacitors in the signal path should not be class-II (or class-III) ceramic capacitors as these are quite bad in many respects. For example, they distort much more than any other common capacitor type. Foil capacitors are much better, their distortion ranges from hard to measure (metallized MKT) to nearly impossible to measure (polystyrene, film-and-foil polypropylene).
Just ran across this thread. I, too, use KEF 101 and subs. I did the active crossover thing about 15 years ago.
One tidbit about the 101s most don't know. There's a large capacitor in series with the B110 that augments the bass output, rather like porting...
I had good luck with rolling the sub out at 24db/oct at 100Hz, and the 101s at 12db/oct at 100Hz. This is still what I use, though I've gone for a digital crossover since.
I used the basic schematic as shown in Linkwitz' original article: SB1980-3way
One tidbit about the 101s most don't know. There's a large capacitor in series with the B110 that augments the bass output, rather like porting...
I had good luck with rolling the sub out at 24db/oct at 100Hz, and the 101s at 12db/oct at 100Hz. This is still what I use, though I've gone for a digital crossover since.
I used the basic schematic as shown in Linkwitz' original article: SB1980-3way
Thanks Marcel, I understand the pot now! there is always a 47K to ground and the wiper just splits the reistance above and below the output - like a voltage divider... Thankyou
what approximate 'fraction' of the voltage goes to the opamp (negating the folowing resistor to ground)?
My caps are all Polypropylene
@jplesset - great to hear someone else enjoying the 101's... thanks for the info too.... I will be building this little circuit first as I've spent some time trying to get my head round it, but I understand that you are saying the 101's might not respond as well as we thought to a 2nd order LPF @ 80Hz
Marcel, thanks for all your help - does the attached look correct? specifically the polarity of the final cap?
what approximate 'fraction' of the voltage goes to the opamp (negating the folowing resistor to ground)?
My caps are all Polypropylene
@jplesset - great to hear someone else enjoying the 101's... thanks for the info too.... I will be building this little circuit first as I've spent some time trying to get my head round it, but I understand that you are saying the 101's might not respond as well as we thought to a 2nd order LPF @ 80Hz
Marcel, thanks for all your help - does the attached look correct? specifically the polarity of the final cap?
Attachments
Surfstu, the 101s do very well with 90 or 100Hz crossover in my place. I've not actually tried them with lower. I do play them pretty loud at times, and my living room is pretty large, so the higher crossover helps keep the excursion down. I am currently using my dad's old Phase Linear 700 on them...
Thanks Jp. Once I have sign off from the opamp experts that the attached cicuit above is not going to disobey any laws of electronics that I don't know about (which is alot) - I will build it and let you know how it sounds.
My room is small (I Live in the UK - London) and an odd shape. I will also be driving a 10inch car sub box tucked away in a corner and not central to the stereo speakers so I know its going to be luck as to the sound. But I've enjoyed building it and learning a little.
My room is small (I Live in the UK - London) and an odd shape. I will also be driving a 10inch car sub box tucked away in a corner and not central to the stereo speakers so I know its going to be luck as to the sound. But I've enjoyed building it and learning a little.
At maximum gain you get (10 kohm//47 kohm//150 kohm)/(10 kohm+(10 kohm//47 kohm//150 kohm)) ~= 0.4387 times the left input voltage plus (10 kohm//47 kohm//150 kohm)/(10 kohm+(10 kohm//47 kohm//150 kohm)) ~= 0.4387 times the right input voltage, including the resistor to ground.
The circuit looks OK to me. With 150 kohm you get a higher cut-off than with 300 kohm, but it is still low enough.
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