Hello. This is my first post here, after years of being a member.
I am trying to get a handle on the pros and cons of the following scenarios when running "bookshelf" speakers (6 inch woofer+tweeter) and a subwoofer:
1. Subwoofer with built-in high pass filter,
2. High pass filter or "bass blocker" between the power amp and the speakers
3. High pass filter between the amp and preamp, with the subwoofer still getting a full range signal.
I am interested in advantages and disadvantages (and implied considerations) of each approach, but with a particular interest in whether or not there is a difference in the power demand on the main amp.
I have seen debates in other forums as to whether or not a high pass filter inserted between the main power amp and the primary speakers significantly changes the demand on the power amp, with some arguing that this is only the case when the filter is ahead of the power amp and others arguing that the effect on the power amp is the same in all of the three scenarios.
I guess it comes down to whether or not the power amp has to produce a full range signal when the filter is implemented downstream from it (vs before it)?
Thanks in advance.
EDIT: To clarify, I am referring to a passive filter in all scenarios. Also, unless I am mistaken, scenarios 1 and 2 are effectively the same.
I am trying to get a handle on the pros and cons of the following scenarios when running "bookshelf" speakers (6 inch woofer+tweeter) and a subwoofer:
1. Subwoofer with built-in high pass filter,
2. High pass filter or "bass blocker" between the power amp and the speakers
3. High pass filter between the amp and preamp, with the subwoofer still getting a full range signal.
I am interested in advantages and disadvantages (and implied considerations) of each approach, but with a particular interest in whether or not there is a difference in the power demand on the main amp.
I have seen debates in other forums as to whether or not a high pass filter inserted between the main power amp and the primary speakers significantly changes the demand on the power amp, with some arguing that this is only the case when the filter is ahead of the power amp and others arguing that the effect on the power amp is the same in all of the three scenarios.
I guess it comes down to whether or not the power amp has to produce a full range signal when the filter is implemented downstream from it (vs before it)?
Thanks in advance.
EDIT: To clarify, I am referring to a passive filter in all scenarios. Also, unless I am mistaken, scenarios 1 and 2 are effectively the same.
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Electrically they are the same. The large electrolytic that would be necessary for the post-amplifier filter
is inferior to the small good film capacitor that would be used for the pre-amplifier filter. The film capacitor
would not wear out with use like the electrolytic will, and can have much better precision of value.
With the filter after the amp, full amplitude bass would still be amplified, so there would be less headroom.
The amp would just not have to source a full range current.
The pre-amplifier filter is better in every respect except one. A post-amplifier blocking capacitor would help
to protect the speaker in the event of amplifier failure.
is inferior to the small good film capacitor that would be used for the pre-amplifier filter. The film capacitor
would not wear out with use like the electrolytic will, and can have much better precision of value.
With the filter after the amp, full amplitude bass would still be amplified, so there would be less headroom.
The amp would just not have to source a full range current.
The pre-amplifier filter is better in every respect except one. A post-amplifier blocking capacitor would help
to protect the speaker in the event of amplifier failure.
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Thanks very much for your reply.
When you say that the filtered low frequencies ("full amplitude bass") will still be amplified, I assume that the reduced headroom you are referring to is as opposed to a filter ahead of the power amp? In other words, in the case of a filter past the amp, the voltage would be the same regardless of existence or absence of the filter?
Also, can I assume that the decrease in current drawn by the main speakers - again, in the case of post-amplifier filter - is a function of increased impedance due to the filter? Would this decrease be minor (versus non-filtered signal), or significant, or is it situationally dependent? If situational, is there a relatively simple means to determine its scale?
Intuitively, given that the increased impedance affects only a narrow band of frequencies, it would seem that effect on total load (total amperage drawn by the filtered signal vs a non-filtered signal) would be quite small...but sometimes intuition and reality are quite different.
Again, thank you.
When you say that the filtered low frequencies ("full amplitude bass") will still be amplified, I assume that the reduced headroom you are referring to is as opposed to a filter ahead of the power amp? In other words, in the case of a filter past the amp, the voltage would be the same regardless of existence or absence of the filter?
Also, can I assume that the decrease in current drawn by the main speakers - again, in the case of post-amplifier filter - is a function of increased impedance due to the filter? Would this decrease be minor (versus non-filtered signal), or significant, or is it situationally dependent? If situational, is there a relatively simple means to determine its scale?
Intuitively, given that the increased impedance affects only a narrow band of frequencies, it would seem that effect on total load (total amperage drawn by the filtered signal vs a non-filtered signal) would be quite small...but sometimes intuition and reality are quite different.
Again, thank you.
Oh...one other question.
When you say that they are electrically the same, I'm not sure I understand.
My understanding of the remainder of your reply is that in the case of a filter ahead of the amp, both voltage and current are attenuated across the affected frequencies. But, when the filter is after the amplifier, the full range signal is still being amplified, but with the same attenuation of current as when the filter is ahead of the amp..
If I have this right, I don't see how they are electrically the same?
In any case, I think I do understand that with the filter ahead of the power amp, there is some gain in headroom (available voltage), but with the filter after there is not.
When you say that they are electrically the same, I'm not sure I understand.
My understanding of the remainder of your reply is that in the case of a filter ahead of the amp, both voltage and current are attenuated across the affected frequencies. But, when the filter is after the amplifier, the full range signal is still being amplified, but with the same attenuation of current as when the filter is ahead of the amp..
If I have this right, I don't see how they are electrically the same?
In any case, I think I do understand that with the filter ahead of the power amp, there is some gain in headroom (available voltage), but with the filter after there is not.
Filter ahead of the amp is best.
Something worth noting is that your bookshelf speakers will not have a linear impedance curve towards the bottom of the operating range, which means the speaker-level capacitor won't work as intended.
Other notes:
- Ideally, you want at least a 2nd order highpass to cut down excursion at very low frequencies.
- If you raise the impedance of a speaker (ie, by inserting a capacitor and assuming linear impedance - see above), less current will be demanded from the amplifier. It will still have the same voltage output, though.
- A highpass filter before the amplifier reduces the output voltage at low frequencies, which means the amplifier has more voltage swing available for the rest of the range.
- Electrolytics work fine, but they can degrade over time.
If you want to use a subwoofer to simply extend the LF cutoff of your system, you can proceed without a highpass filter. That assumes your bookshelf speakers are currently having no excursion troubles.
If you want to use a subwoofer to increase the output of the system by reducing LF demand elsewhere:
- If your amp is the limiting factor, you need a highpass before the amp
- If your speakers are the limiting factor (but the amp still has plenty of headroom), then you could put a highpass at speaker-level. NB - caveat above still applies.
In conclusion, putting a highpass filter before the amplifier is the best approach. It gives the amplifier and speakers an easier life. A highpass isn't always necessary, though, depending on the SPLs you demand of your system.
Chris
Something worth noting is that your bookshelf speakers will not have a linear impedance curve towards the bottom of the operating range, which means the speaker-level capacitor won't work as intended.
Other notes:
- Ideally, you want at least a 2nd order highpass to cut down excursion at very low frequencies.
- If you raise the impedance of a speaker (ie, by inserting a capacitor and assuming linear impedance - see above), less current will be demanded from the amplifier. It will still have the same voltage output, though.
- A highpass filter before the amplifier reduces the output voltage at low frequencies, which means the amplifier has more voltage swing available for the rest of the range.
- Electrolytics work fine, but they can degrade over time.
If you want to use a subwoofer to simply extend the LF cutoff of your system, you can proceed without a highpass filter. That assumes your bookshelf speakers are currently having no excursion troubles.
If you want to use a subwoofer to increase the output of the system by reducing LF demand elsewhere:
- If your amp is the limiting factor, you need a highpass before the amp
- If your speakers are the limiting factor (but the amp still has plenty of headroom), then you could put a highpass at speaker-level. NB - caveat above still applies.
In conclusion, putting a highpass filter before the amplifier is the best approach. It gives the amplifier and speakers an easier life. A highpass isn't always necessary, though, depending on the SPLs you demand of your system.
Chris
I myself am trying to understand what high pass filter dose my SW design need.
can someone link an example of a pre amp high pass filter?
can someone link an example of a pre amp high pass filter?
The system frequency response of both cases is the same, but it still matters where
in the system the filter is located.
If Chris is correct, it seems like the response might not be the same.
A related question...assuming the amp does not have a perfectly regulated power supply, wouldn't there be at least a small amount of headroom gained with a downstream filter? I am guessing that the effect would be minor unless the power supply is especially weak.
A related question...assuming the amp does not have a perfectly regulated power supply, wouldn't there be at least a small amount of headroom gained with a downstream filter? I am guessing that the effect would be minor unless the power supply is especially weak.
That's correct. The PSU would have to be really squishy for a speaker-level cap to increase amplifier headroom.
It's easy enough to simulate the effects of a large series capacitor. The results aren't pretty unless designed-in. See Brian's thread on the subject w/sealed boxes.
Chris
It's easy enough to simulate the effects of a large series capacitor. The results aren't pretty unless designed-in. See Brian's thread on the subject w/sealed boxes.
Chris