High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread

[SoulReflection]

Well, right. You said pretty much what I said in about 1/50th of the bandwidth.

I learned that trick after translating my British wife's lengthy stories into one, short sentence !

 

[dabis]

Note that I didn’t say under the stage. Generally they are in front of the stage edge or left/right depending on the staging, need for pattern control and the FOH engineer’s preference.


Typically around 25-30Hz but sometimes a little higher.

Last week I've been working in two events, where the subwoofers configuration were completely different:

6 flown and 6 down per side. 24 units of Clair CP 218 if I'm not wrong. This was the first show of the European Stadium Tour of a US born rocker:

2 rows of 12 pieces, stacked, with a separation of about 0,75m. 24 units of L'Acoustics SB218 if I'm not wrong. The local enginner tipically uses this configuration to achieve a cardiod pattern, where the singer/band is not feeling too much the sublows. This was the 50th anniversary concert of a catalan folk-rock band:

 

[31HZ]

Top photo is interesting; there are going to be huge phase issues that result in lobing patterns (power alleys) from having subs in multiple locations. I wonder what the simulator software shows.

Bottom is an end-fire array. Very effective at "aiming" the output.

The End Fire Cardioid Subwoofer Array Made Visible - ProSoundWeb

 

[jb90]

Ok. Everybody talk about how great HPF/LPF is but what about phase issues and other issues which those filters introduce to the sound? PFs are very strong filters.

 

[agedhorse]

Top photo is interesting; there are going to be huge phase issues that result in lobing patterns (power alleys) from having subs in multiple locations. I wonder what the simulator software shows.

Bottom is an end-fire array. Very effective at "aiming" the output.

The End Fire Cardioid Subwoofer Array Made Visible - ProSoundWeb

In reality, once boundaries are added into the calculations, any lobing tends to average out (especially in a large, live room)

Ok. Everybody talk about how great HPF/LPF is but what about phase issues and other issues which those filters introduce to the sound? PFs are very strong filters.

HPF/LPF phase response is very well predicted and behaved, and the significant phase shift occurs as the signal is attenuated rather than in the flat portion of the pass band. In reality, unless you are summing two identical signals, one high passed (or low passed), it won’t matter.

 

[Taustin Powers]

I bought a HPF after reading through most of this thread.

For years, I wondered what the big deal was, it didn’t seem to make a very noticeable difference to me.

Then I plugged in a 5 string bass, played the low B string, and it finally all made sense.

YMMV, but I would not recommend spending money on one if you’re only playing fourbangers in standard tuning.

 

[SoulReflection]

I bought a HPF after reading through most of this thread.

For years, I wondered what the big deal was, it didn’t seem to make a very noticeable difference to me.

Then I plugged in a 5 string bass, played the low B string, and it finally all made sense.

YMMV, but I would not recommend spending money on one if you’re only playing fourbangers in standard tuning.

Nice to see you added YMMV. It has been my experience and that of others that the number of strings isn't the only determining factor. Basses, pickups, strings, amps and rooms they are played in all are factors. The main benefits are to reduce boom/mud and frequencies that tap into amp power and to protect drivers.

 

[ddnidd1]

YMMV, but I would not recommend spending money on one if you’re only playing fourbangers in standard tuning.

An HPF also works well on 4 strings.

 

[Btbp]

Nice to see you added YMMV. It has been my experience and that of others that the number of strings isn't the only determining factor. Basses, pickups, strings, amps and rooms they are played in all are factors. The main benefits are to reduce boom/mud and frequencies that tap into amp power and to protect drivers.

+1

 

[taketwo]

I recently completed some modifications to my 5-string which added built-in fixed HPF and LPF by taking advantage of the existing preamp: Jackson Spectra: details, electronics, and some fixes/upgrades

The 1st order HPF at 48Hz attenuates the fundamentals of the B and E strings, hopefully balancing them with the rest of the strings, and also to attenuates any subsonic junk which enters the preamp. It also prevents the bass shelf control from making things muddy.

The 2nd order LPF at about 2.5kHz matches the (retuned) pickups' inherent LPF to create a 4th order (-24dB/8ve) LPF to really eradicate the excessive string and fret noise I was hearing when playing into studio headphones, with or without a cab sim. I can now use the treble shelf control to increase string brightness without increasing noise.

Thanks to the many who posted their preferred settings for their LPF/HPF pedals, so I had real numbers to work with.

 

[agedhorse]

There are some serious limitations to passive filters where input and output impedances are not well defined. You are at the mercy of the outside world’s parameters and they are not usually very friendly.

 

[taketwo]

There are some serious limitations to passive filters where input and output impedances are not well defined. You are at the mercy of the outside world’s parameters and they are not usually very friendly.

Very true, but the outside parameters don't vary enough to matter here:

The HPF is a simple RC filter which depends only on the input impedance of the preamp. I guessed it's about 1M, and it looks like I wasn't wrong (but I have not measured it).

The LPF will be affected by the cable capacitance and the amp input impedance, but not enough to matter for all but really extreme cases (where a clean boost/buffer pedal would have been necessary anyway). The LPF is a plain RLC filter with 1kOhm, 100mH, and 40nF, so its capacitance is an order of magnitude above that of even a long cable, and its resistance is two orders of magnitude below that of the 500k volume pot in parallel with the 1M amp input impedance. (The preamp doesn't seem to have much output impedance itself.)

That all said, analog electronics are not my specialty, and I know I missed some details in my calculations (e.g. the 210 Ohm resistance of the inductor). If you think I have missed something important, I'd love to learn about it.

 

[walterw]

I recently completed some modifications to my 5-string which added built-in fixed HPF and LPF by taking advantage of the existing preamp: Jackson Spectra: details, electronics, and some fixes/upgrades

oh neat!

i hope you added a bypass switch option for all that, i could see situations where the amp you used had that stuff already and being stuck with it on the bass too might constrict the overall tone too much

 

[taketwo]

oh neat!

i hope you added a bypass switch option for all that, i could see situations where the amp you used had that stuff already and being stuck with it on the bass too might constrict the overall tone too much

Also a fair point, and it would be easy to do, especially if I removed passive mode and re-used the push/pull DPDT switch.

However, in my situation (headphones) I can keep it "always on", and I'm thinking it would likely be fine with most any amp+cab as the HPF is already set to a point where most cabs can't reproduce the sound well, and the LPF is well below the sharp 4-5 kHz cutoff of bass speakers (not tweeters, of course). If I do need some more high end, turning up the treble shelf control on the preamp does that. But, this might not do for someone else.

 

[agedhorse]

Very true, but the outside parameters don't vary enough to matter here:

The HPF is a simple RC filter which depends only on the input impedance of the preamp. I guessed it's about 1M, and it looks like I wasn't wrong (but I have not measured it).

The LPF will be affected by the cable capacitance and the amp input impedance, but not enough to matter for all but really extreme cases (where a clean boost/buffer pedal would have been necessary anyway). The LPF is a plain RLC filter with 1kOhm, 100mH, and 40nF, so its capacitance is an order of magnitude above that of even a long cable, and its resistance is two orders of magnitude below that of the 500k volume pot in parallel with the 1M amp input impedance. (The preamp doesn't seem to have much output impedance itself.)

That all said, analog electronics are not my specialty, and I know I missed some details in my calculations (e.g. the 210 Ohm resistance of the inductor). If you think I have missed something important, I'd love to learn about it.

Amp input impedances can vary typically from 250k to 1M, which is a 2 octave deviation.

The RC HPF will also load the source, a potential issue with passive pickups.

 

[walterw]

Amp input impedances can vary typically from 250k to 1M, which is a 2 octave deviation.

modern amps?

i thought electric instrument amplifier inputs had kind of settled on 1MΩ at this point

anyway (since i'm not gonna read all two thousand posts in this thread ) what's the consensus on the high-pass sitting at the front vs the end of the signal chain?

my impression is that the further downstream the HPF the more effective at actually keeping the errant low frequencies out of the speakers

 

[agedhorse]

It depends on the amp’s architecture. It can be effective anywhere in the path, but earlier in the path after the input buffer or after the first gain stage seems to be the most useable for the most players.

 

[djmay]

I use an Alembic SF-2 Superfilter, which has 2 filters, each of which can be set to HPF, LPF or bandpass. There is variable frequency and Q boost for each filter. Also, the dry bass signal can be mixed with the filter. The filters can be used in parallel or series. I use the filters in series with the HPF at the end.

With my fretless 5-string (2 pickups) I installed Underhill (underhillbass.com) filter-based preamp with a variable LPF (frequency and Q) on each pickup. On the Alembic SF-2 I only use the HPF, set at about 110. I had been using ~80 but I prefer the slightly higher setting. This is what I use playing with a big band.

I have been trying to relearn my fretted 6-string Pedulla ET-6, after not playing for 17 years. With this bass I set the first filter as LPF (680hz), series to the HPF at 110hz. I am not ready to play this bass in public , but it sure sounds nice.

 

[taketwo]

Amp input impedances can vary typically from 250k to 1M, which is a 2 octave deviation.

The RC HPF will also load the source, a potential issue with passive pickups.

The RC HPF resistance is the input impedance of the preamp, so the load to the pickups is effectively unchanged from stock. (I did check that there is no significant capacitance at the preamp input!)

Yes, amp input impedance does range that much, but there's a 500K volume knob in parallel, so the total LPF load will then vary from 167k to 333k, a one octave deviation, and that's still negligible relative to the LPF's 1k resistance. I don't think this thread is the place for a long discussion about electronics, so I'll summarize by saying that yes, the passive LPF is affected by external impedances, but in the same manner and no worse than a passive bass is.

 

[agedhorse]

The RC HPF resistance is the input impedance of the preamp, so the load to the pickups is effectively unchanged from stock. (I did check that there is no significant capacitance at the preamp input!)

Yes, amp input impedance does range that much, but there's a 500K volume knob in parallel, so the total LPF load will then vary from 167k to 333k, a one octave deviation, and that's still negligible relative to the LPF's 1k resistance. I don't think this thread is the place for a long discussion about electronics, so I'll summarize by saying that yes, the passive LPF is affected by external impedances, but in the same manner and no worse than a passive bass is.

Sorry I offered my opinion.