Amp heads with a controllable low-cut filter near 20-30 Hz?

I'm curious about this too. Seems like the standard EQs don't go low enough. My main amp's 9 band goes from 40hz to 16khz. I'd much rather have one that goes from 20 to 8......

Most decent bass amps include a (24db, I think) rolloff in the neighborhood of 30-40hz I think. Most Genz amps have one that's roughly 40, and adjustable to 30, IIRC (either pushbutton or a knob).

What you should do is get a pre-power rig with a sweepable high pass. Most good power amps have a 30-50hz switchable at least, if not a sweepable 20-200 or so. That'll cut you down having to rack it. QSC PLX's have the 30/50 setup IIRC.

Most amp companies do not document this unfortunately. Seems like an obvious thing to put in a manual.

Personally, what I'd love is a compressor with sweepable high pass. Man, that would be net

I did read your post. I suggested options that would save you rack space.

Also, the Genz one is sweepable down to 30, which fits your bill. I believe that's useful information. You can consider the rest a bonus or ignore it, but calling me out for it when I provided useful info is not going to earn you any brownie points. Prefacing a pointless internet toughguy criticism with "I don't mean to be mean" doesn't make it any less lame.

Edit: The Genz high pass is not documented IIRC, but it is mentioned several times on the forum by folks at Genz you can trust. PM Agedhorse for specifics or search.

Re-edit: The Genz high pass is documented in the GBE 750/1200 manuals

Also, from the GBE 1200 manual:
Global Signal Shaping Switches and Controls — There are 3 separate shaping filters on the GBE 1200. The “L.F. EXTEND” (low frequency extend) lowers the low frequency roll-off response of the preamp (between 30 and 45 Hz) to allow the full effect of 5+ string and extended scale instruments. The amount of extension is adjustable depending on the setting of the corresponding control. Use this function with care due to the power of this amplifier.

Why do you feel that a 40 hz filter is too high? Unless you play with a low F#, Not much will be affected because of the roll off, so you would still have plenty of sub sonic signal

rpsands, I would not have responded so informatively after bongomania's initial response. You are a better man than I.
I was gonna chime in with some info, but I think not!

Take it easy guys.

My post had info that was relevant. Some of it might not be. Who cares?

I posted enough info for you to know that Genz has an adjustable low end rolloff, and from there I think you could figure it out. They are basically the only guys who document it from what I've seen so you're out of luck if you want citations for the numbers for a bunch of amps. You're gonna need to do some legwork if you want to know for more than the couple very few amps that have this.

What you could have done was taken my post for what it was and read some Genz manuals without me having to do it for you, instead you wrote a prickly response that pissed me off for no reason.

Edit edit edit: And once more, I think the suggestion to get a power amp with the feature built in is quite relevant. Your power amp doesn't have it (one of the few things I do not like about the Carvin power amps). If you got the right power amp you could save you rigging a 3-way crossover or some other rack piece to do this job, and not have to buy a ready made amp head. Maybe that's not the way to go but it's a really easy option, that you may or may not have considered in your quest. What the heck does it hurt for me to mention it anyway?

The problem I see with a 20-30Hz filter cut-off is that it's too low for 99% of ported cabs. What will you be using?

Alex

The only amp I know of that has a variable hi pass filter is the AI line of heads. The have a master filter that can be switched either to 'hi pass', variable down to 30 hz, or a to a notch filter cut for DB feedback.

Interestingly, when I had one of these heads and engaged the hi pass at the lowest, supposedly 30 hz setting, I could really hear a thinning of the low end, which suprised me. So, it was impacting more than pure sub-harmonics.

Per your comment regarding the MB heads, several TBers put them on the scope (as did Bass Gear Magazine), and if I remember correctly, the are very even down to about 40hz, and then really drop off, which is the perfect voicing for me for live bass guitar performance.

Finally, the EA iAmp (the 500, 800 and new Pro) have a semi-parametric shelving bass control that sweeps down very low.

This assumes that designers spec a full frequency to begin with, I'd venture that this is more often not the case. In many a bass preamp, the low and hi pass points are built into the circuit so effectively what you have is a bandpassed spectrum anyway, which may be 40 - 12 Khz, if it was designed in the days before 5 string basses.

There are too many variables with this issue to be generic in reply.

KJung said:

The only amp I know of that has a variable hi pass filter is the AI line of heads. The have a master filter that can be switched either to 'hi pass', variable down to 30 hz, or a to a notch filter cut for DB feedback.

Interestingly, when I had one of these heads and engaged the hi pass at the lowest, supposedly 30 hz setting, I could really hear a thinning of the low end, which suprised me. So, it was impacting more than pure sub-harmonics.

Click to expand...

I wonder how steep the slope is? You really need 24dB/octave slopes to knock out those lows that make little beneficial sound but increase distortion (and thus mud and boom) whilst decreasing power handling and maximum output without removing any of the weight from your lowest useful lows. Even a 24dB/octave filter may knock off a dB or so where you can hear it but a tiny tweak of your bass EQ can fix that. Shallower slopes will unfortunately eat into the lows that do matter.

Alex

alexclaber said:

I wonder how steep the slope is? You really need 24dB/octave slopes to knock out those lows that make little beneficial sound but increase distortion (and thus mud and boom) whilst decreasing power handling and maximum output without removing any of the weight from your lowest useful lows. Even a 24dB/octave filter may knock off a dB or so where you can hear it but a tiny tweak of your bass EQ can fix that. Shallower slopes will unfortunately eat into the lows that do matter.

Alex

Click to expand...

+1 It seemed to behave more like a standard 'shelving' bass control, which I assume has a much shallower slope than a true hi pass filter.

The only hi pass filter I've used that really worked the way it should was on a Rane rack mounted parametric EQ way back in the day. I had one of those awful SWR400's at the time, that spent what seemed like most of its headroom pumping out sub harmonics (I've never seem the cones on speakers move so much and yet produce so little volume). Once I cut everything below 30hz (or even lower, I can't remember), the amp's tone didn't change, but the output increased and the cone flapping was reduced.

synaesthesia said:

This assumes that designers spec a full frequency to begin with, I'd venture that this is more often not the case. In many a bass preamp, the low and hi pass points are built into the circuit so effectively what you have is a bandpassed spectrum anyway, which may be 40 - 12 Khz, if it was designed in the days before 5 string basses.

There are too many variables with this issue to be generic in reply.

Click to expand...

Just one comment. The idea that you need or even want that 32hz extension with a 5 string bass is not necessarily true. The 'meat' of the lowest notes on a B string are represented by the harmonics of the note (if I have that term right), and there is very little 'musical energy' in the fundamental. Also, you would only get down that low, even with the fundamental, in the lowest region of the B string anyway (i.e., the lowest few notes).

That 40hz or so drop-off seems to be a very nice tuning for a 5 string, and allows plenty of 'meat' and low end punch and fullness, without getting into all those 'deep low end' issues with headroom, power, blowing through the xmax of many bass speakers, etc., etc., etc.

Anyway, my point is, bass rigs that are designed to go all the way down to that open low B fundamental are not really necessary, and for many of us, not particularly desirable.

I may have some of my terms wrong above, but hopefully I've communicated that general point correctly (or at least understandably!)

totallybacan said:

Why do you feel that a 40 hz filter is too high? Unless you play with a low F#, Not much will be affected because of the roll off, so you would still have plenty of sub sonic signal

Click to expand...

+1, although more pertinent is the intent of the filter, which is to protect the speakers. Driver excursion rises precipitously below the cabinet tuning frequency, which can lead to mechanical damage. The filter's intent is to limit output below the tuning frequency, and very few cabs are tuned lower than 40 Hz. In fact, 45-50 Hz tuning is the most common.

Kind of a bummer that there is so little documented evidence of actual HPF implementation in bass heads. Some have FR that shows a drop at the left side of the graph (if you actually ever get to see a graph) but they don't seem to be steep enough to actually mean much but that the design didn't pursue flat response at the lowest frequencies.

The only thing in bassists' favor here is that typically the lower notes on a bass don't have much energy at the fundamental. But there is still alll the thumpy non-harmonic content that can be below the note's pitch (I hear this sometimes with some EQ profiles when isolating notes played on the G string ferinstance), and there's all the potential that big LF EQ can still be a real bummer.

OK, didn't answer your question, because I haven't seen too many examples where it was obvious the manufacturers took HPF into consideration, but I thought I'd comment on some more observations why it is desirable.

Let me add some clarification to this discussion as it's important enough that it's worth understanding in detail IMO.

1. The typical compact bass cabinet has a -3dB point of around 55Hz. The typical mid size bass cabinet has a -3dB point of around 45-50Hz. The typical large (or extended range) bass cabinet has a -3dB point of around 40-50Hz. I know of no bass cabinet that has even a -10dB response that goes down to 30Hz.

2. Driver mechanical stress increases exponentially when you go below the -3dB point especially as you approach the -10dB response. Bill's post above is worth reading again.

3. Power handling of a speaker falls off very quickly as you approach the -10dB point as the driver unloads acoustically from the enclosure air mass. A "500 watt" driver may only handle 200 watts mechanically at 30Hz. This is probably the most common cause of driver failure today.

4. The steepness of the slope and the -3dB point of the filter affect the response and resulting sound. A shallower slope will need a higher HPF frequency to avoid driver damage but this will affect the sound more too. With a steeper slope, the HPF filter frequency can be much lower while still offering driver protection. A higher slope will generally be less audible, but for many playing styles a more gradual or higher rolloff point is in fact desireable.

5. There is a focus on a simple highpass filter here. In fact, I use compound filters which result in asymmetrical slopes in order to achieve the "best of both worlds". This improves the feel and playability at the very bottom end of the scale. Keeping things from becoming "sloppy" is important to the overall control and refinement of the bottom 1/2-octave IMO.

6. The LF extend function on our amps that rpsands mentioned is a function that does two things simultaneously, as these two variables need to track each other to maintain proper voicing and driver protection. With a typical shelving filter, extending the LF response would result in a large amount of subsonic garbage that would really muddy up the response and in the large amps, destroy speakers. The slopes must be pretty steep too, because we're talking about a pretty tight bandwidth we want to act on. Specifically on the ShuttleMax, there's a combination of 2nd and 4th order filters with carefully augmented response alignments in order to achieve satisfactory voicing. This is harder than it would seem on the surface too.

7. So, when you sweep the LF extend filter, the amplitude response in the bottom 1/2-octave of a 5 string increases while the effective -3dB point shifts down from say around 40Hz to around 28-30Hz. This is not a "flat" response, but a carefully crafted, detailed manupulation of the signal to achieve the "best of both worlds". It's all done in the analog domain The voicing on the GBE-750 and 1200 is similar, though not identical.

8. IMO, there's no practical benefit and PLENTY of potential downside with flat response down to 20Hz. There's no bass cabinet on the market (forget the Bag End ELF concept which uses "integrators" to achieve extended LF response but the power handling falls off quickly proportional to the amount of extension) that has even a -10dB response down to 25Hz. This is why most bass amps are voiced to start rolling off around 40Hz... this works well for the majority of medium sized cabinets on the market.

9. Regarding standard shelving controls (Baxandall and the like), they are a first order filter and while good for overall tone shaping, are virtually useless for any tight control adjustments like I have discussed above.

10. If a true variable high pass filter is desired, generally this can be had by inserting a suitable active crossover that sweeps down that low on the first high pass band and using the 2 way mode high out. Note that the x-ove must sweep low enough, many do not sweep down to 20Hz on the high out, and also, some have a fixed HPF of 25-30Hz anyway. Something like a DriveRack has a variable HPF that's part of the crossover filters module, just be sure it allows for adequate wide bandwidth routing. I use a modified analog 4th order crossover for doing R&D experimentation and testing of power amps where I want to have convenient control over these variables.

We have in fact discussed this (to a much lesser depth) in our manuals, but that means reading the manual which doesn't seem to be a "guy" thing.

Hope this helps

Lots of good points there!

What's the response curve like on the Shuttle 3.0 and 6.0 as I gather from the manual that there is a steep highpass filter on the input? Also how does this curve look after LF extend is switched on?

I was looking at power amps today (investigating gain variations) and I noticed that the sweepable highpass on the Mackie M1400 was only 12dB/oct whilst the switchable (2 choices) highpass on the QSC PLX is 24dB/oct. I wonder if the more complex circuitry means that sweepable highpass filters tend to be less steep?

Alex