Double Bass Help: AI Coda or Clarus 2R plus a Wizzy?

[bolo]

Link Removed

The Gramma pads are okay. Pretty cool for wooden stages and risers like you said. I picked one up a few years ago. But I end up using a pretty Invalid Link Removed much more often.

[ Edit: The Lakland is in the pic just 'cuz I think it's purdy. ]

 

[bolo]

I had a nice long talk with Rick today, and went thru all the HP NF thing.
He said that turning this Notch filter on reverses the phase, turning the HP on then reverses it back, So to be clear: The two filters DO have a very neat phase reversal built in. If you have the notch filter engaged and on its lowest setting it is acting like a mild HP filter, that is, it is cutting out the infrasonic crap.

Affirmative. Sorry it took me so long to loop around on this one. I got the low down face to face today. Also got a new Focus 2R, and a tour of AI headquarters - way, way cool. This is the third location of theirs that I have visited since 1999, each one quite a bit bigger than the previous one as you might expect.

Saw the demo room, including a pretty remarkable looking prototype speaker cab or combo amp that "might never see the light of day" that I probably shouldn't say anything more about, the burn in room for the amps and the cabs, shelves full of neatly arranged amp innards, lots of boxes and shelves full of the Contra shells, a close look at the drivers, and even two specialty items called the "crapper" and the "boneyard." Some day I may try to type it all up.

Rick was a very, very gracious host for a local small-timer like me. He took me into his office and showed me graphs of the frequency patterns for the notch and the high pass filters on his computer. He let me listen to the differences in how they sound by running pink noise through them in one of his tech rooms while he turned the knobs. We talked about the more efficient paper driver in the Corus that makes it sound more "like" a front-firing cab for them thar guitarists. Said some DB players actually prefer the new Corus. He talked about how down-firing cabs are actually kinda misunderstood in some ways, and that for one thing they don't have problems w/ frequency beaming the way front-firing cabs do.

Did I mention that some of this was over my head, and all of it was way more than I expected, and very, very cool?

Bobby K. - He may put some of this stuff up as white papers or PDFs at some point. I think he really wants too, but is just too durn busy.

Anyway (whew), just like Piro said ->

Notch filter invokes a phase reversal. High pass filter reverses it again, effectively nulling it out.

That's it.

 

[tornadobass]

Notch filter invokes a phase reversal. High pass filter reverses it again, effectively nulling it out.

That's it.

Hmmm...it can't just be said that the notch is wired to reverse the phase, while the HP filter isn't?

 

[bolo]

Well, I reckon so. Functionally, that's how I understand it. But strictly speaking, the HP is wired to reverse the phase. Just so happens it's reversing the phase that was reversed once already by the notch circuitry.

Is -1 * -1 the same as 1? Or is it different?

Anyway, don't you wanna know more about the "crapper"?

 

[drurb]

I would not advise using a notch filter to perform the function of attenuating infrasonics. It is a poor way to do that and that's the function of a high-pass filter.

If both the notch and high-pass filter reverse phase, then yes, using both cancels the phase-reversal and brings you back to 1.0. I am not familiar with the AI circuit but from what I gather from what has been posted, the design of these filters is such that they perform an overall phase-inversion when they are engaged. That is mathematically equivalent to shifting all frequencies by 180 degrees.

Keep in mind, however, that the filtering action itself will introduce additional frequency-dependent, phase-shifts of between of between 0 and 360 degrees. The most severe of these are typically right near the "corner" or "cutoff" frequencies of the filters. It's always nice to have a separate phase-inversion switch on the amp so that you can choose the setting that is more desirable in terms of constructive or destructive interference of the acoustic output of the speaker and the DB itself.

 

[drurb]

Hmmm...it can't just be said that the notch is wired to reverse the phase, while the HP filter isn't?

No, because engaging each apparently inverts the signal.

 

[MT Spaces]

If both the notch and high-pass filter reverse phase, then yes, using both cancels the phase-reversal and brings you back to 1.0. I am not familiar with the AI circuit ...

Greetings again Good Doctor

I'm still mentally challenged by all this stuff ...

If I understand my new Coda 2-channel amp correctly, on a given channel I can use either notch or HP but not both since there is pushbutton selector ("out" is notch / "in" is HP). However I can use notch on one channel and HP on the other. That seems to be the way folks like Uncle T, Flatback, Bolo, et. al. use the 2 channels for mic and pup combinations.

I'm thinking that Tornado's statement:
"Hmmm...it can't just be said that the notch is wired to reverse the phase, while the HP filter isn't?"
makes the most sense to me otherwise I have a logical conundrum .

Say I start up the Coda with the filter switch off, then I turn filter on and select notch .... I think I've got notching with phase reversal. Then I select HP filter and phase reverses ... OK. But what if I do the scenario again, start up the Coda, turn filter on with the starting position for filter type to HP. Does this mean I get phase reversal? I don't think so but I could be wrong. Does this logical impasse makes sense to anyone else or did I just stay up too late last nite drinking microbrews?

Yeah Bolo, I do hope the "white paper" or PDF from AI can happen at some point in time. And, heck yes, I want to know about the "crapper" !

 

[drurb]

Hi--

I just got off the phone with Rick at AI.

Here is the information, one step removed from the horse's mouth:

Activating the notch filter produces an overall inversion of phase.

Activating the high-pass filter does not invoke any such reversal of phase.

Thus, the two do not cancel each other in terms of phase.

Remember, however, as I stated above, the filters themselves will produce frequency-dependent phase-shifts in the region of their cutoff frequencies.

Here's the explanation regarding using the notch as "sort-of" high-pass filter. Intuitively speaking, the notch filter produces a "hole" in the spectrum (the frequencies). Imagine placing the "hole" around 40 Hz. Well, it doesn't matter that frequencies below the hole are "accepted" by the filter because there's little energy down there. The frequencies in the region of 40 Hz will be cut out and the ones above those around 40 Hz will be accepted. Voila-- a "sort of" high-pass filter at 40 Hz.

Now, if you are trying to rid yourself of infrasonics, this won't work for you. That's because, in the case of infrasonics, there IS undesirable energy far below 40 Hz that you want to kill. The notch filter won't do that job. For that, you want a true high-pass filter that says, "Nobody below 40 Hz gets through-- nobody!" Yes, I imagine talking high-pass filters.

 

[tornadobass]

Helpful clarification! Tied into this is what happens once the phase is reversed...low frequencies cancel out somewhat then, but in the infrasonic range? Or more random based on the room and the bass, plus distance from amp and reflecting surfaces?

 

[MT Spaces]

.... I just got off the phone with Rick at AI ...

Yes, I imagine talking high-pass filters.

Right on DOC . Thanks, way to take the "filter bull" by the horns. As I said earlier in one of my posts:

"If nothing else, a PDF info page would cut down on the number of phone calls they have to field from all of us asking the same questions."

From Bolo, FlatBack, Tornado, DrUrb, and I'm sure some others ...... Rick has answered the same types of questions a few times now. Probably spent about the same amount of time talking as it would take to type up a PDF . OK, sorry, I will go back to my corner and put a cork in it .

 

[drurb]

Helpful clarification! Tied into this is what happens once the phase is reversed...low frequencies cancel out somewhat then,

Well, not necessarily. Whether there is cancellation depends on the relative phase between the low frequencies radiated from the speaker (and after reflection) and those radiated from the DB itself. Usually, one or the other phase will produce a more desirable sonic result.

but in the infrasonic range?

Well, the DB does not actually radiate anything close to infrasonics. They are sometimes produced as an artifact of using a piezo pickup. Their effect is to rob the amplifier of power and to drive the cone of the speaker crazy. Inverting their phase will not help. That's where the handy-dandy high-pass filter comes in.

Or more random based on the room and the bass, plus distance from amp and reflecting surfaces?

Yes, this is to what I was referring above.

 

[Uncletoad]

Activating the notch filter produces an overall inversion of phase.

Activating the high-pass filter does not invoke any such reversal of phase.

Thus, the two do not cancel each other in terms of phase.

Sorry I've been a bit preoccupied and missed this stuff. It's important to note that on the Focus 2RIII the Notch and High pass filters are mutually exclusive. You can use one or the other but not both at the same time.

Remember, however, as I stated above, the filters themselves will produce frequency-dependent phase-shifts in the region of their cutoff frequencies.

Here's the explanation regarding using the notch as "sort-of" high-pass filter. Intuitively speaking, the notch filter produces a "hole" in the spectrum (the frequencies). Imagine placing the "hole" around 40 Hz. Well, it doesn't matter that frequencies below the hole are "accepted" by the filter because there's little energy down there. The frequencies in the region of 40 Hz will be cut out and the ones above those around 40 Hz will be accepted. Voila-- a "sort of" high-pass filter at 40 Hz.

Now, if you are trying to rid yourself of infrasonics, this won't work for you. That's because, in the case of infrasonics, there IS undesirable energy far below 40 Hz that you want to kill. The notch filter won't do that job. For that, you want a true high-pass filter that says, "Nobody below 40 Hz gets through-- nobody!" Yes, I imagine talking high-pass filters.

Exactly which is why I have been suggesting all along that AI have Phase reversal, Notch filtering and high pass filtering separated out from each other. I use the notch filter because most of the time the phase reversal is more important to my sound than high pass filtering. I would prefer to have both and individually adjustable.

 

[onemother]

Just got my Contra tonight and have been putting it thru its paces. Am using a 20 yr old Underwood PUP with a new Monster 1/4" cable (15') on a German Mathieu Toma bass, with Obligatos on it. I placed the amp on a wooden floor in my dining room.

Sound? Wonderful! Sounds like me, acoustically. No boominess, no feedback, very clean. But I'm very concerned about the volume. I messed around with the various levels, EQ, and positioning w/ and w/o the tilt mechanism on the bottom, and I would have to say, at worst, the volume level was very *subtle* - I had to tap the pickup on the bridge to doublecheck I was being amplified. At best, I would say I was twice as loud, maybe three times louder, than I am unamplified, and I have a reasonably loud bass sound. (BTW, this was with all levels on full tilt, with and without filtering.) There was definitely no rattling of the house as I was playing - maybe the occasional vibration of a glass on a nearby shelf.

Is this in keeping with other people's experience of the volume with the Contra? Is this the tradeoff of volume for purity of sound that people allude to? Does anyone have any suggestions for me? I love the sound but I have to admit that the expression "The Emperor's New Clothes" came to mind after a while.

(BTW, I posted a version of this on another thread, but this thread seems to be where the AI action is at.)

 

[drurb]

Exactly which is why I have been suggesting all along that AI have Phase reversal, Notch filtering and high pass filtering separated out from each other.

Agreed! In fact, from my conversation with Rick, he knows that would be a desirable feature.

 

[drurb]

At best, I would say I was twice as loud, maybe three times louder, than I am unamplified, and I have a reasonably loud bass sound. (BTW, this was with all levels on full tilt, with and without filtering.) There was definitely no rattling of the house as I was playing - maybe the occasional vibration of a glass on a nearby shelf.

I'd say something is VERY wrong. You should be able to shake the walls and crack dishes in your house. Not being able to do any hands-on diagnosis, I'd suspect the pickup first.

What system were you using before?
How was the level with that system?

Try an alternate source. Another pickup would be best. If one is not available, patch the output of, say, an iPod to a 1/4 inch phone-jack. TURN DOWN THE LEVEL COMPLETELY ON THE AI. Plug in the iPod. Slowly inch up the level on the AI. Plenty loud? Then it's the pickup.

 

[bolo]

Hi--

I just got off the phone with Rick at AI.

Here is the information, one step removed from the horse's mouth:

Activating the notch filter produces an overall inversion of phase.

Activating the high-pass filter does not invoke any such reversal of phase.

Thus, the two do not cancel each other in terms of phase.

Okay Doc, I guess I agree, kind of. But bear with me please. I acknowledge you are the master.

Functionally, that’s correct. When you are using the HPF, you are not getting an overall phase reversal. But ... that’s because the phase has been reversed twice, again as I understand it.

It's important to note that on the Focus 2RIII the Notch and High pass filters are mutually exclusive. You can use one or the other but not both at the same time.

Right. Very important point.

So just to be pedantic, the top button is labelled Notch/Cut, and the bottom button is labelled Off/On. And maybe I misunderstood, but here is how I recall Rick 'splaining it to me while he was holding one of the units and I was watchin':

a) Both Filter buttons out – Nothin’ changes. Filter is in the OFF position.
b) Top button out, bottom button in - Now you’ve got a notch w/ phase reversal.
c) Top button in, bottom button in - Now you’ve got a high pass filter and a second phase reversal, which is ~ to no phase reversal.
d) Top button in, bottom button out – Nothin’ happens. The filter is in the OFF position.

That enumerates all of the permutations I believe. Basically OFF, notch w/ phase reversal, or HP with no phase reversal (or if you prefer, phase reversal x 2 to be technically complete & accurate).

There is a schematic on their web site. Means nothin’ to me. But maybe others would like to pore over it.

Exactly which is why I have been suggesting all along that AI have Phase reversal, Notch filtering and high pass filtering separated out from each other. I use the notch filter because most of the time the phase reversal is more important to my sound than high pass filtering. I would prefer to have both and individually adjustable.

Wouldn’t it be nice? - Brian Wilson

 

[Uncletoad]

c) Top button in, bottom button in - Now you’ve got a high pass filter and a second phase reversal, which is ~ to no phase reversal.

A technicality. Phase is only reversed with the notch filter.

As such I'm forced to find a spot for the notch I don't want to get the phase reversal I do. In this case I'm choosing 30hz as the most benign.

I'd trade fx loops, reverb and the whole tone control section for separate, phase reverse, notch and high pass filters.

 

[drurb]

Okay Doc, I guess I agree, kind of. But bear with me please. I acknowledge you are the master.

Functionally, that’s correct. When you are using the HPF, you are not getting an overall phase reversal. But ... that’s because the phase has been reversed twice, again as I understand it.

The master? Well, I don't know about that. Not being an owner of one of these amps and based on what has been posted here, I thought that the user was able to choose the notch filter, the high-pass filter, or both. I now understand that one can choose the notch OR the high-pass filter.

Still, what I said was correct. When you select the notch filter, the filtering circuitry that is implemented is one that reverses phase. When you select the high-pass filter, the circuit that is implemented does not reverse phase. That is the plain truth.

Now, I think I know from where the idea of reversing the reversal might have come. According to Rick, and as I suspected, the overall phase-reversal from the notch filter comes about because of the use of inverting op-amps and the particular circuit topology employed.

The notch filter employs an odd number of these inverting op amps while the high-pass filter employs an even number. Now, given the fact that the user can choose EITHER a notch or a high-pass filter, I suspect, but do not know, that the design is akin to something like the following:

The notch filter is created by combining a low-pass and a high pass filter with separate corner frequencies. For example, if you want a notch around 50 Hz, you might low-pass at 40 Hz and high-pass at 60 Hz. In this example, 50 Hz falls in the "stop-band" (the part rejected by the filter) of both the low-pass and high-pass filters. Thus, you end up with a notch in the 50-Hz region.

When you choose the high-pass filter, you are selecting only the high-pass section of the pair.

Now, suppose that there is what is called an inverting "buffer" op-amp sitting in front of the filter section. Well, that will flip the signal once. Now, suppose each filter, that is low-pass and high pass, is implemented with an odd number of op-amps. Let's say the number is three. When you select the notch filter (activating both the low-pass and high-pass filter circuits), the total number of inverting op-amps is odd. In our example, 3 (low-pass) + 3 (high-pass) +1 (buffer amp). Now, when you select the high pass filter, the total number of inverting op-amps is even. In our example, 3 (high-pass) +1 (buffer-amp). Now whenever the number of inversions is odd, the net effect is a phase reversal; when it is even, the net effect is no phase reversal.

Again, I do not know this is how the AI is designed but my example illustrates how the inversion and non-inversion could come about.

There is a schematic on their web site. Means nothin’ to me. But maybe others would like to pore over it.Wouldn’t it be nice?

I found the signal-flow diagram on the site that is within the manual. If you showed me a real schematic, I could be much more definitive.

 

[Adrian Cho]

High falutin' technical mumbo-jumbo

Does it really matter? Personally if Rick says that's the behaviour, I'd be inclined to agree with him.

 

[drurb]

Does it really matter? Personally if Rick says that's the behaviour, I'd be inclined to agree with him.

Nowhere did I disagree with Rick. I was explaining what are the facts concerning what the circuitry does (notch filter inverts, high-pass does not) and how that may come about. Judging from the thread, a number of folks seemed genuinely interested.

If you consider it "High falutin' technical mumbo-jumbo" and unimportant, that's fine with me.

Now, does it matter? Well, yes it does! The fact that one filtering operation provides an overall inversion of the signal while another does not means that, beyond the filtering performed, the acoustic output of the rig can end up adding, in one case or the other, to the acoustic output of the bass in a constructive or destructive manner that can alter the sound.