Weird "extra" note. Low-freq intermodulation? Able to fix?

[Matt_B]

I have not yet bypassed other effects other than compressor to see if it is the amp block causing the issue. I just assume it is. Maybe if you are trying to duplicate, you can try the amp block on its own. Power Amp is Mesa Boogie 20/20. That’s it Good luck. Maybe just try the amp block first…

Ok, if you are actually going to try and track down this issue that you perceive than you must isolate the problem yourself. No offense but you're asking for help from others when you haven't even tried the simplest things yourself such as:
- Bypass every block in the chain except the Amp and Cab blocks.
- If I understand your set-up correctly, your using a powered speaker and guitar cabs with a tube power amp. To simplify things you should remove the guitar cabs and tube power amp from the equation and just use the powered speaker.

 

[javajunkie]

hey Java,but cutting 434hz in the amp block and then adding back in 107hz at the end,could that have something to do with it?just asking,because i'm not sure.

I seriously doubt it. It isn't much a boost and the q is standard.

 

[joegold]

Thanks, I'll try that. Also, it was suggested to me off-forum that old strings might be a factor. My strings are overdue for a change too, so I'll try that also.

How close are your pickups to the bass strings?
If the neck pickup's pole pieces are too close to the low E string you'll get problems like you're describing when playing in the upper frets on the bass strings. Plus it will make your intonation impossible to set up correctly.
When fretting the highest fret there should be at least 1/8" between the bottom of the low E string and the neck pickup's pole piece.

 

[fearlessflier]

I have not yet bypassed other effects other than compressor to see if it is the amp block causing the issue. I just assume it is. Maybe if you are trying to duplicate, you can try the amp block on its own. Power Amp is Mesa Boogie 20/20. That’s it Good luck. Maybe just try the amp block first…

Ok, if you are actually going to try and track down this issue that you perceive than you must isolate the problem yourself. No offense but you're asking for help from others when you haven't even tried the simplest things yourself such as:
- Bypass every block in the chain except the Amp and Cab blocks.
- If I understand your set-up correctly, your using a powered speaker and guitar cabs with a tube power amp. To simplify things you should remove the guitar cabs and tube power amp from the equation and just use the powered speaker.

Yes I know, that is a valid comment and no offence is taken. I didn't want to create any unnecessary fuss and this started as an enquiry about whether the issue I was experiencing was a common thing or might be to do with the settings used. I do feel a bit bad that other people are onto it, without me having done much investigation myself, only bypassing compressor and substituting amp with Plexi. But also I am very grateful that this community is giving it some thought.

I moved my rig from practice venue back home over the weekend and have to set up in my garage again before I can try these things. So tonight I will 1. Change strings and see if that alone affects the preset. 2. Try stripping the other effects out so that it is amp only. 3. Try the amp block with default settings. 4. Try other amp blocks with default settings 5. Try some of the parameter tweaks suggested 6. Once I've got it as good as I can, A/B it with a tube amp. I expect to do that tonight (Australian time), maybe part of it tomorrow. Oh yes and I'll do all this through the powered speaker as you suggest.

Phil

 

[Matt_B]

Please excuse me FF. I wasn't trying to bust your cajones. I was just trying to help you think through the logic of the problem, that's all. The best any of use can do is try the patch settings but we can't replicate your guitar, your power amp and cabs (some folks might have the same power amp), or your touch. In other words, we can only debug so much. I know how frustrating things like this can be as I've been wrestling with getting the editor working with my netbook.

All you need to do is try to recreate the issue in the simplest case. In other words, eliminate as many variables as possible.

 

[fearlessflier]

No, we're good. And you are totally right about eliminating variables. Will let you know results a.s.a.p.

On the other point you mention, I've been having editor trouble too. What MIDi interface or adaptor are you using?

 

[fearlessflier]

PROBLEM SOLVED!! Or understood, anyway.

The first thing I did was to drag out my Peavey Delta Blues. Plugged in and played the open D plus 16th fret B on the clean channel. All clean with no noticeable intermodulation. Then switched channels and dialled in a similar amount of crunch as my FAS crunch preset. And guess what - the ugly intermodulation sound was there in spades!!! So, I wrong about saying my tube amp didn't do it. It does.

So I plugged back into FAS crunch preset again and reduced drive. Intermodulation dropped significantly. And here's where I realised something really embarrassing. I told you all that I had drive set to 5.12. Well I did, but I forgot to mention the modifier which switches between 56.7% and 72%. It is only at 72% (Drive= 7.2, I suppose) that the intermodulation is there.

Clearly I am going to be eating some humble pie.

So where did I go wrong? Well, just a bit too much crunch for this song and that combination of notes, I think. I'll change my strings and look for an amp block which has less of the effect, but clearly it is not an AxeFX problem since my tube amp does exactly the same thing. I actually looked up a video of the Cult playing live and Billy Duffy gets a bit of it as well at the same part of the song. I'll chalk this up to experience.

So to all who've offered advice and tried to duplicate (with my dodgy drive setting info), I apologise. And to Cliff too. Thanks for all your kind advice and spirit of helpfulness. Cheers, Phil

 

[joegold]

PROBLEM SOLVED!! Or understood, anyway.

The first thing I did was to drag out my Peavey Delta Blues. Plugged in and played the open D plus 16th fret B on the clean channel. All clean with no noticeable intermodulation. Then switched channels and dialled in a similar amount of crunch as my FAS crunch preset. And guess what - the ugly intermodulation sound was there in spades!!! So, I wrong about saying my tube amp didn't do it. It does.

So I plugged back into FAS crunch preset again and reduced drive. Intermodulation dropped significantly. And here's where I realised something really embarrassing. I told you all that I had drive set to 5.12. Well I did, but I forgot to mention the modifier which switches between 56.7% and 72%. It is only at 72% (Drive= 7.2, I suppose) that the intermodulation is there.

Clearly I am going to be eating some humble pie.

So where did I go wrong? Well, just a bit too much crunch for this song and that combination of notes, I think. I'll change my strings and look for an amp block which has less of the effect, but clearly it is not an AxeFX problem since my tube amp does exactly the same thing. I actually looked up a video of the Cult playing live and Billy Duffy gets a bit of it as well at the same part of the song. I'll chalk this up to experience.

So to all who've offered advice and tried to duplicate (with my dodgy drive setting info), I apologise. And to Cliff too. Thanks for all your kind advice and spirit of helpfulness. Cheers, Phil

Could it be that what you're hearing is just difference tones?

http://www.google.ca/search?q=diffe...s=org.mozilla:en-GB:official&client=firefox-a
http://en.wikipedia.org/wiki/Combination_tone

A high gain amplification setting will increase the audibility of difference tones.
Certain intervals will produce a more pronounced difference tone.
With pure intervals (i.e. the types of frequency ratios that exist within the harmonic overtone series) the difference tone will be "in-tune" with the tones comprising the interval. It will be their "acoustical root".
Eg. C#550 plus E330 would produce the difference tone A220.
So, the acoustical root of that min 3rd interval (C#-E) is A.

But the intervals we use within the 12-tone equal tempered scale system, except for octaves, are not pure intervals.
So their difference tones will seem quite out-of-tune with the tones creating the interval.
Eg. In 12-TET, it's C#554.365 and E329.628, so their difference tone would be 224.737 (554.365 - 329.628) which is a very sharp A (In standard tuning it's A220).

This might be what you're hearing. Just a guess.

The open D string is D146.832.
The 16th fret B on the G string is B493.883.
In order to calculate the acoustical root of this interval we need to transpose the D up so that it's in the same octave as the B. So we'll use D293.664.
493.883 - 293.664 = 200.219 which is about half-way between the 12TET G and G#. (G195.998 is the 12TET G.)

The actual audible difference tone produced by B493.883 plus D146.832 would be a few octaves below the D146.832, possibly 50.05475 (200.219 div by 2, div by 2).
[Please forgive any math mistakes.]

If we were working with pure intervals, then the maj 20th interval formed between the low D and the high B would at a ratio of 10:3, with an acoustical root of G.
I.e. The acoustical root of this interval is G, but the acoustical root of the 12TET interval is a very out-of-tune G, so any audible difference tone produced from this interval will sound kind of "bad" in most musical situations, especially if this interval is being sounded as part of some sort of a Dmaj or Bmin chord.

Just thinking out loud here. This may all be irrelevant bullshit.

 

[mortega76]

Anyone have a clear example of this "Low-freq intermodulation" that I can hear? Thanks!

 

[Sidivan]

PROBLEM SOLVED!! Or understood, anyway.

The first thing I did was to drag out my Peavey Delta Blues. Plugged in and played the open D plus 16th fret B on the clean channel. All clean with no noticeable intermodulation. Then switched channels and dialled in a similar amount of crunch as my FAS crunch preset. And guess what - the ugly intermodulation sound was there in spades!!! So, I wrong about saying my tube amp didn't do it. It does.

So I plugged back into FAS crunch preset again and reduced drive. Intermodulation dropped significantly. And here's where I realised something really embarrassing. I told you all that I had drive set to 5.12. Well I did, but I forgot to mention the modifier which switches between 56.7% and 72%. It is only at 72% (Drive= 7.2, I suppose) that the intermodulation is there.

Clearly I am going to be eating some humble pie.

So where did I go wrong? Well, just a bit too much crunch for this song and that combination of notes, I think. I'll change my strings and look for an amp block which has less of the effect, but clearly it is not an AxeFX problem since my tube amp does exactly the same thing. I actually looked up a video of the Cult playing live and Billy Duffy gets a bit of it as well at the same part of the song. I'll chalk this up to experience.

So to all who've offered advice and tried to duplicate (with my dodgy drive setting info), I apologise. And to Cliff too. Thanks for all your kind advice and spirit of helpfulness. Cheers, Phil

Could it be that what you're hearing is just difference tones?

http://www.google.ca/search?q=diffe...s=org.mozilla:en-GB:official&client=firefox-a
http://en.wikipedia.org/wiki/Combination_tone

A high gain amplification setting will increase the audibility of difference tones.
Certain intervals will produce a more pronounced difference tone.
With pure intervals (i.e. the types of frequency ratios that exist within the harmonic overtone series) the difference tone will be "in-tune" with the tones comprising the interval. It will be their "acoustical root".
Eg. C#550 plus E330 would produce the difference tone A220.
So, the acoustical root of that min 3rd interval (C#-E) is A.

But the intervals we use within the 12-tone equal tempered scale system, except for octaves, are not pure intervals.
So their difference tones will seem quite out-of-tune with the tones creating the interval.
Eg. In 12-TET, it's C#554.365 and E329.628, so their difference tone would be 224.737 (554.365 - 329.628) which is a very sharp A (In standard tuning it's A220).

This might be what you're hearing. Just a guess.

The open D string is D146.832.
The 16th fret B on the G string is B493.883.
In order to calculate the acoustical root of this interval we need to transpose the D up so that it's in the same octave as the B. So we'll use D293.664.
493.883 - 293.664 = 200.219 which is about half-way between the 12TET G and G#. (G195.998 is the 12TET G.)

The actual audible difference tone produced by B493.883 plus D146.832 would be a few octaves below the D146.832, possibly 50.05475 (200.219 div by 2, div by 2).
[Please forgive any math mistakes.]

If we were working with pure intervals, then the maj 20th interval formed between the low D and the high B would at a ratio of 10:3, with an acoustical root of G.
I.e. The acoustical root of this interval is G, but the acoustical root of the 12TET interval is a very out-of-tune G, so any audible difference tone produced from this interval will sound kind of "bad" in most musical situations, especially if this interval is being sounded as part of some sort of a Dmaj or Bmin chord.

Just thinking out loud here. This may all be irrelevant bullshit.

That's precisely where my mind on the first page. The temperment of a guitar isn't perfect. With that much of a difference in frequencies, it's almost impossible to get it to sound tight. Gain amplifies this wobble effect just as it does when you've got a string out of tune.

 

[PacoCasanovasOld]

PROBLEM SOLVED!! Or understood, anyway.

The first thing I did was to drag out my Peavey Delta Blues. Plugged in and played the open D plus 16th fret B on the clean channel. All clean with no noticeable intermodulation. Then switched channels and dialled in a similar amount of crunch as my FAS crunch preset. And guess what - the ugly intermodulation sound was there in spades!!! So, I wrong about saying my tube amp didn't do it. It does.

Dear Phil, of corse it does, because as I said, it's not a fault....it's a feature. And therefore...once again

Well...that's just a wanted feature of the Axe's "ultra realism" which some of you like to have

Remeber, almost every amp in the 70s had this kind of intermodulation. In the old plexiglas amps the power supply capacitors aren't sufficient enough to filter out all of the 60Hz (in europe 50Hz) hum, so the supply rails will ocillate up and down in voltage which has an modulation effect on the ammount of clipping. More clipping...more intermodulation! It's not a fault, it became a trademark specially for Marshall plexiglas amps....

Quote Ken Fischer (who needs no introduction )

weakening filter capacity (50uF -> 20uF in preamp, 100uf -> 50uF or 70uF in B+), this may possibly introduce ghost notes/subharmonics in the signal path

 

[FractalAudio]

Intermodulation is different than "ghost notes". IM occurs whenever two sine waves are passed through a nonlinearity (i.e. distortion). The nonlinearity causes the sum and difference of the input frequencies and their harmonics.

The human ear is slightly nonlinear so we hear a "beat" when playing two notes together. Put these two notes through distortion and the beat frequencies are amplified greatly.

Some RF circuits actually exploit IM as a method of demodulation. The RF input and a carrier are applied to a nonlinearity. The difference frequency is the desired baseband signal. This is mostly done at microwave frequencies where conventional mixing techniques aren't viable.

Ghost notes are a form of amplitude modulation . The cause of ghost notes is excessive AC ripple on the B+. The ripple modulates the gain of the output stage. Ghost notes are especially undesirable because they are harmonically unrelated and occur even when playing single notes. The cure is improved power supply and/or screen grid filtering. Some modelers model ghost notes. It is the opinion of this designer that any amp exhibiting ghost notes is poorly designed and/or needing repair and hence I don't model them.

 

[fearlessflier]

Wow, there is certainly some brainpower in this group!

Joegold, I won't use up half a page by pasting quote of your whole post here. From my somewhat hazy memory of university physics, what you say makes sense. I'm guessing that "difference tones" and "low frequency intermodulation" are two terms for the same effect -or similar? And it sounds like the ghost notes are something else again. But I am no audio engineer and I may have my terminology and understanding wrong. But I would not be surprised if there is a whole lot of different things going on - I do know that non-linearity in systems creates all sorts of effects, potentially even chaos (in the mathematical sense) depending on on how far it is driven from linearity. From Cliff's post, it is clear he understands what is going on at a deep level and I'm sure he doesn't want to turn this into a full-blown audio engineering tutorial. Certainly I am out of my depth, even if others are not!

But I will say that the "extra" note did seem lower, and it was unpleasant. And it DID go away as I reduced distortion. Which is really what I was after.

BTW, I checked my string height and have more than 1/4in clearance. But the point is somehwat moot now that I know I just have to reduce drive. It goes away even with my gammy old strings. The answer was simple, really - I just panicked a bit.

Anyone have a clear example of this "Low-freq intermodulation" that I can hear? Thanks!

Well, I'm a little confused about whether "Low-freq intermodulation" is the correct term or not for what I was hearing. Sorry I can't post recordings, but to duplicate the effect, I think you can take any amp or amp block, turn the drive up to get some distortion, and play a B+ and a D together. It souds like this is a common effect, and if you increase distortion it will become more noticeable, if you decrease it, it will disappear.

 

[fearlessflier]

PROBLEM SOLVED!! Or understood, anyway.

The first thing I did was to drag out my Peavey Delta Blues. Plugged in and played the open D plus 16th fret B on the clean channel. All clean with no noticeable intermodulation. Then switched channels and dialled in a similar amount of crunch as my FAS crunch preset. And guess what - the ugly intermodulation sound was there in spades!!! So, I wrong about saying my tube amp didn't do it. It does.

Dear Phil, of corse it does, because as I said, it's not a fault....it's a feature. And therefore...once again

[quote="Mr.PC":2lir00f7]Well...that's just a wanted feature of the Axe's "ultra realism" which some of you like to have

Remeber, almost every amp in the 70s had this kind of intermodulation. In the old plexiglas amps the power supply capacitors aren't sufficient enough to filter out all of the 60Hz (in europe 50Hz) hum, so the supply rails will ocillate up and down in voltage which has an modulation effect on the ammount of clipping. More clipping...more intermodulation! It's not a fault, it became a trademark specially for Marshall plexiglas amps....

Quote Ken Fischer (who needs no introduction )

weakening filter capacity (50uF -> 20uF in preamp, 100uf -> 50uF or 70uF in B+), this may possibly introduce ghost notes/subharmonics in the signal path

[/quote:2lir00f7]

JEEZ, DON'T RUB IT IN! :roll: :lol: Of course, you were right. And I never meant to imply there was a fault. I was just interested in getting a particular sound i.e. some drive, without any funny extra tones on the high B and open D. Now I know a bit less drive makes it work and in fact the tone is better that way. So all good.