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Fizz when using HELIX into Seymour Duncan Power amp into cabinet

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I finally managed to persuade my friend to splash out on a HELIX. He wants the "cab-in-room"  sound so purchased a Seymour Stage 170 to power his Matamp 2 x 12"  cabinet from the HELIX.  we went to one of the biggest guitar and amp stores in the UK in Birmingham - where he got to try both the above solution  *and*  some FRFR  and PA type options.

He has of course removed any speaker model block in his presets.   He's currently new to HELIX but a long-time blues and tube amp player/owner - currently with a Two-Rock.

He messaged me recently concerning  fizz. An old subject here. But one that has to be sorted. 

I had my own thoughts - which are here in blue - but  i'm not sure whether my analysis is correct and was hoping some of the Line6 team on the technical side might be able to provide some illumination on  this. 

Here's what my friend wrote - 

 

"Been playing with assorted 'almost' clean models.

Arch, soldano, boogie, matchless.
Think I've settled on the matchless with a bit of grit in there.

But that fizz on some models is driving me round the bend. Can cut it with a high cut around 3k but then you are losing a lot of the high end. And it just shouldn't be there in the amp models. Real amps don't have it. Actually my two rock had it when it had a broken reverb valve.
It's not part of the tone, it's an overlying fizz, like they added some loud dithering on top.
I played with input impedance.
Also played with adding a compressor in front and cutting the gain going into the amp. That seemed to help but... you can go hot into a valve amp."

"It's like an overtone Almost like if you hit a hard clean chords it fades in as the chord fades out. Yes through cab. I had it when recording with USB with cab models one time too."

"I did watch a video on fizz but all the examples the guy played were from metal sings Anyway I think some models are fine and usable. Matchless seems very good."

 

 

He has misgivings about simply rolling of the HF frequencies because he feels he'd lose some of the top end tone.  This got me thinking along the lines here in blue.... which may or may not be erroneous :)    

 

 

i suspect that since youre using a transistor clean power amp, maybe the "connection" from your cabinet to the modelled power-amp isnt taking into account the load and inductance and impedance etc of the amp output transformer that is there in a real tube amp. Which would naturally give you a HF roll-off. So when you say you might lose some of the HF content by rolling off the high end - the reality is that this happens naturally when the speaker gerts connected. So you shouldnt worry about cutting off the high end.

A transistor - and class-D power amp presents a much lower output impedance into a speaker - than a power stage that has an output transformer in it..

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Have him try an actual EQ instead of the filters in the cab block. An EQ has a steeper roll-off than the cab filter, may be able to remove some of the highs he doesn't like while still leaving enough mid-highs to sound and feel good. Maybe.

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well, regarding the frequency response of a typical guitar speaker as the Celestion G12M Greenback (measured by celestion), there ist a lowpass behaviour at about 4 KHz with 24db/octave (on axis). Perhaps this information can help finding a base for EQ settings. It also shows a peak of about 5 db between 2 and 4 KHz.

 

If using FRFR or PA type options without cab simulation, the reason for the fizz is clear: There is no "equalizing"  as a typical guitar speaker does and even more, the high frequencies are spread in a wider angle than a single 12" speaker.

Try to power a FRFR speaker directly with a good tube amp and listen directly in front of the cone middle - won't be much better ;)

 

Maybe, the interaction between poweramp and speaker is an additional influence - i can't judge this.

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If you are using FRFR/PA you need to leave the cab blocks active or it'll be a fizz-fest. You can easily test by repeatedly turning on/off the cab block while playing through the FRFR/PA system and you will notice a significant difference in fizz with the cabs on (it should sound much better). Also, if you are using the cab blocks, disable the EQ as the high frequency rolloff (fizz rolloff) will be done by the cab blocks.

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He shouldn't need the power amp if he has PA/FRFR gear.  The Power Amp into the cab could be a lot of things.  Doesn't that SD have its own EQ stacks?  May want to check that as well.

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If using FRFR or PA type options without cab simulation, the reason for the fizz is clear: There is no "equalizing"  as a typical guitar speaker does and even more, the high frequencies are spread in a wider angle than a single 12" speaker.

Try to power a FRFR speaker directly with a good tube amp and listen directly in front of the cone middle - won't be much better ;)

 

He's not using a FRFR speaker.  As i said in the OP - he is using a normal 2 x 12"  Matamp guitar cabinet. The same cab he uses with his Matamp tube amp or Two Rock tube amp.

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He shouldn't need the power amp if he has PA/FRFR gear.  The Power Amp into the cab could be a lot of things.  Doesn't that SD have its own EQ stacks?  May want to check that as well.

 

As I said in the OP - he isn't using a PA or FRFR system   He is using a 2 x 12" guitar cabinet - driven by a  Seymour Duncan Stage 170 power amp. Which is designed specifically for pedalboards and modellers. 

 

https://www.seymourduncan.com/power-amp/powerstage-170

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I use my Helix into the FX return of a Tubemeister but I still use full amps and cabs/irs.

I don't find the need to use hi/lo cuts this way but then again I'm not going into a PA with it.

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If he's running full models tell him to increase bias and decrease drive until he finds a balance. If there's an annoying frequency don't shelve the high frequencies, notch and cut the offending frequencies with a Parametric EQ

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I was in the Chicago GC store a few days ago, and they have a Helix LT set up in the middle of the room, connected (in mono) to a Powerstage 170 into a Line6 4x12 cab.

 

I liked the sound much better with the cab sims off... Not like they sounded bad on; just, seems like it would be a good way to run a split rig, and put the cab sims after the send to the Power Block.

 

That being said, they made me come turn it down - twice! :) In their defense, I was probably playing loud as &%#F%(&*#%$

 

Definitely pleasantly surprised. I've been gravitating towards trying my Helix as pedalboard only into my tube amp(s), though this could be a cool way to keep the amp modeling and split out a stage rig that could be mic'd and also have a direct send with cab modeling to personal monitor and to FOH.. Could be a best of both worlds for the picky soundguy that "must" mic your rig..

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The interaction between a power amp and speaker is actually quite complicated. A guitar speaker has a nominal impedance usually specified as 4, 8 or 16 Ohms. However, the actual impedance of the speaker depends on how it is mounted in a cabinet, the design of the cabinet (open back, closed back, ported, etc.), the loudness of the sound, and the input frequency. As the speaker voice coil moves in its magnet field, the inductance changes based on the excursion and elasticity of the cone. The impedance variance can be quite dramatic, and is reflected back to the output of the poweramp. How much it effects the power amp depends on the amplifier's damping factor. Larger output transformers with more iron in their cores, power supplies with more filtering, etc. help improve the damping factor - the ability of the amplifier to control the movement of the speaker in the face of its changing impedance and inertia.

 

Amplifier designers attempt to address this issue with negative feedback. This is another complex parameter. Adding negative feedback will allow the amplifier to have a higher damping factor, smoother frequency response, less noise and less distortion. These are generally good things in an amplifier, but not necessarily for a guitar amplifier. The issue is that when an amp with lots of negative feedback and a high damping factor reaches saturation, the transition from clean to distorted can be quite abrupt, not smooth and warm. For this reason, some guitar amps don't use any negative feedback.

 

To make things even more complicated, the presence control in most amplifiers removes negative feedback for high frequencies. This changes the damping factor at higher frequencies and tends to make the amp sound brighter when its not distorted, but looses the high boost when the amp is distorted (negative feedback is eliminated when an amp distorts since it has no remaining headroom). This also works nicely for guitar amplification since the clean guitar is brighter, but when the power amp distorts, the high boost is lost and so you don't get the fizz.

 

If you're getting fizz from Helix, try playing with the presence control along with a high cut. Increasing the presence while also adding more high cut could keep the overall clean tone bright, but not introduce fizz when the amp is distorted since the presence high boost is lost when the amp distorts.

 

Note also that none of this applies to using preamp distortion into a clean power amp. In this case the distortion has to be controlled by the EQ voicing before clipping (often low cut, high boost) and after clipping (often low boost, high cut). This is the art form of modern overdrive and distortion pedals. What these distortion pedals often loose is that high boost/high cut that happens when a power amp with a presence control goes from clean to dirty.

 

S-Gear has a Hi-Cut control in the power amp section that nicely captures this amplifier dynamic. You can set the gain and amp drive up pretty high, and set the high cut high. Then when the volume control is turned down on your guitar, you get a nice bright clean tone since the hi-cut only comes into play when the amp distorts. Turn the guitar volume up and you get a nice warm, fizz-free distortion from the power amp as the hi-cut takes effect. Hopefully future Line6 amps will include this dynamic hi-cut in the poweramp section to help tame fizz.

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