Need help understanding the nature of Bias gain


  • A2

    Trying a Bias FX iPad setup with FRFR setups at band volume, I notice there is a lot of breakup on the really low end that I can't quite understand and can't really get rid of if I wanted to. I tried a bunch of different heads, and for the most part things were similar. I noticed all of my FRFR speakers, of different brands did have some serious resonances, but this wasn't quite all of it.

    On mixes and recordings, I'm filtering out the bottom often so its not something I notice, and any boomy resonances, I can get rid of with parametrics or multiband compressors or dynamic eq's.

    I went back into the control room and launched the same presets I made on the desktop version and turned on the subs and noticed that it is there also, just minus the specific cabinet resonances.

    I guess I don't understand Bias Amp especially enough, and don't really know how to control the low end breakup.

    Are there any presets on the cloud of vastly different sorts so I can get an idea of how exactly the heads' settings affect things?

    Any really smooth yet high gain ones you can recommend?



  • What kind of amp model are you using? Is it a matched one? Or a stock one? Have you made any changes to the actual gain staging in the amp within bias amp editor?

    I noticed that it is really easy to get bad an improper distortion if i mess with the setting the wrong way on the different amp models.

    Also, what are you putting into the amp model? It could be a pedal in your chain that is producing unwanted low end issues.


  • A2

    If I just load up Amp, the stock amps "insane 5153, Mk2C, Positive Insane, Triple Treadplate, SLO SP-88 have this issue, while RB 101B and British Lead 800 have it to a lesser, and slightly different degree.

    High passing before the amp changes the nature of it somewhat, but the bottom breakup is still there. I know writing about music is like dancing about architecture, so I am trying to use more specific terms, but, now I'm second guessing a lot.

    I'm going to go to Honolulu in a bit and pick up a few piles of amps and preamps I am familiar with recording and see if I'm not just running into a phenomenon that actually exists in the real world and for whatever reason, is just being more noticeable in this case.

    This really speaks to the incredible coding of Bias, that it can make a seasoned engineer question everything he thought he knew about the nature of distortion.

    Do you have any really smooth starting point reccomendations, maybe a stock amp that you really think behaves properly in the under 200hz range?



  • I haven't played enough of them in the real world to know for sure. I feel like if I set up my tubescreamer into a good amp model I can get a great tone. I use the tube screamer to tighten low end so i am actually opposite of you.

    I play a lot of proggish metal stuff so I am always having to add bass.

    I found in bias amps there is a really specific sweet spot on gain. Too much and the distortion is really bad. Too little and there's no saturation. Gotta get it perfect.

    That's why I just want to get a Kemper lmao


  • A2

    In the real world, we often use tube screamers as high pass filters, with a variable volume boost. I tend to do that in Bias FX as well, as you seem to, but even with the tubescreamer, its still sort of there.

    When Krank was still modding Sovteks and Marshalls at Central Music, I used to bring preamps and things of artists so he could fix or mod them, and I would trip out at the amount of filtering he'd do on the front of some of these. There was a Marshall 900 he had on the scope and was tweaking it to high pass from 700 hz on down! And with a steep enough filter that it actually peak boosted pretty hard at around 800hz. I was aghast and said something like "surely, you are going to end up with emasculated mosquito tone killing that much lows"

    But no, he put a tube screamer on the scope, and though it was a much shallower filter curve, it also was around 700hz

    I dont know that much about amplifiers, wish i did, and it seems like Bias is the ultimate toolbox for this



  • Well I usually use no gain on the tube screamer and sweep the tone so that it is cutting out lows and boosting volume to tighten it all up while driving the amp. So lower gain on the amp model too.

    Hey does master on these bias amp models affect the power amp sim?


  • A2

    Well, this is interesting. Here's PG's screamer vs a sweep. Green is the screamer

    alt text


  • A2

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    Here is a claimed theoretical response

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  • A2

    And PG's

    alt text


  • A2

    Is there a way to open the stock bias fx pedals in bias pedal?



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  • I don't care for the PG version of the TS808 (though I haven't found one I really care for anyways). Clips too hard, and takes away too much bass from the original tone, then adds its own through amplification. This is why being able to dictate the harmonic profile from within the tube, rather than before and after it, would be ideal - and why isn't everyone and their grandmother's dog's girlfriend not interested in this??

    Speaking of low end, I'm curious what you hear in the low end character of my tone in my posted video clip? What does it sound like through different monitors?


  • A2

    Which video clip?

    Have you tried the various ones available in Bias Pedal? I don't know enough about it to really make a tube screamer properly, if it even allows it. I don't mind it HPF'ing the bass, but the wah wah like curve it gives always sounded weird...Wasn't sure why until I did that sweep. Ill see if I cant plug the iPad in and sweep some of the pedal models


  • A2

    Here's the Bias Pedal Screamer:

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  • A2

    And, Bias Pedal Tube Screamer

    alt text



  • @pipelineaudio Just to be sure, are you listening to the amps through the default "mic-placement-on-cab" solution or though the IR Loader with an IR you know well ?

    I hear the same low-end issues you're talking about but it seems to be coming from the cabinet/mic modelisation that i suspect to be a mix of different Impulse Responses.

    I only know some basics about IR technics, but there is a "known" issue => since you use a sweep (and not white/pink noise) to record the hardware response, you'll stay at specifics resonnant frequencies that will be recorded louder than they should be in the IR profile (depending on how long your sweep stay at the resonnant frequency).

    In short, IR are magnifying glasses of all the good and bad points of a recorded hardware/signal chain (when there's acoustic/mechanical resonnance involved).
    I suspects the low-end to be due to the mics (sm57/c414) proximity effect that were used when making the cabinet/mic modelisation.

    Sadly, i don't have the pro version of Bias AMP yet (want to try it out completely before upgrading) to use the IR Loader to hear if it fix the issue or not.


  • A2

    It seems to be always there, but different cabs or impulses magnify the audibility of it differently, they don't change the nonlinear behaviour



  • Seems like there is something at work with the way they merge the cabs (acoustic domain) with the amp signal (electrical domain).
    I went and tried 3 high-gain amp and there are some troubling similarities once you activate the cabinet simulation.

    Bypassed
    Fire Amp
    Mark II Amp
    Rectifier Amp

    With the Cab (same settings => 4x12 treadplate + c414)
    Fire Amp
    Mark II Amp
    Rectifier Amp

    As we can see, the amps all have different curves when bypassed but the moment we start to use the cab simulation we get the same 3 issues on all of them.

    alt text
    .

    • 95 Hz resonnnace
      This one is easy to guess, it is most likely the proximity effect of the C414 at the used coordinates

    • High Frequencies issue
      In which acoustic world do you get "nothing" above 15KHz when recording from a microphone ? Actually, the signal is still there but goes from -120 dB to -175dB (55dB drop on one octave). The 5KHz-10KHz is already a 48dB drop on one octave so i guess there is some sort of lowpass filter at work.

    • Low Rumble
      Now i'm not to sure how to understand that one. There is a lot of energy under the 70Hz point but we can see, as expected, that there was some kind of highpass filter at 80Hz applied from the C414 directly. Still, if had a constant energy signal of that sort during recording, i would ask myself if either i have some voltage issue on my preamp/recorder (maybe it's due to the sound pressure level applied to the mic).

    Conclusion, the cab sim affects a lot the sound rendition and it would be interesting to see what happens in the low frequencies when using an IR with no low end (something like an announcer speaker).


  • A2

    I wouldn't be at all surprised if those are the curves the mic emulations impose


  • A2

    It would be nice to compare these results with a sm57 placed on the same spot of the AKG