BIAS

The world of iOS for guitarists just got a whole lot more fun with the release of Positive Grid’s BIAS. There are several amp simulators on iOS (JamUp, AmpKit and Amplitube), but BIAS is a whole ‘nother ballgame folks. Imagine having an amp workshop chock full of vacuum tubes, transformers, EQs, and speaker cabinets. But instead of giving up your garage for a dedicated place to solder (and avoid electrocuting yourself), you can build your own amps virtually within BIAS.

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I think it helps to think of BIAS as working at two levels. The first level is the familiar catalog of amp presets and some “front-panel” customization. Any knobs that you can twist on the front of an amp, you can tweak here (bass, middle, treble, gain, presence, etc.) The presets BIAS comes with sound great, but where the real magic happens is when you get to the second level and enter “BIAS mode”. Tapping the little gear button on the right-hand side takes you behind the front panel and into a complete customization of the entire amp circuit.

The Signal Chain

Once in BIAS mode, you get a visual representation of the signal chain which moves from left to right.

The first item is the look of the amp preset. There are lots of popular looks here to choose from allowing you to create some truly bizarre combinations (tread plate panel with chicken-beak knobs and a tweed grille, anyone?) This is the only part of the chain that is looks-only, it doesn’t affect the tone.

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As you move through each section of the signal chain, all but the speaker/cabinet section offer high-level presets in the lower left-hand side. Since each module can be pretty complicated, these presets are a nice way to get a tone that’s close to what you’re looking for before you start tweaking the knobs.

Preamp Section

The first item that affects the tone is the preamp section. On the far left is a basic tone-shaping stack which then passes to the pre-amp tubes. Here you can select different kinds of tubes, adjust the gain and even setup multiple gain stages à la Mesa/Boogie and other modern high-gain amp designs. Next, you can bias those tubes (with no risk of killing yourself!) and finally the signal passes out to another tone-shaping stack.

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The great thing about both tone stacks is that you can bypass them completely, so if you’re trying to replicate a simpler amp design from days of yore, you can remove these from the circuit altogether. If you enable both stacks, you have a lot of control over the tone, but you’re also introducing more variables. BIAS is such a complete amp modeler that it also accurately models the chaos theory-like way that tube amps work: tweak a parameter somewhere and it cascades to a huge effect elsewhere.

All of these parameters are interdependent in some way, so you’ll often find yourself circling back to other parts of the signal path as you tweak. Now, honestly, this is what makes the app so fun. All of those earnest discussions you’ve had at the local guitar store about EL34s vs. 6L6s can now be settled by wiring up a tone any way you see fit.

EQ Sections

BIAS mode contains two optional, movable EQ sections. These are highly customizable graphic equalizers that you can drop anywhere in the signal chain. The other parts of the chain (preamp, tone stack, transformer, etc.) are all fixed in their positions. You can also pull them out of the chain altogether for a simpler signal chain.

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Since there are so many tweak-able parameters in BIAS mode, I’d think of these EQ modules a bit like Photoshop adjustment layers (if that analogy means anything to you). Use them for ever-so-slight tweaks and colorations, but not major tone-shaping, otherwise you’ll be constantly tweaking them anytime you adjust the signal anywhere else in the chain.

In my experiments with BIAS I haven’t used them much except for occasionally engaging the EQ in the post-cabinet section of the chain. This is typically after I’ve dialed just about everything else in and just need a little cut or boost somewhere. Again, like Photoshop, I’m just covering up a few blemishes, not performing major surgery.

Tone Stack

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The tone stack section maps to the tone controls on the front panel. Here you have two basic things you can adjust: the tone stack topology and the individual parameters for that topology. This is where we start getting into some pretty deep amp-design history. Switching tone stack topologies will give you a handy description of each with historical context and some suggested uses. 

One word of advice, don’t dismiss certain topologies because they don’t match your playing style. If you’re a metal-head and you see the words “country” and “blues” under “America Tweed” topology, it’s still worth trying it out. I’ve stumbled across some pretty cool tones that I wouldn’t have tried if I stuck to the descriptions.

The presets at this level are the intersection of a particular topology with specific settings for each knob. If you want to start fiddling with the tone stack, I would suggest choosing a preset that gets you in the neighborhood then start tweaking. What’s great about the presets is that once you find one you like, you can save it as your own preset which makes it available in other amps.

Power Amp

In the old days, before we had all kinds of control over multiple stages of gain you had to crank an amp up to get that warm distortion we all love. This involved pushing the power tube section of the amp and is often considered a critical part of getting some of the all-time classic guitar tones. Maybe you aren’t in a place where you can crank up a vintage Fender Champ, but BIAS will let you do the same thing virtually.

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There are several different power-amp configurations you can choose from. A classic Fender champ uses the relatively simply single-ended style power-amp section, whereas a bigger, more modern amp will use “push-pull”. The master knob and distortion knobs let you dial in power-amp distortion. The splitter gain and power gain knobs let you dial-in how the power-amp interacts with the transformer. You can also tweak the bias of the power-amp section and you can also dial in some presence and resonance here.

If anything, it’s more tweak-able parameter than any sane person needs. But, if you’re a tone-chaser, sanity left the building a long time ago.

Transformer

The transformer section of an amp is responsible for converting the high-voltage, low-current signal to low-voltage, high-current signal that can drive the speaker. In BIAS you can choose from three basic styles of transformer. This is also where you can dial in the “sag” of the amp using familiar compressor-like controls.

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Once I had a tone dialed in, I was surprised at how much of an impact the transformer type had on the final tone.

Cabinet and Mic Placement

It seems like in the endless parsing of amplifier minutiae, the impact of speaker and cabinet choice is often forgotten. But make no mistake, it makes a huge impact on your final tone. BIAS comes with a great interface to select from a number of cabinet and speaker combinations. Once you have one selected you can select a SM57 or AKG 414 from the virtual mic closet and place it in a virtual 3D space on that cabinet.

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If you don’t believe what an effect the speaker has on your tone, try listening to the line-out from your amp. It’s pretty harsh sounding and sounds a lot like when you bypass the speaker simulation in BIAS. Even with the same speaker cabinet, the mic choice and placement can change the tone quite a bit. BIAS does a fantastic job modeling all of this while providing a straightforward interface.

Plays Well With Others

On its own, BIAS would be worth the price of admission. But what really makes this a must-have tool is that it plays so well with other apps. First, you can export your amp/cabinet into Positive Grid’s JamUp app. You can even round-trip those tones back into BIAS for further tweaking.

BIAS is also Audiobus-compatible which means it works with any of the hundreds of other Audiobus-enabled apps out there. You can even combine BIAS with AmpKit or Amplitube if you have some favorite effects you like to use in those apps BIAS also supports iOS 7’s new Inter-App Audio which makes it a snap to use with GarageBand.

Summary

If you’re a guitarist on iOS that wants to tweak tone, there’s nothing out there like BIAS. I’m pretty fussy about tone, but I’d use BIAS for recording. I can dial in a great tone without having to setup an amp, fiddle with the mic, etc. etc. Since I’ve gotten into the habit of regularly getting a totally clean, dry signal for all my guitar parts, I can even re-amp my parts back through BIAS.

BIAS is also a great way to learn how tube-amps work and how the various parts affect your tone. Just playing with this app and reading Dave Hunter’s “The Guitar Amp Handbook” I’ve learned so much more about these odd, but essential, critters.

You’d be hard-pressed to find a better use for $20.