How to build a VANDAL guitar amp…

Although we’ve planned to offer a guitar amp AND a bass amplifier, only the guitar amp is currently implemented. Unless we’re sure we’ve nailed it, all talk here remains highly guitar-biased for the time being.

If you’d unmount the case of a VANDAL guitar amp, you’d notice it contains typical tube circuitry to some extent and is in many ways comparable to popular designs known from famous stuff. But we’ve also thought of going an own route whereever it seems worth it, in order to create new tones or expand the amp’s flexibility.

Of course there’s no ‘case’, there are no ‘tubes’, no ‘circuit’ and other things we could touch (or burn our fingers at). This is all virtual. This is analog modelling.

Taking measurements on a THD Flexi-50

While these days, everybody’s claiming to do such kind of DSP wizardry, we’ll hopefully manage to prove that we are serious here, by presenting the underlying technology in its whole beauty. Let’s hope this doesn’t get too boring…
The reason why we only concentrate on one amp currently is that we want to get its interiour right.

The main concept of the VANDAL guitar amp is a virtual circuit that models 3 different preamps and 2 different power amps, accessible via mode switches and a single clearly laid-out front plate:

• 3 channels (clean / crunch / lead), each with pre & post gain controls. The controls ‘memorize’ their settings upon switching channels, so that simply stomping on your floorboard and switching channels doesn’t result in embarrasing level drops or blasts the audience heads off. I do not think one always needs an entire preset change here. The ability to play the amp live is a must in our design philosophy.
  [Well, I should be careful here, as we haven’t made precise concepts for midi implemetation at all at this time… lalalalala…]
• pre-distortion ‘voicing’ control: in order to shape the tone prior clipping, the user can set the center frequency & gain of a mid peaking filter. Comparable to guitarists using a eq pedal before the amp, typical ‘smile’/’frown’ curves can be created here with just two knob twists. This is one of the key features of a great guitar tone.
• post-distortion tone stack. The user-selected ‘model’ decides over the actual implementation, but this tone stack is basically a model of a typical Fender/Marshall-type passive circuit. Unlike most analog & digital filters,these guitar controls are highly interactive. They are everything but perfect engineering-wise, but are perfect in terms of musicality and expressiveness.
• spring reverb. The VANDAL guitar amp houses a classic 2-spring reverb tank, delivering a faithful emulation of that springy, old-school vibe that these units are known for. We currently use delay times of 34 and 41ms and medium echo dispersion for that matter.

• master volume. The power amp can be driven very hard and highly interacts with the preamp, so there may be situations where one would just crank it up. However, for some hard rock/metal styles, this isn’t so desireable. A master volume can sometimes be your friend.

VANDAL guitar amp view (with 'tweak' view open)

Preamp

The preamp is actually 3 different models. Their internal design differs greatly. However, the user controls apply to all the three.
A preamp consists of 1 to 4 tube stages internally, all having different amplification factors and different pre- & post filtering (due to DC decoupling and signal band limiting / tone shaping). A speciality of VANDAL is the ‘voicing curve’ eq prior to stage 2 or 3 (depending on the model).
All preamps are oversampled (4x at 44/48k) to ensure that we don’t get aliasing when cranking up the gain. This sucks a bit more CPU cycles but latest comparisons revealed we are still doing fine here. [My Development machine (old P4 3GhZ, hyperthreading) shows around 3% per plugin instance, for the entire amp. In-depth optimisation is still on schedule, although I’m not that sure what the outcome might be.]

• classic mode. Not based on a specific design, but think of Fenders, early Marshalls or the first Boogies. This preamp has 1 tube stage for the clean channel, cascaded by a 2nd for the crunch and a 3rd for lead. The preamp is generally suited for mild tones, having a smooth transition from clean to distortion. The tone control digs a hole in the lower-mids upon turning the mid control counter-clockwise.
• british hybrid. Going the JCM route here (mid 1980s until now). Many typical UK rock tones have this raspy, gritty distortion tone due to a silicon diode clipping network between two tube stages. Very versatile with 3 channels here; the lead channel is sort of an extra to the traditional layout.
• modern high gain. This one has 4 stages in series, while bypassing part of the 2nd-stage output to the 3rd or 4th stage output for the crunch or lead channel. This allows for large amounts of gain while still keeping the transients intact. The overall character is pretty lower-mid oriented. We aim for thick & famous US stadium rock tones here.

Power amp

VANDAL’s guitar amp has two different power amp circuits. Although their basic tone is similar due to the saturation characteristics of the underlying tube model (quite EL34-like), they behave differently when treated the hard way. The user will be able to select between the two modes:

• class A (low power). This can be driven into distortion quite easily. As a class A design can only operate on a positive halfwave, the incoming (bi-polar) signal has to be offset along the operational curve of the power tube. But since this biasing is only valid for steady-state and because of tube nonlinearities, a class A circuit adds more even harmonics with the transients phase of a signal. This dynamic clipping is a very sought-after effect. We definitely don’t want to miss that in a virtual amplifier.
• class AB (high power). Such a circuit has 2 power tubes (or 4), operating in a push-pull fashion. Each tube stage amplifies one half wave of the signal. Inside the VANDAL amp, we’d let the circuit operate in class AB mode, which means that the biasing is adjusted in a way that both tubes always conduct a little (otherwise we’d get crossover distortion, which sounds just plain awful).
  AB delivers a bit less than the double amount of volume of the class A mode. We’ve recently adjusted it in a way that the perceived volume is a bit less than double, but the overall headroom is still higher than class A.
  With this mode, there are almost no even harmonics on the transients, as the distortion character is mainly symmetrical. However, this circuit reacts very realistic to power supply drain. Just as with real amps containing rectifier tubes in their PSU, the virtual power supply in the VANDAL amp gets loaded by the power amp, too.

  Out of curiosity, I’ve added a sag control to the class AB mode. Initially, I wasn’t that sure if the tube model really behaved like it was supposed to do. But luckily, it does: The more you turn this up, the more you allow the power supply to go down. Currently, from 250V to almost 50V. I’m talking plate voltage here, which means we change the tube’s internal amplification factor as well as the overall characteristics. A starved plate is introducing lots of distortion as well as volume drop. As this happens mainly on the transients here, we get a cool crunchyness on pick sounds and a very ‘compact’ or ‘dense’ sound. Responsive might be the right word.

  The actual charge/discharge charater of the sag action depends on the power supply’s capacitor size(s) (to filter out any AC residue); this isn’t user-adjustable. We’d probably do more in-depth playing and listening here soon to find the right values. This is a privilege with modelling, as you can tweak parameters beyond actual physical specifications. It’s all in the name of tone… *cough*

A word on sag for class A: I’ve tried that. I was expecting it to have some impact on the transient response. So I temporarily connected the sag control to the class A circuit, too. Indeed it had an impact. Surely, the transients led to large amounts of even harmonics, just as in theory. But it didn’t please one’s ears (at least not mine). I don’t think class A sagging is that desirable. So the idea was discarded quickly.