Connecting an SBUS/UK to an external amplifier

 This post is really specific to owners of Quilter Superblock amplifiers (US and UK). 

Connecting a Superblock to an external amplifier

There are several ways to do this depending on the type of amplifier you are connecting to. In all cases, the best way is to use the XLR direct output of the SBUS/UK.

(1) SB to a powered speaker or Mixer that has an XLR input – using an XLR mic cable, connect the SB DI to the XLR input of the powered speaker or mixer. You’ll want to use the speaker sim for this (not FRFR) for the best sound. This is the easy one.

(2) SBUS/UK to Guitar amplifier Instrument input – If the guitar amp you are connecting to does not have an XLR input or an Effects loop, then you will use this method. You need an XLR to TS (1/4” plug) adapter like the Neutrik NA2FP or build your own as shown in the schematic below. Make sure the guitar amp is set to a flat EQ, turn the gain down so the guitar amp is not overdriving, and turn all effects off in the guitar amp. Start with the volume on the guitar amp fairly low, and adjust as needed. Keep the Master on the SB on the low side to start until you achieve your desired volume. I suggest you initially set the SB to FRFR and test it with overdrive pedals or settings. You can try one of the speaker sims, but the tone may suffer (you won’t break anything, and do what sounds best)

(3) SBUS/UK to Effects return of the guitar amp – You’ll need the same Neutrik NA2FP adapter (or your home made one) for this. Plug the XLR cable into the SB Direct Out XLR, and plug the TS end into the Effects Return of the Guitar amp. Start with the SB Master at minimum and slowly raise the volume to where it needs to be. Some guitar amp's Master volume is after the loop, some may be before. If you’re not sure, start with the Guitar amp’s Master at minimum as well (for example, the Micropro Mach2 Master is before the loop). Set the SB sim to the desired cab setting (FRFR will likely be too bright and harsh in this situation). 

I’ve tested all of these methods with a couple of amps I have here, including the Mach2 and they all work (I use a homemade adapter). So, why not use the headphone output? The jack itself has two signals, one for the left and one for the right, you’d need to leave one of those floating to avoid possible damage (it should not be shorted to ground). Why not use the Effects Send of the SB? Well, you might have effects in the loop, plus the Effects Send signal is before the speaker cab sim, so you wouldn’t have that sonic option. 

But Wait...Doesn't an XLR connector have a Balanced Signal?

Yes, under most normal audio conditions, it does. A balanced connection uses three wires, a shield, a non-inverted audio signal (in-phase), and an inverted audio signal (out-of phase). On the receiving end, these two out-of-phase signals are recombined and effectively cancel out any noise that was picked up on the cable (see this link  How Balanced Audio works  ) If we just connected a truly balanced signal in the way shown in the schematic, we would short out the inverted signal.

However, in Quilter amplifiers, the XLR signal on the Direct Out is "pseudo-Balanced" and not truly balanced. I found this out by measuring the signal, and Pat Quilter confirmed it for me (my Mach2, Phantom Block, and Superblock are all the same in this regard). What this means is that in Quilter amps, pin 2 of the XLR carries all of the signal, while pin 3 is just a zero-volt reference signal. Because of this feature, we can connect Pin3 to the shield of the cable (and thus ground) as shown in the schematic and everything will work just fine