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I want to modify my sound with VST before sending it to my amplifier. My audio interface is the Focusrite Scarlett 2i4 and my amp is the Ampeg BA108.

So it will look like this: Bass -> Audio Interface input -> PC -> Audio Interface output -> Amp

My question is what input on my amp should i use and what cable do i need?

Amp inputs:

  • Effect Loop RETURN & SEND
  • CD Input
  • Instrument Jack input

Audio interface outputs:

  • RCA
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There appear to be more than one version of the Ampeg BA108 on the market. I'm not sure which one you have.

You want to use a pair of the unbalanced RCA outputs from your Focusrite audio interface, connecting this to the pair of "CD Input" jacks on your Ampeg amp. This will provide the correct impedance and line signal levels for the amp's inputs.

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You need a basic RCA stereo pair cable, like this:

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Take note of the fact that by doing this, the signal from your bass, processed through your PC and output from the Focusrite, will go into the Ampeg amp's auxiliary input, which will bypass the amp's own "Treble" "Mid" and "Bass" tone controls. The "CD Level" knob will control the volume of the signal coming into the "CD Input" jacks, and the "Volume" control on the left may function as a master volume.

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You have a couple of options here. Consider that by plugging your bass directly into the Focusrite, you're not really using the bass amp for what it was designed for (amplifying an instrument signal), and instead having it do what would be better served by a set of full-range monitor speakers. You'd be effectively using the Focusrite as a DI box and your bass amp as a monitor. The Focusrite is perfectly good at that, but you might want to get some more benefit from your bass amp.

One option would be using the effects send and return jacks on the amp to create an "effects loop" using your Focusrite and VST processors. You'd just need regular TS 1/4 inch patch cables. Your signal chain would go from your bass to the regular input on your amp, then connect the amp's effects send to a line level input on the Focusrite, then from a mono line level output of the Focusrite to the effects return on your amp. (You can always plug a balanced jack into an unbalanced one or use a TS cable instead of TRS -- it just means the whole connection is unbalanced, so don't do it over long distances.)

I'm an amateur at reading circuit diagrams, but I believe that the effects return on your amp will create a bypass of the existing signal, so you get the benefit of the preamp inside your Ampeg, you can do whatever you want to your sound on your PC, and have that come out of the amp speaker. You still get to use all of the controls on the amp, they just affect the signal before it goes into your effects processing.

  • Also, based on your question, you have the original version of the BA-108 with the effects send/return on the back panel. And your Focusrite has 1/4" balanced outputs next to the RCA jacks. If those are in use, you can still use one of the RCA outputs to go to the effects return, you'd just need one half of a stereo RCA cable, and a 1/4"-RCA adapter on one end. – NReilingh Sep 13 '15 at 3:52

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