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Digital piano and audio newbie here.

I picked up a Korg SV2 and JBL 104 active monitors.

I hooked them up with 2 1/4" jacks but the output is very low when maxed out on both the keyboard and monitors.

Is there a way to increase the volume?

I assumed I might need an audio interface but I get the impression that interfaces are used to connect to a computer.

The monitors work fine connected to my computer directly via the RCA cable. I'm trying to create a computer-free practice area though.

My 1/4" jacks were pretty cheap and the connection on the monitors says TRS Balanced. Are not all 1/4 inch to 1/4 inch jacks TRS?

Thanks!

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From the JBL 104 setup manual

Connect your sound source to the master speaker using one of the following inputs:

  • The AUX (3.5mm) input can be used to connect to personal devices such as cell phones, laptops, and MP3 players.
  • On the back panel, you have two input options. You can connect professional equipment with high (+4 dBu) outputs, such as audio interfaces and monitor stations, to the 1/4” (TRS) inputs.
  • RCA Left and Right Inputs can be used to connect consumer equipment with low (-10 dBV) outputs, such as CD players or turntables.

There are three "reference" levels for audio. In general 'pro' audio runs at +4dB, 'semi-pro' runs at -10dB & 'consumer' runs at -20dB. Connecting the wrong ones together is never great, though rarely immediately damaging, so long as you watch your levels.

The expected levels for your three options above are -20, +4 & -10 (fortunately they expected these to have semi-pro use).

The +4 inputs on the monitors expect balanced line, TRS jacks.
Not all jacks are TRS, in fact in most semi-pro & pro applications only TS jacks are used. That includes everything from guitars to keyboards (with some exceptions).

The aux jack will almost certainly be expecting -20 from phones, laptops etc.

That leaves the RCA connectors.

From the Korg SV2 spec

Audio Output L, R connector (XLR balanced), L/MONO jack, R/MONO jack (6.3 mm [1/4"] unbalanced)

So, that gives you two options for cabling to match levels.
2x XLR to 2x TRS balanced.
2x TS (standard guitar jack) to 2x RCA phono.
Jack to phono will be much cheaper if less rugged. For practise, durability may not be a factor.

As you've discovered, plugging TRS from a TR socket at -10dB to TRS expecting balanced +4dB is mismatching your levels by a factor of 14dB or so depending on how you measure it… that's why it's so quiet.

I'm not considering the differences in dBu & dBV standards for this, just to simplify. More info at biamp-Gain structure: input and output levels for the inquisitive.

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    Clear and thorough answer. Great case for: “Just because it fits doesn’t mean it’s going to work well.” +1 – John Belzaguy Jan 10 at 18:05
  • @JohnBelzaguy - thank you. i should have used that as my tagline ;)) – Tetsujin Jan 10 at 18:07
  • You just have to watch out for other interpretations, :0 – John Belzaguy Jan 10 at 18:14
  • @JohnBelzaguy - I had already chuckled at that idea & decided I ought to be grown-up enough not to mention it in public '))) – Tetsujin Jan 10 at 18:18
  • Well, at least one of us was! – John Belzaguy Jan 11 at 3:14

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