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Q. How do I hook up guitar pedals to my synth or keyboard?

Published January 2017
By Hugh Robjohns

When interfacing with guitar pedals, attenuating a  digital keyboard’s output externally is less likely to result in reducing the signal-to-noise ratio to an unacceptable degree than simply turning the keyboard’s onboard volume control. If you can solder, this simple cable is a  very affordable means of providing that attenuation.When interfacing with guitar pedals, attenuating a digital keyboard’s output externally is less likely to result in reducing the signal-to-noise ratio to an unacceptable degree than simply turning the keyboard’s onboard volume control. If you can solder, this simple cable is a very affordable means of providing that attenuation.

I have a Nord Electro 4HP keyboard, which I’d like to run seamlessly through guitar pedals (namely a Digitech Polara and a TC Electronic Flashback X4) in real time and in stereo. I haven’t had any luck so far; the signal distorts quite badly at higher keyboard and pedal volumes. Can you suggest a device that would remedy this? I’m not too audio-savvy but as far as I know I have to match the impedance of the keyboard and the pedals. I already own a Radial ProD2 if that helps.

Timotej Jerman via email

SOS Technical Editor Hugh Robjohns replies: The fundamental problem is that the guitar pedals expect a typical instrument-level signal such as that from an electric guitar, which is nominally around -20dBu, whereas the Nord is capable of generating a line-level output which will be some 20dB louder. So it’s not really surprising that you are overloading the effects pedals very easily.

Impedance isn’t an issue here at all. The golden rule for voltage-matched interfaces like this is that the destination’s input impedance should be at least five times, and more usually 10 times (or more) greater than the source’s output impedance. Electric guitars are very finicky about the impedance they have to drive, and so most effects pedals and guitar amps will have an input impedance of 250kΩ or higher, and most active DI boxes will present an input impedance of 1MΩ or more (ie. four times higher still).

However, with an active electronic output, such as that from the Nord, the output impedance will be a few hundred Ohms at most, so anything above a few kΩ will be absolutely fine — there really are no impedance concerns when connecting an electronic keyboard like this to an effects pedal, all the issues are related to the signal level alone.

The practical problem, then, comes down to the best way of attenuating the signal level to something that the effects pedals are happy with. Obviously you could just turn down the master output level control of the Nord, and for many situations that will work perfectly well. However, if you need to turn the output level down substantially it’s possible that this simple approach will reduce the signal-to-noise ratio to an unacceptable degree, meaning that the Nord’s output will appear to become excessively noisy when routed through the effects pedals. In such cases, a better solution would be to turn the keyboard’s master level control up to a more conventional setting, and attenuate the signal externally prior to feeding it into the effects pedals, as this approach more or less maintains the Nord’s original signal-to-noise ratio.

If you are competent with a soldering iron then you could simply make up a couple of custom attenuating jack-jack cables to connect the Nord into the effects pedals, with a couple of resistors in the destination jack plug to form a simple voltage attenuator. For example, a suitable configuration would be as shown below, with a 10kΩ series resistor and a 560Ω parallel resistor, giving around 25dB attenuation.

Increasing the parallel resistor to 1kΩ will reduce the attenuation to 20dB, and if you swap it for 330Ω the attenuation will rise to around 30dB. This is certainly the approach I’d take, but you would need to mark the cable in some way to make sure you recognise it as an attenuating cable and know which way to connect it — plugging it in backwards wouldn’t actually damage anything, but it wouldn’t provide the correct attenuation either!

If soldering cables isn’t your thing, then you need a ready-made attenuator. For example, the JBL Nanopatch. While this is intended for passive monitor systems, it would be a perfectly workable solution in this situation. Alternatively, some form of re-amping unit could be used. These are usually passive boxes designed to accept a balanced line-level signal from a mixer or DAW interface and attenuate it to instrument level to feed a guitar amp or guitar effects pedals, while also providing galvanic isolation with a transformer to avoid ground loops. In your case you have an unbalanced source, but the re-amper won’t mind — just use a suitable adapter or cable. My recommendation would be the Orchid Amp Interface (http://sosm.ag/orchid-amp-interface).

Published January 2017