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Q. Why do my keyboards sound thin and weedy with some preamps but not others?

Roland Juno 106 synthesizer keyboard.

When I’m recording an old Roland Juno 106 synth and I connect it to a UA Apollo soundcard, it sounds fine. However, I’ve also tried running it into some other preamps and got some strange results. For example, I plugged it into a pair of Chandler TG 500‑series preamps and the sound instantly went thin and weedy. Switching between mic/line level had no effect and neither did toggling the input impedance. The line input of a Neve 1073 gave me the same weird, thin sound with no low‑end at all. But I tried a pair of Shadow Hills Gama preamp modules and, hey presto, up popped the Juno, sounding great again!

Oddly, when I tried the same preamps with the outputs from a Nord Stage 3, everything sounded fine. All the plugging to the UA and external preamps is via a bantam patchbay and I’m using TRS quarter‑inch cables. What’s causing these differences?

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Technical Editor Hugh Robjohns replies: A common problem with keyboards that have a built‑in chorus or stereo effect is that summing to mono will often make them sound rather flat and lifeless, so I initially wondered if you’d forgotten to pan the outputs of your preamps… However, this doesn’t usually attract descriptions of ‘thin and weedy’ or ‘no low‑end at all’, which sounds much more like a fundamental connection issue.

Given the types of preamps that exhibited the problem, the age of the Juno, and the type of cables you’re using, I think I can hazard a good guess at what’s actually going on. The Juno has unbalanced outputs and, given its age, its quarter‑inch output sockets probably only have tip and sleeve connections, whereas more recent keyboards often use TRS sockets and ground the ring along with the sleeve. Normally you’d hook up a keyboard with an unbalanced TS instrument cable, which would work perfectly well in either type of socket. But, as you’ve used TRS cables, the ring contact on the plug was probably left not connected to anything at all.

Top: Electronically balanced inputs are inherently ground‑referenced so, although there is no connection to the preamp’s cold input (because the ring terminal is not connected at the source end), the signal sent between hot and (grounded) sleeve is still detected. Middle: The cold side of the transformer is not connected to anything at the source end, and without a complete circuit path no signal is detected by the preamp. In practice, capacitive‑coupling will pass some high‑frequency content, resulting in a low‑level signal with no bass. Bottom: By using a TS (unbalanced) instrument cable, the ring and sleeve terminals are shorted together in the preamp, so the cold side of the input transformer is now ground‑referenced and the source signal is detected correctly between the hot and (grounded) cold inputs.Top: Electronically balanced inputs are inherently ground‑referenced so, although there is no connection to the preamp’s cold input (because the ring terminal is not connected at the source end), the signal sent between hot and (grounded) sleeve is still detected. Middle: The cold side of the transformer is not connected to anything at the source end, and without a complete circuit path no signal is detected by the preamp. In practice, capacitive‑coupling will pass some high‑frequency content, resulting in a low‑level signal with no bass. Bottom: By using a TS (unbalanced) instrument cable, the ring and sleeve terminals are shorted together in the preamp, so the cold side of the input transformer is now ground‑referenced and the source signal is detected correctly between the hot and (grounded) cold inputs.

Meanwhile, at the other end, you were connecting to the preamps via a balanced patchbay. Some of those preamps had direct electronically balanced (and thus ground‑referenced) inputs, and some had transformer inputs, and it was the latter type that didn’t work properly. In the preamps with electronic inputs, like the UA and the Gama’s dedicated instrument input — the input circuitry is ground‑referenced. So, the signal from the keyboard is passed as a voltage between the tip (hot) connection and the (grounded) sleeve to the electronically balanced preamp input. As the electronics are inherently ground‑referenced, the circuitry can detect the signal voltage even though there’s nothing on the cold (ring) terminal.

Always use TS plugs in keyboard output sockets, unless you know for certain that it is a proper balanced output.

In contrast, the input transformers in vintage preamps are normally ‘floating’, meaning that neither the hot nor the cold sides of the input are connected to ground at all. Consequently, while the tip connection from the keyboard is routed through to the hot side of the preamp’s transformer winding, the cold side is connected to the ring terminal of the jack plug and that goes nowhere at all. So there isn’t a complete circuit path for the signal to flow around. You effectively had a ‘broken’ cable, and the resulting quiet signal with no bass was due to some residual capacitive coupling along the way! The diagram shows simplified circuit schematics of the two configurations.

The simple, practical rule of thumb here is always use TS plugs in keyboard output sockets, unless you know for certain that it is a proper balanced output!

Published April 2021