XR Spectra v2: Redesigning the Preamp

I love designing stuff! It’s an obsession!

The picture above is the Modula preamplifier I designed in 2017. I decided to go back to this form factor for the XR Spectra v2 update. The design makes sense. It is simple, modular, and reusable. It can be connected anywhere in the signal chain; right after each pickup, after the switch. etc. You can even place it inside a stomp box if you want to keep the instrument purely passive (no batteries), but still want to have that active kick! One neat idea is placing this preamp together with one or more Tone Blocks in an external stomp box, with the switching scheme I proposed in the Tone Block v2 article.

The XR Spectra v2 pickup can be active (powered with a preamp), or passive (no preamp, no batteries). In passive mode, our latest tests show that it has an output comparable to off-the-shelf passive, single coil pickups. With moderate picking, we get a 520mV output when hitting the low-E string, compared side-by-side to a DiMarzio Injector neck pickup, which gives 456mV, more or less the same as a stock Fender Strat pickup, producing 512mV.

XR Spectra Preamp Headers

Using the same test setup, the EMG85 neck pickup generates 2.16V. To get the same output level, the XR Spectra v2 preamp needs only a gain of 4 to 5 (14dB). This gain is quite modest compared to the previous XR Spectra version. Lower gain translates to a significantly lower noise floor. The preamp is still differential, like before, but thanks to the lower gain requirement, I can now use a single OpAmp, and get rid of the first stage differential amplifier in the original design. And that means lower power consumption too (125 µA).

The preamp accepts Lo-Z balanced inputs —a 3-pin header for +IN, -IN and GND, and single-ended output, plus supply — another 3-pin header for OUT, GND and V+. The headers are at the bottom of the PCB (image above right) while the active components are at the top (image below).

 

 

 

 

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