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Sapphire Line Preamplifier - My Build Notes

Posted 28th July 2016 at 03:15 AM by rjm
Updated 4th December 2016 at 11:55 PM by rjm (add measurement data)

Introduction

This is my build log for relatively basic line preamplifier based on rev. 30f boards of the Sapphire3 headphone amplifier. I modified the circuit to run at lower currents (about 10 mA output bias) and adjusted the gain settings to 10/16 dB.

It is built in a Hammond 1550 cast auminum chassis, with an external Plitron 160VA 2x12VAC rectified power supply. The volume control is a 50k Goldpoint V24 stepped attenuator, while the RCA jacks are rhodium plated from Oyaide. The feature set is limited to two switchable line inputs and an output mute.

Chassis Layout Notes

Audio components are conventionally designed as rack-mounted equipment with all controls on the front panel and all connectors on the rear panel. To try and keep internal cabling to a minimum I'm modelling my preamp more like a recording console with both the controls and I/O on the top plate.

Build Notes

All components mounted in place with no problems. The volume control blocks access to the screws holding the boards in the standoffs however. This means disassembly is going to be more of a pain than I imagined.

Chassis wiring done. Blech. Not fun, but straightforward. Note toggle switches connect the to the tabs opposite the switch position.

Test. Runs, works fine, all functions operational. The mainboards were pulled from my previous Sapphire headphone amp build, so the only worry was messing up the switches somehow. Quiet.

16 dB was too much gain, but fortunately the boards were enabled with a 10/16 dB switch. Change to 10 dB setting which seems close to ideal.

My 47 Labs amp really likes to be driven hard. Sounds very happy connected up to the Sapphire Pre. Massively better than passive pre, and more energy than with the bbaord buffer too.

Despite what I thought would be an oversized chassis, the build got surprisingly tight and messy with all the wiring. If you want to make it easy on yourself, go with the biggest chassis you can possibly live with - and carefully plan all the wiring in advance down to the last ground terminal.

The mute switch engages with a bit of a thump the first time after power up, silently afterwards. There is a slight potential difference since the preamp has a small DC offset, but I don't see why the de-mute is it only audible the first time.

note: attached image of 30f board is shown with stock component values for reference

Measurements

According to Rightmark the preamp measures 0.0009% THD, -98.0 dB A-weighted THD+N. Not too shabby.

The broadband noise is about -140 dB with a few power line harmonics and some digital interference from the computer about 10 dB above that. There is a curious peak at 24 Hz of unknown origin, but even that is below -110 dB.

Thoughts

You don't see a lot of solid-state DIY line preamps being built these days. I'm really glad I (finally) went ahead with this project, though. It was something I'd been meaning to get around to for ages and ages, finally happening only because I had the spare set of Sapphire boards left over. Everyone deserves a good preamp.
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Attached Files
File Type: zip Sapphire Line 30f vol -10dB RMAA report.zip (32.3 KB, 93 views)
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