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Ultra-LD Stereo Preamplifier & Input Selector, Pt.1

Designed for use with the Ultra-LD Mk.3 amplifier modules, this high-quality stereo preamplifier features a motorised volume control potentiometer. It is teamed with a 3-Input Selector board and both are controlled by the same infrared remote.

By John Clarke & Greg Swain

By now, most readers will have realised that we intend describing a complete stereo amplifier in coming months, based on two Ultra-LD Mk.3 120W power amplifier modules. As well as the amplifier and power supply modules (July-September, 2011), we’ve also described the Loudspeaker Protector module (October 2011) and this month we are presenting the Pre­amplifier/Volume Control and Input Selector modules.

The preamplifier is a slightly modified (and improved) version of the circuit described in the August 2007 issue for our 20W Stereo Class-A Amplifier. It’s a minimalist design delivering ultra-low noise and distortion.

The basic configuration was originally used our Studio Series Stereo Preamplifier described in October 2005. It employs a dual op amp IC in each channel, the first stage providing the gain and the second stage acting as a buffer for the volume control, to present a low output impedance to the power amplifier modules. In addition, the preamplifier PCB carries an infrared receiver, a PIC microcontroller and the motorised potentiometer to provide the remote volume control feature.

The PIC micro on the preamp PCB also provides the necessary decoding for the input selection. The resulting control signals are fed to a header socket and are coupled to a matching header socket on the Input Selector board via a 10-way IDC cable.

Also on the selector board are three stereo RCA input socket pairs, three relays to switch the inputs and a pair of internal RCA output sockets. The latter connect to matching input sockets on the preamp.

Performance

We have tweaked the already excellent August 2007 design for even lower THD+N (total harmonic distortion and noise) by making a few simple changes. Actually, while the changes are simple, the process of arriving at those changes was anything but simple and it took a a great deal of laborious testing of a number of prototypes as we gradually honed in on the final circuit configuration. The improvements in performance are mainly in the frequencies above 5kHz

Fig.7 plots the THD+N for bandwidths of 20Hz-80kHz and 20Hz-30kHz. As can be seen, the THD+N for 20Hz-30kHz (blue line) is generally less than 0.0007% all the way up to 9kHz and is still less than 0.0008% at 20kHz. And for 20-80kHz bandwidth (red), it’s less than 0.0008% all the way up to about 16kHz, with just a very slight rise after that.

Those curves look excellent but that’s not the whole story. As with the Class-A Stereo Amplifier described in 2007, we are limited by the residual distortion in our test set-up. The green line plots the THD+N of the sinewave generator in our Audio Precision test gear and it’s only slightly below the THD+N plots for the preamplifier. For us to make an accurate distortion measurement, the residual distortion in the Audio Precision gear would have be -10dB (about one third) below that of the equipment to be measured.

So we really don’t know how good the preamplifier is. It’s so good that we cannot accurately measure it.

Note that while the above measurements may appear slightly worse than the 0.0005% quoted for the August 2007 design, the two sets of measurements were taken under different conditions. The original measurements were taken at full volume, while the latest measurements were taken at quarter volume which is more realistic given that CD & DVD players have a high output signal level. This also affects the signal-to-noise ratio and the separation between channels.

By any measure, this new design outperforms the original when it comes to THD+N and the other specifications are equally as good. The signal-to-noise ratio is better than -115dB, the channel separation is better than -87dB at 1kHz and the frequency response is virtually ruler flat from 20-20kHz. The accompanying specifications panel and the graphs show the details.

The circuit changes made to the original design and the resulting performance improvements are detailed in a separate panel. As well as these circuit changes, we also substituted vertical RCA sockets in place of the screw terminal blocks for the audio input and output connections. And of course, the preamplifier PCB now carries a header socket (in the remote control section) to interface with the Input Selector module.

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