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20W Class-A Amplifier Module; Pt.3

In Pt.4 this month, we present a high-performance Stereo Preamplifier & Remote Volume Control module. It's a low-noise, low-distortion design specifically engineered for the Class-A amplifier but which can also be used with other stereo power amplifiers.

by John Clarke & Greg Swain

Depending on your requirements, you have several options when it comes to using the new Class-A Stereo Amplifier. Basically, the unit can be combined with a high-quality external preamplifier or used as a standalone unit.

Typically, an external preamplifier will be necessary if you want to connect several signal sources and switch between them; eg, select between a CD player, DVD player and a tuner. The Class-A Stereo Amplifier would then function simply as a power amplifier, with the signal from the external preamp fed directly to the inputs of the power amplifier modules. In this case, all you would need to build into the chassis are the left and right-channel Class-A Power Amplifier modules (May & June 2007), plus the Loudspeaker Protector & Muting Module (July 2007).

If you do elect to use an external preamplifier, then the SILICON CHIP Studio Series Stereo Preamplifier (October 2005, July 2006) makes the ideal companion unit. By the way, don’t be put off by the 102dB signal-to-noise ratio quoted for that unit in the July 2006 issue. That was a misprint – the correct figure is 110dB, so the Studio Series Stereo Preamplifier is an excellent performer that’s quite up to the job (especially considering its distortion is typically less than .0005%).

Alternatively, many readers will want to use only one signal source, typically a CD or DVD player. In that case, the Class-A Stereo Amplifier can be used as a standalone unit but you do need to add a volume control. If your CD player is already fitted with an output level control, you may be tempted to dispense with a volume control on the amplifier but that could be a mistake.

Preamplifier Features & Performance

Main Features

  • High performance design – very low noise and distortion

  • Designed for the Class-A Stereo Amplifier but can also be used with other power amplifier modules

  • On-board remote volume control circuit with motorised potentiometer and muting

    Measured Performance

    Frequency response ............................................................. flat from 10Hz to 20kHz, -3dB @ 100kHz

    Input impedance ................................................................................................................................ ~22kΩ

    Output impedance ............................................................................................................................... 100Ω

    Harmonic distortion ......................................................................................................... typically <.0005%

    Signal-to-noise ratio .............................................................................. -125dB unweighted for 1V input

    Channel crosstalk ............................................................................................................... typically -125dB

  • Just imagine what a blast you will get from the amplifier and loudspeakers if you turn on the CD player and it has been inadvertently set to full output level. The result would not only be deafening but it could easily blow your tweeters.

    The simplest solution which we would recommend is to feed the signal in via a dual-gang 10kΩ log pot and we’ll show you how to do that next month, if you want to use that option. This simple scheme does have its problems though. First, the input signal level may be insufficient to drive the amplifiers to full power output, even when using a CD player. The amplifier modules have an input sensitivity of 625mV for full power but some recordings may give average output signal levels well below this.

    Second, using a simple volume control varies the input impedance to the power amplifiers, thereby slightly degrading the signal-to-noise ratio. Admittedly, we’re splitting hairs some-what here but this is after all a true audiophile’s amplifier.

    So how do you eliminate those problems and achieve the level of performance we want? The answer is to incorporate a high-quality preamplifier module into the Class-A Stereo Amplifier chassis. This will result in an attractive self-contained package that we think will appeal to many people – particularly those who just want to use a single CD/DVD player.

    The preamplifier module described here meets the above criteria. It’s a minimalist design, which delivers ultra-low noise and distortion, but with more than enough gain (with the "wick" wound right up) to drive the 20W Class-A Amplifier modules to full power output. In fact, if you were to wind the wick up too far, the amplifier will be driven well into clipping and horrible distortion.

    That pretty much defeats the purpose of building a high quality amplifier, so don’t do it!

    This preamplifier is almost identical in configuration to our Studio Series Stereo Preamplifier (October 2005). It’s a 2-chip design employing 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 constant low output impedance to the power amplifier modules.

    Low-noise op amps

    The original Studio Series Preamplifier was based the high-performance OPA2134 op amps from Burr-Brown. These are specified at an extremely low 0.00008% harmonic distortion at 1kHz and the harmonic distortion (THD) for the completed preamplifier was typically less than 0.0005%.

    This time we’ve gone even one better and specified the National Semiconductor LM4562 dual op amp. This new device is specified at just 0.00003% harmonic distortion at 1kHz, which is even lower than for the OPA2134. In fact, it is far below the measurement capability of any commercially available equipment, including our own Audio Precision test gear. Just how the low distortion is verified is revealed elsewhere in this issue.

    As a result, the performance of the module on its own is actually far better than the completed stereo amplifier. Just running with its own regulated supplies (and not connected to the amplifier), the preamplifier module delivers harmonic that’s typically less than 0.0005%, a measurement which is more or less meaningless because it is about the same as the Audio Precision equipment. Furthermore, its signal-to-noise ratio with respect to a 1V input signal is around -125dB unweighted (22Hz to 22kHz bandwidth) and separation between channels is also around -125dB.

    Ultimately, it is not possible to get this fantastic performance from the completed stereo amplifier. And why not? The main reason is that residual noise and hum from the power supply degrades the overall measured result, even though the finished amplifier is extremely quiet.

    Click for larger image
    Fig.1: each channel of the preamp is based on a low-distortion LM4562A dual op amp (left channel only shown). IC1a has a gain of two while IC1b functions as a unity gain buffer to provide a constant low-impedance output.

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