Analogue design of the pre-amplifier

Throughout the analogue design of the amplifier, high performance opamps are used. In the past, acceptance of opamps in high-end designs was under discussion. Discrete designs of amplifier stages were able to perform better on noise and distortion. Currently, opamps are widely used since the performance of several audio opamps are at least equal to the best discrete designs. Opamps also offer the possibility to integrate different stages and buffers into the design without a large increase in board space. Besides, during studio recording of music compositions, the signals passes a numerous amount of opamps; then the additional opamps for home reproduction is not limiting quality.display during volume change

The preamp design

Although the preamp is kept as straightforward as possible, it feautres a comprehensive display to show the current state of the amplifier. A few photos are shown, the green display is built around a character LCD module. The blue-ish like display is the newest prototype using a VFD display which can also be used as graphic display for showing different types of fonts. Additionally this VFD display illuminates, so also in the dark it can be seen.

vfd.jpg (76510 bytes)All inputs of the preamp have gold plated busses to reduce contact resistance. The input selector for the four inputs devices is created with a high performance analogue audio switch (Analog devices). These switches feature outstanding channel separation (100dB@1kHz) ultra low noise (1nV/(Hz)) and low distortion (0.003%). The OFF-isolation is 120dB. The selection is done digitally by the microcontroller.

One of the inputs is optimized for SACD. This input features a high quality buffer opamp from Texas Instruments-Burr Brown (channel separation=135dB, THD+N=0.0004%). The available headroom is 24dBu (Vin=0.775Vrms) for THD+N<0.01% which is defined as low distortion. This stage is suitable for SACD.block_preamp.gif (9034 bytes)

The bass output is created by summing both channels using two precision opamps (Analog Devices). These opamps are optimized for low voltage noise at the expense of higher current noise. Generally, both voltage noise and current noise contribute to the total noise in a system. Voltage noise dominates in the low frequency range whereas current noise dominates at the high frequencies (generally, the sum of both is minimum at 1kHz).
For the low frequency bass output, voltage noise is the dominating noise source. It makes sense to take a low noise type for voltage noise in this case. This opamp also has very low offset (10V) which reduces the need for AC coupling with capacitors in the signal line. The slew rate of these opmaps is not very high (3V/s) but a simple and straightforward calculation indicates that this is sufficient for low frequency signals. Besides, it makes the circuit less sensitive to capacitive loads and noise glitches. photo of preamp print, click to see an enlarged view
After this summing an additional opamp is used to create a phase inverter on the bass response (180 degrees). This phase inverter can be beneficial for some acoustic situations. In the calculation of the filter this phase shift is accounted for.

Separate from the bass, two channels are available after the input selector. These signals are further processed by high performance buffer and amplifier stages (Burr-Brown). The balance and volume settings are determined by a programmable gain amplifier (Burr-Brown). This chip uses a serial digital interface to read the code words for the volume settings. The dynamic range of this chip is 120dB and volume settings can be changed in 0.5dB steps. Furthermore, crosstalk is -126dB and THD+N<0.001%. This chip is the central part in the preamp. It processes all audio changes including the MUTE function.
Before driving the filters of the speakers, the signals are buffered through fast acting opamps. To some extent, voltage amplification is performed through these opamps. Here, the slew rate is 20V/s, more than sufficient for any audio frequency to be amplified with a headroom of 24dB typical. A bandwidth limiting capacitor is used with this fast output stage to avoid the introduction of high frequency noise from digital sources. These opamps are chosen for their low voltage noise in combination with current noise since they amplify the full audio range. Offset is <2mV, generally small enough to avoid AC coupling techniques. Any existing DC voltage will utlimately be blocked by the active filters in the speakers. Finally, these opamps are able to drive high capacitive loads. This is important because a relatively long interconnect cable will be connected to this output stage (the active filters are build in the speakers). Finally, the output of the pre amp is protected for power up/down (DC) clicks by using a timer controlled switch.

The noise levels throughout the preamp are very low. As a consequence, all resistances are kept below 5-10kohms to avoid that resistance noise becomes dominant.

Slew rate and headroom

What slew rate is required for an amplifier to perform correctly for the whole audio range? A straightforward and simple calculation provides some insight in slew rate.
A sinusoidal tone of 20kHz, will have its highest voltage after wavelength. In time, after 12.5s. Take as reference a 1V input signal, then voltage must raise 13V to reach its maximum available headroom. This results in a slew rate of 13/12.5 ~ 1V/s. For SACD, 40 kHz signals can be of interest but even then a slew rate of 2V/s would suffice. For low frequencies such as bass, slew rates can be even very low. An input signal of 1V is generally enough to fully drive any amplifier. Purchasing an amplifier with a slew rate larger than 10V/s is of no use for the normal audio bandwidth. In this calculation, the maximum available headroom is used. Headroom is here defined on the dBu-scale. This is a standardised scale for which Vin=0.775Vrms. But more important is at what distortion level this headroom is defined. Larger headroom values are often available but it is important to understand what THD+N is for that headroom. In this case it is defined at THD+N<0.01%.

When additional gain is applied some amount of voltage gain is needed. This would indicate that the amplifier should be faster to handle the complete voltage span. This situation occurs for power amplifiers where large voltage gains are needed to get the required power at the speaker terminals. Power amplifiers need therefore higher slew rate to match with the preamp but a slew rate of 10V/s also suffice for any power amplifier.

 

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