Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part II - Design of High-Performance AVRs and Pre/Pros


Multiple DACs Combined to Produce a Single Channel Output (Mono Mode)

Multiple DACs assigned to one channel can improve the signal-to-noise ratio and, in some cases, reduce distortion. The concept was discussed in Part I of this article, but I am presenting the block diagram for the first time. Mono mode is typically used only for DACs at the top of the IC vendor's line.

Connection for a current mode DAC in mono mode simply involves attaching the leads at the output of the DACs together. This configuration allows the currents to add together and flow into the current-to-voltage (I/V) converter.

Connecting all the DACs in a mono mode configuration doubles the number of DACs for the complete Pre/Pro. Some multi-channel products use the mono configuration for the left and right channels only.

ESS DACs contain up to eight single DACs in one chip. It is possible to expand the concept by tying four of the DAC's current outputs together to form a stereo DAC or even tying all eight together to form a mono DAC.

All the application information I looked at for TI DACs with current mode outputs, on the TI website, showed circuits in which each current output of four individual DACs were converted to a voltage before the summation process to mono. The current outputs were never directly shorted. This requires two added I/V converter stages. The topology is applicable to any current-mode DAC. The TI evaluation board documentation below shows the technique:

Accuphase developed an interesting circuit to operate with the ESS octal DACs extending the concept shown in the TI application note. For stereo, there are four DACs per channel, but they do not sum all inverting (or non-inverting) current outputs together. Instead, Accuphase connects pairs of current outputs together. Two I/V converters are needed to convert the current output pairs (four I/V converters in total for the balanced signals). According to Accuphase, "A combination of current summing and voltage summing is used, for optimized operation."

Accuphase does not produce multi-channel products. It is unclear if the complex topology Accuphase uses will be used in multi-channel products using ESS parts. Since the performance improvement is likely small, this circuit would be used only with the top of the line ESS DAC.

Stereo DACs may have a digital pin that puts the DAC in mono mode. Often, the LPCM digital data to the second DAC is inverted when the DAC is in mono mode.

The current pins are configured so the DAC with the inverted data has the pins of opposite polarity connected. This ensures the current levels are doubled when the extra digital inverters are introduced into the digital LPCM stream.

Assuming correlated distortion exists between the two channels, the process of inverting the LPCM data and then connecting the current pins as shown in the figure above, may cancel correlated distortion. This is only feasible with closely-matched DACs in the same package. Data sheets I have examined do not indicate the distortion is reduced in mono mode when connected as shown above, suggesting the reduction in distortion, if any, is small.

The connection is free to the designer, since the digital inverter and its activation are done internal to the DAC chip. No reason exists not to use the configuration even if the distortion improvement is small.

DACs with the internal digital inverters do report the expected improvement in signal-to-noise ratio. The SNR is unchanged regardless of which current summation topologies is selected to create the mono DAC.

A voltage-mode DAC can be configured for mono mode. It is not possible to short the pins together as with the current-mode DACs. Since a voltage mode DAC has a low output impedance, connecting voltage outputs is similar to shorting them. The same issue occurs when one tries to parallel a pair of batteries.

For the voltage-mode output mono DACs, the balanced-to-single-ended converter must be modified so that it will perform the summation of the two in-phase voltage signals from the DAC and the two out-of-phase voltage signals from the DAC. In its simplest form, this added task is assumed by the same op-amp that performs the balanced-to-single-ended conversion. Since this change is at the circuit level of the balanced-to-single-ended converter, I am not showing a figure. The schematic of the Wolfson evaluation board documentation shows the complete circuit of a voltage output DAC in mono mode:

Like all voltage mode DACs, no I/V converters are required in the mono mode configuration.

A more complex topology to create a mono DAC distributes the balanced-to-single-ended conversion task among three op-amps. This topology is sometimes called double-balanced.

In a double-balanced DAC, all electronics of a stereo DAC are present, including a separate balanced-to-single-ended converter for the left and right channels. The output of the left channel single-ended converter produces the in-phase signal, and the right channel single-ended output produces the out-of-phase signal with the LPCM digital input inverted. Finally, an additional balanced-to-single-ended converter combines the outputs of the two preceding balanced-to-single-ended convertors.

An example where the double balanced approach is useful is an AVR with 13 (11.2) DACs. If only seven (5.2) channels are required, each DAC switches to mono mode except the DACs assigned to the subwoofer. The additional balanced-to-single-ended converter switches into the signal path of each DAC IC, creating five mono DACs from the ten stereo DACs. These balanced-to-single-ended converters (five in total) and the suite of balanced-to-single-ended converters already in place form the double-balanced topology.

The topology is compatible with current-mode DACs if four I/V converters are added. I have not supplied a figure for this configuration.

A single 9.1 AVR can operate as 7.1 with the left and right channels configured as double-balanced. An Integra product offers this option.