DACs and the Differential Dilemma

Most high-performance modern DAC ICs have differential outputs, even those targeting single-ended consumer applications. It’s a great way to reduce even-order nonlinearities in the chip’s conversion and output stages, and it gets you around 3dB better SNR.

To design for a differential (“balanced”) application, you can simply pass each side of the DAC chip’s outputs through a combined filter and buffer and let the downstream electronics do the summing.

Differential output stage design from the AKM AK4493 English Datasheet.

To get a single-ended output, application guides and evaluation boards often show an added a summing stage to the above. But experience has shown that with voltage output DACs at least, things seem to work best when you use a single gain stage that both sums and filters the outputs of the IC.

Single-ended output stage design from the AKM AK4493 English Datasheet

When taking this approach, there’s a good amount of optimization you need to do that might not be initially apparent. But when the needed care is taken, it works very well. I suspect it works better than the filter-then-sum approach because the signal passes through fewer active cells (typically opamps). This leads not just to the potential for lower accumulated nonlinearity but also for less complex distortion spectra. These stated reasons are incredibly speculative at this point, but I am certain of what my ears consistently prefer.

So, there’s an easy solution when you want a differential output, and an almost as easy solution when you what a single-ended output. But what do you do when you want both? Initially it seems you have to decide which output gets priority: You either prioritize the differential output and convert that to single-ended or you prioritize the single-ended output and convert that to differential.

It turns out there’s an approach that as far as I know no one is yet using commercially that solves this dilemma perfectly.1I recently completed a design for a client using this approach that I’m expecting to be released soon. I’ll cover what that is in my next post.

Joys of Low-Voltage Audio: Avoiding input stage crossover

high-voltage lines

In this installment of the Joys of Low Voltage Audio series, I want to present some considerations and techniques to help you work with one of the biggest quirks found in RRIO opamps.

Continue reading “Joys of Low-Voltage Audio: Avoiding input stage crossover”

Joys of Low-Voltage Audio: R2R terms and techniques

high-voltage lines

In the third installment of this series, I want to talk about one of the device types you’re almost guaranteed to encounter in low-voltage audio: the rail-to-rail opamp. More specifically, I want to talk about the terminology around rail-to-rail opamps and the techniques used by IC designers to achieve rail-to-rail behavior. So let’s start by answering a fundamental question.

Continue reading “Joys of Low-Voltage Audio: R2R terms and techniques”

More module talk

Schematic of module interface

A few years ago, I developed an audio gain cell that was exceptionally fast for a fully discrete circuit and quite clean. That design ended up being adopted commercially, including by Audio by Van Alstine, who are using it in their DAC MK 5 and Vision preamplifier. I like to think that this gain cell is a key factor behind why the owner of a well-regarded manufacturer of luxury loudspeakers called the AVA DAC MK 5 one of the best sounding DACs he’d ever heard and a model for other manufacturer’s to live up to.

Continue reading “More module talk”

Joys of Low-Voltage Audio: Power strategies

high-voltage lines

In an earlier post, I talked about how I entered the world of low-voltage audio and my commitment to delivering the best possible performance subject to that constraint. In this post I’d like to consider some strategies for generating power in your LV application.

Continue reading “Joys of Low-Voltage Audio: Power strategies”

Doug Self’s “The Design of Active Crossovers”

I just discovered that I got a mention in Doug Self’s book The Design of Active Crossovers for the work I did a while back on loudspeaker crossovers. If you don’t know who he is, he’s one of the big names in British audio engineering. He’s done work for Cambridge Audio, TAG-McLaren Audio, and other respected brands. Feeling warm and fuzzy.

Joys of Low-Voltage Audio: A journey begins

high-voltage lines

It turns out the world of low-voltage audio is a lot of fun, and I’d like to start sharing some of my journey through it.

I suspect everyone has a different reason for entering this world, lending each story a different color and set of priorities. Mine goes something like this.

Continue reading “Joys of Low-Voltage Audio: A journey begins”

What is a single-ended amplifier?

Single-ended amplifiers, whether made with triodes (as in the single-ended triode, or SET, amplifier), pentodes, or solid state devices, entered the high-end consumer audio consciousness a couple decades ago, and they continue to have a particular pull for a certain camp of audiophiles. This may lead some of us to wonder whether these folks are onto something that we should pay attention to.

However, there seems to be some confusion regarding what exactly single-ended amplifier are. So I thought I’d try to clear things up a little.

Continue reading “What is a single-ended amplifier?”

AVA preamp chassis

AVA “SLR” remote controlled preamplifier chassis

A recent chassis redesign project I undertook for Audio by Van Alstine is now in production.

This project pushed “constraints as creative resource” to the limit. The client specified that the design language and elements from the product’s predecessor be maintained—down to the knobs, faceplate treatments, and typography.

The project brief revolved around electronic and industrial design work to bring the client’s preamplifer platform up to functional parity with current market offerings within a framework that fits with the client’s existing manufacturing capabilities. The result is a platform that is significantly more capable than what it replaces yet easier for the client to manufacture. It is also amenable to comprehensive appearance changes if and when the client deems the timing is right.

So while it might not seem there’s much innovation on the outside, there is a lot of innovation for the client on the inside.

Thoughts on balanced audio for hi-fi

Balanced audio was developed to fight noise and other issues common in professional audio contexts. But it was only a matter of time before high-end consumer audio companies picked up on it as a gotta-have marketing differentiator. Below I explore a number of issues any manufacturer should think about before deciding that they need to support balanced inputs and outputs in their consumer hi-fi equipment.

Balanced doesn’t mean differential

There’s a fair amount of confusion, or at least assumption, regarding what balanced audio is. The primary defining feature of a balanced topology is that output signals consists of two legs — a hot and a cold with identical output impedance — and inputs that differentially sum the hot and cold legs. This produces a net signal where any common mode noise picked up in the interconnection is canceled out.

The above doesn’t require that a balanced output be differential. The noise cancellation will work as intended if only one leg of the a hot/cold pair is driven, as long as the leg that isn’t driven is terminated with the proper impedance. Before you cry “Foul!” many classic AKG mics are configured this way.

So, a truly balanced output doesn’t need to be differential. Further, it’s expected that a truly balanced device will produce identical or nearly identical output behavior whether it receives a fully differential input or an “AKG-style” input. Contrast this with a topology that I refer to as “pure differential.” In a pure differential system, it’s assumed that both hot and cold are actively driven with differential signals all the way from the source to the system output (typically loudspeakers). This distinction is important in the discussion that follows.

Typical pro-audio style balanced topologies are useless for consumer hi-fi audio

A lot of line-level balanced stuff in the wild consists of a single-ended core with balanced to single-ended and single-ended to balanced converters on the front and back. This works well for professional sound setups where the main motivation is to eliminate E.M. noise and other issues resulting from long cable runs. But for consumer hi-fi, where cable runs are so short that E.M. noise pickup is negligible, this approach offers no benefit whatsoever. Worse, it likely introduces signal degradation owing to the additional balanced receiver/driver circuitry. In fact, the requirement that a balanced device produce identical output whether or not the input is active differential raises all sorts of constraints that mean the highest level of fidelity while maintaining pure balanced behavior may not be possible or practical.

Maintaining the highest levels of fidelity with a pure differential approach is much easier. Further,  a pure differential approach when implemented for the highest levels of fidelity may result in better performance than its non-differential equivalent. This is because a fully differential configuration has the potential to null even-order nonlinearities in the gain stages, something not true in general with pure balanced approaches. The problem with a pure differential approach is that it introduces use limitations the customer is not likely to fully understand.

Supporting balanced or differential audio gets very complex very quickly.

Will preamp inputs take only balanced/differential inputs or will there be provisions for single ended inputs as well? Supporting both potentially leads to a crazy lot of circuits and/or circuit switching.

Will outputs be both balanced/differential and unbalanced? Doing both probably means additional circuitry.

Will inputs and outputs be “truly balanced” or “pure differential”? Are you ready to educate your users in the proper use of pure differential devices?

Is it better to try removing the need to balance rather than use balancing to cover up the limitations of existing circuits?

The only thing balancing offers hi-fi, and then only when it is implemented as a pure differential topology, is the potential to null even-order nonlinearities generated by the electronics. Which is to say, any benefit seen by going to a pure differential setup means something in the system isn’t working as well as it could. This means you may be able to achieve the same benefit at much lower cost by optimizing your existing circuitry for better overall linearity.

Of course a pure differential topology will get rid of even-order nonlinearities even if they are small, and that might bring some added benefit after you’ve optimized things as much as possible. However, as with anything, be critical and weigh the costs. Directly following from this …

Is balancing line level signals worth it?

Running a pair of power amps differentially often results in better sound because power amps tend to be the most stressed devices in a system. They tend to operate more deeply in their nonlinear regions than other equipment. But you don’t need a balanced system to experience the benefit of differential amps — you only need to bridge your power amp setup. Note that many switching amp topologies are already bridged.

While it’s not uncommon for power amps to generate audible even-order nonlinearities in typical use, even-order nonlinearity in your line level designs may already be so low that turning them into a fully differential topology won’t give you any audible benefit. If there is a benefit, you may, as pointed out above, be able to achieve the same benefit at much lower cost by optimizing your existing line level circuitry for better overall linearity.

Are balanced systems still a thing?

There was once a lot of buzz (pun intended) in high-end hi-fi circles about balanced systems. But things have changed a lot in the last couple decades. What percentage of the market do the balanced-happy or balanced-sympathetic now represent? If it’s 10% or less, you will need to be become quite a hero in that circle if you’re going to make back your development costs. The alternative is that you have to commit yourself to convincing those outside the balanced circle that they need to get in. In deciding to do this, keep in mind that those folks abandoned the temptation at least once already.