Combining analog and digital

August 9, 2015
 by Paul McGowan

I think the train may have slipped off the tracks a bit. In these latest round of posts I wanted to simply explore a few of the emerging product trends, like the DAC/PRE. But exploring subjects does not necessarily mean I am changing product direction for the company. Several have written to me in a bit of panic convinced the upcoming BHK preamplifier will be a DAC/PRE, instead of a pure analog control center as originally announced. Nothing could be further from the truth. The BHK preamplifier is on track and working its way through the design process as a tube based analog preamplifier. No, we are just exploring things new in these posts.

There have been a number of DAC/PREs, though, as I mentioned, the category never seemed to really take off. We went from DAC with a volume control to full boat integrated, like the Devialet, without stopping at the DAC/PRE category for too long. I actually think we, as an industry, may have lingered too little in this fascinating category.

A DAC/PRE combines an analog preamplifier and a DAC in one box. And from a manufacturer’s standpoint, it’s not a terribly difficult challenge: take an existing DAC circuit, add an analog preamplifier – which consists of an input selector switch, volume control and line stage – and call it a DAC/PRE. The advantages of this approach are many if the potential customer is mostly digital. For example, take me (please! My wife’s sick of me). I listen mostly to digital audio using either my Mac Mini server or PerfectWave Transport as a source. I occasionally listen to phono and I like the sound of digital going through analog for control. Bingo. All I now need is a sweet sounding power amplifier and life’s good. I no longer require the extra chassis and interconnect cables required of a separate preamplifier; it is built in.

The possible downside to this approach is inherent in the skill and intent of the designer. Use a great analog preamplifier and DAC, combine them together with an excellent power supply, and few compromises are made. In fact, one could make the argument it’s better than separates because we’ve eliminated the interconnects. The flipside is often the case, however. Many manufacturers may be tempted to hope the sum betters the parts and use lesser quality designs to save money.

So, as always, the end result can be both excellent or just fair, depending on goals set. I have always been appreciative of adding just the right bits to help eliminate another box in the chain, as long as it makes sense, and is done well.

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16 comments on “Combining analog and digital”

  1. Hallo Paul,
    you wrote: “… And from a manufacturer’s standpoint, it’s not a terribly difficult challenge: take an existing DAC circuit, add an analog preamplifier – which consists of an input selector switch, volume control and line stage – and call it a DAC/PRE.”
    If you’re into digital, why not doing all – even volume-control – in the digital domain?
    Especially if you are using an FPGA this should be considered as a possible solution.
    Sounds more convincing to me.
    Greetings from Germany

    1. You wouldn’t do this because now you need an ADC to get the analog into the digital domain. A DAC/PRE is an analog preamp first, a digital processor second.

      What you are describing was what we did years ago with the Reference Link – an all digital product with an analog twist.

    2. I agree with Bernd here. Why not upsample everything and control the volume digitally? Adding an analog preamp stage between the DAC and the amp is not (in many cases) the best way to go. I do it with Pure Music. The dithered volume control works great (and as you know there are other programs out there that will accomplish the same trick). I can even control the volume on my iPhone and iPad via the Remote app. Caveat: If the gain of the DAC is too high — and therefore you have to reduce the volume considerably in the digital domain — or if you’re reducing the volume on a 16-bit source without upsampling, then an analog preamp would be audibly superior.

      Not all DACs are capable of driving all power amps properly, so that’s a consideration also. Then there’s the problem of accommodating analog sources like LP. Some DACs have an analog input and will perform high-res A/D conversions on the fly, which solves the problem. A dedicated, optimized phono stage can feed the analog input on the ADC/DAC.

      So the high-end product category I wish would take off is not the DAC/Pre but the ADC/DAC (with or without a “pre” on the end). I know many LP lovers get nervous about digital conversion, but it is possible to make that operation transparent. In the best recording studios, DSD or 24/192 recordings can sound exactly like the feed from the mic preamp, so the same thing can apply to converting the signal from a phono preamp.

      1. It occurred to me later (when I was playing my system) that I misspoke when saying that “upsampling” is required when controlling the volume digitally. Obviously it’s the resolution of the processing (in this case, a dithered volume control) that matters. If working at 24 bits (the software I use allows for either 24 or 32 bits), then any volume reduction down to about -12 dB should be transparent. The gain of my DAC is user selectable, so I have it set where I’m almost always listening at between -4 and -12 dB.

        So that’s another feature I need on a DAC: the ability to change the output gain to optimized the gain structure of the whole system. I note that the DirectStream DAC takes this to another level in a very unique, superior way, achieving bit-perfect performance.

  2. Paul your idea is a good one
    I own three such devices but they only use one analog Input.
    Two are just a dac but one is an amp as well. And there are many more Dacs are in fact amps for headphone users but this is something little here read or know about.
    One thing you should consider is an analog output for the dac separate without or with a volume control. Meaning it by passes the pre output. ?
    Is the new bhk pre going to be a hybrid or pure tube product.

    1. I think you’re right Al, though I am not currently planning on any products launches like this, just rambling on.

      The BHK preamp is a pure analog device, but a hybrid – tube input stage, MOSFET output stage.

      1. Paul, with those of us with limited space would you ever consider an integrated amp consisting of your new pre and your new amp? Just a thought.

        1. Not 200/400 watt per channel as the big one but perhaps someday and somewhat smaller one might be an option. Integrated haven’t traditionally sold well – though that doesn’t mean times can’t change. I’d like one too.

          1. I have not ordered a BHK because that huge chassis won’t fit in my equipment rack, even on the larger bottom shelf. If you were to make a smaller and sonically identical unit, I’d get one!

  3. In most modern recording studios, the first thing microphone outputs see is an A/D converter. They are installed right at the microphone. From this point on they are immune to induced hum, noise, and distortion which is not the case in the analog domain. The entire recording process is handled digitally including post production editing. There are exceptions by boutique studios but the trend is clear. When these recordings are distributed whether by streaming or on CD they see a D/A converter before the preamplifier input. The rest of the playback is usually handled in the analog domain except in sound systems where the user converts it into the digital domain for further processing. Generally high end audiophiles don’t do this in the belief that it degrades their results.

    I think in the short term, manufacturers of high end audio equipment will continue on as they have in the recent past refining their existing concepts at great cost and difficulty to get very marginal gains at best. But in time, digital circuits will continue to improve to the point where they will challenge and then beat the best analog can offer. The advantage is that once the technology develops towards that end, mass production quickly amortizes the initial investment to the point where the product becomes increasingly cheap to manufacture. Personal computers, TV sets, and smart phones are examples. As a result, the digital equivalent of today’s best analog equipment will be available at low cost to the mass market. It will be harder and harder for the high end manufacturers to justify the vast differences in the price between their products and those of the mass market as the performance differences narrow and then disappear altogether. I don’t know if it will take 10 years, 25 years, or 50 years but the long term prospect for what we now know as high end audio electronics in the form we know it is bleak. Future electrical engineers will look at the most prized equipment today and marvel at how well today’s engineers did with what will be seen as primitive concepts and technology.

  4. I’m feeling overwhelmed. It’s partly the rate of change as digital and analog try to work out the necessary coexistence in spite of the relative stability of analog compared to digital…particularly in the post-CD era.

    I feel a great need for a DAC/PRE as described above. I’m seldom sure whether this or that product…as described at various manufacturers’ websites…actually will give me what I want. In fact, I’m not really sure what I do want or what I should want when it comes to the digital domain.

    Your discussion of NAS and USB makes it clear to me that NAS is my way to go. (I spend much of the year in Arizona and the rest in New York. The New York location will hold the NAS.) But after that, I’m flummoxed. I’ll have power amps and speakers in both places, Roku in both places, networking (wireless in AZ, wired in NY) in both places. What choices do I have for DAC/PRE functions?

    I recall fondly the good old days of the annual Audio magazine directory issue which came close to achieving its goal of listing EVERY available component neatly divided into understandable categories. I can’t imagine right now what the categories for digital gear would even be. It seems like every product is sui generis.

  5. Hallo!
    I agree with what Mark said about the ADC/DAC in his 7:49 am post.
    Well one could name it so and it is exactly what I wanted.
    The phono-stage can be done digital, too.
    Even all the different cutting stylus frequency curves could be equalized in the digital domain such as RIAA, NAB, DIN, CCIR, AES, Columbia, London whatsoever.
    The FPGA in the ADC section could do this for you.
    I am really not afraid of converting the small signal of my cartridge to digital.
    Have done it by myself building an ADC that does it.
    The only problem I have is that I still need a connection between the seperate ADC and the DAC.
    I admit the ADC does not convert to DSD but PCM 24/192 and has only RIAA standard equalization but it sounds more as good enough for me. Neither my vintage ARC SP-3 nor the even vintage ARC SP-11 sound better, maybe any of the successors do but that is quibbling imho.

  6. It seems that combining digital and analog in the same chassis, particularly if the power supplies are also in that chassis, is a recipe for increased noise. How would that problem be dealt with? Or am I missing something here?

  7. I am currently using a DAC/Pre with an analog volume control designed along the lines of that in an Ayre preamplifier, and class A line out. I can compare it to putting various components through a M-L preamp, and it clearly outperforms using the preamp. This has nothing to do with extra interconnects, as I can take various high quality XLR cables and plug them together in series in various ways and lengths, even a different way at the same time on each channel, with zero change in the sound. It probably has more to do with the extra line out and line in circuitry added, and how the line out to the power amp and the line in of the power amp interact.

  8. For me, simpler is appealing which is why I purchased a DirectStream DAC. It’s an FPGA-based digital-to-analog converter. It features L/R controls so I can center the sound stage to my tastes; a software-based volume control, and my choice of RCA or XLR outputs. That’s a whole lotta versatility to turn one’s nose up at. Though it has not simplified my system, it does afford me the opportunity to scale my D-to-A conversion needs in step with technology…or at least I hope. That translates to not having to sell my CD player for one with newer D-A conversion hardware or software. But as I always say, your mileage may vary.

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