The Gain Cell

December 22, 2016
 by Paul McGowan

The critical nature of a preamplifier’s heart and soul, the volume control, has vexed designers concerned with sound quality for years. Time and again we face the dilemma posed by it: What’s the best device possible for the smallest degradation? Turns out, the answer to that is no device.

Instead of compromising audio quality by inserting an attenuator in a purely analog preamp, better results are obtained by eliminating the device altogether. Instead, it is possible to reduce a preamp’s essential elements from three to two by varying the gain of the output stage, rather than attenuating the signal into a fixed amplifier.

But how do you design a transparent sounding gain-variable output stage in a preamp? You turn to the recording industry who has wrestled with this problem for years.

In the heyday of analog mixing boards, (they’re mostly digital now) engineers faced the same problems as we in audio: How to remote control potentiometers without sacrificing audio quality. They first used motors. It didn’t take long before that solution became unwieldily so they turned to something else. A variable gain amplifier. Based on an older circuit design known as a Gilbert Cell, this unique amplifier topology uses multiple differential pairs in a balanced configuration and a voltage to vary the gain. According to Wikipedia, “The Gilbert cell was invented by Howard Jones in 1963 but usually attributed to Barrie Gilbert (before joining Analog Devices) in 1968.”

My first experience with this unique topology was gratifying. I had long wondered what sonic compromises might be attributed to this device and set about testing a number of them. At the time I was interested more in sound quality than functionality (they all worked well for gain setting—not all sounded good). I wanted to find a Gilbert Cell that had the sonic richness and transparency I insisted upon to go into one of our products. None lived up to my expectations until our chief engineer, Bob Stadtherr made a suggestion to me. “Maybe it’s not the cell but the way you’re using it.”

Aha! Of course. It took me six months of hard work to design a fully balanced input to output amplification stage whose gain could be varied by simply changing a voltage, but hard work was rewarded with amazing sonic performance.

And thus, the Gain Cell was born.

You’ll be reading much more about this sonic wonder in the next few weeks as we launch the Stellar Gain Cell DAC for beta testing next month.

Stay tuned.

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19 comments on “The Gain Cell”

  1. Paul, was under the impression that you designated this new product the “Stellar Control DAC” not wanting to use the term preamplifier even though incorporating three analog inputs. However, your language of late has been the Stellar DAC, Stellar Gain Cell DAC,
    please clarify …

    PS > who’s idea was the product name “Stellar”?

    1. “Stellar” as a product group was named by my son Scott. And yes, originally we were calling it the Stellar Control DAC and the more we thought about it the more we thought that name might be confusing – leading people to believe it was more just a DAC rather than an analog preamp and a DAC – but wanted to not just call it a preamp/DAC.

      Sigh. Naming is quite an art, one we get right sometimes. But, Stellar Gain Cell DAC it is

      1. Paul, please excuse the double posting, this is an extension of yesterday’s conversation that inadvertently was posted in your “silver ears” post today.

        Yes, product names/designations can be a tough one, though on occasion, divine inspiration and serendipity find mutual destiny.

        When asked what’s a Sprout? It’s simple to respond and categorize Sprout as a stereo integrated amplifier. “Stellar Gain Cell DAC” sounds like it belongs in the category of digital to analog converters obfuscating it’s preamplifier functionality when in essence it’s a stereo preamplifier w/digital to analog conversion, or (stereo pre-dac) which “Stellar Control DAC” nomenclature implies to a certain extent.

        I dig the Stellar concept, S300 for (300w stereo) and M700 for (700w mono) are clear as day. Gain Cell terminology is more of a technology description when compared to functionality.

        Obviously, the basis of success with this new product category will be the value/performance ratio and the optimization of market penetration towards target demographic within your omni-channel marketing.

        Back in the day we would reference a great performance, recording or tape as “Stellar”.

        Cheers!

    1. The Gain Cell itself has been revamped from the G Series and sounds even better. There’s no power amplifier in this and no B and O module like there was in the G Series.

      The Stellar mate to the Gain Cell DAC, the Stellar S300 and Stellar M700 amplifiers have B and O modules in them for their power amplifier outputs, and what we call an Analog Cell for their inputs. I’ll explain what that is in future posts. It really makes them quite magical sounding.

          1. Paul,
            How about an answer to Bernd’s final question in his article from yesterday’s posting?
            My other concern is that you use the term “full” in all descriptions of the sound you prefer, but live music isn’t always presented that way, so what gives.

            1. I answered Bernd’s question. Thanks for the nudge.

              I do like full and agree not all music is such when you listen live. But, that said, I don’t want false full, I want real. So I listen to a variety of music to determine if there are any consistencies that aren’t natural.

              Full, as opposed to “then and threadbare”.

              1. Hi Paul,

                it’s difficult for a German to understand what is “false full”.
                If you’ve read my answers to jb4 you would have seen that I too listen to a variety of music and that I took part in the recording of music many times.
                The only difference between you and me that I see is that we have different audio systems at home and that I don’t have to sell any product not even the old ones that are still here in my flat and ask for dust off every week

                Regards

  2. Just curious. You mentioned in a prior post that you used your PS Audio stepped attenuator in the BHK preamp. As described, this sounds superior to an attenuator. Why was a gain cell not used in that application?

    1. It’s close. The BHK preamplifier uses a variable gain amplifier too but done differently than the Gilbert Cell I just described. That;’s because the Stellar is a very cost-conscious design where every penny is argued over to get the kind of performance we want at a price point. Here the Gain Cell is an affordable alternative to the way we do the variable gain amplifier in the BHK – which ain’t cheap!

  3. Interesting series of posts over the past week, although technically way over my head!

    So from a practical standpoint can someone explain what exactly I am supposed to notice with the different types of volume controls. I always struggle with this as well as file types (and cables and amp types and DAC chips just to name a few). It may be my preferred choice of music but whether I listen with a system that uses pots or stepped attenuation I just hear no difference.

    Sample music on the playlist this week:
    Neil Young “Peace Trail” 2016
    Wishbone Ash “New England” 1972
    Poppy Family “Best of” 197?
    Iron Butterfly “Best of” 197?
    Jamestown Revival (the whole catalog) 2014+
    Kacey Chambers (the whole catalog) 2000+
    Many other similar rock or pop or folk bands from 1970s to current.

    So am I unlikely to hear differences as many of you do since this is not opera or classical or piano or similar live voice?

      1. Thanks Paul. This was exactly what I was thinking would be the case, which explains a lot why over the years I have failed to detect differences on my kit that others feel quite clearly are evident on theirs. It’s all about the provenance I know.

        I agree as well that many of these are not great recordings, from a technical sense, but that does not make them bad music. Many of them meet the “toe tapping” criteria from start to finish!!

        The new Neil Young effort is a puzzle. The music draws me in at times, but, the overall production value is fair, almost organic. I read where the album was recorded over a two day period or something like that. At one point it sounds like someone ran into a chair or something in another part of the studio. I was listening to it first on headphones while in the dark having a nap and jumped up thinking someone else was in the room!!

    1. A lot of what constitutes “great sound” is subtle differences in time. These need to be coherent to a physical sound generator and acoustic space to make sense. Any recording that utilizes multi-tracking, mixing and processing, basically every knob in a studio, mangles those time cues and gives you less reality in the recording.

      Reducing distortion and flattening frequency response are always worthy goals because distortion compounds geometrically, so a certain level of playback distortion is audible even on rather distorted recordings, but for systems over $999 you need clean, unprocessed, unmixed, unmastered recordings of acoustic music made one microphone per speaker to test their worth.

      1. Acuvox, too much Christmas cheer, family visiting time, football and New Year’s celebrations left me way behind in responding to emails and posts, so, sorry for the late reply to this one. Thank you so much for the response, and, I guess I would agree with you. Once the studio console gets involved with all its twists and turns it certainly does distort what the original music would have been.

        As Paul said, it does not necessarily make it bad music, just not the best recording. I may have to do some AB testing with music of the type you describe, with unmastered material, and see what I think of my various systems. I doubt that I will change my listening preferences based on what I find, but, it will be good to know in my mind what my system is capable of or not.

        Have a great 2017 and I look forward to many many posts on this site over the next year.

        Larry

  4. This series of posts has been an eye opener. Most enjoyable. This is off topic, Paul, but some months ago you mention that the BHK preamp was under review with TAS magazine. Any sense of when the review might be published ?

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