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Thread: Possibly the most frugal high-end sounding amp?

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    Possibly the most frugal high-end sounding amp?

    I've recently diverted my attention from the Ozone DAC building onto designing a simple and frugal amp which matches the tonality and soundstage depth of my modified Xindak thus doing justice to the Ozone. Having the same level of output power isn't a requirement though, so I have been searching far and wide for chipamps which might have the timbral accuracy and 'bloom' I desire.

    Having built chipamp based amps for a number of years, they produce great but not stellar sound in the main, when issues like grounding and decoupling are addressed fully. I have the hypothesis that what holds them back is the input stage, being as it is typically a LTP and a bipolar one at that. Added to that, they do tend to have inadequate PSRR by virtue of not having power supply separation between the signal and output stages. So they pollute their own patch, so to speak and filters can't be included because the necessary wires aren't brought off-chip.

    I have up until fairly recently, played with the TDA7293 because its fairly unique in allowing separation between the voltage and current amplifying stages, especially so when the amplification function is split between two chips. This device is the only one I know of that permits that so the power supply issue can be fully fixed. It does however still have the ubiquitous LTP input stage which has given me so many problems in opamps in terms of excluding HF interference.

    The search for a chipamp sans LTP has been running for a few weeks now and recently I've come upon some very interesting parts from Philips - the TDA856X series. The reason it has taken me so long to find these is I was looking in the 'audio amplifier' sections of manufacturers' websites, whereas these are in the 'automotive' section by virtue of being designed for car radios - thus they're not considered 'audiophile' in any sense and aren't particularly high powered. No matter for this application, its the SQ that's important, and based on a couple of days' listening only it seems I might have come across a diamond in the rough.

    Details on my prototype TDA8561 amp to follow sometime, stay tuned

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    Site Founder And Administrator Steve Williams's Avatar
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    Hi Richard

    I am always interested in reading about your projects
    Steve Williams
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    Before I get to talk about the amp itself, first a bit of back story about the chip. Here's a block diagram culled from its datasheet:

    Name:  TDA8561-block.jpg
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    At first blush you might say - "well it looks like there are opamps inside the chip, so what's making this any different from other chipamps?" 'Look closer!' I retort - those triangular shapes on the left side aren't your normal LTP input opamps. How I know this is because when I saw this block diagram I did a deja-vu with another amplifier I recently pondered over - this one :

    Name:  AD830-block.png
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    The AD830 is a largely unsung innovation from Barrie Gilbert, one of analog design's true artists. Analog Devices really hasn't developed this architecture very much, only having about 3 chips based on it in their range. Linear Technology also has some chips using the same topology, for example the very frugal (but seemingly totally unknown in audio) LT6552.

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    Quote Originally Posted by Steve Williams View Post
    I am always interested in reading about your projects
    Thanks Steve - you might be interested in building this one then? I figured I'd start this project on here as its far less a daunting one for beginners than the Ozone DAC. Then once someone has this under their belt they might be ready for the Ozone as a next step. I was surprised how easy it was to wire up one of these chips and get superb sound. A kid could do it

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    Quote Originally Posted by opus111 View Post
    Having built chipamp based amps for a number of years, they produce great but not stellar sound in the main, when issues like grounding and decoupling are addressed fully.
    How can the sound be "great," but not "stellar?"

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    I'm not sure if your question is about my listening experience or about the circuit details - care to clarify?

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    Quote Originally Posted by opus111 View Post
    I'm not sure if your question is about my listening experience or about the circuit details - care to clarify?
    Take your pick. You made the statement that you have built chipamp based amps for a number of years and they produce great but not stellar sound. So you need to clarify what you mean by that. Whatever the case, I was just wondering how something could produce "great but not stellar sound."

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    I'll talk about the listening experience then - the chipamps I've made in the past have been LM3886 and TDA7294 types (and the variants thereof). These I've driven with DACs which haven't been top-flight because only recently have I designed a transparent enough digital source to reveal their shortcomings. So 'great sound' means good but not top-notch soundstage depth, good but not top notch tonality and good but not top notch dynamics.

    Any clearer now?

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    A couple of important documents on opamps

    A chipamp is really just a powerful opamp, so these two papers concerning opamps and their non-linearities have considerable bearing on the use of chipamps in high-end audio.

    The first is by Barrie Gilbert who I've already cited - its quite a technical one and I'm not going to go into those arcane details he covers. Suffice to quote his second-to-last paragraph because there he's talking about the architecture I've already introduced. Here 'OPA' refers to the traditional opamp architecture (including the problematic LTP) -

    Later, another fundamental building block, which I call the
    Active Feedback Amplifier, or AFA, will be discussed. In my
    opinion, this structure, which, unlike the OPA, has high
    open-loop linearity and excellent closed-loop linearity due
    to its distortion-canceling topology, and a very high degree
    of versatility arising from its dual fully-differential inputs,
    has the potential to eclipse the OPA in all applications
    involving the manipulation of purely voltage mode signals.
    Being a super-set, it can do anything that a conventional
    op amp can do, plus a whole lot more.

    http://www.linearaudio.nl/Documents/...y%20Linear.pdf

    The second paper is much more recent and covers real-world issues with LTPs in opamps which Barrie Gilbert doesn't address, maybe he wasn't even aware of. These issues have probably only come to light because of the greater degree of electromagnetic pollution in today's world - we live bathed in GSM and WiFi signals.
    Attached Images Attached Images

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    Here's a quick shot of the first prototype in action. I've decided to baptize it 'Nitro' for the dynamics. The power supply (offstage left) at present is a 19V laptop switching unit, providing around 60W max

    Name:  Nitro1.jpg
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