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Thread: Inside of an Audio/Video Receiver (AVR)

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    Inside of an Audio/Video Receiver (AVR)

    Hi guys. This is an article I wrote for Widescreen Review Magazine a few months ago. Hope you find it useful.

    Inside Your AVR
    Do you know what is inside your electronics? What is a DSP? The output stage? Power supply? FPGA? ASIC? Do these terms have any meaning? I suspect for many of you these are obscure terms. Yet, pick up the brochure for any audio/video product and such buzzwords abound. While it is impossible to convey the true nature of these technologies in an article like this, I think we can become more educated and at least have some high level understanding of what goes on inside your electronics. For this article, I am going to open the top of a premium AVR I purchased back in 2007. At the risk of stating the obvious, you are not going to become a design engineer from reading this one article. But rather, get you started on a journey to know more about what is under the hood and what type of design trade off a manufacturer may make. I hope to do the same with other electronic components in the future, helping explain some of the other commonplace technologies used in them.

    Block Diagram
    I have annotated the major subsystems of the unit in Figure 1. Let’s go through them one by one:
    1. Power Transformer. This is the easiest part to identify because it is by far the largest and heaviest part of the unit. Its job is to convert the high voltage incoming supply from the wall outlet, to the lower voltages needed for the various circuits in the unit. It can be either square as in this one or round (called toroid). While it is hard to miss in this case, transformers are easy to find by tracing the wires from the AC plug on the back of the unit which after a few detour, find their way to it.

    Since the transformer is the gate to the incoming power to the unit, its capacity directly impacts the total power the unit can produce. So all else being equal, the larger and beefier the transformer, the more powerful the unit, and the less attempt to cheapen the design. So pick up the unit and feel the weight. That might tell you something about the quality of the unit.

    If you look inside an AVR but don’t find such a large beast, then you likely have a “switchmode” or Class D amplifier. The topology of these units is far more complex and beyond the scope of this article. For now, I will just mention that they are far more efficient and produce a lot less heat. They are also smaller and lighter. Their efficiency will make them ideal for closed in cabinets where traditional amplifiers may cook themselves to death. I have a Pioneer Elite AVR in my entertainment cabinet for this very reason. It generates a fraction of the heat of the Onkyo AVR that it replaced. Note that there can be some fidelity issues as the performance of class D amps could have dependency on the load (or even speaker wire!). They are commonly used in subwoofers where the amplifier and speaker can be designed as one.

    2. Input filtering and power control. This is not a major component of the unit. Indeed, it is a tiny subcomponent but I point it out because it does something people go and buy external devices to do. Namely, it implements a powerline filter. Products sold in the west must pass regulatory compliance tests for radiated emissions. With so much going on inside modern electronics, considerable amount of electronic noise works its way backward through the power supply and can then transmit on air, causing interference with TV and radio reception (although in this day and age with cable/satellite receivers and lack of AM/FM use in the home in US, this is not a real concern anymore). Regulatory compliance is a condition of sale in the country so companies spend great effort to make sure they meet the limits of radiated power and hence the inclusion of this functionality in every piece of audio/video electronics you buy. A benefit of this is that the filtering works both ways: it both gets rid of noise that wants to get out, and noise that may want to enter from the AC input.


    Figure 1: Onkyo TX-SR805 AVR Block Diagram

    Figure 2 shows a close up of the input circuit. You can see a protection fuse and a gray box. The box is a safety rated capacitor (0.1 microfarad for the electronic hobbyists among you). The safety rating is there because this capacitor is designed to not become dangerous in failure mode. All of those markings on it are the safety/regulatory compliances.

    A capacitor is a basic electronics component with many different uses. In this case, it is being used to create a “low pass filter.” The incoming power into our homes and electronics is called AC or alternating current. In the U.S. the frequency of this alternating current is 60 Hz or 60 times a second. That is the desired frequency we want to let in. Anything above that we would ideally filter out. And that is what a lowpass filter does: it lets in lower frequencies but filters out higher ones. While better filtering can exist in external power conditioners and filters, know that your AV device already has such a component in it.


    Figure 2: AC line filter and control which lives in between the AC plug and the main transformer.

    I was impressed to see flexible sealant on the capacitor to keep it from moving during shipment. Usually this is used for heavy component where such shock could dislodge them. In this case, the capacitor is a lightweight part so having the white sealant or “goop” as we call it, is “good design hygiene.”

    I was less impressed however when I tried to tilt the capacitor back to take a picture of it. The connecting pin pulled straight through the PC board (the green thing that holds all the components) as you can see in Figure 3. Granted, no one is going to put that kind of force on the component and the sealant would have helped keep the part there. Still, I worry about the quality of soldering to allow such a component lead to pull through. Normally I expect that bond to be so strong as to cause the pin to detach from the component, not for it to slide out and pull the solder with it.

    This board also has an additional piece of functionality, namely, remote power on/off. A relay (black box barely visible in Figure 3) either allows the power to go through or not, as instructed by a small microprocessor (little computer) that receives the Infrared signal from the remote control. Because that part of the AVR needs to always stay on, or else it could never act on the remote signal, there is a small transformer and power supply on this board, surrounding the relay module. The power control relay can also be part of the protection circuit, shutting the unit down, or keeping it from powering up should a fault be detected in the amplifier section.


    Figure 3: Close up of the input filter capacitor.

    While I have not marked it, below the AC Input module there is one of many different power supplies in the unit. Their function is to take the lower AC voltage from the transformer and turn it into DC, or non-alternating current that electronic circuits use. A fundamental electronic component called a “diode” or in this case, “rectifier” is used to change the negative current cycles into positives. Another type of capacitor, called an electrolytic capacitor, is used to smooth that out so that it resembles smooth, DC voltage. The larger the capacitor, the more reserve power it has to handle sudden peaks of power demand and keeping the supply voltage flat and smooth. So if money were no object, you would want to see very large capacitors or an array of them in parallel.

    As essential as electrolytic capacitors are to design of our audio and video electronics, they are also one of the most unreliable. The chemical material in them can dry out over time, leading to catastrophic failure of the capacitor and often, the rest of the circuits around it. This is the main cause of failure in vintage electronics. The top of the capacitor has a slit in there that allows out gassing due to pressure build up as the device is being used/heated up. Usually a bad capacitor can be found by simply feeling the top of the capacitor (with the device off!). If it is bulged up, it usually means the capacitor has gone bad. Heat is therefore the enemy of capacitors and reliability of the unit.

    Unfortunately we see a recipe for a accelerated failure of a capacitor in this unit. As you see in Figure 4, the rectifier, the large flat device, is touching the capacitor. The large size of the rectifier indicates that it is designed to dissipate fair amount of heat. Due to direct contact, the heat directly couples to the capacitor causing its operating temperature to be much elevated had there been an air space between the two components. While there was an easy fix of simply bending the rectifier away from the capacitor, this is the kind of flaw that should have been caught during assembly or better yet, more distance designed into the PC board that holds the components between capacitors and heat generating sources.

    More disappointing though sadly very common is the temperature rating of the capacitor. As I have marked, the capacitor is the common 85 degree C rated type. While this is plenty high for most electronic circuits, it is not in the vicinity of high power devices and inside of a cramped AVR with so many channels of amplification all generating heat. I would like to see capacitors rated at 105 degree C in such scenarios. To wit, next time you are shopping for PC desktop power supply, you may want to peer into its venting slots and see if you can read the temperature ratings of the caps. If they are 85 degrees and not 105, I would walk away from it. High quality ones will always have 105 degree C capacitors given the very close proximity of them to the high power components.


    Figure 4: Power supply rectifier and filter capacitor touching, leading to shorter life.

    3. Let’s now focus our attention on the amplifier section. I have shown two parts of this, the driver stage and output. The latter is connected to your speaker wires and directly powers them. You can identify this stage easily by the massive heatsinks in the form of the shiny (or black) extruded aluminum fins. Mounted on them are a series of transistors that perform the final function of amplification. Pushing them to do their work is the “driver” stage. This is lower power version of the output stage and in this case, are organized into little vertical modules. If we count them, we arrive at total number of amplifiers inside this AVR which is seven (7).

    Nestled in between the driver stages are two more large electrolytic capacitors. The output stage has its own power supply due to its need for high voltages relative to rest of the circuits. I was surprised to see custom capacitors there (marked by the name of the AVR manufacturer). Looking at its specification, it has an odd, 71 volt rating. Likely the next higher standard voltage rating would have been much higher, causing the capacitor size to grow substantially. Having a custom part rated at a lower voltage allowed them to save space and possibly cost. I was yet again disappointed to see 85 degree C rating. This part of the amplifier is going to be cooking and cooking good. The generated heat will cause shorter life for these capacitors.

    The biggest disappointment is not the capacitors but the two fans. They are buried under the two metal square boxes one each side of the power supply. They are slanted 45 degrees, blowing sideways onto the heat sink. You don’t need a degree in physics to realize that air blown across the heat sinks is not going to uniformly cool them. The fins closest to the fan will get the bulk of the air movement, and the rest get substantially less. And the air that does get through is turbulent due to obstruction of the fins.

    Proper cooling would call for smooth air movement between the fins which would call for the fans to be above or below the heatsink. That would make the unit much taller which was likely the reason it was not used. This compromise means some of the amplifier channels will run hotter than others, and in theory, have shorter life.

    To add insult to injury, the fans kick on based on specific volume level rather than heat generated. Better design calls for a temperature sensor to keep the fans off as much as possible if the unit has ample circulation. But no, as soon as you turn up the unit, the fans click on saving a few cents in component pricing, but bringing with it the higher noise of the component, and shorter fan life.

    Actually the right design would have called for a much larger heatsink so that no fan would have been necessary. That of course costs more money as heatsinks are expensive as is space to house them and shipping cost to get us the unit from far away manufacturer. So compromises were made, hoping one wouldn’t notice. And probably no one does.

    I am being harsh here since there is an unwritten rule that in the home consumer market that amplifiers are fanless and silent. Professional units use fans because they live in much harsher environments of cramped racks and outdoor applications (e.g. for live sound). But home units usually live in homes with less hostile temperature ranges that allow passive cooling if one does not aim to reduce cost. For the $1,000 that I paid for this AVR, it would have been a fair expectation to get an amplifier without fans.

    As a way of comparison, let’s compare this amplifier to a much higher end unit, namely the Proceed AMP5 (Figure 5) I have on hand (a long discontinued amplifier from circa 2000). We immediately see a difference in how entire amplifiers are replicated rather than having shared power supply, heatsink, etc. as was the case in our AVR. There are five of everything starting with the toroidal transformers lined up in the front, each with their own independent pairs of output supply capacitors (blue cylinders) feeding independent amplifiers. What this means is that there is little “crosstalk” between channels. In a design where there is one power supply shared among multiple amplifiers, high amount of current consumption may cause insufficient power to be available to another channel. Indeed measurements of most AVRs shows that they output more power in stereo than multi-channel. While this brings great efficiency and cost savings, it can cause activities of one channel to bleed into another as the shared power supply voltage fluctuates. Whether this is audible or not is a matter of much debate and a function of a how much power you attempt to pull out of the unit. What we do know with certainty is that if you have completely separate amplifiers, this problem goes away.

    As is usually the case, there is no free lunch. The Proceed AMP5 costs six times more than the entire AVR and weighs some 120 pounds! It is an investment I am happy to have made as amplifiers don’t become obsolete and quality brings peace of mind and a more quiet unit here, in the form of a fanless design. But not everyone can justify the increased costs in the face of unquantified improved performance other than totally silent operation (which you could also get in an AVR).


    Figure 5: Proceed AMP5, five (5) channel amplifier.

    4. Moving on digital components, clearly visible on top, sporting four HDMI connectors, we can easily identify the video subsystem (Figure 6). Here the design is comprised of three “VLSIs.” This is a technical term for custom integrated digital integrated circuit (although these components also have small analog circuits in them). Another name for this type of device is an ASIC. Either way, it indicates a purpose built electronic circuit made for a specific functionality.

    The core function of an AVR is to bring audio/video into the unit, separate them into independent streams, process them, and then output over HDMI. The interface to HDMI is implemented in this case using the Silicon Image SiL 9135 receiver and companion SiL 9134 transmitter.


    Figure 6: Video Input/Output and Processor board.

    Silicon Image is the company behind the original specification which became the computer monitor interface, DVI. Their proprietary design of transmitting the color components over very high-speed serial interface over a twisted pair serial link then became part of the HDMI interface which added audio and copy protection (HDCP) to the DVI interface. That same serial interface which was originally designed for short distances of computer to monitor, is also behind the very tricky nature of HDMI when distances become larger.

    Many devices today have built-in HDMI interface and that functionality is pulled directly onto the main processor in a processor called SoC or System-on-a-Chip. SoCs are major building blocks of Blu-ray players and cable/satellite set-top boxes which I hope to cover in a future article.

    In addition to our HDMI interfaces we also have a video decoder and processor. This being an older unit, it sports a Gensis FLI 8125 VLSI which is able to decode and scale both analog and digital video. Finding the video processor in an AVR is a good way of learning of its scaling and de-interlacing properties. With the advent of UHD/”4K” video, scaling has become part of our lives again and by finding the part number and searching for it online, you can find many discussions of features and capabilities of these subsystems.

    5. Last major subcomponent is audio processing. This part was buried into the vertical board that was tied up with way too many connectors for this lazy author to try to disassemble. At high level though, the design is simple. A DSP or digital signal processor (a form of a processor optimized for numerical computations) handles such things as decoding of Dolby and DTS audio formats. Modern AVRs today sport streaming/networking functionality implemented using an SoC which likely has swallowed the DSP functionality into the main processor.

    So there you have it. Your first overview of the anatomy of an AVR. Notice how much we could glean from just looking at the unit with no technical documentation but with some help from search engines to find part numbers of major components. AVRs are incredible combination of components pushed into a rather compact package. At this mid-priced to higher end price level, some compromises are apparent and common in the category. Learning what makes these devices tick allows you to better able to identify these and help choose a better unit for your needs.


    Amir Majidimehr is the founder of Madrona Digital (www.madronadigital.com) which specializes in custom home electronics. He started Madrona after he left Microsoft where he was the Vice President in charge of the division developing audio/video technologies. With more than 30 years in the technology industry, he brings a fresh perspective to the world of home electronics.

  2. #2
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    Cool Amir; I had the Onkyo TX-SR805 AV receiver some time ago, my brother has it now. Back in 2007 they used a rudimentary video chip (Genesis), now they are much better.
    Back them the 805 had Audyssey MultEQ XT, now they have the new AccuEQ system (in my opinion...inferior).
    The 805 weights 51 pounds and was $1,099 (MSRP - US) back then, but many people bought it on sale for half. * I paid $800 Cdn for it.
    The two fans never activated, they are useless because they are set to only activate when the temperature is way way too hot...a design flaw in my book.
    And there were other flaws in that receiver too, even if it was quite powerful for a receiver, with a fair heat sink and main transformer; the two caps are rated @ 15,000uF each.
    But it's still working fine according to my brother.

    Onkyo has a new line of AV receivers (the RZ series)...very nice looking face...and still nicely looking inside (but looks is like women...):

    ♦ Onkyo TX-RZ900 => http://www.areadvd.de/tests/test-onk...iver-tx-rz900/
    {Use your translator feature.}

    ________

    <<<>>> I started a thread way back on AV receivers and pre/pros...more for the home theater enthusiasts...but that never really took off.
    So I go where the wind blows for that. I'm also big on movies (Blu-ray and 3D), which is a lonely thread. I understand though...IMAX is much better.

    Music is WBF forte...and DACs...and platine tournantes. ...Some loudspeakers too...quality monitors and the big boys...powered by the cool tubes (hot actually). ...And all the other usual suspects; sport section, stock market, scientific research, galaxy and the universe, ...all that jazz.

    The Proceed amp is nicely designed, compact (like my IBM ThinkCenter) and heavy @ 120 pounds. ...More money too.

    Denon had a receiver back in the days, the AVR-5805CI, and it weighted 97 pounds, and was eleven inches high.

    In the 70s the stereo receivers, the top guns, from Pioneer, Marantz, Kenwood, ...were gorgeous and well designed, and had good heft too. I miss the days, but still own some antiques for nostalgia, and still use my Marantz analog tuner occasionally.

    Today, it's Dolby Atmos, DTS:X, Auro-3D, Audyssey Platinum...in Denon/Marantz top guns (receivers and pre/pros), in the affordable range...less than four grands.
    Also, Anthem is in that category with new receivers (their flagship with no less then eleven internal amps) and ARC EQ system. ...Plus a new pre/pro ($3,000 or so).

    Audio Control and Arcam have also some strong contenders in the higher echelon of AV receivers...up to $9,000. ...The new 2015-16 models.
    Last edited by NorthStar; 10-28-2015 at 11:11 PM. Reason: Can you believe it! ...One typo
    All the Very Best, - Bob --------- "And it stoned me to my soul" - Van Morrison --------- AudiophileAudition

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    Some more details about the Onkyo TX-SR805 AV receiver (and the 705):

    http://www.thefreelibrary.com/Onkyo+...s.-a0178450770

    * Just click on Skip survey
    All the Very Best, - Bob --------- "And it stoned me to my soul" - Van Morrison --------- AudiophileAudition

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    Quote Originally Posted by amirm View Post
    ...
    This board also has an additional piece of functionality, namely, remote power on/off. A relay (black box barely visible in Figure 3) either allows the power to go through or not, as instructed by a small microprocessor (little computer) that receives the Infrared signal from the remote control. Because that part of the AVR needs to always stay on, or else it could never act on the remote signal, there is a small transformer and power supply on this board, surrounding the relay module. The power control relay can also be part of the protection circuit, shutting the unit down, or keeping it from powering up should a fault be detected in the amplifier section.
    ♦ Higlighted in red; it should say; in Figure 2 ... and not in Figure 3. ... The small black relay box. ...Of the Onkyo TX-SR805 AV receiver.

    As a way of comparison, let’s compare this amplifier to a much higher end unit, namely the Proceed AMP5 (Figure 5) I have on hand (a long discontinued amplifier from circa 2000). We immediately see a difference in how entire amplifiers are replicated rather than having shared power supply, heatsink, etc. as was the case in our AVR.
    There are five of everything starting with the toroidal transformers lined up in the front, each with their own independent pairs of output supply capacitors (blue cylinders) feeding independent amplifiers. ...
    ♦ Not five toroidal transformers in the Proceed Amp 5 (lined up in front) but three toroidal transformers total. ...Figure 5.

    1. One toroidal for both the Left Front and Left Rear channels (#1 & #2).
    2. One toroidal for the Center channel (#3).
    3. And one toroidal for both the Right Rear and Right Front channels (#4 & #5).

    ______

    * Those are just two small details that I detected. ...Nothing much to sweat over.
    All the Very Best, - Bob --------- "And it stoned me to my soul" - Van Morrison --------- AudiophileAudition

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    A very interesting article about what's inside a receiver:
    AV Receiver and Amplifier Power Ratings Trends: Manipulating Wattage Ratings

    by Patrick Hart — July 11, 2015
    Originally published: Nov 9th, 2004 by Patrick Hart

    http://www.audioholics.com/audio-amp...-power-ratings
    Kevin

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    All the Very Best, - Bob --------- "And it stoned me to my soul" - Van Morrison --------- AudiophileAudition

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