What's in those guts? 8ight's Amplifier Analysis! Part I

[8ight]

Hello everyone, today I ask that you please take the time to learn something new or add to knowledge you are already building on the subject. I wish to offer some of my knowledge as a potential aide to the community.

I have very little to contribute to the forum compared to many of the brighter minds here but one area in which I am more knowledgeable than most is circuit design. This is due mainly to my unhealthy obsession with amplifiers. We all need amplifiers to run our systems but not all of us know what to look for. Everyone loves gut shots of their favorite amps and may just wonder what all those guts do for the amp. This is why I am posting this thread, without getting too into specifics I would like to write up a short guide on how to pick amps based on their internal structure and components. I will not be explaining the functions of any individual component in too much detail but I will do the best I can to educate anyone reading on the importance of these parts and what they mean about the amp. Above all else in importance is the actual design of the amplifier, which is what the primary focus of this write-up will be. I will try to keep it simple, please offer your criticisms, advice, and ideas so I can make this better!


Table of Contents:

Class A/B selection: ...Draft
1.1.....Class A/B power supply design
2.1....Class A/B output design

Class D selection: ...COMING SOON!
3.2....Class D power supply design
4.2....Class D output design

Other related topics for discussion: ...COMING SOON!
5.3....Terminal and I/O quality
6.3....Heatsink design
7.3....On-board crossovers
8.3....Power ratings

Spotting the cheap stuff: ...COMING SOON!
10.4....(Section title)


Class A/B amplifier selection:

1.1 : Power supply design

The first thing I look at in any amp is the power supply. In a class A/B power supply there are three main components that jump out immediately that I can judge quality and capability by: transformers, capacitors, and transistors. The job of the power supply is to deliver the current that the output section demands when it demands it. The power supply takes the input voltage (usually +12v in a car) and steps it up to the amplifier rail voltage. Let's look at some good power supplies and some bad ones and see just what we find...

Here is a PPI Art Series A600.2, a legendary SQ amp with an extremely well constructed supply.
This power supply does its job well, it sucks down current quickly enough to keep up with the demands of the output section giving the amplifier a tight, quick, and clean response through transient notes.





The components:

Red - Excellent explanation courtesy of Wicks!

Contrary to popular belief, the aluminum electrolytic capacitors used in basically all amplifier architectures are not for energy storage.
They're for something called "decoupling" or filtering two different areas of circuitry.
Electronic devices are built with the assumption that the supporting electrical supply is stable.
There is no possibly way for those caps inside the amp to hold enough energy to actually power the amp if the supply current drops significantly which happens regularly when people try and use huge amps on stock electrical systems.
The only amp (that I know of) that can support itself with internal caps is the Fosgate T15K which has a MASSIVE amount of capacitance inside it.
A capacitor acts like a short circuit at high frequencies. When paralleled into a circuit, it helps to eliminate (short-out) higher frequency noise.
Also, the voltage across a capacitor can not instantaneously change, therefore its great for voltage stability (noise smoothing).
There are two main capacitor sections in an amp, before the power supply and after (Class D amps will have some in the output but that's another topic).
The "Before" caps:
These are physically the smaller of the two and are likely rated up to 25V. Its common to use a cap with double the voltage rating for reliability, when possible. They are directly connected to the power terminals on the amp and "decouple" the input of the power supply from any noise and whatnot in the vehicles 12V electrical system.
The "After" caps:
These are the larger of the caps and are typically placed in the middle of the circuit board. They are usually rated a certain percentage above the voltage of the power rails in the amp and will give you a rough idea of what voltage the amp's power supply is capable of.
If you understand how transformers work then that is what's going on in the amp's power supply.
The initial ~12V is pulsed into the input of the power supply transformer by the switching MOSFETs.
This pulsed or switched voltage is now a form of AC which the transformer can use to step up the voltage.
The transformer outputs this higher voltage "AC" signal which must be transformed back to DC for the output section of the amp to use it.
The higher voltage AC is fed into a pair (or more) of rectifying diodes which remove the unwanted polarity of the AC wave.
The rectified waves are then fed into the large capacitors which smooth the voltage into DC.
The amount of capacitance and number of caps used depends on the amount of ripple voltage that needs to be smoothed and the output current of the amp.
As you would expect, the larger the amp, the more capacitance is needed in the output of the power supply

Blue - MOSFETs just like any others, there are 10 here which is more than adequate for an amp of this power level. Larger amps use similar amounts of supply transistors but keep in mind the package size of these transistors is very small compared to those found in larger high-current amplifiers.

Green - Transformers, there is not alot to say about these. Transformers are big and flashy by design and have a tendency to convince buyers that their presence means the amp is powerful. This could not be further from the truth, the job of the transformers is like the job of a piece of wire, they handle what load they can and everything below that level because they are purely passive components. The transformers in this amp are tightly wound and feature larger-gauge wire typical in amps from this time period. These parts are also mounted flush and not slathered in glue (overuse of glue typical of even the best Chinese and Korean construction.) The quantity of transformers also has little to do with performance, every transformer beyond the first in the power supply only serves a different purpose such as isolation or is run in parallel with the first to distribute the load more evenly (known as a dual power supply system.) In such a dual power supply system each power supply often handles half of the channels, exceptions exist in some amplifiers notably in five and six channel amplifiers.


Now that you have some idea of what you are looking at I will give you some examples of quality power supplies and those that are less than adequate for the application, you will have to classify these yourself. Remember, nothing is necessarily wrong with any of the designs I post some are just great while others are OK. Answer by categorizing your choices in a copy of the table below! I will give a 4 ohm rating and channel count for every amplifier, do not forget this in your consideration!


Score Cards for 8ight's Amplifier Analysis
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The Good (power supplies):
Amp#'s

The Bad (ps):
Amp#'s
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The Good (output sections):
Amp#'s

The Bad (os):
Amp#'s
----------------------------------











1.2 : Output Design

Not a lot has changed from the power supply to the output section, namely it's the lack of transformers and capacitors that one immediately notices. Looking at the output section I do a few things: find the FET/bipolar transistor count per channel, look for output inductors, and look for isolation of the output section. An isolated output section will be on a physically different PCB, coupled to the power supply by an isolation transformer, or optical coupling. Any combination of the above methods may be employed but it is not common to need more than one. Isolation of the output section helps reduce the amount of noise from the power supply entering the output stage. I will again go through the A600.2 and point out just what makes its output section rock.




The components:

Red - Output MOSFETs/Bipolar transistors. This is where the magic happens, these transistors pulls current from the power supply to amplify the signal. The number of outputs should always be EVEN! There are 5 FETs per channel 20 total in this amp, well more than enough for 600w RMS any day, this amp was obviously built with some headroom (this is what most call 'underrating' when in fact more than the necessary amount of output devices is present on purpose so the rated output can be reached well before THD begins to spike)

Green - Not so important but still very important is the gate drive circuitry. I have highlighted the part of this circuit (the resistors) usually responsible for failure and subsequent failure of connecting output devices. These resistors are responsible for keeping your output section alive (without getting into details just know that if they go so does the connected FET/transistor) and higher quality amps usually feature very large ones such as those shown here. The larger resistors are more expensive and take up more room on the board but are capable of dissipating more heat.

Blue - The speaker outputs, I highlighted these to point out that there are NO output inductors found in this amp, it is a straight output.

The rest - There is a lot more to the output section I cannot begin to attempt to explain. The best you can do when analyzing the other portion of the output section is check for quality construction such as good solder joints and flush components, component count may indicate quality as well but it does not directly relate to it in any way. When comparing high-end amps that I know are well-engineered I take component count more seriously.


Now I will give another example of a fine output section, this one in from my own personal amplifier: a Zapco Reference 1000.4. This amplifier features output inductors I will point out. This is probably a bad example to use as a baseline because this amp is overbuilt, use this example only to help identify and understand components.





The components:

Red - Nothing new here, more output devices. This particular amp only makes use of two per channel but they are large package high-current devices that will easily generate the 150 x 4 @ 4 ohms this amp is capable of.

Black - Amplifier output, notice the little coils you see.

Blue - Yes, those little coils, very important to me. These inductors are placed before the output of each channel. These parts are usually found on audiophile amplifiers and immediately signify quality as I have never seen them in a cheap amp, they have no place there. The purpose of these inductors is to isolate the amplifiers internal feedback loop from outside RF noise coming in through the speaker wires. You will find that they are present on nearly every high-end amplifier that calls itself sound-quality oriented simply because there are almost no design obstacles that prevent their easy implementation.

[Carbon]

Great thread man!

[newls1]

is this a copy of another thread? I remember seeing this exact thread or one very similair to it here about 2 years back......

[ric006]

Its nice to show the others here about amps

[Tecomah]

Awesome. Good info

[8ight]

Great thread man!

Thanks! There is much more to come, if you want play the games post your answers and I will score everyone after enough people have a chance to discuss and pick answers.

[xx1]

this is awesome, been trying to learn this kind of stuff for a while, thanks.

[MrEwubbsZ]

I don't have time to read the entire thing but what I did read was awesome.

It helped me understand amps a lot better.

[8ight]

I know this is a long post, I will do my best to keep it organized as I add new content. Glad to be hearing positive things already!

[8ight]

I am working on some illustrations to show signal path and organization in many common amplifier designs. Stay tuned! There are only so many ways to build an amplifier even though they are all "different" ...