Reading An Amplifier or DAC Layout
This started from a simple discussion on the lounge, where Dean asked about PCB layouts. Dean was asking on how he can go to identify what parts of an amplifier or a DAC, and what function do they do. I think we all have asked such questions before, and while I’m typing the reply to Dean’s question, I find myself writing something close to a full length article. So I thought, I’d make it a full article on the website.
I’m gonna start with saying that all Amplifiers and DACs consist of mainly two parts: Power supply (PSU) section, and the actual amplifier/DAC section. Fancier units may also have a separate section for user interface controls that covers anything from volume adjustments, input selections, display LCD, and other functions on the user interface.
All electronic devices need a PSU section to supply clean power to the actual amplifier or DAC circuit. The power supply circuit can be on a separate enclosure (i.e on the big amps like the Beta22. Check out the Beta22 article for internal shots.), or in a form of a wallwart, or joined together in one enclosure as the amplifying or DAC circuit. Normally if you see an amplifier or a DAC internal shots with a toroidal travo inside the enclosure, then that means that the power supply is housed on that circuit. A PSU circuit can be very simple, consisting of travo, diode, capacitors, and regulator chips, to something as complex as the Sigma22 power supply (check out the Beta22 amplifier photo down below). Simpler doesn’t always mean poorer performance, and more complex better, but the goal of a good PSU section is to supply a clean and consistent power to the circuit to be powered. Some amplifiers run on single rail power supply, and some on dual rail power supply. The requirement is normally specified on the design of the amplifier circuit.
After the PSU circuit comes the actual amplifier, or DAC circuit. There really is no bottom line rule to this, as the design of amplifier or DAC circuits varies significantly from one model to the next. Amplifier circuits are quite varied, and the layout will vary as much as the design and topology of the amplifier. Vacuum tube circuits, solid state circuits, and hybrid (vacuum tube & solid state mixed design) circuits. In the realm of solid state circuits, opamp-based (or chip-based) circuits are normally simpler, as the the opamp is designed to replace quite an array of parts that would otherwise be required in an all-discrete circuit. You’ll later see how the amplifier circuit of the M-Stage (opamp based) is much simpler than that of the Beta22 circuit (all discrete).
Now, let’s take a look at some real circuits to try to see what’s going on.
Here is the Matrix M-Stage layout that has the PSU section also enclosed in the same enclosure. Notice how the toroidal comes with its own enclosure (blue color) that’s designed to suppress noise level on the electronic circuit. Left of the toroid are two big capacitors (the gold/silver colored circles) which should be the power supply capacitors, and next to it are two black small U-shaped heatsinksfor the power supply regulators (along with some red color wima capacitors for the regulator). Those are the power supply circuit. Next to the then you have the amplifier circuit to the left of the power supply circuit. You have four black U-shaped heatsinks that house the power transistors for the amplifier circuit, and the opamp is placed below it, on the right side of the blue box–the potentiometer that controls the volume.
On the other hand, this is the Beta22 amplifier, and it comes with two separate enclosures for the Amp section and the PSU section. The amplifier section is far more complex and more serious than the M-Stage. It comes with four separate PCBs to handle a fully balanced stereo signal: L-, L+, R-, R+. On the center is a volume control, mounted on a shaft that links it to the volume control knob. This is an all discrete design, hence you will find no chips anywhere in the PCB, either on the amplifier circuit or the PSU circuit. An all discrete design normally takes up more space than a chip/opamp design, but you get better sound quality in return for the space disadvantage. This is why good amps are usually big in size.
On the back panel (top section in this view), are signal input connectors, as well as power input connectors (remember that the PSU is on a different enclosure).
Like the amplifier section, the PSU section of the Beta22 is all discrete, and the complexity of the circuit is far higher than a typical PSU with a chip regulator. There are two identical power supply unit with two travos for supplying power independently to the L and the R channel of the amplifier.