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• A deeper dive into the amplifier design

  Marco Tabini • 3 days ago • 0 comments

  
  
  It has occurred to me that, in the Design Walkthrough, I never really explained how the amplifier itself works from an overall design perspective. I thought it would be useful to do that, so here we are.

  To understand why Amply is designed the way it is, we need to start with our desired result. At the output of the amplifier, we ultimately want to drive a certain amount of power into the speakers. Assuming that our desired power is 10W, and that the speakers have an impedance of 8Ω, we can calculate the required voltage and current using the following formulas:

  This means that our amplifier has two distinct requirements: It needs to take our input signal (which, at full volume, is around 1Vrms) and amplify it to around 9Vrms, and it needs to be able to provide around 1.12A of current to the speakers.

  In addition, we also want to:

  • Present a high impedance at the input, so that we don't load down the Bluetooth receiver.
  • Manage the amplification loop so that we can control its gain, and prevent it from distorting the signal or going into uncontrolled oscillation.

  There is no single device that can do all these things at once, so we need to break down the problem into smaller pieces. Hence the use of an op amp, which provides the voltage gain, and a power stage, which provides the current gain. The op amp also has a very high input impedance, and can use negative feedback to control the gain and stability of the amplifier.

  Voltage gain

  As mentioned in the design walkthrough, the op amp is configured as an inverting amplifier. This is needed because, while the input signal includes both a positive and negative component, our amplifier is powered by a single supply, which means that the output can only swing in one direction relative to ground.

  We solve this problem by creating a “virtual ground” at the mid-point of the supply voltage, and then allowing the output to swing around that point. The inverting configuration has the distinct advantage of allowing us to inject the virtual ground directly into the non-inverting input of the op amp, which means that we don't need to use a separate buffer stage to create this signal.

  The gain of the inverting amplifier is determined by the ratio of the feedback resistor (RV1) to the input resistor (R1):

  The negative sign indicates that the output is inverted relative to the input—for all practical purposes, we can ignore the sign in our application, so long as both the left and right channels are inverted in the same way.

  In our case, the maximum possible gain with a 50kΩ feedback resistor and a 1kΩ input resistor is:

  Current gain

  While the op amp has no trouble providing the voltage gain we need, it really cannot provide more than a few tens of milliamps of current, which is nowhere near the 1.12A we need to drive the speakers. If we plugged its output directly into the speakers, the only sound we would hear is probably that of the op amp going up in smoke.

  The power stage of the amplifier is responsible for providing the necessary current to drive the speakers. In our design, we use a Class AB push-pull configuration, which consists of two complementary transistor pairs (one NPN-based and one PNP-based) that work together to amplify the current. I have already mentioned how this works, so I don't want to spend too much time on it here.

  The key point, however, is that the power stage primarily amplifies current, not voltage. As you can see here, we need to use two transistors for each leg of the push-pull stage because power transistors typically have a much lower gain than small-signal transistors like the 2N3904 and 2N3906. By using a compound pair, we effectively multiply the gain of the two transistors, which allows us to achieve the necessary current gain to drive the speakers. The Sziklai configuration is particularly advantageous in our case because it requires less base current than a Darlington pair, which means loading the op amp less and...

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• Drawing first smoke

  Marco Tabini • 6 days ago • 0 comments

  In the Design Walkthrough, I mentioned that the R15 resistor on the output stage of the amplifier was a compromise between the needs of the circuit and the available board space. Good design practice would have been to use two separate resistors, one for each branch of the push-pull output stage, but I wanted to fit everything on a 100×100 mm PCB to stay within the size for which most PCB manufacturers charge a flat promotional rate.

  In this update, I wanted to dig a bit more into this particular part of the circuit and show you that, even though a 0.33 Ω resistor may seem like an insignificant component, it is actually a critical part of the design.

  Transistors are weird, man

  Transistors are inherently “unique” devices: even if you pick two transistors of the same type, and even if they come from the same batch, they will have different characteristics—sometimes wildly so. This is because of the way they are manufactured: they are made by doping silicon with impurities, and the exact amount and distribution of these impurities can vary from one transistor to another.

  To add to the problem, the characteristics of a specific transistor also change based on the operating conditions. For example, the gain of a transistor (the ratio of the output current to the input current) can vary with temperature, and it can also vary with the amount of current flowing through the transistor. Left unchecked, this not only makes it difficult to design a circuit that works reliably, but it can also lead to a phenomenon called “thermal runaway,” where the transistor gets hotter, thus increasing its gain, which in turn causes it to draw more current, which makes it even hotter, and so on until the transistor is destroyed.

  Thus, the golden rule of transistor circuit design is that the reliability of the circuit does not depend on the characteristics of the transistors; instead, the passive components around them are designed to ensure that the circuit operates within a specific envelope, is stable, and so forth.

  Degenerates live in your circuit!

  One of the most common ways to ensure that a transistor operates within a specific envelope is to use a technique called “degeneration.” This involves adding a resistor in series with the emitter (for a BJT transistor) or the source (for a MOSFET) of the transistor. (The term “degeneration” comes from early vacuum tube and transistor amplifier theory, where it referred to a mechanism that degrades or reduces the gain of an amplifying device through feedback.)

  The resistor creates a voltage drop that is proportional to the current flowing through the transistor. This means that if the current increases, the voltage drop across the resistor also increases, which in turn reduces the voltage across the transistor and thus reduces its effective gain. This negative feedback mechanism helps to stabilize the operating point of the transistor and makes it less sensitive to variations in its characteristics. Obviously, the resistor also limits the maximum current that can flow through the output stage and dissipates power of its own, so you typically choose a small value: large enough to prevent thermal runaway, but small enough not to waste too much power or reduce the efficiency of your circuit.

  (Note that this is not the same kind of “negative feedback” that is commonly used in amplifier design, where a portion of the output signal is fed back to the input to reduce distortion and improve linearity. Degeneration is a form of local negative feedback that is applied directly to the transistor itself, rather than to the overall amplifier circuit.)

  In general, you want each branch of the push-pull stage to have its own degeneration resistor so that each output transistor can be stabilized independently. In our case, R15 is therefore not a true degeneration resistor; instead, its role is primarily...

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• Design walkthrough

  Marco Tabini • 05/06/2026 at 04:41 • 0 comments

  Amply uses a very common design—in fact, the project constraints more or less require the use of a fairly old-fashioned amplifier topology that is far less common in modern designs.

  Basic design philosophy

  Despite its low price tag, Amply was not designed primarily to minimize cost. In several places, the design is intentionally conservative in order to maximize performance and reliability. The idea is that you should be able to assemble the amplifier with minimal technical knowledge and still end up with something that sounds great and is built to last.

  In practice, a fair number of components can be omitted while still producing a perfectly good-sounding amplifier, if at the cost of reducing the design's safety margins and potentially resulting in an amplifier that is less robust or more prone to instability or failure.

  For those feeling particularly adventurous, I will point out which components are required for basic operation and which ones can probably be omitted without too much risk.

  Power

  Amply can be powered either from a DC supply between 9 V and 30 V or from a USB-C power supply. In the latter case, any USB Power Delivery (USB-PD) profile of 9 V or higher will work.

  Using USB-C power requires a small USB-PD trigger module that negotiates the appropriate voltage from the power supply. I use a generic module purchased from AliExpress, which cost about $1.30 at the time of writing.

  A pair of Schottky diodes (D1 and D2) prevent backfeeding between the two power sources and provide reverse voltage protection. They also introduce a small voltage drop—typically around 0.3 V—but ensure that the amplifier is protected if both power sources are connected simultaneously or if the user commits a wiring “oopsie.”

  If you intend to power Amply from only a single source, the diodes are unnecessary and can be replaced with wire bridges to eliminate the voltage drop and slightly improve efficiency and available supply headroom. Likewise, if you do not plan to use USB-C power, the USB-PD trigger module can simply be omitted and the DC supply connected directly to the circuit.

  Single-rail woes

  Audio signals are AC, which means the amplifier must be able to process signals that swing both positive and negative. One of Amply's design constraints, however, is that it operates from a single positive supply rail for the sake of simplicity.

  To work around this limitation, the amplifier creates a virtual ground at half the supply voltage and references the audio path to that midpoint. With a 12 V supply, for example, the signal reference point sits at 6 V above the negative rail. This allows the amplifier to process signals that swing both above and below the midpoint, effectively providing up to roughly ±6 V of signal swing in theory.

  The upside of this approach is a much simpler power supply design. The downside is that the available output voltage swing is reduced compared to a true split-rail supply, which in turn limits the maximum output power. For Amply's intended use case, this is a perfectly acceptable tradeoff.

  The virtual ground is generated using a simple voltage divider (R3 and R4). In this amplifier topology, the midpoint reference only supplies very small signal-level currents associated with the op amp input and feedback network, so there is no real need to buffer it with an active circuit despite its relatively high impedance.

  I also added a 10 µF capacitor (C17) to reduce noise and lower the AC impedance of the reference node. Realistically, the amplifier would probably still function perfectly well without it.

  5 V regulator

  U2 is a bog-standard 5 V linear regulator that provides power for the Bluetooth module. Since the Bluetooth circuitry only draws a modest amount of current, a simple linear regulator is perfectly adequate here, provided the input voltage is not excessively high.

  The LM7805 can technically tolerate fairly high input voltages, but power dissipation quickly becomes...

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