Vary power supply voltage of Class D Amp as volume control - Page 1
Forum section: electronics / projects

author: jusaca
author_rank: Unknown
timestamp: June 22, 2025, 02:39:56 pm
content: When building class D amps (especially in the home stereo market), is it a thing to modulate / vary the supply voltage to change volume?I was just thinking about class D designs in generel and came to the conclusion, that you throw away quite a bit of PWM resolution if you also use it for volume control.If instead one keeps the PWM modulation the same regardless of volume and just changes the supply voltage of the output stage, one can use the entire PWM resolution for the actual audio signal. But I'm not sure whether that really is a good idea or if it is already used in commercial class D hifi amps?I know that envelope tracking is a thing in RF amps, but with this question I'm specifically concernd with hifi gear.

author: oPossum
author_rank: Super Contributor
timestamp: June 22, 2025, 04:01:47 pm
content: The TacT Millennium did that. I don't know of any others.

author: jusaca
author_rank: Unknown
timestamp: June 23, 2025, 10:46:36 am
content: I wonder why it is not more common, it seems like an obvious solution to me. It would reduce EMI, stress on the power switches (lower dV/dt) and the need for ultra-short pulses.Only downside: You need a power supply able to regulate over a wider range. But even most USB-C chargers nowadays support PPS, which offers a programmable output voltage between 3.3 V to 21 V in 20 mV steps. So this does not seem like a big hurdle?

author: mtwieg
author_rank: Frequent Contributor
timestamp: June 23, 2025, 12:26:25 pm
content: Sure it's technically doable, but the benefits are questionable. In order to really hear the difference, you would have to:1. Operate the amplifier/speaker at a very low volume, and be far closer to the speaker than normal. i.e. you're one user error away from blowing out your eardrums.2. Use a much smaller/more sensitive speaker than is appropriate for the amplifier. i.e. you're one user error away from blowing out your speakers (and also maybe your eardrums).

author: tooki
author_rank: Super Contributor
timestamp: June 23, 2025, 12:44:31 pm
content: Quote from: jusaca on June 22, 2025, 02:39:56 pmEnvelope tracking to reduce power consumption is a thing (i.e. voltage tracks volume to save power); I’ve never heard of envelope tracking being used to reduce volume.

author: CaptDon
author_rank: Super Contributor
timestamp: June 23, 2025, 01:57:56 pm
content: Perhaps a high/low toggle switch would be the better solution. For sustained 'background' listening the switch could be set to low which would cut the power supply rail voltage by 50% or so but would you still be able to use all the bits? I guess a lot of the design will still fall on the digital to PWM converter? How to take advantage of using all of the available bits without clipping the Class D output stage running at a lower voltage Seems like the loop gain of the DAC to analog to PWM would also need to shift with the Hi/Lo switch? There is a system often used and seen in AM shortwave stations. They incorporate a delay of perhaps .5 seconds and then use a look-ahead circuit to ramp up the RF Final voltage when needed to pass large peaks. They did the same in vinyl record recording. The look-ahead circuit would speed up the lead screw driving the cutter head to allow more groove spacing to accommodate large peaks in the audio signal.

author: jusaca
author_rank: Unknown
timestamp: June 23, 2025, 03:27:47 pm
content: Quote from: mtwieg on June 23, 2025, 12:26:25 pmThe benefits I see are actually quite huge. Drastically reduced wear on the power FETs (slower dV/dt and smaller transient voltage peaks) in addition to a drastically reduced effort in creating high resolution pulses in the control loop. To play back an audio signal at least something around 16 bit are used. For a 500 kHz PWM this already means pulses as short as T / 2^N = 2 µs / 2^16 = 30 ps ?! Not sure if I made a mistake, but this already seems improbable.If we now assume a typical hifi amp has around 100W per channel and in most use cases typical playback power in every day scenarios is more like 1W or so? We would have to use 100 times shorter pulses to achieve the same resolution.And we could entirely eliminate this difficulty by just varying power supply voltage.Don't get me wrong, I don't assume that I'm a genius and everybody else is doing stupid things, I'm just curious where the knot in my brain is and what exact reasons manufactures might have to "go the hard way" by choosing to need a higher resolution PWM.

author: tooki
author_rank: Super Contributor
timestamp: June 23, 2025, 03:56:18 pm
content: Quote from: jusaca on June 23, 2025, 03:27:47 pmI don’t know the answer, but if we push my sampling theory knowledge to its limits, consider this: in the audio recording, each sample is not independent of the adjacent samples, because (thanks, Shannon and Nyquist!) the input signal must, necessarily, be bandwidth-limited.Consequently, assuming a halfway competent DAC, the signal going to the class D amp’s modulator is also bandwidth-limited.

author: oPossum
author_rank: Super Contributor
timestamp: June 23, 2025, 07:18:37 pm
content: The TacT Millennium used Equibit technology from Toccata. Lars Risbo was the founder of Toccata and the creator of the Equibit technology. Equibit is a class D topology that is sometimes called a "Power DAC" or "Digital Amplifier". It creates PWM using DSP. Digital audio in, digital signal processing, digital out. Texas Instruments acquired Toccata and implemented the Equibit technology in some of their Class D amplifier chips.The most popular class D topology currently used is SODA (Self Oscillating class D Amplifier). One of the best SODA designs is UcD (Universal class D) created by Bruno Putzeys at Philips. One of the benefits of UcD is the use of post filter feedback rather than pre filter feedback. UcD amplifiers take an analog input and use analog circuitry to create PWM. The PWM is not quantized by a fixed clock. UcD amplifier modules are made by Hypex and Purifi. They are used in many brands of audio equipment.So Lars and Bruno are two very innovative and influential creators of Class D amplifiers. What if they worked together to create the ultimate class D amp? What topology would they choose? They chose UcD.https://purifi-audio.com/about