low/medium power HiFi audio amp  - Page 1
Forum section: electronics / projects

author: bonelli
author_rank: Contributor
timestamp: November 16, 2022, 09:54:38 pm
content: Hello,When I was student, I built an audio amp based on the well-known TDA7293. The input stage is based on a CS8416 (optical TOSlink and SPDIF to I2S) and a DSD1794A (24bits 192kHz audio DAC). After 15 years of daily use, it is still working, but I'm dreaming to something better.THD+N measurements are quite bad, I have 50Hz noise, and also some spurious at a few kHz. I think the layout is not really good and I have also made some wrong choices in the power section.Today I'm planning to build a better design, I will try to use a XMOS CPU to have both TOSlink and especially an USB async audio input. I will keep my 2 speakers that are very good for theirs size. I'm questioning myself about the power amplifier section.The TDA7293 is designed for 100W. A lot of people are using it for home amp, honestly I don't know what they do with 100W. Mine outputs 100mW when working with a background music, and a few watts when pushing the volume up. My speakers probably have a good sensitivity. I have designed the I/V converter between the DAC and the amp to limit the output power to something like 15W but I never use it at full power.The problem is, even if the datasheet shows a quite nice THD at 100mW power, this is not the THD+N of the whole design. The TDA requires a minimum gain of 26dB (x20). The reference design uses a gain of x33 (30dB). 100mW at output, means 36mV RMS at the amp input. And it's quite hard to have a decent SNR with a such low voltage (70dB SNR = noise floor at 10µV, 90dB SNR = noise floor at 1µV).Note : ok, it's is absolutely useless to have 90dB of SNR at 100mW, because 100mW of electrical power in my speakers probably equal to 40dB (audio level). 40dB-90dB of SNR is -50dB below the lowest audible audio level for the best ears. So, I'm maybe in the wrong, but I would like to say that the required SNR for an audio amp is not more than the audio level in dB you want to have.Ideally, I would like to use something like a low-power version of the TDA7293, that will impose a lower gain. A power stage with a unity gain would be perfect. I was hoping that manufacturers like TI/AD/linear have some more modern versions of the TDA, but all modern devices I found are class D amp, low cost (and high THD) amp or headphone amp.I don't have enough analog skills to build a good amp based on discrete transistors.I find the OPA564, which is a power op-amp capable of outputing 1.5A at unity gain. This device is used on high-end headphone audio amp. Some people say that its output impedance is to high to properly drive a 8-ohm load.Feel free to give any suggestion about this subject. I know this could be a hot subject

author: Benta
author_rank: Super Contributor
timestamp: November 16, 2022, 10:08:46 pm
content: I'd look at the National (now TI) series of Overture amps. Unfortunately, they seem to live in the shadows at TI today.LM4752 or LM4755 might fit the bill, 2 x 5 W.Top of the line is of course the legendary LM3886 amp.

author: coppice
author_rank: Super Contributor
timestamp: November 16, 2022, 10:15:06 pm
content: Quote from: Benta on November 16, 2022, 10:08:46 pmThere's just no demand for amps like that any more. Every wants class-Dish amps in that class.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: November 16, 2022, 10:19:58 pm
content: I designed and built a nice 2x150W class-D amp based on the TPA3255 and switched all the old LM3886 module amps out and put these in, with better power supplies (resonant mode SMPS).I have about 12 of these running here now in various systems, including one with 3 stereo modules in it for a stereo 3-way active speaker system.

author: jonpaul
author_rank: Super Contributor
timestamp: November 16, 2022, 10:23:33 pm
content: See AP.com for Audio Précision Analyzers.Their many fine app notes give all information on measurement of THD, IM, , Noise floor, dynamic range, and CMRR test.Modern design power amplifier IC are class D, see TI.COM or AKM for a wide rangeBon courageJon

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 17, 2022, 02:08:26 am
content: Consider the Infineon MA12070, similar audio quality as TPA3255 but requires less voltage to operate.

author: David Hess
author_rank: Super Contributor
timestamp: November 17, 2022, 03:35:14 am
content: Quote from: bonelli on November 16, 2022, 09:54:38 pmModern speakers have smaller enclosures which for otherwise the same performance have lower efficiency, but this is not a problem because solid state amplifiers are so powerful.

author: DrGeoff
author_rank: Frequent Contributor
timestamp: November 17, 2022, 04:01:24 am
content: Quote from: David Hess on November 17, 2022, 03:35:14 amAlso allow for dynamics unless you listen to highly compressed muzak. Typically these can be 12dB or more above the RMS in some genres. But yes, unless you need it very loud periodically, you probably won't peak above 50W. Dance music genres with a lot of bass energy will push the amplifier higher, however using larger efficient speakers allows 120dB SPL easily.