Super easy composite amplifier utilizing LM3886/76! - Page 1
Forum section: electronics / projects

author: pet_rushi
author_rank: Contributor
timestamp: July 19, 2024, 05:41:13 am
content: Hello guys,For a year or so I put a lot of effort into building a replica of a famous composite amp (https://www.kaltecs.com/60w-ultra-low-distortion-kit/) utilizing TDA7293/4 but as I failed to find a suitable op amp and after a few tossed power chips I shifted my attention to LM38xx series power chips (1).The main difficulty with TDA chips was the lack of open loop gain/phase/frequency graphics in the datasheet.As TI provides such info for LM chips it took me just a few days to build a very stable and really nicely sounding composite.The principal circuit is the same (https://www.kaltecs.com/tda7293-composite-amplifier/), just the power chip is different.So I got lucky with LM power chips. The op amp which fits best is from TI (4). It's an excellent op amp, not very popular among DIY circles.The gain is 11 and the amp is absolutely stable. With +/-33.5V PSU it provides around 50W. The lack of a snubber is not accidental. It is not needed and degrades the performance near the clipping.The open loop gain is boosted by ~50dB and the THD goes below -115 -118dB.The sound is sonically neutral and extremely pleasant.If someone wishes to design a PCB I may help.(1) -https://www.kaltecs.com/60w-ultra-low-distortion-kit/(2) -https://www.kaltecs.com/tda7293-composite-amplifier/(3) -https://www.ti.com/lit/ds/symlink/lm3886.pdf

author: moffy
author_rank: Super Contributor
timestamp: July 19, 2024, 06:27:09 am
content: There is a published paper:https://www.omicron-lab.com/fileadmin/assets/Bode_100/Articles_UseCases/Jan_Petrik/OpampSpiceModel.pdfin which the author, Jan Petrik, does detailed frequency response measurements of the TDA7293, I was able to take those curves and reconstruct a SPICE model following the advice in the article. I ended up with a reasonably close facsimile of the TDA7293 for modelling purposes, I didn't bother with the output impedance just the gain/phase/frequency relationship.

author: pet_rushi
author_rank: Contributor
timestamp: July 19, 2024, 06:33:02 am
content: Wow, that is great! Thanks

author: macboy
author_rank: Super Contributor
timestamp: July 19, 2024, 12:06:30 pm
content: Interesting.How are you measuring the distortion? At what power level and frequency?I have quite a few of these chips waiting for a worthy project.

author: pet_rushi
author_rank: Contributor
timestamp: July 19, 2024, 06:38:01 pm
content: I measured the distortions using my own setup:1. this oscillator -https://www.kaltecs.com/ultra-low-distortion-oscillator/2. -70dB active 1kHz notch13. low-noise 66dB gain amp4. -70dB active 1kHz notch25. low-noise 60dB gain amp6. 60/120Hz -30dB passive notch7. FFA oscilloscope (BW-10kHz)8. Excel spreadsheetThe measured distortions -> -116.5dB +/-2.5dBA friend of mine measured the distortion in a local college lab (BW=22kHz AP2700) -> -114.5dB

author: quadtech
author_rank: Regular Contributor
timestamp: July 20, 2024, 03:59:40 am
content: These threads may be of interest -https://www.diyaudio.com/community/threads/high-loop-gain-composite-op-amp-circuits.45794/https://www.diyaudio.com/community/threads/my-audiophile-lm3886-approach.54571/https://www.diyaudio.com/community/threads/miniref-schematic-and-pcb-layout.184165/page-11#post-7009480

author: pet_rushi
author_rank: Contributor
timestamp: July 20, 2024, 06:07:08 am
content: I know all those threads.Diyaudio.com banned me for no reason when I posted this circuit there.I'm sure they have financial interest in some of the amplifiers discussed there and don't allow anything to interfere to that.Very, very sad!

author: pet_rushi
author_rank: Contributor
timestamp: July 23, 2024, 12:15:02 am
content: Anyone willing to design a PCB?

author: moffy
author_rank: Super Contributor
timestamp: July 23, 2024, 12:55:30 am
content: Quote from: pet_rushi on July 23, 2024, 12:15:02 amMaybe, what schematic?