Audio amplifier supply voltage vs. ouput power LM1875

A look at how changes in supply voltage can affect output power of an audio power amplifier. More can be said, but the video was getting long.

[@Elecifun]

I will rebuild my supply for my LM1875's using 12v 0 12v AC. I did use a 24v 0 24v and regulators but now realise now that's to much. Wish I had watched this video again before building. GREAT video's John, Thank you.

[@JohnAudioTech]

+Elecifun Glad you are getting the amp sorted.

[@Elecifun]

+JohnAudioTech Cheers. I have made the changes and it is much better. The amp runs cooler and no regulators to cool. I am quite impressed with the LM1875's such a nice punch for such a small circuit. Just got to figure this 12v 12AX7 pre amp out and build a supply for it.

[@KissAnalog]

Nice video a John!

[@omsingharjit]

Pretty Interesting video

[@adryanyork3581]

Thanks JohnAudio

[@Elecifun]

Are there major differences if using a single rail supply?. I have set up LM1875 with single rail at about 10.5 volts and am building a dual rail supply so I can try at + - 12 volts.

[@JohnAudioTech]

+Elecifun I assume you know the difference in components and setup if you used the single supply diagram example on the data sheet. The output coupling capacitor will roll off the deep bass if you don't select a large enough value. 10.5 volts won't get you a lot of power. +/- 12 will get you much more. Of course, that is 24v in single supply mode and will yield the same power..

[@Elecifun]

JohnAudioTech Cheers that explains it well for me. I look forward to your next video. I have tried to find a video on setting up both sides of a 12AX7 running at 12 volts and biased for mp3 type player input, with a circuit diagram. Would be happy to donate £10 to you for this ? Cheeky I know, but don't ask don't get ;)

[@Spentelectrons]

Im having a little trouble figuring what power supply to buy for a stereo lm1875 board. I have a split supply that reads +/-26V on my meter with only the meter as a load. When I measure the rails at idle on the board rectified and filtered it reads 70V at idle so i shut it off. I see that the lm1875 has a maximum rating at 60v. Im concluding that that would have to be 60v at idle and less then 60v at clipping. If i was to get say a 200volt amp torroid transformer what would be a good estimated voltage to purchase so that once rectified it reaches but doesn't exceed 60v at idle? I would like to push it to the limit. I have all genuine parts and realize listening to music instead of a sine wave and at less then 100% volume won't hurt it with it pushed to the limit in terms of supply voltage for a given load

[@JohnAudioTech]

For 8 Ohm loads, I'd go with a transformer with an 18-0-18 volt secondary 100va would be plenty for 2 channel. If your using 4 ohm speakers, use 12-0-12v secondary because you have to work within the current limit range of the output. Use 10,000uf capacitors to help with good dynamic power and don't skimp on the heatsink size.

[@Spentelectrons]

@@JohnAudioTech thanks John. Much appriciated. I'm going to have to seriously think about this. I have a nice rack mount case, massive heatsinks and a variety of speakers that are between 2 and 8 ohms. I started experimenting with series speakers splitting the power but increasing the total cone size. That's going well. I guess I don't know for sure my load yet and didn't want to leave power on the table.

[@Spentelectrons]

@@JohnAudioTech I finally understand. I expected the rails would sag more under load. The info was all their in your videos, I just needed to piece it all together.
Would their be any issue in using a 20-0-20 transformer rectified to 28.28v powering parallel 1875s. If I'm not mistaken this would allow 26v peak with 2.28v headroom. That would 18.382v rms over 8ohms is 42.237w. My idea is to have a voltage divider that is a 200k switched in series with a 1meg. This would attenuate the 18v rms down to about 16v allowing me to run it at 64w over 4ohms. My question is if their is a problem with too high of rails for 4 ohm load if the input is limited in amplitude to keep things in check?
I understand if i was to have the switch oriented wrong for a 4ohm load it would fail. This would be a more permanent setup with a fixed phone input and a fixed load depending on what speakers i decide to connect.

[@MrSamAli]

will u do a diy video on its power supply?

[@PetruV]

It is very easy to trip the current limit on speakers bigger than 6", My 6ohm 8" speakers makes the amp fart at high volumes.Worth mentioning that my supply voltage is +-24v

[@rich1051414]

That is everything to do with circuit design. It is possible the amp was only designed for 8 ohm loads or higher. It is simply bad design to use enough gain to push an amp into distortion on a load it claims it can handle.
This is assuming you aren't boosting the input voltage above input levels. That would force the amp to effectively have a higher gain than it was limited to.

[@amitghosh6966]

Please make a similar video on tda2030A too