Briangt rev 2 LM3875 build advice

[ultrachrome]

I built two rev 1 LM3875 amps back in the day and was gifted a pair of rev 2 boards that I never built. I have most of the parts necessary to build these save for the transformer and I need an extra amp so I'm looking at building these.

Looking at chipamp.com on the Wayback Machine I came across the rev 3 / carlosfm unregulated PSU. I've come across a few variations of the schematic. Is this rev 3 PSU still the way to go?

My rev 1 builds are using 330VA 25+25V transformers. I don't currently have particularly difficult speakers to drive but would it make sense to use a lower voltage transformer for this build? With Avel Lindberg through parts-express, it seems my only other option would be 250VA 18+18V. Searching Digikey gives basically the same result.

Thanks.

 

[chermann]

Hi ultrachrome



You wrote that you have no "difficult" speakers. If you look at the Figure 27 in the datasheet you get an overview which maximum operating supply is recommended to use different speakers impedance.


so for 8R speakers its fine to use the maximum operating voltage (all diagrams are done at +/-35V into 8R)...dont use the maximum ratings!


for 4R speakers i would recommend to use your planed 18-0-18VAC transformer
to avoid the chip getting too hot and starts the current limiter....etc..


chris

 

[rabbitz]

I've always used a 18-0-18 transformer on these as I never know what speaker will be used. The lower voltage also reduces heatsink size and current requirements.

Looking at the Overture Design guide (used default standard component values):
35V rails 57W into 8R, 50W into 4R (IC current limited and required heatsink too large... see attached)
25V rails 29W into 8R, 47W into 4R

I've attached Rev 3 details but I've used my own power supply on the last build.

 

[ultrachrome]

Thanks. I forgot about that spreadsheet.

I also just found the carlosfm regulated supply so now I'm considering whether to do that instead. Approaching analysis paralysis.

 

[chermann]

Thank you Rabbitz


Where you get this excel sheet? i am searching around but without success


thx
chris

 

[ultrachrome]

This site apparently has a version.

A Complete Guide to Design and Build a Hi-Fi LM3886 Amplifier - Circuit Basics

 

[rabbitz]

Overture Design Guide attached zip file.

 

[chermann]

Overture Design Guide attached zip file.

thanks rabbitz- you are my hero

 

[ultrachrome]

I was looking at the attached schematic and don't understand why the positive lead of the bottom electrolytic cap is connected to the positive end of the bridge.

Is there an error in this schematic?

 

[Mark Whitney]

Yes, there is an error in the schematic. The diodes are wrongly drawn.

 

[rabbitz]

This is what I found in the instructions for Audiosector LM3875 kit.

 

[ultrachrome]

Thanks for confirming, Mark and Rabbitz.