It didn't explode....... YET (aka niges amp)

[jkeny]

Thinking about the distortions that we heard on this amp running into ESLs in Tony's last Sat, I wonder are you using the compensation capacitor across the inputs recommended in the LM3875 datasheet? It is intended to reduce the AC gain at higher frequencies - which seems to match our interpretation of what we were saying -clipping/distortion at HF?
If it's based on Peter Daniel's LM3875 kit, it doesn't look like he used this cap?
Worth a try.

[nige2000]

Did some investigating at frans couple days ago with 57s
Primarily the buffer can't take anything above 1v input without distortion
So needs revision
Confirmed the ac coupling output cap is loosing some detail and Hf
Distortion was not a trafo fault

All is not a problem if I stick to original design
So think I'll do that for GB

[jkeny]

Good work - glad you got to the heart of the issue

[nige2000]

testin battery pack all looks good :)

[Fran]

looks very neat Nigel.

[nige2000]

looks very neat Nigel.

a friend called over as i was assembling it
i couldn't help demonstrating what happens if you short across the output posts with a piece of wire
well there was a flash, a bit of smoke as the wire was evaporated and a very startled look of the rabbit in the headlights

he stated "you could weld with that fxxker "
i think he shouldn't be giving me ideas
coz im tempted to try it

[tony]

a friend called over as i was assembling it
i couldn't help demonstrating what happens if you short across the output posts with a piece of wire
well there was a flash, a bit of smoke as the wire was evaporated and a very startled look of the rabbit in the headlights

he stated "you could weld with that fxxker "
i think he shouldn't be giving me ideas
coz im tempted to try it

Once it don't fry 57's that will be fine for me.

[nige2000]

the amplifier chip this amp is based on lm3875 is discontinued and is no longer available from the usual reliable suppliers farnell, mouser, digikey etc
although i already have enough of them to fulfill the gb
i thought it would be wise to stock up some spares from other sources before they get particularly hard to find so i ordered some

why not use a different chip?
i tested lm3886 chip which is a close relative to lm3875 in the same circuit implementation
its not near as transparent as the 3875 and sounds quite ordinary...... at least in that implementation
the gap is quite large, so im not taking any chances with other chips

the rest of the printed circuit boards are ordered
hopefully will arrive in a week or two

[rickmcinnis]

Could not believe my eyes about the LM3875.

Went MOUSER and DIGIKEY - looks like they are long gone.

When did this happen? Did not think they would ever stop making the LM3875.

Wonder if TI has something better in the works or was 3875 one of those brilliant mistakes?

[nige2000]

Could not believe my eyes about the LM3875.

Went MOUSER and DIGIKEY - looks like they are long gone.

When did this happen? Did not think they would ever stop making the LM3875.

Wonder if TI has something better in the works or was 3875 one of those brilliant mistakes?

the lm3886 is still active
ti killed off all other variants afaik
was the wrong one me thinks