LM3875 dual mono kit - a few questions.

[Peter Daniel]

Tighter tolerances have no effect on DC offset or quality of the sound. They will only influence the matching of both channels (of gain for instance).

[quickshift]

Originally posted by Peter Daniel
Tighter tolerances have no effect on DC offset or quality of the sound. They will only influence the matching of both channels (of gain for instance).

OK, cheers.

That's saved me a few quid.

[Vikash]

Hi quickshift, offset on one of my amps is 120mv and I have no real issue with that.

To quote good ol' Carlos:

To avoid high values of DC-offset, sometimes very different from channel to channel, you need to use the Ci caps (Look at Fig. 1 on the LM3875 datasheet).
Good electrolythics bypassed with quality poly 0.1uf caps will bring DC-offset to a couple of mv, without affecting the sound.

more here: http://www.diyaudio.com/forums/showt...773#post469773

[quickshift]

Hiya,

Ah! Now I know where I know your name from.

Excellent, thank you.

Sounds like a plan, I'll have a go.

Cheers,

Q.

[peranders]

Originally posted by quickshift
I have measured it with the inputs shorted, the high side reads 57.6mV and the low side reads 13.1mV. If it's down to the chip I'll get a replacement, would it also be worth getting premium components with tighter tolerances?

In other words you have 1.8 mV and 0.41mV in input offset voltage (pretty good) but you have also a current flowing into the non-inverting input of the amp (see datasheet about input bias current). It's this current which adds on the extra offset.

[quickshift]

Originally posted by peranders

In other words you have 1.8 mV and 0.41mV in input offset voltage (pretty good) but you have also a current flowing into the non-inverting input of the amp (see datasheet about input bias current). It's this current which adds on the extra offset.

Thank you.

OK, that's not so bad then.

I'll put the fire extinguisher away now.

Actually, joking aside, such is my faith in my abilities with electronics that the first time I switched the amp on I used a bit of wood at arms length while holding a fire extinguisher across my face in case it caught fire or exploded. Mind you I made the loom for my track bike but that's just wires so it's easy.

Thank goodness people have had the time, skill and good sense to make kits so people like me without the ability to create an amp from scratch can also enjoy them.

Three cheers for the kit suppliers

[quickshift]

Hello again.

Right, I have looked at the diagram, it's very nice but I don't understand it.

I have looked up bypass caps and it says connect the cap between the power and the main ground - that seems simple enough in itself.

I'm assuming I need to buy 0.1 uF poly caps and 22uF electrolytic caps - one per amp board but what do I do with them?

I have the rev 3 boards and haven't implemented the zobel.

Sorry for not really understanding much about all this stuff yet.

[quickshift]

Right, I have worked out where to put the 22uF caps - I unsoldered ground side end of R3 (681 ohm) and lifted it from the board a bit, popped the - of the cap in the hole and then soldered the free end of R3 to the + side of the cap.

Results...

DC offset is now 1.7, 0.5, 0.5, and 0.5 YAY!!!!

I did 1 ch first and had a listen for a bit, adding the cap has further smoothed the sound, there was a tiny touch of grittiness or edge on the channel without the cap compared to the channel with the cap, I didn't notice it before but when switching between l + R on the preamp it was obvious. It's certainly made an improvement to the sound without any negative side effects. I'm assuming this is because the speakers have to work against the DC as it will slightly push the cones and offer more physical resistance in one direction of travel.

Many thanks for the info and help everyone.

I have also made the caps and snubber section of the carlosfm PSU (using a diagram!) from the decibel dungeon site (excellent BTW) and will add that between the PSU and amp boards next.