LM38xx : Left behind by all those chips ! :D

[youyoung21147]

Oh, I was just looking at the LM4780, which seems to be a double LM3886 in the same package.

I'm very interested in it because it has the good SNR ratio of the 3875 (97dB mini) and the driving capabilities of the LM3886.

My purpose would be to use it in parallel mode, the simplest config and it is said to reduce distorsion (is it true BTW ??). I assume the parallel mode cannot increase the noise, as only the currents (and not tension) are added, isn't it ?

I've also seen the bridged mode operation, it looks more complicated. Has it got advantages ?

Has anyone tried it this way ?



By the way, what do you think about putting 2 LM3875 or 3886 in parallel, connected on the same components ? In the LM4780 datasheet, they use separate feedback resistors, but if they both receive the same signal, why not connect them on the same components ?

[Nordic]

I don't mean bridged, but literaly stacking 2 and connecting their pins...

[youyoung21147]

OK thanks Joensd, I'll go for the -25 0 25 trafo, it was my basic idea.

I don't really want the watts, but want what the watts "reserve" implies : good damping factor, easy dynamics and lower distorsion.

Anyway, I'm sure I would destroy my FX-120 with 40watts !

Concerning "parallel" operation I can see two solution : literally "stack" 2 chips together, or use the National's recommended parallel operation, which is quite similar apart from the fact that both opamps have their own feedback network.

What solution would you recommend me ? I don't mind using more one than the other.

[goudekim]

I recently completed a 50Watt rms bass amp using the 3886. Had a little trouble with the tone controls, but it is all working now. This is my own design.

3886 is reasonably clean (I don't have good test gear except my ears). It does thermal shutdown altho there is not much hysteresis between on/off states.

This is one of the easiest chips I have used. Like any op amp, esp. power, be careful of layout and ground loops.

Other info:
+/- 36 volt power @ about 3 amps
preamp all TL082 opamp
tone ctrls each +/- 15 dB (more or less) bass, mid, treb
total gain is about 60 dB with 10dB gain across the power amp

If you want to know more, ask and I'll try to answer soon.

[youyoung21147]

I read carefully the AN-1192 from National and I think I will order the LM4780 because it is the easiest way for me to get virtually two LM3886 work together.

This IC has good SNR ratio though it still has this annoying mute function.

Moreover, if I order samples from National, I'll get the equivalent of 2 times more amplifiers

I assume I'll have to find the best association between bridge and parallel operation, or in the worse case let one half of the chip "idle".
It is not clear in the datasheet which association mode is the best : they show THD graphs for both but they are not taken in the same conditions. It sucks

I googled for LM4780 bridged and paralleled and found some designs, but I couldn't find anything that would look like pin-to-pin stacked amps !!

Goudekim : you use your amp for a sub don't you ? Did you compare it to more "solid" amps ? Do you think it can control a big driver firmly ? What speaker do you have on it ?

[dariusku]

& tried LM1875 and LM3875 chips (24+24V Toroyd toroidal transformer,2 bridges MUR820,Elna Silmic 1000,0 or batteries 12V 7,2Ah(2 or 4,resistors Caddock MP930 and Allen bradley).The best results & achieved with LM1875 NIGC,3k and 130 ohm Allen Bradley,10,0 uF BG-N,till now Alps RK27112 40k pot(not the best solution).LM1875 sounds much cleaner,natural than LM3875.With standard resistors(22k and 680) and 4 batteries sound is brighter but & feel a lack of liveness.LM3875 is a little more dynamic,more bass and is more suitable for rock lovers.For jazz.classics LM1875 is the best.

[youyoung21147]

Thanks a lot for your report !

I assume the components are :
- 10mfd for the input decoupling
- 3k for the feedback resistor out->in
- 130R for the feedback loop to ground

What are the values used for the input resistors ? Did you use a buffer ? Is there a resistor between the input and the source ? Are there any decoupling caps apart from the input one ?

What components did you use on the LM3875 ?

[dariusku]

& connect a potentiometer directly to input(the best value 10k-20k).As signal source & use upgraded Marantz cd43 without decoupling caps(10 mV DC at the output).So there are no decoupling capacitors in signal path(& don't like them).With my loudspeakers(Seas,91db) in normal volume dc at the output don't reach 50 mV.If uou need decoupling capacitor BG-n(not less 2,2 uF) is good choice.LM3875 sounds better with small resistors and 2* (25-30)V DC.

[goudekim]

I use the amp for a bass guitar. The driver is a 12inch (300mm) cheapo. If this works, you should see attached photo.

[goudekim]

Here's another photo of the box.