LM3886: Questions about the LM 3886 single-supply application schematic in the datasheet

AndersG

Part Number: LM3886

1. Mute circuit - If the idea is to unmute after a time interval, then Cm will do exactly the opposite, right? It will be discharged initially and then create a current rush when powered on. Shold it not be connected to Pin 7 and not ground to work as advertised?

2. The circuit will exhibit quite a lot of (mainly 2nd order) distortion at low frequencies because of the Vcc/2 circuit, Ra, Ra, Ca. Putting a 100uF cap from Pin7 to ground helps somewhat, but parallelling Ca with 100uF lowers that distortion by a factor 10 as the capacitor value in that case is multiplied by the Hfe of Q1. Filtering there will also help getting a cleaner bias through Ra

Graph below shows measurements taken with a HP8903A on two PCBs. "Mine" is one that I did, following the ideas in the DiyAudio thread where Tom Christensen (Neurochrome) provided some excellent advice. The other PCB is a cheap one I got from eBay for prototyping. Vanilla is as is (schematic on page 6 of datasheet) and "ebay cap" is the same but with 100uF across Ca. The difference is quite big.

I used a single-supply because I wanted an SMPS, because the amp will sit in a small loudspeaker box on a pedestal and I wanted to keep the weight down.

Any comments??

over 5 years ago

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Anders,

can you show a schematic and a photo of your setup?

The LM3886 was not designed to be powered by a single supply voltage. It can be done if absolutely necessary, but I wouldn't do that.

Powering the LM3886 by a SMPS is no good idea, either. The LM3886 (as all other amps as well) needs a stable, clean and noisefree supply voltage. So, you should low pass filter the supply voltage a bit so that the SMPS's switching noise is hitting the LM3886 not all too much.

Have you thought about using a SMPS providing a bipolar supply voltage for the LM3886? But don't forget the low pass filtering...

Kai

0 AndersG over 5 years ago in reply to kai klaas69

Prodigy 80 points

The schematic is straight from the LM3886 datasheet, page 6:

My capacitor added in red. The measurements were taken with a lab power supply. A good SMPS can provide stable and noiseless power as well and at the fraction of the weight of a linear PSU. In this case it is a tradeoff. I need low weight and SMPSes with dual outputs, say +/- 18V are extremely rare.

So, yes, a single supply has it's tradeoffs, but there is no reason not to try to get the best possible result. The amp provides quite respectable values for THD with this simple modification.

0 Andy Liu (BMS) over 5 years ago

TI__Guru 52115 points

Hi Anders,

Check the following thread. Hope it would help you.

e2e.ti.com/.../388055

Andy

0 AndersG over 5 years ago in reply to Andy Liu (BMS)

Prodigy 80 points

Thanks, but it does not really answer my question. I know how the mute circuit is supposed to work, but from my understanding does not the mute circuit in the single-supply schematic not work as intended.

0 AndersG over 5 years ago in reply to AndersG

Prodigy 80 points

This is not a life-and -death issue for me, but I still feel that the LM3886 datasheet has incorrect info and would appreciate a clarification.

0 Andy Liu (BMS) over 5 years ago in reply to AndersG

TI__Guru 52115 points

Hi Anders,

LM3886 is a very old part and was originally inherited from National Semiconductor. To be honest, we don't have a very good knowledge base on this part either. I will see what I find to help you, to the best of our knowledge.

Andy

0 kai klaas69 over 5 years ago in reply to AndersG

Guru 140200 points

Hi Anders,

do you know this link:

www.circuitbasics.com/.../

Kai

0 AndersG over 5 years ago in reply to Andy Liu (BMS)

Prodigy 80 points

Yes, I know that the part originally came from NS. It is old, but still very much in use. Just look at the discussions on the DIY Audio forums. I appreciate if you can dig out something!

0 AndersG over 5 years ago in reply to kai klaas69

Prodigy 80 points

Yes. I have read all of it. It is a very good walkthrouugh, but does not really answer any of my questions. As indicated earlier was my main inspiration this www.neurochrome.com/.../ plus the associated discussion in the DIY Audio forum. The PCB layout does make a tremendous difference in THD.

0 Andy Liu (BMS) over 5 years ago in reply to AndersG

TI__Guru 52115 points

I went through quite a few existing posted e2e questions today and didn't yet find any good answer.
Also, I used to know one expert who knew the part well. However, he left a few years ago. I would keep looking for answers and let you know once I find them. Thanks.

0 kai klaas69 over 5 years ago

Guru 140200 points

Hi Anders,

1. Grounding CM at pin 7 of LM3886 instead of pin 4 in the single supply circuit should work. The only disadvantage I see is, that Q1 must deliver a more abrupt current change when de-muting. Especially in combination with a small CA this could cause an audible click. Here, the conventional circuit with the smooth current change:

And here, your circuit with the more abrupt current change:

anders.TSC

2. Increasing CA looks like a good idea when it comes to distortion and PSRR. But a big CA increases the time pin 7 needs to settle to the final value during power-up. Me thinks, that the designers of this chip wanted to keep this period short. This might be the reason why CA look so small. A fast settling time seems to be the more important the higher the supply voltage is, because this means a higher stress to the internal circuitry which is directly referenced to pin 7.

Kai

0 AndersG over 5 years ago in reply to kai klaas69

Prodigy 80 points

Thanks for taking the time to simulate this. I did the same a while back and remember I do not have a mute switch, all I want is mute when powering on and off. Here is my schematic with Cm (C3 here) connected to Vcc/2: