LM3886 silent? (Ive searched many threads for a fix)

[audiomodder]

hey all,
I know there is a bazillion topics on this chip but I've searched through dozens and dozens of threads and web pages and cannot find a fix for mine. There is *very* little info on the single supply setup which is what I am working with.

I attached the schematic I built, which is the same as the official application minus the optional components. However, I do not understand the Rm equation very well since I am not using VEE...I've tried 10k and 22k values but nothing works. I'm using a 24v power supply.
The base of the transistor has a 12v bias (which I also read on pin 10), the output (pin 3) reads 23v. But all I get is a loud pop when I power it up and no sound at all. Any ideas?

[kevinkr]

You do know that Ci is not optional in a single supply application despite the asterisk next to it? (Omitting it will result in a dc gain of 21 instead of unity, and the output stage will slam to the positive rail.) Hope this is the issue with yours. I've used these in a number of applications with bi-polar supplies and have found them to be pretty trouble free..

[Bill_P]

For single ended operation Rm<(V7-2.6)/I8 where V7 is the voltage at pin 7. I8 is 0.5 mA or more. For your example V7 is about 11.5 and if we choose 1 mA for I8 then Rm is 8.9K. Since I8 can be as small as 0.5mA, Rm=10K should work. Cm creates a time delay and for now I would remove it until your problem is sorted.

I don't understand your DC voltage readings. Pins 3, 9, and 10 should all be the same, around 12 volts. It might be worthwhile to measure the DC voltage at all the pins and post the results here.

[audiomodder]

Thanks for the quick replies! I am using a 10k from pin 8 to pin 4 without Cm. Thanks for the info on Ci, it looked necessary so I included it (weird they have that asterisk there) Here are the voltage readouts I am getting:

1- 24v
3- 23v
5- do I connect Vin here?
7- 12v
8- 0v
9- 23v
10- 12v

good to know I'm looking for 12v on pins 3 and 9...I wonder why they're different? Maybe the chip is blown
Also the schem doesn't show pin 5 but in the first schem on the data sheet its connected to Vin...I tried this but got the same results.

[Bill_P]

Besides pins 3 and 9 being the wrong voltage, pin 8 should be around 9 volts, not 0. Unless there is a wiring error, the chip may be blown. Turn the amp off and try measuring the resistance from pin 3 to pin 1 and then from pin 8 to pin 4. If either or both readings are near zero the chip is probably bad.

You can connect pin 5 to Vin but the datasheet indicates pins 1 and 5 are both Vin. From that it seems either or both pins connected to Vin is ok.

Ci is an optional component only for + and - supplies, not for a single supply. It should not be listed as an optional component for a single supply application.

[audiomodder]

So I checked for shorts and there weren't any however, I desoldered the entire chip and just tried plugging in the supply voltage and behold- 23v on pin 3. So it would short but only on startup.

Fortunately I have another LM3886 here that I swapped in-- and it works! and boy does it sounds great... only one small problem and that is that is the output seems far below what it should be...is there any way to adjust the gain or is the output for a single 25v just really low?

Thanks for all your help!

[Bill_P]

The gain is 1+Rf1/Ri which is 21. You can reduce Ri to raise gain but maybe the real problem is your input voltage is low.

With Vin at 24 volts you will only get 5 watts or so output, depending on the load impedance. If your speakers are not efficient that will not drive them very loud.

[audiomodder]

I tried reducing Ri to 680ohms but then the bass section began to get distorted...

Only 5 watts output though?? I thought this amp could pump out a lot more than that, even at low voltages....I was looking to replace my LM4752 which does 11W per channel on a 24v supply.

Unless there is a way of reducing the distortion on higher gains this sucks
Very nice sounding chip but maybe it is only for really high volt apps :/ Maybe I'm dreaming too big with my 12v battery converted to 24v 3A output.

[Bill_P]

Increasing Ci will bring some of the bass back. I'm not familiar with the LM4752 - maybe I will get some time to look it up tomorrow.

[audiomodder]

Thanks for the Ci tip, that seemed to help. The LM4752 is really an interesting one, single supply only though, but I've enjoyed it for quite some time because it sounds very nice. The 3886 is better though, and if I can get 2 of them into my setup and the gain adjusted better I'll drop my LM4752. Is there no way to get more power out of it at 24v?