LM3875 comes 'back to life' after turnoff transient suppression

[Beggar]

Originally posted by zeno
+/- 24 V AC, little bit more than 32 V DC.

I am not so sure that the caps can be the reason. Standard types (Philips), better ones (panasonic) and best (?) BG. All behave in the same manner. And it is only 1000mu.
Just to make it clear : I cut the mains power. The amp failrly quicly silences and than, all of a sudden, the sound comes back. Distorted.

It seems to happen when the caps are almost 'empty', so some voltage regulation activity could be the reason.

I do have the same behavior in four chips uptil now.


Marc

Yeah thats exactly the same thing as me, ive never had a problem with my LM1875's but these 3875s.....

[zeno]

The noise has (until now) only occured in test set-ups. No shielding, proper grounding etc..

I hanven't finished your immaculate chassis yet, Peter. I will know more than.

Marc

[SheldonD]

This happened to all of my 8 channels Brian GC, ( 2 amplifiers) This is only of course with input after power off. when I later added large caps at the rectifier I got as much as 18 20 seconds for the second "play" of a second or 2. I fixed this with a bleeder resistor across the rails. There seems to be 2 voltage levels that the 3875 will perform, standard full and a much lower one.

This was with 1500 pana caps at the chips.

Now using only 47ufd at the chips but 10,000 ufd at the rectifier and regulate with 338s there is no second "play"

[Beggar]

well ive been poking my multimeter around in places, and found some interesting but seemingly pointless results

On an amp that doesnt second off...
The +ve rail discharges much slower than the -ve and seems to sit around 5.1v
The -ve rail discharges faster, and seems to sit at around 3.5v

The same is true of an amp that does second off, however, when the negative rail is around 3.5v and the positive rail is around 5.5v it second offs, leaving the pos rail at 4v and the neg rail a 0v.

Right now im not sure what to make of this?? although it would seem to hint that its a difference in the 3875's rather than the power supplied to them

Nik

[coredump]

Beggar,

do you use additional caps? I remember having had similar problems using two additional caps (well, one has to try) with 10.000uF each.

Best regards,

Oliver

[peranders]

The best you can do is to track the supply voltages when you are turning off power. From this you can see something. Can you borrow a digital oscilloscope?

Here is a picture of my amp when turned on and off.

Positive voltage, signal out and negative voltage

Power on

Power off

[Upupa Epops]

All what you describe here is normal behaviour of typical amp which have not allsymetrical connection and you can't do nothing with this. All amps which have not output relay can cause problems by switch on or by switch off. If this amp have not problems with DC voltage on output by switching off, so let it be, 'cos I say again, it is normal process. Clear solution is ( if you can't let it be ) output relay, which immediately switch off output by switch off mains - but it will be quite different amp .

[Peter Daniel]

Originally posted by Beggar
well ive been poking my multimeter around in places, and found some interesting but seemingly pointless results

On an amp that doesnt second off...
The +ve rail discharges much slower than the -ve and seems to sit around 5.1v
The -ve rail discharges faster, and seems to sit at around 3.5v

The same is true of an amp that does second off, however, when the negative rail is around 3.5v and the positive rail is around 5.5v it second offs, leaving the pos rail at 4v and the neg rail a 0v.

Right now im not sure what to make of this?? although it would seem to hint that its a difference in the 3875's rather than the power supplied to them

Nik

Could it be that the caps discharge unevenly, which could be caused by not keeping the same charge, which causes inbalance in the rails (when switching off)? Are they new or used?

As I mentioned before, this is not a normal behaviour with those chips. The relay shouldn't be required (to mute those transients).

[Upupa Epops]

Yes, Per have thruth - help shorting of input by switch off .

[KCHANG]

Hi,

Just my guess of what might have caused the "come back to life after power down" problem. I don't know the circuit layout of your amps, so here is a question for you: Do you have a resistor in series with the input pin?

If you look at the suggested application on the first page of the data sheet for LM3875, this resistor is called "RB" and has a value of 1kohm. On page 6 of the data sheet is an explanation of why RB is needed : "Prevents currents from entering the amplifier's non-inverting input which may be passed through to the load upon power-down of the system. … This phenomenon occurs when the supply voltages are below 1.5V." This sounds like the phenomenon you are experiencing with your amps.

Ciao,

Kurt Chang