Hi everyone, sorry if this has been asked before, but i really couldnt find anything.
Ive just built 4 GC based on the BrainGT PCBs, and on 3 of the 4 amps i get a strange phenomenom, where after power is removed the amps mute, however about 3-5 seconds later the amps then breifly (less than a second) unmute and play a distorted signal and then turn off properly.
I'm not even sure if this is a problem as such, and im confused why it happens on 3 of 4 "identical" amps, just wondered if anyone has this happen to their amps, and if so how they did, or did not fix it.
Thanks, Nik
Ive just built 4 GC based on the BrainGT PCBs, and on 3 of the 4 amps i get a strange phenomenom, where after power is removed the amps mute, however about 3-5 seconds later the amps then breifly (less than a second) unmute and play a distorted signal and then turn off properly.
I'm not even sure if this is a problem as such, and im confused why it happens on 3 of 4 "identical" amps, just wondered if anyone has this happen to their amps, and if so how they did, or did not fix it.
Thanks, Nik
hello
I have 26.8v on both rails, its very strange though, i am running 2 transformers, each powering 2 amps each. On one of the transformers, one of the amps makes the coming back to life problem, where as the the other does not. Im going to try and see what the rail voltages are when the problem occours, although with only a DMM i think this is going to be hard to do.
Nik
I have 26.8v on both rails, its very strange though, i am running 2 transformers, each powering 2 amps each. On one of the transformers, one of the amps makes the coming back to life problem, where as the the other does not. Im going to try and see what the rail voltages are when the problem occours, although with only a DMM i think this is going to be hard to do.
Nik
Bas Horneman said:
Hey
Im begining to think it may have something to do with the caps, maybe as the thing approches a certain level of offness the amp actually 'thinks' its on again, maybe as the voltages on the rails become within some threshold of each other? Im not really sure, the main issue for me is if this is normal behaviour or my 3 of 4 amps are an exception?
Nik
+/- 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
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
second off
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"
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"
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
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 themNik
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 .
.
Beggar said: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??
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).
Do you have RB at the input?
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
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
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