| Beggar |
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 :D |
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| zeno |
I have built NIGC and IGC, both have the same problem. NIGC on the boards of Brian.
Maybe a zobel stops this????
Marc |
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| peranders |
| Have you checked how the power supply voltage looks like? It may have something to do with the undervoltage protection. I'll believe that this protection has certain conditions in order to work well... my amps are completely silent.... |
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| Upupa Epops |
| Which voltage have you on rails and which value has resistor on mute pin ? |
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| peranders |
| LM3875 hasn't got a mute pin :( |
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| Bas Horneman |
| Isn't that quite common behaviour? Most amps do that...like the caps that discharge...and then give a final effort...after which they are really discharged and the amp is then silent.... |
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| Beggar |
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 |
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| Beggar |
| quote: | Originally posted by Bas Horneman
Isn't that quite common behaviour? Most amps do that...like the caps that discharge...and then give a final effort...after which they are really discharged and the amp is then silent.... |
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 |
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| 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 |
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| Peter Daniel |
It is not a common behaviour and the amps should be completely quiet when switching on and off.
It is probably cased by ground and power connections in your amps, try to change them. |
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| Beggar |
| quote: | 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..... |
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| 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. :D I will know more than.
Marc |
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| 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" |
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| 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??:confused: although it would seem to hint that its a difference in the 3875's rather than the power supplied to them
Nik |
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| 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 |
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| 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 |
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| 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 ;) . |
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| Peter Daniel |
| quote: | 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??:confused: 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). |
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| Upupa Epops |
| Yes, Per have thruth - help shorting of input by switch off ;) . |
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| 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 |
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| Upupa Epops |
| Peter, it IS normal behaviour and you will observe it in case, when you have large value PS capacitors - when you use only 1 G cap, all process will be too short for " hearability ". |
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| peranders |
| Pavel, the pictures show my QRP01 with LM3886 and from what I know the undervoltage protection is the same...but it may be a difference that I have a buffer.... |
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| Upupa Epops |
| You can make the same courses with auxiliary circuits connected on input ;) ( with LM 3875 ). |
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| Beggar |
thanks everyone for replying :)
Whatever the reason for it doing this, it doesnt seem to affect the quality of the amp, so for now at least i think im going to live with it, also as i intend to use these amps with a cross over (buffer) on the same power switch it maybe that i never notice this 'problem' again. The only thing that worries me is that the cross over turn off transient may be played through the tweeters of my active system, but i guess with rail voltages of 3.5v i dont have too much to worry about :)
Thanks everyone... also look out for pics of my finished system which fingers crossed should be done tonight or tommorow :D |
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| Beggar |
Hi peter and kurt,
The caps are new (ones that came with the basic kits), so i dont think there's anything more i can do in the cap department,
I tried increasing the value of Rb to 1k22 however this made no difference, i think this is a different problem however as it occours when the supply voltages are >3v
Thanks. Nik |
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