roccus
Chief Journeyman
Going back to this wouldn't it be a caps job to filter subsonic frequencies out?
DCP going south??
- Thread starter roccus
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Going back to this wouldn't it be a caps job to filter subsonic frequencies out?
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On Dons board it b does not act like a filter, it's acting as a trigger, in conjuction with the 1237. In essence it acts as a cutoff point, so yeah its a filter....but with caps they work, or they don't (generally). This, being intermittent , I would check all solder joints on the ic and wiggle it during operation...
roccus
Chief Journeyman
I'm pretty sure the DCP is all good at this point... I'm going back to the beginning and having a look at the U1 chips looks like that is the first thing that gets hit with the input I'm wondering if that opamp can magnify subsonic frequencies and send them down the line if damaged
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Some amps actually amplify from DC to above 80khz. The opamps on the driver boards will pass on anything that comes to em. Dons board is only active on the output...
roccus
Chief Journeyman
The power going into U1 is DC? And don's board gets it's signal from the U1 opamp pin 2?
If you give the amp D.C., it will pass it through. It won't amplify it but will pass it to the output.
roccus
Chief Journeyman
The power going into U1 is DC?
Hi Ray
U1 is indeed powered by DC, +15V and -15V to be exact.
U1 will amplify anything put on its input be it DC or AC up to 8MHz.
Your preamp should AC couple the signal into the amp if you do not wish it to pass DC. That means it should have a high quality film capacitor on each of its outputs to block any DC that may be inherent in the design of the preamp from reaching the next amplification stage (namely the PL400 power amp).
roccus
Chief Journeyman
OK.... so if U1 gets damaged can it leak DC into the output and if it can, can it be intermittent? I don't know a lot about op amps except usually when they are on the way out they can cause distortion and noise at an output I'm not getting distortion and noise
That is not what I said Ray.
I said if you give U1 DC, it will amplify it...
If U1 was bad, you would know it in the audio
roccus
Chief Journeyman
I kind of got ya Joe U1 is powered by DC voltage but the input on U1 should be AC and there is no way the DC power could ever get into the output. Also an opamp could not take a low frequency input and amplify it to a sub sonic frequency that the dcp could see as DC? ... I'm thinking it is not the preamp... unless I just happen to have 2 preamps that went bad at the same time
It will amplify subsonic frequencies just fine Ray. It will amplify all frequencies right down to DC. The amp is very capable of amplifying signals too low for you to hear.
Folks here are telling you that if you have very low frequency input to the amp, you can trip the DCP.
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Hey Ray. What Joe is saying is if you do have a DC level on the input to the 400, since all of it is direct coupled then essentially that DC is going to appear on the outputs of the amp. Kind of like the dreaded "garbage in/garbage out" axiom of computing. Some preamps like those Adcom GTP's have two outputs, one labeled as "lab" and the other "normal". The difference between the two is the one labeled "lab" does not have a capacitor sitting between the preamp's output and the RCA jacks, but the one labeled "normal" does. The capacitor's purpose for being there is to effectively "block" the DC level if there were any.
Perhaps a good way to check yours of this condition is to take a DC volts reading with the preamp connected but measure it between pads 1 and 2, left and right, on the PL14_20 board. No music.. preamp at idle. If you see DC there, it's the preamp.
Billboard
Journeyman
Ray, at this point I would recommend that you "connect" with one tech on this board, Lee, Joe, Don, or someone else. Then do exactly as they say, nothing more.
Go back and forth with ONE person and they will get the amp fixed, I do not recommend changing parts in a shotgun approach. I wish I was closer, I would be happy to help you out. I am on the Mass NH border, still a haul to get to you.
Bill
Go back and forth with ONE person and they will get the amp fixed, I do not recommend changing parts in a shotgun approach. I wish I was closer, I would be happy to help you out. I am on the Mass NH border, still a haul to get to you.
Bill
roccus
Chief Journeyman
I will take a look at that tomorrow I was looking at that earlier today but with music playing pad 1 and 2 L and R were showing some DC very low something like ,012mv DC as I recall
roccus
Chief Journeyman
Thanks Bill yea you are about a 2 1/2 hour treck away we moved up here from Epping NH I'm from Salisbury Newburyport area
roccus
Chief Journeyman
Did the DC voltage reading with preamp on but no music or anything turned on connected to the preamp doing DC readings was all over the place both channels I also did DC and AC reading on the outputs at the speaker terminals
Left pad 1&2 bouncing between 2.10 & .055 mv DC
Right pad 1&2 bouncing between 2.80 & .600 mv DC
Left output (@ speaker terminal) bouncing between 2.10 & .148 mv DC and 7.501 mv AC
Right output (@ speaker terminal) bouncing between 2.80 & .139 mv DC and 7.488 mv AC
Left pad 1&2 bouncing between 2.10 & .055 mv DC
Right pad 1&2 bouncing between 2.80 & .600 mv DC
Left output (@ speaker terminal) bouncing between 2.10 & .148 mv DC and 7.501 mv AC
Right output (@ speaker terminal) bouncing between 2.80 & .139 mv DC and 7.488 mv AC
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What meter ya got Ray??
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Pre-amp output turned all the way down..
roccus
Chief Journeyman
Yes Lee this is with preamp turned all the way down right now I am using the PL 3000 for testing only have an XM receiver and a TT connected to it power was off on both of those... my C-1 right now is hooked up in my main system controlling my stock 400 till I can put the wopl back...
I have a Fluke 189 true RMS multimeter....
