24-11-2016, 07:18 AM
I've been working on a big 900w amp for a while. I've got it working but it has a few niggles, one being a stuck on "signal present" LED, see schematic. Pin 6 comes from a relay which is normally open, but is closed by a 555 timer circuit. This connects the amplifier OP to the spkr terminals. I'm in the process of seeing if the relay is stuck on, but I think not.
I'm a bit uncertain as to how the circuit works. When the relay sw is closed a voltage would be present on the inverting IP of IC6 (TL071) pin 2 I suppose. This "turns on" the op amp leading to a current on the base of Q24, Q24 turns on lighting D24. AFAIK, an op amp does what it can to keep the IP's the same, so I'm confused by the non inverting IP being grounded. I presume D20/21 are for protection and see there's FB from the emmiter of Q24. So how does this circuit work? What does C40 do, is this Q bootstrapped? I don't understand how this circuit can cope with up to 425w from the amp OP. Q24 is a BC517 darlington, I presume needed as D24 is a big LED so needs more current. I've checked it, it's ok as is the op amp and all R's.
I also have a query regarding the quiesent current of the amplier. It says to "Adjust VR1 for 0.560v across R35 (TP1)" I measured up to 400mv at TP1, adjustment wouldn't go higher. At 400mv the amplifier got VERY warm; 160mV seems more sensible, a typo perhaps or am I doing the adjustment wrong? Thinking about should I have shorted the input? R22 on the schematic is VR1.
Andy.
I'm a bit uncertain as to how the circuit works. When the relay sw is closed a voltage would be present on the inverting IP of IC6 (TL071) pin 2 I suppose. This "turns on" the op amp leading to a current on the base of Q24, Q24 turns on lighting D24. AFAIK, an op amp does what it can to keep the IP's the same, so I'm confused by the non inverting IP being grounded. I presume D20/21 are for protection and see there's FB from the emmiter of Q24. So how does this circuit work? What does C40 do, is this Q bootstrapped? I don't understand how this circuit can cope with up to 425w from the amp OP. Q24 is a BC517 darlington, I presume needed as D24 is a big LED so needs more current. I've checked it, it's ok as is the op amp and all R's.
I also have a query regarding the quiesent current of the amplier. It says to "Adjust VR1 for 0.560v across R35 (TP1)" I measured up to 400mv at TP1, adjustment wouldn't go higher. At 400mv the amplifier got VERY warm; 160mV seems more sensible, a typo perhaps or am I doing the adjustment wrong? Thinking about should I have shorted the input? R22 on the schematic is VR1.
Andy.
