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Old 17th May 2012, 11:29 PM   #1
Kei is offline Kei  Wales
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Default LM4562 Oscillation

I was advised that LM4562 were the best choice for the preamp in my headunit. I have fitted them today and although they sound excellent i am pretty sure they are oscillating as they are running very hot. (just shy of 60 degrees celcius) This is the circuit diagram for where they are at present. I don't know whether the opamp on the tone control is also getting hot as it's inaccessible whilst the unit is on.

Click the image to open in full size.

A common mod i've seen done is to replace the 10uF capacitors (C450/457/470/477) with 47uF. Would i be better off trying to find a more suitable opamp or should i try to get this one to work properly?
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Old 18th May 2012, 12:28 AM   #2
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I doubt changing those caps will help. The circuit is already bypassed, though I don't know where the caps are located The 4562 is fast and the bypass caps need to be very close. Though one of the best out there, if the circuit isn't laid out well or has some quirk that leads to oscillation, it's not a good choice of part. If you have a scope and some time to experiment, you might sort it out with bypasses located closer, or some series resistance in a particular location. If not, a different part might be better.
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Old 18th May 2012, 02:05 AM   #3
Bonsai is offline Bonsai  Taiwan
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It's a unity gain stable device. If the layout is decent and suitably decoupled, your problem is in overall loop compensation - and that's not the opamps fault.

Can you post a scope shot of the output - we may be able offer more advice looking at it.

BTW I assume the devices are in the right way around ;-)
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Old 18th May 2012, 08:02 AM   #4
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This excerpt is from the LM4672 data sheet pg 24 top

The LM4562 is a high speed op amp with excellent phase margin and stability. Capacitive loads up to 100pF will cause little change in the phase characteristics of the amplifiers and are therefore allowable. Capacitive loads greater than 100pF must be isolated from the output. The most straightforward way to do this is to put a resistor in series with the output. This resistor will also prevent excess power dissipation if the output is accidentally shorted.

I dislike schematics with 'pictorial' ICs rather than the conventional triangle symbol as it's difficult to envision where the output is. Your circuit has LOTS of outputs directly driving the next stage. Some strategically placed low value resistors (27-100 ohms) on each output directly at the pin may very likely settle it down. Improved bypassing would be a plus.

G
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Old 18th May 2012, 08:24 AM   #5
Kei is offline Kei  Wales
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Devices are the correct way around. Sadly i don't own a scope and can't actually prove they are oscillating but they are running far too hot compared to the original devices which ran almost at ambient. (the originals were 3x uPC4570 and 2x NJM4580. I ran this with the NJM4580's swapped out for OP275 from a japanese version) Adding components to the circuit isn't really possible as the circuit is too small and cramped to make it possible. This is the pcb design we are dealing with. Its a high voltage preamp for a car radio. (4V out and 9V maximum unclipped with EQ)
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Old 18th May 2012, 09:13 AM   #6
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Hi, please try the smallest 10 nF cap you can find with the right voltage specs directly from the + to - pin of the opamps. So directly under the PCB or above the opamps. Prefab the caps lead wires with pliers so that lead wires are very short.
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Old 18th May 2012, 09:24 AM   #7
PChi is offline PChi  United Kingdom
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There might be nothing wrong. The LM4562 quiescent current is quite high at typically 10 mA. With a + 15 V / - 15 V supply the power dissipation is 300 mW so I would expect an SO8 package to run warm to the touch. If you were here in England in the spring that we are having any heating is a bonus.
I haven't compared the quiescent currents of the parts replaced but they are likely to be lower.
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Old 18th May 2012, 11:20 AM   #8
Kei is offline Kei  Wales
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The operating current of NJM4580 is typically 6mA (max of 9mA), uPC4570 is 5mA (max of 8mA) and OP275 says max supply current of 5mA.

Jean-paul is that 10nF across pins 4 and 8? I don't have any 10nF capacitors in my collection. Closest value i have in any quantity are 6.8nF, but they are quite large styrol caps. (7mm leg spacing and the legs are 5mm long) Only other values that i have lying around are 1nF, 47nF, 56nF and 100nF.
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Old 18th May 2012, 01:54 PM   #9
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film caps won't help anything. Too much inductance. Electrolytic caps won't help anything, too much inductance. You need to put 0.1 uf ceramic caps between 8 and 4 of the back of the socket (power pins) and you need to put about a 22 pf ceramic caps between 7 and 6 and also between 2 and 1, Unless they are a direct short. I did this on my ST33078s, killed the oscillation. The way I knew I had oscillation before I put the scope on , the power amp fan was running full blast. The op amps were in the disco mixer, had been slow old 4558s.
I actually put the 0.1 uf's about an inch away from each pair of op amps by drilling new holes in the PCB, but the nearness to the op amp is important. LM4562's are faster slew rate even than ST33078's.
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Old 18th May 2012, 02:28 PM   #10
kaos is offline kaos  United States
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Not sure I can reliably read the part number locations accurately, but theres some 56K resistors going to ground from pins 3 and 5 of the OP275 locations. Might want to try putting some 100 pF caps across those to see if it helps.
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