LME49810 pcb layout - Troubles - diyAudio
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Old 6th April 2011, 05:01 PM   #1
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Default LME49810 pcb layout - Troubles

Hello DIY'ers,

I've finally made my first try-out pcb for the LME49810. I tried just to apply 26-28 Volt to the board to check if everything was working out. But when I apply the supply I can read >20 Amps are being drawn from the supply (2 screw driver batteries). I don't know what's wrong. I've posted my schematic and I can take some voltage measures on different spots on the pcb if you need it. Nothing is burning out. I only recognize some heat from the output transistors, so I guess the problem lays there. Also when I connect a speaker it just make a little pop noise.

Regards,

Simon H.A.
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Old 6th April 2011, 05:54 PM   #2
svokke is offline svokke  Belgium
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Did you check if all transistors are isolated from the heatsink?
I'm intrested in making an amplifier with the same chip. Could you post a schematic, not a layout?
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Old 6th April 2011, 07:10 PM   #3
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Sorry for the late response, was away for a momement. Yes I've done a lot in making sure none of the transistors are short cutting each other on the heat sink. I bought some good heat pads @ Digi-Key.
Here is my schematic used for this board.

Edit: The potentiometer is 200 ohm
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Last edited by Cyberzim; 6th April 2011 at 07:13 PM.
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Old 7th April 2011, 04:23 AM   #4
sasmit is offline sasmit  India
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you're having some obivious mistakes in the design.

you're using multiple transistors in the output but do not have current sharing resistors, so one of the transistors in the output will end up doing all the work the rest will be just sitting there.

You need to have a Zobel on the output of your amp.

A small film cap like 0.1 uf across the collector and emitter of the Vbe multiplier is required.

If your POT is 200ohms then 1.2/(300 + 200 ) > 2 even when the pot is at it's full resistance you'll still be having a good amount of bias, instead use a 680 + 500 ohm pot

Last edited by sasmit; 7th April 2011 at 04:31 AM.
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Old 7th April 2011, 05:13 AM   #5
routhun is offline routhun  United States
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Hi Sasmit,

The schematic and the PCB has 0.22 ohm current sharing resistors from emitters to ground. Yeah this amp might be oscillating hence drawing lots of current.

CyberZim,

Disconnect the power transistors and leave the driver transistors and check the voltages. For temporary testing, also break the emitter resistor on the driver transistors into two equal values and take the feedback from these resistors. This setup will allow you to measure DC voltages without burning power outputs. The emitters of these driver transistors should have +0.6v and -0.6v (or even less) then you'll have an idea whether there are any faults in the layout.

If the voltages are good and large currents are drawn after connecting the power transistors back, check for oscillation. A zobel network help suppress the oscillations.

Thanks,
Routhun
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Old 7th April 2011, 05:36 AM   #6
Johno is offline Johno  Australia
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The voltage gain of the circuit is 150 (Rf/Rin) and is way too high for the 10p compensation cap. Cc should be silver mica by the way, not a Z5U chip ceramic. Recommended values are gain 30 (Rf = 6k8) and Cc 30p.

Therefore suspect your circuit is oscillating - hence the high current drain.

Query if the wiper of the pot is open circuit (it is on your schematic).

Cheers
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Old 7th April 2011, 09:13 AM   #7
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Okay, as far as I understand I have to do the following changes:
Replace potentiometer with a 500 ohm pot.
Also replace the 300 ohm resistor in the bias connection with a 680 ohm.
Replace Rf with 6k8 ohm.
Also when I have to test for the voltages I've added a schematic with all the things I think I understand what to do.
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Old 7th April 2011, 09:23 AM   #8
AndrewT is offline AndrewT  Scotland
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do not connect a speaker until AFTER you have proved that the amplifier is working correctly for all conditions.
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Old 7th April 2011, 09:41 AM   #9
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I'll remember that
But are the things I'm going to change for testing, correctly?
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Old 7th April 2011, 04:32 PM   #10
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I've changed the components and also cutted the power transistor off from the drivers. I get a -0,7 volt on the npn driver and 0,7 on the pnp driver transistor.
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