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Old 21st February 2008, 10:41 PM   #1
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Default + supply volts on my speakers,help!

I have just made an amp using LM4766T.Made it exactly to the datasheet except I added 1u caps on inputs.Using a sig gen with only one output(1khz 200mv sine) I fed the signal to each input in turn.What happens is this.
1 Input on R,No output from R speaker,no dc,nothing.
2 The L speaker(with no input signal)goes up to full plus supply voltage with heavy ripple,same heavy ripple on plus supply,negative supply is ok.
3 If input signal is applied to L input,same problem occurs on R output.
4 One test sig applied to both inputs,no dc on speaker outputs and no tone output even when test sig. is increased.

The amp is only on for a couple of seconds at a time while I troubleshoot as I know this will fry speakers.LM4766 stays cool all the while.Supply is +/- 32VDC,6800u smoothing caps and transformer are from the dvd player I got the ic's from.
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Old 21st February 2008, 10:50 PM   #2
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mute pin mute be connected to neg supply thru 10K resistor. Add a resistor from + signal to ground.
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Old 21st February 2008, 10:53 PM   #3
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#1 You can't leave the mute pins unconnected.

#2 How did you actually build it? Did you make all connections to both A & B channels per the datasheet, or did you follow your hastily drawn sketch? For example, in your drawing, the only connection to the chip's B channel is the Vcc connection on pin 15. If you only connected Vcc and not Vee for the B channel, that would explain your problem.
The whole problem with the world is that fools and fanatics are always so certain of themselves, and wiser people so full of doubts. Bertrand Russell
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Old 21st February 2008, 11:58 PM   #4
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I built both channels but drew only one for clarity.Pin 2 and 15 are + supplys for both channels but this ic only has one negative supply pin(pin 4) for both channels.
I will try the mute pin thing but I am sure when I checked the pcb the ic's came from the mute pins (6,11)were just shorted together.
Thanks for your advice guys.
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Old 22nd February 2008, 01:49 AM   #5
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Where is the input-to-ground resistor???
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Old 22nd February 2008, 04:51 AM   #6
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when i built my previous one , I was stupid enough to have my +Vcc touch the speaker out pin.

I built it on vero board, and a pin pushed through the isolation of the +vcc wire.

check and double check for shorts everywhere. Especially when you are using veroboard or point to point.
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Old 22nd February 2008, 04:26 PM   #7
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I just checked the datasheet again and this time READ the application notes,not just the diagram.
The diagram shows the mute switch as normally open and I assumed this was un-muted.I was wrong as switch is closed for un-muted operation.Silly me.
Does the input to ground resistor go directly from + input pin to ground or from between input cap and input 1k resistor?
Is the absense of this resistor the reason for unused input making the corresponding output "go high"?
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Old 22nd February 2008, 07:27 PM   #8
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Connected mute pins 6,11 together and grounded them through 12k resistor.
Connected 47k resistor from junction of input caps and resistors to ground.
Everything works fine now,thanks for the pointers everyone,no doubt I will be posting again here soon.
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