Weird amp problem

[DJduck]

Hey, I recently built an amplifier based on the AN7125 chip. I used the sample schematic included in the datasheet. I built it inside an old power supply case and it worked fine. Now I built a new case for it and had to take it apart, to put it into the new case. I put it all together and powered it up. There is no sound in either of the channels. Once I turn it off from the mains power switch and the voltage drops to 0v, the sound comes on for about half a second. I measured voltages on the transformer: the main supply coil reads 14v and the 5v coil reads 4v, -35v coil reads -35v (not used). Voltages seem fine after bridge rectifiers. I have no idea what the problem could be, it seems to turn on only for a moment when the voltages drop down before going to 0v. Ideas?
Datashit: http://www.alldatasheet.com/datashee...IC/AN7125.html

[Mrx3750]

Re: Weird amp problem

IC could be shutting off at higher voltages for some reason. Can you reduce the voltage going into it by half and see if it does anything?

[DJduck]

Re: Weird amp problem

I don't think that this is the case, since I used the same transformer before, and it is reading 14v but the maximum for the IC is 24v

[DJduck]

Re: Weird amp problem

All the components in my amp are the exact ones in the diagram, except for the main power filter cap, mine is a 25v 6800uf Rubycon if I remember correctly, and the two other caps are Nichicon and Panasonic.

[Mrx3750]

Re: Weird amp problem

Only other suggestion is to recheck all your components. Also check your case to see if it's grounding anywhere it shouldn't. Herd of that issue on a PC motherboard, worked fine in one case, went berserk in another. Turned out it was making ground contact where it shouldn't have.

[budm]

Re: Weird amp problem

According to the spec: :Operating voltage range 6 V ∼ 18 V (12 V typ.)" max input is 24V, STBY pin is 5V.
14V transformer will put out about 19V, 5V winding will be about 6V. You may want to check the MUTE pin 9 and STBY pin5 voltages.

[DJduck]

Re: Weird amp problem

I think I know. Before i used 4 diodes with low voltage drop to convert 5V for standby. Now i use a bridge rectifier and it drops the standby voltage down from 5V to about 3.9V or so. So the standby off 5V is not getting enough power and is not turning off from standby. But since I do not want to go back to 4 diodes, I'm going to put a 7805 after the rectifier to pump the voltage back up to 5V. But i can't do it now, since my shitty soldering iron died and i have to get a new one. Probably another cheapo one, since i am poor as fuck and can't even afford 40€ pair of speakers

[lti]

Re: Weird amp problem

A 7805 won't increase voltage. It can only drop a higher input voltage to 5V. The minimum input voltage is around 6.7V.

[budm]

Re: Weird amp problem

The standby pin only draws about 100uA (worst case, 5V feeding 51K resistor), very very small, you can easily use the resistors voltage divider and cap and Zener Diode. to drop the main power supply, or use half-wave rectifier from the 5V winding. That 5V winding should not drop down to 3.9V, 5V - 1.2V (Vf for 2 Diodes on each half cycle) = 3.8V, 3.8V x1.414 = 5.37VDC with only 100uA current draw.