Yamaha Stagepas 300 powered mixer, loud pop at shut off

[Randall]

I have a friend's Stagepas 300 that I changed out the fans on. I didn't bother to listen to it before I did the replacement. Now, I find a loud pop when switching it off, even with all volumes controls down and no input. Anyone have an idea what might do this, because I have not a clue. I include the service manual, but it is all about board replacement, and how to disassemble everything. Not much help.

http://www.go-gddq.com/upload/2014-0...2721337905.pdf

[J M Fahey]

First try the simplest: add a line rated .022uF or .047uF across each power switch contact which opens on turning amp off (hint: 1 or 2)

If not, and it's a power amp turn off Thump, (instead of a switch Pop, they sound different), I VERY much doubt the amp left Factory that way (specially a Yamaha) , they always include some type of mute, from a simple FET to a dedicated pin in the chip amp (if used) or fast output relay switchoff before the amp has time to thump.

In that case you'll have to get the schematic of that section (post it here) and debug it.

[Randall]

I'm pretty sure it is power amp turn off thump, it drives the cone a lot. If the schematic is not in the factory service manual, what chance do I have of finding one?

Edit: My bad, the schematics are indeed included in the attached service manual. Is it the TOP249Y switching controllers where I should be considering on p. 50? I don't understand how they work.

[mozwell]

Ask your friend if the unit ALWAYS had this turn off thump, if so, not much you can do about it.

The TOP249Y is a primary side switch mode power supply control ic with inbuilt switching mosfet
A thump at turn off can be common in solid state power amps
i can see no relay to disconnect the speakers at turn off, but there is an ENABLE line to each power amp, controlled by Q808 (page 2)
Page 1, Q806 & Q807 turn off the enable as the power supply collapses
It may be the +/-15V or the main power amp +/-Vcc rails are no collapsing at an uneven rate....

Unless some of the circuitry is "not ok" around Q806, Q807 or Q808, you may not be able to do much about the turn off thump.

WARNING, all the primary side of the power supply, ie around the TOP249Y ic's is LIVE at mains potential, do not touch anything here

[J M Fahey]

Worst case, the best and cheapest thump killer is to add a standby switch ... actually opening the speaker out connection.

Instead of a digital intelligent controller, use an intelligent owner, instructions are:

1) at turn on, first switch power on, count to 10 and switch "standby" on.

2) at turn off, switch standby off, count to 10 (actually not necessary, but for symmetry and mnemonics) and turn power off .

I always use this in absolutely silent places, such as: meditation/New Age/Yoga places (no kidding) , live theaters, churches, hospitals, etc.

Works like a charm.

Easy, huh?

[Enzo]

The turn off thump is no louder than the loudest kick drum hit you will amplify through the system. Not likely to hurt anything.

[Jazz P Bass]

Of all the manufacturers, I find it hard to believe that Yamaha would allow 'turn off thump'.

From the manual:
Powering off
- all active inputs should be turned off first.
- master volume should be all of the way down.

[Randall]

The owner says it has always made a slight turn off noise, but what I am experiencing is quite a thump. I don't think this is normal. Jazz, it does it with no input, and with all vols turned full off, so what is wrong here? This system is used for speaking engagements, not for a band.

[Jazz P Bass]

What you have there is a switching power supply a two Class D amp modules.
(be carefull in there, the modules are not available)

If both channels are thumping at turn off, the only item that I can think of to check is the 'Enable' circuit, which is common to both channels.

The circuit starts at Q824 & the final ouput is Q808.

It appears to have startup delay capacitors but what is it doing at turn off?

Stagepas 300 output.pdf

[Randall]

I know Enzo won't like this approach, but I have ordered the five SMD transistors in that section, and will replace them all at one swoop. The cap and resistors measure OK. If that doesn't work, I will advise my friend to simply turn down the master vol and unplug the speakers before powering down.

[Jazz P Bass]

Have you attempted to 'monitor' the ENABLE pin of the output modules.

I would like to know what is happening there at "Turn Off".

[Randall]

No Jazz, I have not. What would you suggest?

[Jazz P Bass]

Measure the voltage at the Enable pin.

It looks like it is a 'mute' circuit.
When it is a low voltage, it is muted.

Look at the pin at Turn On and it should ramp up.

Then look at it at Turn Off.
Hopefully, it drops rapidly.

[Randall]

When turned on, voltage at the enable pin rises quickly to about 3.5 volts, then ramps down and snaps to about 25mV at about the two second mark. When powered off, it jumps down to about -3v at the sound of the pop, then continues to drop slowly to about -5v, then slowly rises to ground over the course of maybe 10 seconds.

[Jazz P Bass]

Which ground reference are you using?

Kind of odd, that -5Vdc, seeing that the circuit operates off of the +15Vdc supply.