Fender Passport 500 Pro SMPS question

[JP-Stereo]

I have increase C679 to 470 uf, the charging of MUTE ON at Power Up is a little longer.
But no MUTE ON signal at Power Off, the noise occur before the Mute On begin to rise...

I also replace C678 for a 10uf but i still have voltage at R663 for long after Power Off.
Look like Q610 do not drain the line, it was replaced a few step before... ?

[JP-Stereo]

I had a broken trace between R663 and D638 once repaired, the Mute On at power on is OK.

But the response is too slow at power off, the noise happen before the Mute On raise.
I still have a 10uf at C678.
I tried to remove C677 to make the response faster at power off, but it's more of a filter and Q610 will not work without.

At this point the noise is weaker and no a loud Pop anymore.
It more like a "schlick"...

I have to accelerate the action of Q610 in order to get a solid Mute On at power off.
Maybe next is to lower the value of R664 ?

I begin to think that this circuit only mute at Power on.
Q610 is there only to drain C678 for the next startup...

[g1]

I begin to think that this circuit only mute at Power on.
Q610 is there only to drain C678 for the next startup...

This is as I have always assumed for all mute circuits.
I don't know how a circuit can be fast enough to avoid turn-off thump unless it contains a time machine.

[JP-Stereo]

OK so after a fair testing everything is working fine exept for the Power Off noise.
I change C679 to 470uf and C678 to 10uf and it's now a loud "Click".
I figure that it will be difficult to make better with the original circuit.

What is causing these Power Off noise ?

Is it really unavoidable ?

[x-pro]

С678 47uf!

I wrote above that Q610 is needed to quickly discharge C678 and quickly open the upper transistor when the device is turned off. Mute in this mode will last as the 330uf capacitor discharges.

[JP-Stereo]

Even if C678 is lowered at 10uf, the Mute On is way too slow at Turn Off.
Q610 is draining C678 at Turn Off but only to prepair it for the next Turn On mute.

There is a delay in the action that is preventing the Mute to hide the Turn Off noise.

How to implement a momentary push button that will manually Start a Mute On cycle of a few second before Turn Off.

A momentary switch that will open the AC8V line before turning off the amp ?

[JP-Stereo]

No hope ?

[JP-Stereo]

If triggering the Mute via a push button is not a good option, what else may shorten the Mute action at Power Off.
I've already tried to decrease the value of C678 to 10uf.
I've also tried by removing C677 but my test didn't work because i had a broken trace at this step that i found later.
Maybe C677 is the only improvement possible ?

A lot of these and similar amps have this Power Off problem from my research.

[g1]

In my opinion the 'key' is not the issue of turning on the mute circuit fast enough, rather turning off the amp slow enough.
The mute will engage almost instantly because of C677 quick discharge. But at the same time, the power supply is discharging and any imbalance between + and - discharge rates can create a pop or thump. They need to discharge very evenly, and slower than mute engagement.
Having supplies with discharge times perfectly balanced is expensive and can change quickly with age. This is why we consider turn-off thump a fairly normal thing, not really economical to chase down unless very severe.
If you wanted to design something with zero turn-off thump, a better solution would be 'soft turn-off', using relay or software to disconnect speaker first. More like how a modern computer shuts down.
The mute line is usually at the input of the power amp. It can only prevent a thump from passing through. It can not control any event further on it the power amp circuit.

[JP-Stereo]

The mute line is usually at the input of the power amp. It can only prevent a thump from passing through. It can not control any event further on it the power amp circuit.

If i understand ok, the Mute operate on U703/U704 via CDS1/CSD2.
This IRS20957 have the following parameters:


Is anything can delay the shotdown of these drivers in order for the Mute to engage ?

Or the problem come from the B+/B- lines themself after U703/U704 ?

In the last scenario,is it a question of capacitors matching and real values or else ?

[g1]

Or the problem come from the B+/B- lines themself after U703/U704 ?

In the last scenario,is it a question of capacitors matching and real values or else ?

Yes, the mute circuit can not do anything about the collapse of the supplies.. Also the VCC supply for the VB pins could be a factor.
A question would be, how bad is the thump, and was it there when unit was new?
For me, an amp that is perfectly quiet when shutting down is the exception, not the rule.

[x-pro]

......What is causing these Power Off noise ?

Is it really unavoidable ?

He's on both exits? There's two channels of amplification.

[JP-Stereo]

About the pre-existing state of the turnoff noise;
The customer is not certain because the amp was about always on...
May had such a noise but not at an annoying level as i am earing.

Yes it is on both channel and i can see the power rails imballance on the scope at power off.

What can be the most probable cause of one rail collapsing faster than the other, ?
Those big caps on the main switcher ?

[g1]

Possibly those big caps. But x-pro asked an important question, do you hear the thump from both amplifiers?