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Onkel Alex 17th October 2008 07:26 PM

Problem with PPI amps - POP on power on and power off

I have a problem with some of my PPI amps (2030AM, 2050AM, 2075AM and 2150AM).
They all do a "pop" when turning on or turning off.
It tested them installed in my car (propper power-supply) and at my test-setup.

Currently the 2030 lies naked in front of me on the desk.
I already replaced all capacitors (low ESR types and special audio caps) and the OP-amps but it changed nothing.
Maybe there is a problem at the mute-circuit?
Unfortunately I do not have any schematics.

If someone has an idea what I could look for, please feel free to reply to my post.

Thanks in advance...


Perry Babin 17th October 2008 07:31 PM

Does it pop if no RCAs are plugged into the amp?

Onkel Alex 17th October 2008 09:42 PM

Yes, it happens with and without RCAs connected and on both channels.

Perry Babin 17th October 2008 10:15 PM

Can you post a few photos of the inside of the amp?

Does this amp have two 2N5638 transistors near the op-amps?

If so, is the voltage on leg 3 of those transistors near 0v for a few seconds after remote voltage is applied?

If so, does it then drop to ~negative 15v?

Do the +15v regulated and the -15v regulated supplies both reach their respective voltages at approximately the same time?

Are both at the proper voltage (15v)?

Onkel Alex 18th October 2008 04:33 PM

I will take some pictures maybe tomorrow.

There is no 2N5638, but i think I identified the mute circuit.
There are two J108 (already replaced by new ones), connet to signal ground, the second OP-amp and a photo-transistor which is near the PS control.
I measured the voltage, but it seems to be correct. The voltage at leg 3 dropps to -18V (PS output voltage) a few seconds after turning the amp on.

The +/-15V are generated by a pair of 7815/7915, the actual voltages were about +15,2V and -14,7V. So I replaced them.
...same behavior as before!

As I only have an old single-channel-scope, i cannot measure the exact behavior of the voltages when tuning on/off.
I soldered two 6k-resistores in series to the +15V and -15V rails and connected their commen junction to te scope.
When turning on, the signal jumps once a little bit (simultaneous to the "pop")
When turning off, the signal dropps about 2V and slowly returns to zero.
So obviously the positiv rail is "faster".
I allready replaced the capacitors, tried different values (220F originally, but currently 100F), but the behavior is the same ever time.

I also measured the behavior of the PS output voltages the same way. Here everything seems to be O.K..

...any ideas? ;)

lumanauw 18th October 2008 04:57 PM

I found this the hard way. Replacing all cap and transistors don't help. In my case, it happen because +15 and -15 are not rising and falling the same time--result opamp's output is drifting at on and off.

Check output of opamps (many of them, easy with DC scope), when on or off, at which direction it drift, to +DC or to -DC (don't have to be big voltage, small drift is enough for pop)? Step2, check the polarity of elko attached to this output, should be in right polarity.

For example, if opamp's output node drift to +DC, then check the polarity of the coupling cap. If the polarity of this elko is +/- then it's OK. But if this opamp's output is connected to -/+ of the next audio signal coupling elko (polar type), it will pop at loudspeaker's output.

This happens because when headed with wrong polarity (+elko to -DC and -elko to +DC), it will make DC at elko's output. Not a problem when the polarity is right (no pop).

The easier way, find out every single audio coupling cap, replace them with non-polar elko.

Perry Babin 18th October 2008 07:25 PM

Is the pop the same on all of the amps or is does it vary in volume between the amps?

Is the pop the same volume on both channels of the 2030?

Are you applying remote only 'after' the B+ and ground are connected?

Are the op-amps the original parts or have you 'upgraded'?

Does the volume of the pop vary if you vary the time between removing remote and reapplying remote voltage (1 second, 5 seconds, 10 seconds...)?

Does the audio mute release gradually or does it go from fully muted to fully released instantaneously?

Onkel Alex 19th October 2008 06:52 PM

The volume of the pop is different on every amp. I tried the 2030 and 2050 at the tweeters and the 2030 pops loudest.

The volume is the same on both channels. In bridge mode the volume is less.

B+ and Ground are connected all the time. I apply remote only for the test.

The OP-amps are LM833 (original). I already replaced them and tried TL072, NE5532 and some Burr-Brown.
Even without OP-amps there is a pop. But I can only hear it when I hold the speaker close to my ear.

The pop on turning off is only, when I release remote AFTER the mute circuit turns off.
The pop on turning on is every time I apply remote even it was turned off for less than 1 second.
The volume is the same every time.

And the pop is independent from the mute circuit. It pops just when the PS powers on and long before the mute circuit releases.

The issue with the reverse poled capacitors is a good point, but I replaced the capacitors in the same direction than the original parts. The direction is also printed on the PCB.

I think, the main problem is that the PS does not power up/down correctly.
I checked the output of the PS again and even here, the behavior of both rails is unequal.
Now the question is: Why?
And why do all of my amps have this problem?
The only thing they have in common is the age... ;)

Perry Babin 19th October 2008 07:48 PM

It sounds like a power supply problem.

You said you tested the rail voltage and they came up evenly. Has that changed or are you referring to the regulated voltage?

If it doesn't pop without the op-amp, that would tend to eliminate leaking transistors.

Do you have an external power supply that could be connected to the regulators to keep them powered up before and after the amp is switched on/off? If that eliminates the pop, you would know that the problem is related to the op-amp (likely the one driving the audio amplifier stage) or the offset of the regulated voltage. The J108s should be able to clamp virtually anything driven into the last op-amp when the muting circuit is active so the problem almost certainly lies with the last op-amp.

If you try this, you'd need a supply with an isolated ground that could be connected to the secondary center-tap. You'd lift the input terminals of the regulators and feed them with your power supply.

If you're careful and you fuse all leads with 1 amp fuses, you should be able to use the rails of the 2050 to feed the regulators in the 2030. If the rails of the 2050 are higher than 35v, you should use the 2030 to power the regs in the 2050 and try to find the problem using the 2050 instead of the 2030.

The amps pop when the voltage on pin 3 of the J108s is at or near 0v. Is that correct?

Do any of the amplifiers have the original (installed when the amps was manufactured) op-amps?

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