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 Post subject: Home stereo question
PostPosted: Mar Tue 13, 2018 3:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19321
Location: Warner Robins, GA
I have a smaller home stereo consisting of a 70Hz 24 db/octave two way active crossover, a plate amp for a sub and a Lafayette LA-375 amp for the main speakers.

I have a problem with the crossover in that it makes a pop in the speakers upon turn on and an ungodly noise in the speakers when the system is turned off and the B+ on the crossover gets to around 9Vdc.

The temporary solution is to set the main/aux speakers to aux before turning the system off.

I am tired of doing that and want to find the problem as I'm sure that shouldn't be happening.

Here's the schematic.

Attachment:
Power supply.jpg
Power supply.jpg [ 49.19 KiB | Viewed 471 times ]


Attachment:
HP LP filter LP sum.jpg
HP LP filter LP sum.jpg [ 91.33 KiB | Viewed 471 times ]


Is there anything with the circuit that could be causing an issue?

If the circuit is fine is there a simple way to solve the problem?


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 Post subject: Re: Home stereo question
PostPosted: Mar Tue 13, 2018 4:08 am 
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Joined: Jan Fri 02, 2009 11:32 pm
Posts: 566
Location: Equinunk PA 18417
How about a relay or jfet mute circuit from this page?
http://sound.whsites.net/articles/muting.html

edit the trick is to make a control voltage that delays at turn on, and mutes instantly at turn off, before the supply caps discharge significantly. A separate tap from the supply, off the transformer, can do it.

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 Post subject: Re: Home stereo question
PostPosted: Mar Tue 13, 2018 4:29 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 19321
Location: Warner Robins, GA
The relay wouldn't work in my situation as I need a delay on turn on and instant mute on turn off.

The JFET might work provided I can figure out how to delay the - voltage applied to it and instantly remove the - voltage when the system is turned off.

Now I could use one of those delay relays that delays the turn on and then turns off instantly when power is removed, but the only way I could do that is with a 120vac one and I don't like 120 volts being that close to audio signal lines and I cannot install the 120 volt delay in the amp due to using an external power transformer with it.

While looking at that website I did see that some OP-AMPs do misbehave under a certain supply voltage.

When connecting the output of the crossover to my scope I can see the output when the voltage gets low enough and it goes positive.

I'll take the crossover to work tomorrow and monitor the + and - voltage to see if both drop at the same rate when I remove power from the crossover.

The ideal solution is a circuit like some power amps use where a relay switches the speakers on after a short delay.

Thinking about it taking the crossover to where I work (civil service on an air force base) would not be a great idea since I built it in an old ammo can.

I might take the amp to work and experiment with a relay and figure a delay circuit. Wonder if a resistor and capacitor would delay the turn on and provide a quick turn off of the relay?

EDIT:

This page describes my problem.

http://sound.whsites.net/project104.htm

That requires extra circuitry though.




EDIT 2:

Decided to experiment with a small DPDT relay and came up with this circuit.

Attachment:
Mute.png
Mute.png [ 3.6 KiB | Viewed 423 times ]


It has a 2 second on delay and a 5 second off delay. Both controlled by the 100uF cap.

Would like it to be the other way around, but that would require extra circuitry.

The relay connects to the left and right inputs and just shorts them to ground when the relay is off.



EDIT 3: Tested the simple circuit and it works quite well.

No turn on pop from the crossover and no turn off noise either.

Also after adjusting he idle current of the Lafayette LA-375 amp this circuit is in I had to increase the resistor to 3.95K to keep the relay voltage around 12.4Vdc.

I had thought of switching the speaker outputs, but that would have required a larger relay, wouldn't have worked with that simple circuit and wouldn't have done anything to prevent the turn on pop and turn off pop from going through the amplifier.

The relay I used was the small round PC board type with long leads and I re-purposed the two terminal strips that held the resistors for the rear channel outputs to mount the relay, resistor and capacitor. I just used regular wire to run from the relay to the inputs since the distance was short.


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