LM3886 loud pop when turned off - Page 4

nige838 · 6th October 2011, 04:06 PM

Quote:

Originally Posted by madtecchy

Hi .
Are you still using the preamp section of the realistic amplifier ??
Ian

Hello,
Yes, I am still using the preamp, EQ, and FM radio as well as the original connections. Essentially all I have done is replace the power amplifier circuit with the LM3875s.
I'm a big fan of the tone controls on these old Realistic amplifiers. That and they look clean, nice wood finish. Not to mention that they were hand made in America.

Anyways, I've been too busy to work on it much. The more I listen to it, the more problems I notice. I think that I'm going to take the amps out, desolder the components, and use a PCB. Also, I want to use a better schematic...

Thanks for all of the advice. I have a much better understanding of what I'm doing now...

Michael Bean · 6th October 2011, 08:26 PM

Th3 uN1qU3, I don't know what kind of speakers Nige838 is using and niether do you, and while the point you make about limiting low frequency response for small speakers may be valid, it's a bad idea to limit frequency response in the feedback network to achieve that end, it should be done at the input of the amp.

Mike

Th3 uN1Qu3 · 6th October 2011, 11:24 PM

Of course, but that's what's spec'd in the datasheet, and if he gets the circuit as presented to work fine, then he can modify it to his taste afterwards. It's always good to have a solid foundation to start upon.

Michael Bean · 7th October 2011, 12:26 AM

It's been said many times on this forum that National's data sheets are not well witten in many instances, and thier chip amp data sheets are no exception. They are quite flawed with respect to the designed high pass roll-offs for input and feedback networks. The inputs have Fc set at .13 Hz, and the feedback Fc is set at 15.9 Hz, so if a sub-15 Hz signal finds its way to the input, the feedback network won't work as expected, resulting in instability and distortion. The main point I'm trying to make here is that it makes more sense to do it correctly in the beginning rather than do it wrong and then go back and fix it after the fact. Besides, what's the problem with putting in a 100uF cap instead of the 22uF on the data sheet? It won't cost much more and size won't be too much larger.

Mike

madtecchy · 7th October 2011, 05:47 AM

Quote:

Originally Posted by nige838

Hello all,
I just completed my first gainclone. I gutted the main amplifiers on a old realistic stereo and implemented 2 LM3875s. I have a problem where when I turn the stereo off, it makes a POP and the speaker cone moves A LOT.
I'm running it a 55 volts, using this schematic
but instead of 5600uf caps on the + and -, I'm using 10000uf caps...

Why is the amplifier doing this? What can I do to fix the problem?

Thanks

Nige838 I would bet my bottom dollar the loud pop that you hear
is caused by the preamp section. Before you dismantle it disconect the input signals to the power amp and short its inputs to ground.
Then switch the amplifer on and then off there is a very good chance that
there will be a big reduction to the level of the pop you hear.
Would any of the thread hijackers like to comment and help provide suggestions to a solution.

Regards Ian

nige838 · 7th October 2011, 07:32 AM

Quote:

Originally Posted by madtecchy

Nige838 I would bet my bottom dollar the loud pop that you hear
is caused by the preamp section. Before you dismantle it disconect the input signals to the power amp and short its inputs to ground.
Then switch the amplifer on and then off there is a very good chance that
there will be a big reduction to the level of the pop you hear.
Would any of the thread hijackers like to comment and help provide suggestions to a solution.

Regards Ian

Many thanks Mad Tecchy. This makes sense to me. I will try this tomorrow. For tonight, this amp is pushing the jams for my birthday.

Much Appreciated.

I'm researching it..

Michael Bean · 7th October 2011, 11:44 PM

Nige838, after you've gotten your amp re-built correctly and have figured out that Madtecchy is right about the source of the popping problem, you might want to use this or something like it. The circuit is a speaker controller that will delay connecting the speakers to the amp output at power-on, immediatly disconnect speakers at power-off, and protect them from amplifier faults.

SPKR PROTECT.JPG
Speaker Protect BOM.txt

Mike

6th October 2011, 11:24 PM	#33
Th3 uN1Qu3 diyAudio Member Join Date: Feb 2010 Location: Bucharest	Of course, but that's what's spec'd in the datasheet, and if he gets the circuit as presented to work fine, then he can modify it to his taste afterwards. It's always good to have a solid foundation to start upon. __________________ "Audio grade" components simply means that they failed at a more critical job.

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6th October 2011, 08:26 PM	#32
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Th3 uN1qU3, I don't know what kind of speakers Nige838 is using and niether do you, and while the point you make about limiting low frequency response for small speakers may be valid, it's a bad idea to limit frequency response in the feedback network to achieve that end, it should be done at the input of the amp. Mike

7th October 2011, 12:26 AM	#34
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	It's been said many times on this forum that National's data sheets are not well witten in many instances, and thier chip amp data sheets are no exception. They are quite flawed with respect to the designed high pass roll-offs for input and feedback networks. The inputs have Fc set at .13 Hz, and the feedback Fc is set at 15.9 Hz, so if a sub-15 Hz signal finds its way to the input, the feedback network won't work as expected, resulting in instability and distortion. The main point I'm trying to make here is that it makes more sense to do it correctly in the beginning rather than do it wrong and then go back and fix it after the fact. Besides, what's the problem with putting in a 100uF cap instead of the 22uF on the data sheet? It won't cost much more and size won't be too much larger. Mike

7th October 2011, 11:44 PM	#37
Michael Bean diyAudio Member Join Date: Sep 2004 Location: Minneapolis, Minnesota, USA	Nige838, after you've gotten your amp re-built correctly and have figured out that Madtecchy is right about the source of the popping problem, you might want to use this or something like it. The circuit is a speaker controller that will delay connecting the speakers to the amp output at power-on, immediatly disconnect speakers at power-off, and protect them from amplifier faults. SPKR PROTECT.JPG Speaker Protect BOM.txt Mike