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-   -   LM3886 & miniDSP turn off transient (http://www.diyaudio.com/forums/chip-amps/206346-lm3886-minidsp-turn-off-transient.html)

Bazukaz 9th February 2012 02:49 PM

LM3886 & miniDSP turn off transient
 
3 Attachment(s)
Hello,

Recently I have purchased a miniDSP kit. However after connecting everything with an LM3886 amp I have discovered that DSP is basically unusable.
This is because of very harsh turn off transient when disconnected from mains. I don't think tweeters would last this for long.

So amplifier must be somehow muted before miniDSP turns off. I have a couple of solutions in mind.
One is to use uPC1237 speaker protection IC and drive a relay. However I do not trust relays, especially in the case when it is placed inside a speaker cabinet.
Another solution I can think of is to feed mute pin of LM3886 from a separate bridge rectifier with small capacitance and a filter resistors in such a way that mute current would fall very fast and LM3886 is muted before miniDSP blows its noise to amp's inputs.

I did simulations, so a few questions :

Maybe there is a better(or simpler) solution to this?

If not , then :

Is there a problem with mute current ripple of about +/-0.15 mA(Avg. current is around 1 mA) for LM3886 ?

Do I need to add a reverse diode between separate "Mute PSU" and -Vee so voltage on mute pin can never exceed that of -Vee?

Edit : It looks like I did not take into account the behaviour current draw of mute pin of LM3886, so first schematic may not work as expected.
If mute current ripple of ~20% is acceptable, probably second schematic can be used ?

Regards,
Lukas.

GloBug 9th February 2012 03:34 PM

Post a schematic of the amp if you have it.

Most of the gainclone type amps are missing a few "optional" components that help it behave.

I would check it out anyways, might be easier then your mute idea, keep that for plan B.

Bazukaz 9th February 2012 03:53 PM

Quote:

Originally Posted by GloBug (Post 2899046)
Post a schematic of the amp if you have it.

Most of the gainclone type amps are missing a few "optional" components that help it behave.

I would check it out anyways, might be easier then your mute idea, keep that for plan B.

Hello,

The schematic is a typical application circuit from datasheet with split power supply(without mute capacitor Cm). There are absolutely no problems if miniDSP is disconnected, so IMO this is not a problem with amp.

Regards,
Lukas.

GloBug 9th February 2012 04:07 PM

There is another thread in the Audiosector forum were a dude is also chasing a mini DSP problem. Maybe you two can put your melons together.

Does the mini DSP cause problems with other amps?>Why not fix the DSP then?

Shut the power off on the amp first. (Though not ideal I admit.)

frank1 9th February 2012 09:17 PM

Quote:

Originally Posted by Bazukaz (Post 2898981)
Hello,

Recently I have purchased a miniDSP kit. However after connecting everything with an LM3886 amp I have discovered that DSP is basically unusable.
This is because of very harsh turn off transient when disconnected from mains. I don't think tweeters would last this for long.

I have exactly the same setup.
MiniDSP feeding 4x LM3866 amps.

I completely solved the switch-off noise by connecting a large capacitor (10,000uF) via a diode accross the supply to the the MiniDSP so that the DSP remains powered for several seconds after the mains is removed.
Frank

Bazukaz 10th February 2012 09:46 AM

Quote:

Originally Posted by frank1 (Post 2899521)
I have exactly the same setup.
MiniDSP feeding 4x LM3866 amps.

I completely solved the switch-off noise by connecting a large capacitor (10,000uF) via a diode accross the supply to the the MiniDSP so that the DSP remains powered for several seconds after the mains is removed.
Frank

Thanks for suggestion. I am going to try this.

Regards,
Lukas.

Mooly 10th February 2012 10:35 AM

Other thread,
http://www.diyaudio.com/forums/solid...off-setup.html

Bazukaz 13th February 2012 11:00 AM

Ok,

So I have tried to increase PSU capacitance to 4700 uF. Still miniDSP has been shutting down far too quck. Larger capacitance would have needed miniDSP psu boards to be rebuilt.

Then I bought an upc1237 amp protector and used it to short miniDSP outputs to ground instantly via relay when power is removed. Turn-on delay was also implemented. Although datasheet recommends voltage of 25-60V, the IC seems to be working from 12V supply without a problem with component values adjusted.

I think this solution has several advantages over large capacitance :

1) It works even when power supply of MiniDSP is disconnected but amp's not(like blown fuses, etc)
2) Turn-on transient is also always supressed. While LM3886 has internal muting that normally prevents this until power amp supply has reached proper voltage, I have found miniDSP power-on noise can still be passed in some situations(like when power button is clicked on/off twice within short time).

While disadvantage IMO is somewhat more complexity and need to use relays(I guess gold plated sealed small relays should be reasonably reliable for small currents).

Regards,
Lukas.


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