Please Help!!! Decoupling Zeners

[arupg]

Hi all:

This is related to the schematic of a Quad 606/707/909 amplifier. Each channel amplifier PCB has 3 critical zeners, notably D1, D2 & D12. I was keen to decouple them and someone recommended using Tantalums. Tantalums are in nature noisy so I chose the Blackgate polarized PK compact series cap.


Please click below to see how I connected the cap.




Upon shut-down of the amp, I got a huge low freq & sub low freq vibrations from my speakers & was really scared whether my precious woofers have blown! Lucky they are still alive & kicking.

I used 2.2uF/100V for the D1 Zener while 22uF/50V for the D2 & D12 Zeners.

The amplifier schematic is below for information. (Red circles marked are the 3 Zeners that I tried to decouple).

http://img262.imageshack.us/my.php?i...hematicfp8.jpg



I will appreciate very much if Forumers can be kind enough to advise what and where things went wrong and also what would be the remedy.


Thanks in advance!!

Arup

[DRC]

It may be the fact that your caps are holding the lower voltage rails up for a little longer during power down. maybe a diode across the feed resistors (reverse bias during normal operation) would cause them to discharge back into the collapsing power rails.

In any case you should use a dummy load during testing and probably a series resistor (light bulb) in the ac line as well or you may make an expensive mistake !!

[arupg]

Thanks DRC for taking a look and suggesting a way out. I've marked in BLUE what I consider are the feed resistors. Do you feel they are the ones? Please do take a look at the schematic link below:

http://img205.imageshack.us/my.php?i...cmarkedyt9.jpg


Also, if they are the resistors, do I try the diode across the resistor going into the Zener or away from the zener?

Sorry about the questions..... Appreciate your time & patience!

[peranders]

May I ask why you do this? Are you have troubles and want to fix it or is it a tweak/improvement?

[janneman]

Originally posted by arupg
Thanks DRC for taking a look and suggesting a way out. I've marked in BLUE what I consider are the feed resistors. Do you feel they are the ones? Please do take a look at the schematic link below:

http://img205.imageshack.us/my.php?i...cmarkedyt9.jpg


Also, if they are the resistors, do I try the diode across the resistor going into the Zener or away from the zener?

Sorry about the questions..... Appreciate your time & patience!

Yes the blue ones are the feeds, the diode should point AWAY from the zener.

Do you relize that the impedance of the caps is MUCH lower than that of the zener so they will not 'do' anything except maybe for a 2Hz tone ?

Jan Didden

[Conrad Hoffman]

AFAIK, zeners produce noise up to extremely high frequencies, so a more appropriate bypass would be a small film cap, say 0.1uF polywhateveralene. Or a ceramic. There will always be LF noise as well, but the values required to get rid of that are huge, and would give you more problems than you've already got. Depending on the location and function of the zener, bypassing may not be appropriate at all.

[janneman]

Originally posted by Conrad Hoffman
AFAIK, zeners produce noise up to extremely high frequencies, so a more appropriate bypass would be a small film cap, say 0.1uF polywhateveralene. Or a ceramic. There will always be LF noise as well, but the values required to get rid of that are huge, and would give you more problems than you've already got. Depending on the location and function of the zener, bypassing may not be appropriate at all.

True. And of course D12 is already decoupled by C15 for hi-freq noise.

A 0.1uF decoupling right at the two opamp supply pins to ground may be worthwhile, but then again this is a quad. While not infallible, they DID know what they were doing.

Jan Didden

[arupg]

Thank you all for your responses.

peranders: The idea of the caps on the zeners were to get rid of the nasty noises inherent with Zeners.

Modders said to use Tantalums but we all know that tantalums too are noisy. Hence, Blackgate polarized was the next choice to me keeping in mind their small size for easy mount on the foil side of the PCB.


janneman: Thanks for pointing out the resistors.

A 0.1uF decoupling right at the two opamp supply pins to ground may be worthwhile, but then again this is a quad. While not infallible, they DID know what they were doing.

This is already done with a 0.1uF/100V Vishay 1837MKP - Rail to Gnd on the PS of opamp.

[jackinnj]

Ron Mancini of Texas Instruments addressed this topic in an issue of EDN in 2004 --
http://www.edn.com/article/CA454658.html

Bonnie Baker may also have had something insightful to say about this.

[janneman]

Originally posted by arupg
[snip]This is already done with a 0.1uF/100V Vishay 1837MKP - Rail to Gnd on the PS of opamp.

In that case, forget about the elcaps - waste of money.

jan Didden