GHOST in 3 band graphic equalizer!! SOS! - diyAudio
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Old 14th December 2012, 09:29 AM   #1
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Question GHOST in 3 band graphic equalizer!! SOS!

First a brief history:
I started to build a 3 band Equalizer using NE5532 as per this website. 3 band EQ

First I tested the second part of the circuit on a breadboard and it worked as expected. ( attachment: graphic eq on bread board.jpg )

Then I made a quick PCB and built it.

Upon first power up it worked nicely but I found 2 problems

1. after some 10 seconds the sound started to spark
2. and gain reduced.

I touched my fingers at the input and it became normal again for another 10 seconds.
I added a 100k bypass Resistor infront of the 10uf/63v Electrolytic cap like this below in the input and the problem was solved! (Kindly see the attachment EQ FIX 1.png )

After some days I replaced the electrolytic input caps with a bigger nonpolar 10UF/100v caps. I found the sound was improved . I was happy. But after some 5 minutes the previous problem reappeared. i.e gain reduced and sparking noise.

I monitored the voltage ... +-15v okay, no over heating of the ICs, No pcb foil line is damaged. I tried my best to solve that but alas...
As a last resort I had to use the electrolytic caps again with the 100k biasing resistor.
Could please help me fix this problem so that I can use the bigger caps instead of the el-caps?
eq question how to solve .jpg

Thank you,
Best regards,
Som.
Attached Images
File Type: jpg graphic eq on bread board.jpg (497.3 KB, 259 views)
File Type: png EQ FIX 1.png (57.2 KB, 228 views)
File Type: jpg eq question how to solve .jpg (257.2 KB, 215 views)
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Last edited by csom; 14th December 2012 at 09:37 AM.
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Old 14th December 2012, 09:43 AM   #2
freax is offline freax  Australia
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sounds like an earthing issue to me, possibly a ground loop issue.

Reason for thinking this is that you touched the input with your fingers and that fixed it.

Get some perfboard and put the project onto that.
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Old 14th December 2012, 09:46 AM   #3
Bone is online now Bone  United Kingdom
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The positive input to the 5532 needs a resistance to earth to give it a reference. Move the top of R15 to the junction of C1 & R1.
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Old 14th December 2012, 09:46 AM   #4
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Thank you freax.
I am lost as the el-caps are working perfectly with the 100k resistor but the bigger nonpolar ones ( which are dramatically improving the performance) fail.

I checked the ground continuity too. No problem there .
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Old 14th December 2012, 09:49 AM   #5
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Quote:
Originally Posted by Bone View Post
The positive input to the 5532 needs a resistance to earth to give it a reference. Move the top of R15 to the junction of C1 & R1.
Thank you Bone !
Is the gain of that buffer stage 2? If yes can I use a 47k instead of that 100k ?
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Old 14th December 2012, 09:59 AM   #6
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Quote:
Originally Posted by freax View Post
......
Get some perfboard and put the project onto that.
That was the second thing I did Freax It worked okay . Although the buffer stage was not there. Its the buffer stage that's the culprit here.
Here is the pic
Attached Images
File Type: jpg eq on perf board except the buffer stage.jpg (627.8 KB, 222 views)
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Last edited by csom; 14th December 2012 at 10:11 AM.
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Old 14th December 2012, 10:09 AM   #7
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Your schematic does not correspond to original.
Original shows an inverting opamp config.
You enter into + input. when this one should be grounded.
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Old 14th December 2012, 10:16 AM   #8
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Quote:
Originally Posted by bobodioulasso View Post
Your schematic does not correspond to original.
Original shows an inverting opamp config.
You enter into + input. when this one should be grounded.
OUCH!! A "BIG FALE" for me !!!
Thank you so much bobo for pointing out that!
Re-designing the PCB now!
Sigh!
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Last edited by csom; 14th December 2012 at 10:19 AM.
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Old 14th December 2012, 04:02 PM   #9
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Fixed the prototype. No more sparking or gradual gain reduction now.

Improvement: Noise is minimized to a great extent.

But the stereo image/ sound stage is reduced. weird. Earlier it was better.

Could you please suggest what can be done to increase Stereo image ?

Regards,
Som.
Attached Images
File Type: jpg 4 caps added to the eq prototype.jpg (513.0 KB, 232 views)
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Old 14th December 2012, 04:59 PM   #10
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Problem solved! One of the pots ( midrange 100k ) had a dry solder joint.
All fixed now! Sound stage restored again !
Working great now! Thank you all!

If any newcomer, hobbyist ,like me, wants a quick, simple yet very effective Equalizer solution and add it to your DIY amp, please go ahead and give it a try, you will be surprised to see its performance.


Regards,
Som
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Last edited by csom; 14th December 2012 at 05:07 PM.
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