Weird problem with a preamp with 3 band EQ module - diyAudio
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Old 5th July 2013, 07:28 PM   #1
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Unhappy Weird problem with a preamp with 3 band EQ module

I made a 3 band equalizer following a Spanish Site
Construya un preamplificador con entrada de micrófono, línea y ecualizador

and here is the circuit in PDF
http://construyasuvideorockola.com/downloads/pretonos.pdf

It is a stereo preamp with 3 band equalizer module.

It was working okay for some 3 hours but suddenly it developed problem.

The sound is getting distorted after some 5 mins.
When I increase the bass control to the fullest extent, the sound stops altogether.
And sometimes the sound is distorted.
Disconnecting and reconnecting the power supply is fixing it for another 5 minutes.

They suggested to reverse the 1uf caps between TLO71 and TLO72 if distortion occurs or the sound stops. I tried that too.
But that didn't fix the problem.

I can't ask for help there as I don't know Spanish.

I am frustrated because I already wasted two precious pieces of PCB on it. and rebuilt again thinking I might have made some mistakes. But all my 3 attempts produced the same distorted result.

My PSU is +- 15 volt dual regulated and working great with other circuits.

Is it a design fault of the equalizer module posted in that site?

So you all are my last resort as always.
Could you kindly check the circuit and help me out fixing this problem?

Any help is sincerely much appreciated.
Thank you.
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Old 5th July 2013, 07:38 PM   #2
Mooly is online now Mooly  United Kingdom
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The circuit in the pdf won't work. Whats missing is a DC bias path to two of the opamps. Add a 470 K resistor from the "floating" + inputs on the opamps to ground.
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Old 5th July 2013, 07:59 PM   #3
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Quote:
Originally Posted by Mooly View Post
The circuit in the pdf won't work. Whats missing is a DC bias path to two of the opamps. Add a 470 K resistor from the "floating" + inputs on the opamps to ground.
Thank you so much Mooly!

Something like this?
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Old 5th July 2013, 08:02 PM   #4
Mooly is online now Mooly  United Kingdom
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That's it The value isn't very critical, 470K to 1Meg is fine.

Its these two locations here.
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Old 5th July 2013, 08:07 PM   #5
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Quote:
Originally Posted by Mooly View Post
That's it The value isn't very critical, 470K to 1Meg is fine.

Its these two locations here.
You are such a kind and great resource person!

I will add the resistors and report you back.

Thanks a ton!
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Old 5th July 2013, 08:17 PM   #6
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Alternatively if you had used really poor quality 1uF caps they would have leaked enough current that the Opamp would just have just about had enough current to work, which is probably why they thought it was OK.

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Andrew
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Old 5th July 2013, 09:04 PM   #7
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Quote:
Originally Posted by Mooly View Post
That's it The value isn't very critical, 470K to 1Meg is fine.

Its these two locations here.
I have no 470K at the moment. So I added a 560k. It's running fine so far.
Earlier problem has not come back yet.

Only the bass control response is so so.

I added this module in front of Project 88 Preamp by Rod Elliot.
The arrangement is like the following:

this 3 band Eq module > Rod's preamp > LM3886

1. Could you please tell me if project 88 preamp is still needed with this module or I can directly add this to my Power Amp using LM3886 omitting Rod's preamp?

2. Is there any better free DIY project like this 3 band EQ module with microphone input ( with PCB layout etc. ) that I can build?
( I tried APEX ML3 but microphone is not working as expected )

Thank you again so so much Mooly for your being here for us whenever we need.
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Old 5th July 2013, 09:09 PM   #8
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Quote:
Originally Posted by gfiandy View Post
Alternatively if you had used really poor quality 1uF caps they would have leaked enough current that the Opamp would just have just about had enough current to work, which is probably why they thought it was OK.

Regards,
Andrew
I thank you for your valuable input, Andrew.

I used fresh new good quality caps.

It's sad that they posted such a circuit without testing it much hence leading to utter frustration. But thank god I got experts like you all as constant help.
Much appreciated!.
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Old 6th July 2013, 07:08 AM   #9
Mooly is online now Mooly  United Kingdom
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Pleased to hear its working

I wouldn't have thought you needed a pre amp as well as this because this tone control has gain. The microphone input is high gain (100) but has a really low input impedance (1K) so will not suit all mics.
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Old 6th July 2013, 09:00 AM   #10
gfiandy is offline gfiandy  United Kingdom
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Hi, this circuit should be Ok to drive power amp directly so you can remove preamp.

Is the microphone input working ok on this design as it has no phantom power supply. If you have a fairly good microphone it is likely it expects a phantom supply, even most PC microphones expect a bias voltage to work.

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