Weird problem with a preamp with 3 band EQ module

[csom]

The total resistance seen at the input will determine the input impedance. A 470K is fine for the opamp bias. If you leave the 100K then the input impedance will be around 82K (470K and 100K in parallel at AC). If you omit the 100K then the input impedance is just 470K if that is the value you use.

Either option is fine for modern source components.

Oh great!
I understand it now!

( I deleted this section of my reply as I understood when I read your post again. Sorry. )

Thank you!

 

[Mooly]

Its just two resistors in parallel (the cap can be discounted within the audio band as it appears as a "short" to audio signals)

Resistors in Parallel - Parallel Resistor Network

Series and parallel circuits - Wikipedia, the free encyclopedia

 

[csom]

Its just two resistors in parallel (the cap can be discounted within the audio band as it appears as a "short" to audio signals)
......................

Oh thank you! I didn't know that.

 

[Mooly]

Your welcome.

 

[csom]

Dear Mooly,
I want to build this 5 band equalizer.
http://construyasuvideorockola.com/proyect_eq_5band.php
and here is the pdf.
http://construyasuvideorockola.com/downloads/eq5bandas.pdf
I guess they have the same biasing problem at the gain stage using TLO72?
Could you kindly confirm?

I have 2 more questions.

1. can I use a 22K dual volume control in lieu of 20K used in the circuit?

2. In case of non-availability of 50k Pots, can I add a 100k resistor parallel to the 100K pot like the attached image below?

Thank you.

 

[Mooly]

The pdf won't open but yes, you can use 20K for the volume control. You can't bridge pots like that because although the total resistance is correct, the resistance seen by the wiper isn't but 47K pots would be OK.

 

[csom]

The pdf won't open but yes, you can use 20K for the volume control. You can't bridge pots like that because although the total resistance is correct, the resistance seen by the wiper isn't but 47K pots would be OK.

Thank you Mooly.

Kindly try this link
http://construyasuvideorockola.com/d_eq_5bandas.php

the ">>>Descargue aqui<<" at the bottom of the page leads to the pdf

"Perhaps this might open.

 

[Mooly]

The opamp biasing is OK because they are configured differently (inverting with the bias applied to the + input via the two 10K's to set the mid point voltage)

 

[csom]

Thank you Mooly.

I also want to know more about this differential opamp configuration done by Mr Mile
http://www.diyaudio.com/forums/analog-line-level/167363-mic-line-eq-preamps-3.html#post2198562

I built this some days ago.
http://www.diyaudio.com/forums/analog-line-level/167363-mic-line-eq-preamps-41.html#post3535016

Equalizer section is very good but still the over all gain is low.
And the mic is simply unusable for me. ( very low gain and when I increase the volume the sound of mic becomes distorted. )

Mr Mile has given a solution but that didn't work for me. Perhaps I didn't understand that fully as my microphone has a two pin jack.
http://www.diyaudio.com/forums/analog-line-level/167363-mic-line-eq-preamps-42.html#post3545100
Though many people got good result from this project and praised it.

How can I mod the circuit so that I can increase the gain of the Equalizer as well as the microphone?

We all know that Mr Mile is a very experienced person but I need to learn and understand and fix my problem of a just finished project( ML3).
That's why I am asking you as I feel more at home with your way of explaining things.

 

[Mooly]

A lot of these circuits are OK but not perhaps suitable for your needs. The ML3 circuit has balanced inputs for example which you probably don't want. You also probably need a dedicated separate mic preamp as I think I mentioned before. The first opamp stage can easily be changed to normal unbalanced mode and given more gain.

You need to read and study basic opamp theory and then you will understand how the gain and so on is set by the external resistors.

 

[csom]

A lot of these circuits are OK but not perhaps suitable for your needs. The ML3 circuit has balanced inputs for example which you probably don't want. You also probably need a dedicated separate mic preamp as I think I mentioned before. The first opamp stage can easily be changed to normal unbalanced mode and given more gain.

You need to read and study basic opamp theory and then you will understand how the gain and so on is set by the external resistors.

Thank you really so much Mooly!
This is a great help once again!

I had read about op-amps theory ( and used 741 and LM324 earlier) really long ago ( some 20 years ago or more I guess ) But after that I am totally cut off from electronics, math etc. due to a completely different professional life.

So now is the time for me to re-learn everything. And this time I have a great resource person like you beside me. So I feel fortunate and privileged being here

I will make a dedicated microphone circuit and read about the op-amps theory again after I finish the on going LM3886 tri-amping project and ask for help to you again if I get stuck.

Thank you once again.

 

[Mooly]

You are welcome, good luck