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Old 7th July 2013, 10:49 AM   #21
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Quote:
Originally Posted by Mooly View Post
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!
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Last edited by csom; 7th July 2013 at 10:53 AM. Reason: a second reading made the explanation clear
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Old 7th July 2013, 10:53 AM   #22
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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
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Old 7th July 2013, 10:56 AM   #23
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Quote:
Originally Posted by Mooly View Post
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.
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Old 7th July 2013, 11:23 AM   #24
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Your welcome.
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Old 8th July 2013, 05:48 PM   #25
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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.
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File Type: jpg pot mod.JPG (20.3 KB, 30 views)
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Old 8th July 2013, 05:57 PM   #26
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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.
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Old 8th July 2013, 06:09 PM   #27
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Originally Posted by Mooly View Post
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.
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Old 8th July 2013, 06:49 PM   #28
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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)
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Old 8th July 2013, 07:08 PM   #29
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Thank you Mooly.

I also want to know more about this differential opamp configuration done by Mr Mile
Mic, Line, EQ... Preamps

I built this some days ago.
Mic, Line, EQ... Preamps

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.
Mic, Line, EQ... Preamps
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.
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Old 9th July 2013, 06:38 AM   #30
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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.
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