Why is this mic eq more cuting than boosting? help

Joseph R · 28th May 2010, 09:25 PM

Hi,

I have build this mic preamp with semi parametric eq and it is more cutting than boosting. Cut is around 10db but boost only around 3db. Why?

bob91343 · 28th May 2010, 10:37 PM

Probably because you are using log pots for the controls. Use linear pots and it will likely do what you want. In other words, the 'flat' position may not be in the middle.

mickeymoose · 28th May 2010, 11:05 PM

Well, I must say that I love the connection between pin 2 +3 on the first (left-most) IC (not numbered). Than, I do not know what U1A + B are supposed to do. Please explain.
The tone controls seem to be OK. I would take each section of the circuit and run it through LTSpice individually and see what they do. E

Joseph R · 30th May 2010, 01:51 PM

Thanks bob and mickey

The short between pin 2 and 3 is only on the schematic. Not on PCB.
I think that bob could be right. I think I have put a log pot in place of R20 but must be Lin pot.
I have already spiced the whole schematic and it worked fine so it must be a pot problem.
I will go back tomorrow to my workshop and check it.
Thanks

mickeymoose · 30th May 2010, 05:56 PM

bob is right, when you use the wrong type of pot at the centre pos it will not give you the same result. But bob is wrong, at the end position you should have the same boost/cut as the other pot. So, maybe, there is a wiring or parts problem somewheere. E

Joseph R · 30th May 2010, 09:49 PM

Yep true mickey.
I did some more spice and did find out that if I turn down the input volume R5 the asymetrical behavior is even more pronounced and if I turn full up R5 it is much more symetrical.
I did ad a buffer between R5 and the tone control and it looks to work perfectly. I try this tomorow on the board.

Joseph R · 7th June 2010, 03:58 PM

Finally I had some time to fix it. I aded a buffer between the pot and eq stage and it works perfectly with +-15db of boost/cut.
This is what it looks like.
Thanks for your help.

Relz · 27th November 2012, 05:15 PM

Hi Jospeh you said you got this circuit to work perfectly, I am just wondering did you make any more changes. I also built this circuit with added buffer but i am getting a low frequency hum when using the sweep & bass pot, did you have any other problems?

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28th May 2010, 10:37 PM	#2
bob91343 diyAudio Member Join Date: Mar 2010	Probably because you are using log pots for the controls. Use linear pots and it will likely do what you want. In other words, the 'flat' position may not be in the middle.

28th May 2010, 11:05 PM	#3
mickeymoose diyAudio Member Join Date: Nov 2008	Well, I must say that I love the connection between pin 2 +3 on the first (left-most) IC (not numbered). Than, I do not know what U1A + B are supposed to do. Please explain. The tone controls seem to be OK. I would take each section of the circuit and run it through LTSpice individually and see what they do. E

30th May 2010, 01:51 PM	#4
Joseph R diyAudio Member Join Date: Oct 2008	Thanks bob and mickey The short between pin 2 and 3 is only on the schematic. Not on PCB. I think that bob could be right. I think I have put a log pot in place of R20 but must be Lin pot. I have already spiced the whole schematic and it worked fine so it must be a pot problem. I will go back tomorrow to my workshop and check it. Thanks

30th May 2010, 05:56 PM	#5
mickeymoose diyAudio Member Join Date: Nov 2008	bob is right, when you use the wrong type of pot at the centre pos it will not give you the same result. But bob is wrong, at the end position you should have the same boost/cut as the other pot. So, maybe, there is a wiring or parts problem somewheere. E

30th May 2010, 09:49 PM	#6
Joseph R diyAudio Member Join Date: Oct 2008	Yep true mickey. I did some more spice and did find out that if I turn down the input volume R5 the asymetrical behavior is even more pronounced and if I turn full up R5 it is much more symetrical. I did ad a buffer between R5 and the tone control and it looks to work perfectly. I try this tomorow on the board.

27th November 2012, 05:15 PM	#8
Relz diyAudio Member Join Date: Nov 2012 Location: France	Hi Jospeh you said you got this circuit to work perfectly, I am just wondering did you make any more changes. I also built this circuit with added buffer but i am getting a low frequency hum when using the sweep & bass pot, did you have any other problems?