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General Discussions => The Lab => Topic started by: electrichild on September 07, 2008, 02:41:02 PM

Title: amek m2500 eq Q pot...
Post by: electrichild on September 07, 2008, 02:41:02 PM

Hello,

I'm realizing an EQ based on this schematic

http://www.gyraf.dk/schematics/amek_m2500_eq.gif

and just finished first HF section: frequencies' pot and gain pot work properly but Q pot (R7) must remain fixed otherwise moving it all goes into severe oscillation...any suggestion?

Thankz

Title: amek m2500 eq Q pot...
Post by: 12afael on September 07, 2008, 03:44:56 PM

check your wiring an values. it shouldn`t oscilate.

what op amp are you using?

Title: amek m2500 eq Q pot...
Post by: electrichild on September 07, 2008, 04:52:19 PM

Quote from: "12afael"

check your wiring an values. it shouldn`t oscilate.

what op amp are you using?

Checkin the schems and your version (amek+harrisonFV2) in your old posts I found a little difference in the position of R11 1k (original schem) that is between pin2-C4 and R138 freq pot and your R6 1k that is between freq pot and pin7: can this cause the oscillation???
Other issue is extreme distorsion applying the maximum gain...
I'm using TL074CN opa
I retry check tomorrow

Thankz

Title: amek m2500 eq Q pot...
Post by: 12afael on September 07, 2008, 11:25:33 PM

I don`t think so, it is just a variable resistor.
check your resistors if you have extreme gain an error there could be the problem.

Title: amek m2500 eq Q pot...
Post by: electrichild on September 08, 2008, 03:38:02 AM

Checking the circuit I found a bad solder point between the capacitor trough pin 1 and 2. All seems ok now but I notice something strange: after few minutes of properly working there is a drastically change in the level...too low. I retry powering up and all it's ok and again the level drops...
It's a long way...

Title: amek m2500 eq Q pot...
Post by: gyraf on September 08, 2008, 05:40:28 AM

..check for HF oscillation..

Title: amek m2500 eq Q pot...
Post by: electrichild on September 08, 2008, 01:11:28 PM

Quote from: "gyraf"

..check for HF oscillation..

How? :?

Title: amek m2500 eq Q pot...
Post by: 12afael on September 08, 2008, 10:18:10 PM

mmm is not so easy without a scope. The IC is unusually hot? maybe try a polyester cap between the ic supply pins and ground.

this is more a question than an answer, it`s possible to rectify the output and measuring DC to verify high-frequency oscillations?

Title: amek m2500 eq Q pot...
Post by: electrichild on September 09, 2008, 03:23:43 AM

I placed 2 small caps (0.1uF) between + and - supply rails and ground...