Sontec EQ - Need help Troubleshooting High shelf switching

[elskardio]

Hi guys,

I'm working on an EQ prototype based on the MEP250.

My High frequency band works perfectly in Bell mode but the moment I switch for High shelf I get a lot of hiss and oscillation... The IC are properly bypassed. I ran some simulations in LTspice and everything looks fine.

Any hint on what could be the cause?

Thanks for your help

 

[elskardio]

Any idea? Thanks

 

[jensenmann]

I´m too lazy to search for the original schematic. But what I see is that your 5534 lacks the compensation cap. And IIRC R35 was much lower in later revisions of the 250.

 

[elskardio]

I´m too lazy to search for the original schematic. But what I see is that your 5534 lacks the compensation cap. And IIRC R35 was much lower in later revisions of the 250.

Thanks jensenmann

Here's a link to the original schematic:

http://www.gyraf.dk/schematics/Sontec_MEP250a.GIF

I'll test with a 22pf compensation cap tonight. Also I will try with the Bias circuit disconnected to see if it changes anything.

 

[Harpo]

SW-pin3 connects to the other side of R31 (only one half of the dual rev-log pot is shorted out, the 3k3 R31 remains in circuit) ...

 

[elskardio]

SW-pin3 connects to the other side of R31 (only one half of the dual rev-log pot is shorted out, the 3k3 R31 remains in circuit) ...

Thanks Harpo! That was it

I must have checked the schematic a 100 times and never saw the mistake  :

 

[gyraf]

Elskardio - I can't see the schematic you're referring to (5534?) - can you re-upload?

Jakob E.

 

[Harpo]

Jakob, buggy schematic in 1st.post displays fine here.
Maybe your browser is blocking Elskardios pic source ? https://s28.postimg.org/4m6wq5bvx/schema.png

 

[gyraf]

Thanks Harpo - you're right, it was my browser set too paranoid..

Jakob E.

 

[elskardio]

Quick question...

Should I add the compensation cap to the filter's 5534 ? The original doesn't use it... I know it's essential to run stable at unity gain... but in this particular eq circuit?

Thanks

 

[Harpo]

IIRC additional compensation cap between pins 5/8 for the NE5534 shouldn't be needed.
You are aware, the boost/cut pot is missing the center tap connection ? (pot is at least a 4-pin device)

 

[elskardio]

IIRC additional compensation cap between pins 5/8 for the NE5534 shouldn't be needed.
You are aware, the boost/cut pot is missing the center tap connection ? (pot is at least a 4-pin device)

Thanks Harpo

Yes the Center tap is connected to ground. It was pretty hard to find the pots... luckily found some Bourns that works really well!