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elskardio

Sontec EQ - Need help Troubleshooting High shelf switching
« on: January 09, 2017, 10:18:01 AM »
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

« Last Edit: January 10, 2017, 09:24:50 AM by elskardio »


elskardio

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #1 on: January 10, 2017, 09:25:34 AM »
Any idea? Thanks

jensenmann

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #2 on: January 10, 2017, 10:26:58 AM »
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.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

elskardio

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #3 on: January 10, 2017, 12:17:30 PM »
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

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #4 on: January 10, 2017, 01:40:08 PM »
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) ...
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

elskardio

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #5 on: January 11, 2017, 06:40:05 PM »
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

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #6 on: January 13, 2017, 02:40:22 AM »
Elskardio - I can't see the schematic you're referring to (5534?) - can you re-upload?

Jakob E.
« Last Edit: January 13, 2017, 02:52:24 AM by gyraf »
..note to self: don't let Harman run your company..

Harpo

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #7 on: January 13, 2017, 03:14:38 AM »
Jakob, buggy schematic in 1st.post displays fine here.
Maybe your browser is blocking Elskardios pic source ? https://s28.postimg.org/4m6wq5bvx/schema.png
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

gyraf

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #8 on: January 13, 2017, 09:25:23 AM »
Thanks Harpo - you're right, it was my browser set too paranoid.. :-)

Jakob E.
..note to self: don't let Harman run your company..

elskardio

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #9 on: January 13, 2017, 11:24:40 AM »
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

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #10 on: January 13, 2017, 12:38:00 PM »
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)
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.  -Douglas Adams

elskardio

Re: Sontec EQ - Need help Troubleshooting High shelf switching
« Reply #11 on: January 13, 2017, 12:48:53 PM »
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!