oscillations...

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electrichild

Active member
Joined
Aug 12, 2008
Messages
35
Location
italy
Hello,
I have this problem building an Amek M2500 Eq
http://www.gyraf.dk/schematics/amek_m2500_eq.gif
all seems OK but setting the HF or others frequency, gain and Q pots to max the entire section goes into oscillation: it' s audible at low listening levels and sounds like continuous clicks or pops. Setting the gain to normal levels after few minutes disappears again.
When there is no oscillation anyway the sounds have a little and audible distortion compared to bypassed sound.
Just tried to view in an frequency analyzer (I haven't a scope) and found some pulses 50-60Hz at -70dB.
I leave the circuit unbalanced.
Any suggestions?
Thanx
 
There are thousand and one things to go wrong in such a complicated circuit (the quality of the mains is probably the last thing I'd check though); you need to show at least a couple of pics so that we have a chance for a minuscule idea where to start. If there is audible distortion it might be a sign that there is still some more subtile instability going on. What about noise levels?

Samuel
 
what opamps are you using?  TLO74 are pretty resistant to HF oscillations due to their low speed.   If you used something faster then you might have introduced the instability since the circuit was designed around the TLO74.  Improper decoupling can have an effect here too.  So can wiring.

Can you tell us more about how you made this?

You said clicks and pops..  like how?  Clicks and pops are not the normal sounds you would expect from oscillations.  You would expect that from bad grounding and/or DC on your output.





 
I haven´t oscillation problems with my nemek. not completly a amek2500 eq but very, very close.

can you show us your layout?
 
OK,
I post all I have...first is the eq output noise MP3. I edit the wave with a digitally gain change (+6db) to make it more audible. 
http://www.nipperstudio.net/eqnoisesoscillations.mp3
These are shots of the protoboard, my first project (hope you can magnify) don't kill me...
Used TL074s, some Wima, Panasonic and polystrene caps
http://www.nipperstudio.net/proto5.jpg
http://www.nipperstudio.net/proto7.jpg
Pots are mounted separately and connected for now with...some wires (I know is far from elegant solution!)
http://www.nipperstudio.net/pots.jpg
The PSU toroidal trafo 220/18 (double output) VAC and 2 simple AC/DC converters for 2 channels based on LM317. The variable output is set to + and - 16V.
http://www.nipperstudio.net/psu.jpg
Thanx for your time and help guys!
 
that's a good follow up Electrichild.

Those seem to be some kind of oscillation but not like any I've heard before.  Those are definitely very low speed oscillations, more like slow ramping. 

Let me think about this a little bit.

BTW, that is some serious protoboard work!

 
See if you can read the thread about when I built Samuel's shared-gain preamps on proto board which had oscillations.

Make sure your boards are suitably earthed; use star grounding where possible. The distortion you describe will possibly be the result of oscillation. You may notice the opamps get hot, even when idle.
 
rodabod said:
See if you can read the thread about when I built Samuel's shared-gain preamps on proto board which had oscillations.
I see you have thousand of posts: what can I search exactly?

rodabod said:
Make sure your boards are suitably earthed; use star grounding where possible. The distortion you describe will possibly be the result of oscillation. You may notice the opamps get hot, even when idle.
Grounding was an issue because my rack case had varnished parts (all exept the front aluminium panel) I removed for good connections. The opas after many hours of checks are cold.
Thank you
 
The oscillation changes with time--did the circuit decide so or did you change EQ settings? I've seen/heard similar effects when opamps went into a multivibrator state due to internal thermal feedback, but I doubt this is the case here. Would be interesting to know if there is additional HF oscillation going on; any chance you can get your hands on a scope?

The wiring of the pots frightens me. I think you need to temporarily replace them with local resistors (chosen for values simulating the pot positions giving oscillation). If the oscillation dies then you need to think about the pot wiring (placement and perhaps screening). Otherwise I'd check the grounding and decoupling next.

Samuel
 
Samuel Groner said:
The oscillation changes with time--did the circuit decide so or did you change EQ settings?
Yes I made changes on pots during the recording so you hear oscillation of single frequency band with all relative pots to max.
Samuel Groner said:
Would be interesting to know if there is additional HF oscillation going on; any chance you can get your hands on a scope?
I don't for now...my next instrument in recording studio will be a scope!
Samuel Groner said:
The wiring of the pots frightens me. I think you need to temporarily replace them with local resistors (chosen for values simulating the pot positions giving oscillation). If the oscillation dies then you need to think about the pot wiring (placement and perhaps screening). Otherwise I'd check the grounding and decoupling next.
Very good idea I'll report the results next days
Thanx a lot Samuel
 
electrichild said:
rodabod said:
See if you can read the thread about when I built Samuel's shared-gain preamps on proto board which had oscillations.
I see you have thousand of posts: what can I search exactly?

Try here:

http://www.groupdiy.com/index.php?topic=18300.0

 
OK,
just rewiring all the things to make pots wires shorter I tried some empirical checks on the protoboard: around the TL074 I found that pin 1, 9 and 14 (with power on) are connected togheter. On protoboard wire (thin copper) that comes from pin 1 to 9 "crosses" the wire from pin 14 one on the front the other on the rear. Oscillation starts when there is no signal. Simply touching one of them with the tester pointer oscillation stops. I suppose there is something wrong at this point and connected with gain pot.
Other thing: powering off and on stops the oscillation too.
Next step I'll try to avoid from the "crossed" wiring and check the powering.
Your suggestions welcome 
 
electrichild said:
Oscillation starts when there is no signal. Simply touching one of them with the tester pointer oscillation stops.

You probably need a small amount of capacitance on that node to ground.
 
Svart said:
electrichild said:
Oscillation starts when there is no signal. Simply touching one of them with the tester pointer oscillation stops.

You probably need a small amount of capacitance on that node to ground.

Do you mean a small decouplin cap (some picofarads?) between the node (opa pin 9-1)  and the ground? Or add some picos to the C1,6,11,16 caps between pin 13-14?
 
Hi,
I' m fixing the things: I found bad pots-wire connections that act as open tracks and the 33pF cap in parallel  between pin 13 and 14 connected only to pin 14. Can it be the cause of oscillations?
Thanx
 
I fixed all the oscillations...
it was a power issue. In fact I tried to increase the capacitance value of decoupling caps used to make 0V on the board. They were 1uF 35V. I used 220uF instead (just because I have this value) and oscillations desappeared.
I imagine a smaller value can go well too...
Thanks for your useful suggestions
Respect
 

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