741 op.amp works, tl 071 is oscillating?

[zoranaudio]

Originally posted by megajocke
It depends on a lot of other things in the circuit, I'd just try with another value and see what happens. I'd guess it needs to have a lower value but I'm not sure.

You could try it and see if it gets less stable or more stable.

Originally posted by wg_ski



That cap is the key to getting this stable. I've made many many many of these to work - including kilowatters. In addition to the 90 degrees thru the opamp and the 90 thru the VAS, you get a few extra degrees through the outputs. Normally you can ignore them, but not here. Tap the lead compensation cap from the output of the VAS, NOT the output of the amp. And the value will need to be determined experimentally, unless you do a Bode analysis of the circuit (in which case you can nail it the first time).

Ok, I will try to fix this problem , and will play a little with C10, lowering the value perhaps desolder it and with higher value, this cap is ceramic, is that the best type for this purpose?

[peranders]

Notice that Jan's Lynx amp is the same as you have built but with a upgraded design.

http://www.audio-circuit.dk/index.ph...ects&Itemid=60
Notice that diyaudio.com doesn't like his URL

[wg_ski]

Originally posted by zoranaudio


Ok, I will try to fix this problem , and will play a little with C10, lowering the value perhaps desolder it and with higher value, this cap is ceramic, is that the best type for this purpose?

I use silver micas for these and the Miller caps. The ESR can be a bit high on micas but it doesn't matter here. Class 1 ceramics are fine, but Class 2 or 3 are absolutely NOT. Trouble is, many times you don't know which you have.

[zoranaudio]

Unfortunately I finished with this , cos nothing' s help.
I am going to make a sand with hammer.

[zoranaudio]

Originally posted by megajocke
It depends on a lot of other things in the circuit, I'd just try with another value and see what happens. I'd guess it needs to have a lower value but I'm not sure.

You could try it and see if it gets less stable or more stable.

Originally posted by wg_ski


I use silver micas for these and the Miller caps. The ESR can be a bit high on micas but it doesn't matter here. Class 1 ceramics are fine, but Class 2 or 3 are absolutely NOT. Trouble is, many times you don't know which you have.

DEAR FRIENDS, YOU DONT'T KNOW HOW MUCH I AM HAPPY
NOW THIS IS WORKING, YES, YES, YES!!!!
SORRY TO BE NERVOUS IN THE PREVIOUS POST, SO WHAT HAPPENED:
FIRST I TRIED WITHOUT THAT CAP C10, WHEN I TURN ON THE AMP, I NOTICED A SMOKE COMING OUT FROM THE R25+L1(OUTPUT HF SUPRESSOR) AND TURNED OFF.
AFTER I TRIED WITH HIGHER VALUES OF THE C10 TILL 1000 PF BUT NOTHING CHANGED!!!
FEW MINUTES AGO, A BIT MORE RELAXED COME BACK TO THE AMP AND PUT 18 PF FOR THE C10 AND AMP IS PLAYING MUSIC AT ANY POSITION OF THE TRIM. POT FOR BIASING!!!!!!!!
NOW HOW CAN I BE SURE THAT 18 PF IS THE RIGHT VALUE FOR MY AMP??????
AND HOW TO MEASURE IF , PERHAPS SOME OSCILLATIONS ARE ABOVE THE AUDIBLE FREQ. ABOVE 20 KHz??????
MY BIGGEST THANKS TO MEGAJOCKE AND WG_SKI, YOU HELPED ME, AND WILL WAIT FOR YOU IF SOMETHING I CAN CHANGE IN ORDER TO GET THE MAXIMUM FROM THIS AMPLIFIER!

[wg_ski]

If it were still oscillating, you'd have either distortion or smoke. The fact that you have neither suggests it's stable now.

To get the max out of the amp, go as low as you can on that cap without peaking in the closed loop response. If you can't measure that, use your ears. Above-band peaking will cause it to sound very "detailed" or "bright" initially, but cause fatigue from extended listening. My guesses say this will happen when you get down around 6-8pF. It may start oscillating before than anyway.

[zoranaudio]

Originally posted by wg_ski
If it were still oscillating, you'd have either distortion or smoke. The fact that you have neither suggests it's stable now.

To get the max out of the amp, go as low as you can on that cap without peaking in the closed loop response. If you can't measure that, use your ears. Above-band peaking will cause it to sound very "detailed" or "bright" initially, but cause fatigue from extended listening. My guesses say this will happen when you get down around 6-8pF. It may start oscillating before than anyway.

Dear friend!
I would like first to thank You for Your help.
No, the sound now is clear and detailed, when the amp. was oscillating, i can hear "bird song when the drums hit"
I will stay at this value of c10=18pf.
Now I must to check the bias of the power tr. .
Also I would try with changing the TL 071 with NE5534, to see if the sound is better?
Do you think that NE5534 will work now ?

[djk]

"Do you think that NE5534 will work now ?"

Sure, as long as you tack the compensation cap across the correct pins on the back side of the board. See the 5534 data sheet for the suggested values.

[zoranaudio]

All was my mistake!
The c10 was "681" cap that means 680 pf.
I solder 60 pf in place of c10 cos don't have 68pf, even amp was working with 18pf, so nothing's wrong with this scheme.

[wg_ski]

Originally posted by zoranaudio
All was my mistake!
The c10 was "681" cap that means 680 pf.
I solder 60 pf in place of c10 cos don't have 68pf, even amp was working with 18pf, so nothing's wrong with this scheme.

And "680" could mean either 680pF or 68, further adding to the confusion. If you're ever in the market for a new DMM, get one with a capacitance function.