Amp design attempt number 2 (simpler) - Page 23 - diyAudio
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Old 14th August 2013, 08:09 AM   #221
mcd99uk is offline mcd99uk  United Kingdom
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Keantoken,

Another couple of ideas to try, thank you.

When you say inject the square wave via a 1K resistor do you mean taking the output of the amp and using that as the source.

Just so I understand why I'm changing the transistors (Q5,6) which characteristic is likely/possible to cause a change in behaviour?
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Old 14th August 2013, 08:13 AM   #222
mcd99uk is offline mcd99uk  United Kingdom
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Magic,

I shall be having a raid of components here at work.

Always feel much happier when I have things to try. Expected to get problems somewhere along the way with this amp. To have a problem like this seems like a minor one compared to what could have happened. At least the amp works, it's just "tweaking" really.

Thank you

Paul
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Old 15th August 2013, 07:43 AM   #223
mcd99uk is offline mcd99uk  United Kingdom
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Well I have made some more progress.

It's not the driver transistors and its not a local oscillation. Slowing the amp down does not help either.

Have worked out that the problem is with the control loop U11,12,13,14. Have also discovered that the situation is helped but not solved with a capacitor between the emitters of the VAS/TIS outputs.

Just got to work out how to stabilize this loop.

Last edited by mcd99uk; 15th August 2013 at 07:49 AM.
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Old 15th August 2013, 11:58 AM   #224
astx is offline astx  Austria
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If I read your schematic correctly you introduced a diode based capacitance directly between the two gates.
Have you removed D9_B - D12_B just to see if this helps?

BR, Toni
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Old 15th August 2013, 12:11 PM   #225
mcd99uk is offline mcd99uk  United Kingdom
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Toni,

Thank you for taking the time to look at my schematic. No I haven't removed the diodes. Another thing to try.

One thing I noticed was that if the power rails come up at the same time it oscillates. If you delay one of the power rails by a second or more it does not oscillate. Then I decreased the gate stoppers again and the same oscillation reappears regardless of power rail power up timings.

Paul
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Old 15th August 2013, 07:28 PM   #226
mcd99uk is offline mcd99uk  United Kingdom
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What a rookie error!!!!!

Messed up the ground scheme. Had the signal in ground sharing a trace with the feedback shunt resistor ground. The parasitics of the track allowed a feedback to happen via the signal in shunt capacitor and pulldown resistor back into the amp.

one track cut and new ground added and no more oscillation. Now to rebuild the amp back to its correct state.



This is a great learning curve. Loving it!
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Old 15th August 2013, 07:33 PM   #227
AndrewT is online now AndrewT  Scotland
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Could you explain this further?
Maybe a diagram of the before and after?

Usually the Signal Ground is the correct place to take the NFB, but from my reading of your description you are telling us that was the error !
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Old 15th August 2013, 07:37 PM   #228
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Quote:
Originally Posted by mcd99uk View Post
What a rookie error!!!!!

Messed up the ground scheme. Had the signal in ground sharing a trace with the feedback shunt resistor ground. The parasitics of the track allowed a feedback to happen via the signal in shunt capacitor and pulldown resistor back into the amp.

one track cut and new ground added and no more oscillation. Now to rebuild the amp back to its correct state.



This is a great learning curve. Loving it!
Beautiful! That's fun stuff (after finding it, not before obviously). Great you got it to work
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Old 15th August 2013, 07:51 PM   #229
mcd99uk is offline mcd99uk  United Kingdom
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Andrew,

Here's a diagram. Got some noise minor issues but two smps supplies are not helping.

It was the parasitics of the track that was causing the problem. The current flow of the feedback was changing the voltage at the signal input ground and feedback back into the amp.

Whether this grounding system is correct, I'm not sure. For the real thing the signal ground will go to its own star point away from the PSU grounds.

Magic,

Yes, it gives a sense of satisfaction.
I'm a stubborn type of person. It was only a matter of time.
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Last edited by mcd99uk; 15th August 2013 at 07:56 PM.
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Old 16th August 2013, 10:33 AM   #230
AndrewT is online now AndrewT  Scotland
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What you have done is not what I would do.

Instead, are you willing to try a further change?
Take R11 direct to R12. The bottom of R12 becomes your Signal Ground reference point. All traces around here MUST have lowest loop area: the input pair and the NFB pair. The input references there, R12 references there and R11 references there. NO OTHER signal uses any of that trace arrangement.

Now take a reference from your SIGNAL GROUND to your main Audio Ground (I think you are calling this your "Star point".
As a further experiment insert a small resistor value into this last connection, maybe around 10r.
But you must bypass this resistor (if added) with a pair of inverse parallel diodes that can pass a fault current from Input Interconnect to the Chassis Earth.

Both (Rinsert=10r & Rinsert=0r0) these arrangements should work equally well when the amp is not connected to other equipment.
When you do connect other equipment, you will probably find that the resistor=0r0 or 10r will have a measurable and possibly an audible effect. Choose which ever is better. This can be optional, with a switch, or shorted pins, across the resistor.
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