Re: Amplifier Oscillating Help Please

From: Robert Baer (robertbaer_at_access4less.net)
Date: 10/07/04 

Date: Thu, 07 Oct 2004 00:41:07 -0700

Marlboro wrote:

> Mark <Mark@> wrote in message news:<lgk7m01p79gglrit0i7o0d07aq7esng7ol@4ax.com>...
>
> Sound like you have hard time, it happens to me once a while...
> here some of my thoughs from what you posted
>
>
>>I am more of a digital man than an analogue one. I need help please,
>>the set-up is as follows, I have an serial ADC output feeding an
>>Op-Amp feeding a 12V 40W Amplifier IC. The amp works in Class H to get
>>the output. The trouble is I am getting HF oscillations at high volume
>>levels (in rythem with the signal). You can see them on a scope, I
>>
>
> Look like its your power supply issue, its not trong enough to power
> the final Amp, since it works fine in low volume?
>
>
>>have a common ground plane for all of the circuit, I have thick tracks
>>going to the high power parts, I have a choke in line with the 7805
>>regulator for the logic. The Op-Amp is powered from the 5V logic line
>>(I only need 2V swing to drive output amplifier fully. I have tried
>>putting a heavy choke to the power supply input of the power amplifier
>>section, which did not help, I have tried putting a cap across the
>>input to the output amp, this helped a lot (with some decrease in
>>treble) but did not cure it completely, I have tried doubling the
>>values of the two caps used for the class H operation, did nothing,
>>tried doubling the power supply reservoir capacitor, this did nothing,
>>tried isolating the OP-Amp power supply with a diode and a smoothing
>>cap, this did nothing, also tried adding a choke to the opamp supply,
>>again nothing.
>>
>
> Have you try to power the opam & ADC from a COMPLETE separated supply
> from the big Amp? This may give a clue of what going on. Another
> though is that you may have ground/power plans/tracks improper design.
> Review your PCB and check that if any output may have the "return
> current" to your sensitive components such as opam & ADC...Try to
> isolate the GND plan/or temporary re-route your tracks with wires if
> necessary,
>
>
>>The thing that is throwing me is my Tektronics scope. When I put a
>>heavy connection wire between the Ground input of the scope and the
>>Ground of the circuit under test, and I put my probe onto the Ground
>>of the circuit under test (even at the same point where the heavy
>>ground wire is, I can see the HF oscillations on the screen. Trouble
>>is this is throwing me off the scent as to where the Oscillations are
>>coming from.
>>
>>Can anybody give me some pointers please, I am starting to think that
>>I should have not taken on an Analogue design. Any web sites giving
>>
>
> Hmmm, dont give it up easy cauze you'll learn a great deal when you
> beat the beast. Frankly I do not work with audio but video, and its
> very sensitive to noise, osc,...specially when it is
> hi-res/progressive scan. Most of the time
> I found its the ADC/DAC issues, these chips need to be take care alot:
> clean
> power source, clean clock, good PCB design with proper GND/POWER
> plans. The next important is always the power supply...
>
>
>>hints and tips on this subject would also be appreciated.
>>
>
> Anyway, it's hard to tell cauze can't see your circuit & PCB, but
> believe there
> always an answer to that, maybe other folks have better ideas.
>
> cheers,
>

   Regarding the power supply, adding an inductor to a 7805 may make
things worse.
   Use two 0.22uF mylar caps, one right at the input and the other right
at the output of the 7805.
   Then try disconnecting that power amp (the class H) to see if things
are OK.

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