M
Mark
Guest
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
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.
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
hints and tips on this subject would also be appreciated.
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
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.
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
hints and tips on this subject would also be appreciated.