Well, the resistor was in place then.
I will give it a go. I guess it was just some PPI amps that there was a possible concern that I read about.
I will give it a go. I guess it was just some PPI amps that there was a possible concern that I read about.
What I'm trying to see is if taking the Zobel out of the circuit (and changing nothing else) makes the problem better, worse or the same.
Looking back I see you said earlier, 'with no signal'.
I knew I read somewhere that there was a possibility that some amplifiers would not run properly without the Zobel network intact, so I wanted to make sure I understood what you were saying so that I did not damage something else. I looked it up and saw you were referring to PPI amplifiers.
Rx78 is removed in all channels. The outputs, no input signal, all look identical now.
Very similar to Ch 4 in post #41, with the exception that the horizontal line is flat with the ringing still present every ~ third division.
Also the buzzing I could hear earlier is greatly diminished when a signal is input.
Should I install the resistors in channels 1,2 and 4 and then compare to Ch 3 w/o the resistor?
I knew I read somewhere that there was a possibility that some amplifiers would not run properly without the Zobel network intact, so I wanted to make sure I understood what you were saying so that I did not damage something else. I looked it up and saw you were referring to PPI amplifiers.
Rx78 is removed in all channels. The outputs, no input signal, all look identical now.
Very similar to Ch 4 in post #41, with the exception that the horizontal line is flat with the ringing still present every ~ third division.
Also the buzzing I could hear earlier is greatly diminished when a signal is input.
Should I install the resistors in channels 1,2 and 4 and then compare to Ch 3 w/o the resistor?
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Do you still have the problem of the bias adjustment causing the idle current to go up and not go down until remote cycling?
Do you see less noise if you ground the scope probe to the non-bridging terminal of each channel?
The pulses you see are likely just noise from the power supply switching and would not be audible.
Do you see less noise if you ground the scope probe to the non-bridging terminal of each channel?
The pulses you see are likely just noise from the power supply switching and would not be audible.
It appears to have resolved that problem. I am able to adjust Ch 3 bias without oscillation.
I have been taking all readings with the scope probe ground on the non-bridging terminals.
I have been taking all readings with the scope probe ground on the non-bridging terminals.
They are all ~ the same but not in tolerance of 5.11K 1%.
These were measured in place, but all Rx78 resistors still removed.
Ch1: 4.60
Ch2: 4.61
Ch3: 4.60
Ch4: 4.61
These were measured in place, but all Rx78 resistors still removed.
Ch1: 4.60
Ch2: 4.61
Ch3: 4.60
Ch4: 4.61
All close to the limit of 5% tolerance.
Ch1: 10.50
Ch2: 10.44
Ch3: 10.45
Ch4: 10.53
Ch1: 10.50
Ch2: 10.44
Ch3: 10.45
Ch4: 10.53
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I don't really know which way to go. I don't like to re-engineer an amp. It could be that the circuit, as designed, is perfectly fine and the problem is due to a problem with the layout on the board.
One option would be to leave the Zobel out of the circuit. They had multiple a2 series amps that had nothing but the 5.11k on the output.
You could try to adjust the feedback capacitors to reduce the gain at high frequencies. This would require changing two small capacitors in the channels causing problems.
One option would be to leave the Zobel out of the circuit. They had multiple a2 series amps that had nothing but the 5.11k on the output.
You could try to adjust the feedback capacitors to reduce the gain at high frequencies. This would require changing two small capacitors in the channels causing problems.
Makes sense, especially since like you mentioned and I have seen in my searches there have been several other 400a4s with similar problems, but no definitive resolution, that I have seen.
Please correct me if I'm wrong, but isn't the reason for the zobel network is to allow the amplifier to 'see' the load as resistive at higher frequencies to prevent oscillation? What is the downside of leaving it out?
So, I am thinking Cx15 would be one, would Cx14 be the other feedback capacitor?
Please correct me if I'm wrong, but isn't the reason for the zobel network is to allow the amplifier to 'see' the load as resistive at higher frequencies to prevent oscillation? What is the downside of leaving it out?
So, I am thinking Cx15 would be one, would Cx14 be the other feedback capacitor?
I've seen it stated as being used to stabilize the amp to drive inductive loads but it's all the same.
I The 100a2, 160a2 and 250a2 (at least) have nothing but the 5.11k resistor across the output.
I don't know if there will be a problem if left out but I'd suggest testing with multiple different loads to confirm that it's stable (no oscillation).
Those are the capacitors. It may not take much of a change to make a difference.
Was it always ch3 that caused the problem on the other 400a4s?
I The 100a2, 160a2 and 250a2 (at least) have nothing but the 5.11k resistor across the output.
I don't know if there will be a problem if left out but I'd suggest testing with multiple different loads to confirm that it's stable (no oscillation).
Those are the capacitors. It may not take much of a change to make a difference.
Was it always ch3 that caused the problem on the other 400a4s?
Perry,
Did you mention that you had another schematic of a 400a4?
The one we have been using is exactly like mine in all respects except the crossover and input section is different.
My amplifier uses 11-TL072s and the schematic we have been using shows 8.
Also all of the designations are different using two digit numbering ('common parts') as opposed to specific channel designations.
I was mistaken earlier when I said the audible humming went away with removal of the zobel network.
However, it does go away if I probe the + side of the capacitor (C314)on the input of the output stage with the oscilloscope.
Also the amplitude of the signal from the input stage is not equal between Ch3 and Ch4. So I believe there is a problem in the input stage.
I have one resistor that I think is out, but the top looks damaged and I cannot read the value. I know it starts with 30x and reads 8.4k with the meter. Of course the designation R92 is not on the diagram we have been using.
Did you mention that you had another schematic of a 400a4?
The one we have been using is exactly like mine in all respects except the crossover and input section is different.
My amplifier uses 11-TL072s and the schematic we have been using shows 8.
Also all of the designations are different using two digit numbering ('common parts') as opposed to specific channel designations.
I was mistaken earlier when I said the audible humming went away with removal of the zobel network.
However, it does go away if I probe the + side of the capacitor (C314)on the input of the output stage with the oscilloscope.
Also the amplitude of the signal from the input stage is not equal between Ch3 and Ch4. So I believe there is a problem in the input stage.
I have one resistor that I think is out, but the top looks damaged and I cannot read the value. I know it starts with 30x and reads 8.4k with the meter. Of course the designation R92 is not on the diagram we have been using.
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These are all of the 4ch 400s I have. I treated it being a different file size as being different.
if there's any way i can help you out, i have a fully working 400a4, (non-x-card version) i was about to see how much i could sell it -as-is its not vary shinny outside anymore - but still works 100% .. but i can identify any chips or such for you ...
Attachments
Thank you for the offer.
I still have it opened up on the bench, but have gotten busy with work so I haven't had time to continue troubleshooting it. It will probably be a couple of weeks before I get back to it, unless we get a rainy spell some time soon.
Back to this amp.
I am still getting hum from amplifier with no load that varies with input frequency. It is audible from 400 Hz to 1.7 kHz.
It sounds like this http://www.bcae1.com/temp/cooleditmixedoscillator01.wav
The only thing that gets rid of it is by putting Ch1 and Ch2 crossover switch to low pass.
It also changes pitch slightly when I touch the positive speaker output on Ch1 with the o-scope probe.
The input source does not seem to make any difference. i.e. head unit, signal generator and phone.
I am still getting hum from amplifier with no load that varies with input frequency. It is audible from 400 Hz to 1.7 kHz.
It sounds like this http://www.bcae1.com/temp/cooleditmixedoscillator01.wav
The only thing that gets rid of it is by putting Ch1 and Ch2 crossover switch to low pass.
It also changes pitch slightly when I touch the positive speaker output on Ch1 with the o-scope probe.
The input source does not seem to make any difference. i.e. head unit, signal generator and phone.