I am getting severe Hum on my loudspeakers which I think is coming from my separate 15V power supply for my Pre-Amp.My Power Amp works fine on it's own with no Hum so can't be coming from there.
I've just tested the Power Supply with a scope disconnected from my system and there is no noticeable ac hum so I'm wondering whether it only gives excess Hum under load-ie connected to the Pre-amp input.Does this make sense and how should I test for AC Hum under load.
This has only been a recent problem so some kind of component failure I think.
Here is the schematic of the regulated power supply that feeds the pre-amp.
I've just tested the Power Supply with a scope disconnected from my system and there is no noticeable ac hum so I'm wondering whether it only gives excess Hum under load-ie connected to the Pre-amp input.Does this make sense and how should I test for AC Hum under load.
This has only been a recent problem so some kind of component failure I think.
Here is the schematic of the regulated power supply that feeds the pre-amp.
Attachments
What is the Voltage across C11 and C12? What is the expected load current of all the regulators? You have
a scope so how about some scope photos of C11 an C12 and any of the outputs? What is the secondary
Voltage and current rating of the transformer? We can't help you without info. My hunch is 2200uF is
insufficient and you have too low Voltage from the transformer but scope photos will narrow it down
quickly.
G²
Right, the regulators are likely dropping out due to either high input voltage ripple, or low AC line voltage.
This is a New Problem. I had no hum for a number of years with this set-up.
For a short time the hum was fixed by replacing the filter capacitors C11 and C12.However after a few days the hum returned.
For a short time the hum was fixed by replacing the filter capacitors C11 and C12.However after a few days the hum returned.
Could their voltage ratings be too low, or are these small, low ripple current parts
that are being overstressed? Have you measured the DC voltage across each?
If the voltages are under 18VDC, the regulators cannot work very well or at all.
Does the hum come and go (which could be the AC line voltage varying)?
that are being overstressed? Have you measured the DC voltage across each?
If the voltages are under 18VDC, the regulators cannot work very well or at all.
Does the hum come and go (which could be the AC line voltage varying)?
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Is this a home built or factory built unit? Was there any capacitor acid on the board? I have seen the acid
damage the board with such small voids they cannot be seen with a microscope but nevertheless are open
circuit. This is where the scope is the only tool to isolate the problem. Assume the trace is faulty and check
each point along the trace.
G²
rayma-I replaced with similar caps of same voltage rating.The unit worked in my system for over 10 years with no hum or any other issues.
stratus46-It's a home built kit from Hart Electronics with supplied parts.
stratus46-It's a home built kit from Hart Electronics with supplied parts.
Then replace them again, if possible with caps that are somewhat higher in uF, voltage rating,
and ripple current. If that doesn't work, the AC line may be low now, it happens.
and ripple current. If that doesn't work, the AC line may be low now, it happens.
The AC line voltage can often be too low, and can degrade due to a bad utility transformer,
or a bad junction box in your residence. Measure the voltage on the capacitors to see if it is adequate.
The capacitor voltages should each be over 18VDC.
How could a ground loop problem develop over time unless some wire has come loose and shorted something?
There's another thread somewhere on this site (sorry I don't remember where 😱) describing a significant low line voltage condition. It was widely variable yet persistent, bad enough for the power provider to investigate and solve, and pretty sure (IIRC) it was your country.
Without requisitioning a pencil and bar napkin, 2200uF seems a little light for 3 - 7815's. Even if they have only tiny heat sinks they can easily need enough current to produce a few volts of ripple on the filters.
We could sure use those filter cap voltage measurements -- both DC and AC would be informative. And if possible, check the line voltage within a few seconds of the other two.
Cheers
Without requisitioning a pencil and bar napkin, 2200uF seems a little light for 3 - 7815's. Even if they have only tiny heat sinks they can easily need enough current to produce a few volts of ripple on the filters.
We could sure use those filter cap voltage measurements -- both DC and AC would be informative. And if possible, check the line voltage within a few seconds of the other two.
Cheers
The op has a scope (post#1) and so the obvious test is to scope it all under load. I can't understand why that is not being done.
The measured DC voltage across the caps isn't the whole story, what matters is that the AC ripple voltage on those reservoir caps does not go below the regulators minimum drop out value.
I agree that a low line voltage could do this in a marginally specified system.
The measured DC voltage across the caps isn't the whole story, what matters is that the AC ripple voltage on those reservoir caps does not go below the regulators minimum drop out value.
I agree that a low line voltage could do this in a marginally specified system.
Thanks every one for your suggestions. I'll power up the supply connected up to everything and get some measurements as suggested.
Just hooked the scope to the output of the power supply while hooked up to the pre-amp and humming.
I'm getting a very unstable ripple of around -0.01V which fluctuates from 0V up to -0.01V every 3 seconds or so.It's also negative when it should be positive.
Also I'm getting a faint mechanical hum from the supply if I put my ear right up to it.
Looking through my notes from before It looks like I was getting 22.3V out of the filter capacitors going into the regulators which was then dropping it down to
15V (the correct output for the Supply).
I'm getting a very unstable ripple of around -0.01V which fluctuates from 0V up to -0.01V every 3 seconds or so.It's also negative when it should be positive.
Also I'm getting a faint mechanical hum from the supply if I put my ear right up to it.
Looking through my notes from before It looks like I was getting 22.3V out of the filter capacitors going into the regulators which was then dropping it down to
15V (the correct output for the Supply).
0.01v ripple is 'only' 10 millivolts. Where are you measuring that?
I would expect way more than that across the reservoir caps and quite a bit less than that on all the regulated outputs.
I would expect way more than that across the reservoir caps and quite a bit less than that on all the regulated outputs.
Quite. Sorry not to be clearer.I measured the 10 millivolts ripple at the output from the regulators so quite high.This
ripple was a lot higher when measured under load-ie.connected to the pre-amp.When measured without any load connected the ripple at the outputs from the regulators was barely measurable on my cheap scope.
Any chance this could be the transformer as there is a slight mechanical hum when my ear is right up to the unit?
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