Hello everybody,
I have to state that I am not an expert: my first goal is to understand more.
For some years my Cabre AF-25 power amp has been humming through the speakers so annoyingly that listening at low volumes is really unsustainable. Lately I also noticed that the right side of chassis got very hot after a few minutes even without load, so I went to a professional for solving the problem: he repaired the right final stage (which now doesn't get hot) and replaced the electrolytic capacitors. He reported he didn't hear any humming after the job.
Back home, however, I noticed, with regret, that the buzz is always there. A few more indications:
This last consideration makes me think of the capacitors, which have also been replaced. Actually there are a couple of large ones that always seem the same to me: I don't think they have been changed, if this consideration has any value.
I have never fully understood how impedance works, but the first point suggests that it has something to do with it and can help me pinpoint the cause of the problem in the input signal path. But I can't go further than that, also because I don't have any diagram.
Do you have any ideas? Any opinion will be sincerely appreciated.
Greetings from Italy
I have to state that I am not an expert: my first goal is to understand more.
For some years my Cabre AF-25 power amp has been humming through the speakers so annoyingly that listening at low volumes is really unsustainable. Lately I also noticed that the right side of chassis got very hot after a few minutes even without load, so I went to a professional for solving the problem: he repaired the right final stage (which now doesn't get hot) and replaced the electrolytic capacitors. He reported he didn't hear any humming after the job.
Back home, however, I noticed, with regret, that the buzz is always there. A few more indications:
- the noise is very loud when I disconnect the input signal from the final, and it decreases, even though well perceptible, with the preamplifier connected. If I connect the preamp to one channel only the noise is louder in the detached channel. The intensity of the noise also varies according to the position of the potentiometers of the output levels of the preamplifier (louder in the middle, quieter at the beginning and end of the stroke);
- the speakers have nothing to do with it because I hear the noise even with the Stax from the amplified output and from the headphone level output;
- the noise is heard from all three speaker ways (I don't have an oscilloscope but analyzing the spectrum of the recorded sound it seems to me there are harmonics, I would not say a stupid thing);
- the hum is latent, in the sense that it can occur immediately or at a distance of a few minutes / hours from switching on. I have also noticed that sometimes it can go away by restarting the power amp a few times, maybe disconnecting and reconnecting the preamp between one restart and another, too. I must say, however, that since I got it from service the noise seems more persistent than before, even if slightly quieter.
This last consideration makes me think of the capacitors, which have also been replaced. Actually there are a couple of large ones that always seem the same to me: I don't think they have been changed, if this consideration has any value.
I have never fully understood how impedance works, but the first point suggests that it has something to do with it and can help me pinpoint the cause of the problem in the input signal path. But I can't go further than that, also because I don't have any diagram.
Do you have any ideas? Any opinion will be sincerely appreciated.
Greetings from Italy
If large electrolytic capacitors leak very considerably to cause enough ripple to be audible on speakers, it means, these capacitors must also get hot with the energy dissipation. Energy has to go somewhere, in this case, it transforms itself into a much higher temperature.
Another possibility may be a parasitic signal path that has not been yet found. Corroded copper tracks on the PCB may also contribute to generate a parasitic hum signal. If you have access to the inner circuitry examine the circuitry of the input stage as that is the most sensitive part of the amplifier.
A simple test to do is to connect a shorted input connector to the input, and see whether the hum vanishes. If it remains it would mean, it is the amplifier that is generating it.
You mean shorting L and R amp input? There’s no chance I’d damage anything by doing that? Thank you!
Yes short the input from center pin to ground. It won’t hurt anything. If the hum goes away it is in the input circuit. if it does not go away it is in the power supply. If the unit is over 20 years old I would change out the power supply caps with the same exact value and voltage or higher voltage new cap.
In your opening post you refer to a hum then later a buzz.
Is the noise mains 50/60Hz hum or rectified 100/120Hz which is more of a buzz. Mobile phone app can "decipher" or use online tools like audacity.
50/60Hz points to issues before rectification, eg, ground issues, loops... 100/120Hz points to after rectification, eg, main caps and...
Does the hum increase with preamp volume control?
Possibly you have a bad solder connection on the input RCA's ground connection, you could test this by measuring the resistance between the RCA outer and any black speaker connector. Repeat for RCA outer and amp chassis. Compare readings with reading when probes are touching/short. A difference of 0.5ohms will cause issues.
Is the noise mains 50/60Hz hum or rectified 100/120Hz which is more of a buzz. Mobile phone app can "decipher" or use online tools like audacity.
50/60Hz points to issues before rectification, eg, ground issues, loops... 100/120Hz points to after rectification, eg, main caps and...
Does the hum increase with preamp volume control?
Possibly you have a bad solder connection on the input RCA's ground connection, you could test this by measuring the resistance between the RCA outer and any black speaker connector. Repeat for RCA outer and amp chassis. Compare readings with reading when probes are touching/short. A difference of 0.5ohms will cause issues.
Thank you for your reply. Sorry, English is not my first language: it sounds more like a buzz to me but I am not sure at all on the frequency: I've recorded it, here it is.
The noise does not change when I adjust the volume knob on the preamp, but only when I operate the "Out Levels" knobs, that seem the last components before the preamp output. However, as I reported, the noise is at its peak with the level knobs on the preamp in middle position, and it decreases a bit with higher or lower positions.
I did the measurements: same values as touching probes between outer RCA and black speaker connectors, but no contact at all with amp chassis. The chassis is connected to earth ground, too.
Thanks, I'm now trying some top tier RCA cables from another system hoping it can help.
Hello, I see that you are in Italy and I don't know if your power plugs have a ground connection. If they do, I suggest that you power up the power amplifier with the power plug to the pre-amp disconnected. If the hum disappears, You have a ground loop problem as mbz suggests.
Greetings from England. Mike
I think that plugging the preamp into the same receptacle location as the amp, and leaving the preamp OFF, would determine if there's a problem with the preamp and/or its signal cables.
We do have ground connection in our plugs, but both my final and preamplifier are vintage products with external earth ground, so I made a sort of equipotential node for connecting every chassis to the mains ground. Clearly the first thing I tried was messing up with this setup trying every configuration: nothing has made the noise go away, except for shorting the input as edbarx suggested.
Also, the noise is subsceptible to (small) variations when I operate the "out level" knobs from the preamp even when the preamp is disconnected from the mains (and from earth ground too).
Also, the noise is subsceptible to (small) variations when I operate the "out level" knobs from the preamp even when the preamp is disconnected from the mains (and from earth ground too).
Try not using any "extra" grounding wires, just using the power cord's own ground path.
Ground loops can be an issue with those "extra" paths.
Ground loops can be an issue with those "extra" paths.
That's the point: my power cords don't have any ground path: ground connection is intended to be external on those machines. However the problem persists even if I remove the external ground wiring.
Would it be dangerous to try connecting Earth ground to audio ground and see what happens?
I cannot advise with an accurate answer.
The equipment in question is not familier to me, or their schematic/service info.
I don't like to speculate on something..... I want to know for sure.
Feeding the file into audacity gives,
A peak at 100Hz, so rectified mains. There is a lot of other noise at perceivable levels so understandable that you described it as a buzz. Will focus on the 100Hz for the time being.
While tired main filter caps could be the issue, IMO, it's more likely to be a failing voltage regulator, outside chance it's another cap in the power supply. You will probably need an oscillocope to track this down. Do you have a schematic? Didn't find it online. Posting a few hi-res pix of the internals MIGHT help, looking for obvious signs of stress...
A peak at 100Hz, so rectified mains. There is a lot of other noise at perceivable levels so understandable that you described it as a buzz. Will focus on the 100Hz for the time being.
While tired main filter caps could be the issue, IMO, it's more likely to be a failing voltage regulator, outside chance it's another cap in the power supply. You will probably need an oscillocope to track this down. Do you have a schematic? Didn't find it online. Posting a few hi-res pix of the internals MIGHT help, looking for obvious signs of stress...