2) noise at P11 (P18+P19 shortened), P11 via cable to probe, probe GND to alu strip
right: 15mVpp
left: 10mVpp
pictures (right, left, 1/10/100 kHz)
right: 15mVpp
left: 10mVpp
pictures (right, left, 1/10/100 kHz)
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3) noise LS output, both inputs shorted
right, left channel pictures (right, left, 1/10/100 kHz)
right, left channel pictures (right, left, 1/10/100 kHz)
4) 4x 1900uF blue caps on mains frequency + 1 picture at 0,5Hz, which is same for all 4 (regulator working?)
0,5Hz for all 4 caps
0,5Hz for all 4 caps
Hans, my guess is, that while the current P6 to P7 is normal, the noise on P11 is not. To me it looks like noise not ripple, but somehow it appear not even distributed, but more irregular around 1-10kHz.
BR
BR
Just checked: buffer P11 via C to ground generates extra noise (or louder white noise).
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Thank you for all this information.
A few things are cleared by now.
Current through P6-P7 is O.K.
The 1900uF caps are also O.K.
What I don't get is all this noise.
Noise from P11 should be amplified 21x to the LS output by the gain of the amp but it does not, they are almost the same, so I'm afraid your probe is picking up some interference from a switching power supply nearby, or some other switching device producing EMF noise.
How's the power supply of your scope, by batteries or is it a laptop or whatever ?
And what about ground loops ?
Is your amp connected to the mains gnd and what about your scope ?
And what is the resistance between the LS gnd to the Chassis, is this 10R on both sides ?
Hans
A few things are cleared by now.
Current through P6-P7 is O.K.
The 1900uF caps are also O.K.
What I don't get is all this noise.
Noise from P11 should be amplified 21x to the LS output by the gain of the amp but it does not, they are almost the same, so I'm afraid your probe is picking up some interference from a switching power supply nearby, or some other switching device producing EMF noise.
How's the power supply of your scope, by batteries or is it a laptop or whatever ?
And what about ground loops ?
Is your amp connected to the mains gnd and what about your scope ?
And what is the resistance between the LS gnd to the Chassis, is this 10R on both sides ?
Hans
Hans, what you are looking at are the measurements from 2) P11 from a 2nd, working 27.5, standing in the same spot. I ruled out led lamps with switching etc. in the room. If there is ambient EMF, it's what you see below, and the 5cm line to the probe + 4cm probe gnd line pick it up (?). Can't make it shorter to get to P11.
As you can see, there is also noise, it's less (around 10mVpp), but evenly distributed. Can't hear it from the tweeter.
The noise in the broken 27.5 seems uneven, and that baffles me. Maybe it's a red herring altogether and in the end noise from components AC-8/8.5.
I must say I have very very little experience with scopes and ground loops. Maybe I overlook something.
As you can see, there is also noise, it's less (around 10mVpp), but evenly distributed. Can't hear it from the tweeter.
The noise in the broken 27.5 seems uneven, and that baffles me. Maybe it's a red herring altogether and in the end noise from components AC-8/8.5.
I must say I have very very little experience with scopes and ground loops. Maybe I overlook something.
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same channel (LS Out, but now both inputs shorted)
If I press my ear to the tweeter, can hear this noise vaguely.
The difference to the other seems not in the amplitude (around same Vpp), but in the 'noise distribution'. If I had a spectrum analyser...
If I press my ear to the tweeter, can hear this noise vaguely.
The difference to the other seems not in the amplitude (around same Vpp), but in the 'noise distribution'. If I had a spectrum analyser...
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Input noise is mainly caused by the LM394 input pairs and two 604R resistors, totalling noise with shorted inputs to ca 5nV/rtHz giving roughly 1.5mV rms noise @ 100Khz BW times 21 for the gain is 30mV noise at the LS output.
What I see is even less, so I'm not at all alarmed, almost a bit suspicious.
What I don't understand is that you can hear this through your speakers.
30mV@8Ohm means 112uWatt !!!
Maybe you can connect your speaker and measure the output voltage at the LS output again to see if this causes oscillation.
and when you touch one of both inputs, can you hear it from the speaker?
Hans
What I see is even less, so I'm not at all alarmed, almost a bit suspicious.
What I don't understand is that you can hear this through your speakers.
30mV@8Ohm means 112uWatt !!!
Maybe you can connect your speaker and measure the output voltage at the LS output again to see if this causes oscillation.
and when you touch one of both inputs, can you hear it from the speaker?
Hans
Only barely hear it with ear directly at tweeter, but yes, it is there.
If I touch pos input (RCA via XLR adapter), I hear a loud hum from the speaker (shortening of both inputs removed of course, otherwise I hear nothing)
There measurements above were made with speakers connected (at least for the working 27.5). I will repeat the LS Out measurement to see if it makes any difference.
BR
If I touch pos input (RCA via XLR adapter), I hear a loud hum from the speaker (shortening of both inputs removed of course, otherwise I hear nothing)
There measurements above were made with speakers connected (at least for the working 27.5). I will repeat the LS Out measurement to see if it makes any difference.
BR
(off topic - while the amp is warming up for measurements, and coffee 😉
I had the chance to pick up a 20.6 some years ago in France but in the end didn't, partly because I figured I wouldn't be able to repair it (I hear it gets very hot compared to 27.5) and there is no repair source in my vicinity.
I like the 27.5 a lot, especially the bass. Probably haven't heard so many components like you guys must have, but still. Right now I have a Rowland Model 2 next to it (without the BPS), and while the Rowland seems to have more refined mids, it somehow does not have the same bass as the 27.5
I realize there are serious diy projects going on here, very interesting. Re-engineering the 20.6, wow. But I will focus on the 27.5 now and have a look later...
I had the chance to pick up a 20.6 some years ago in France but in the end didn't, partly because I figured I wouldn't be able to repair it (I hear it gets very hot compared to 27.5) and there is no repair source in my vicinity.
I like the 27.5 a lot, especially the bass. Probably haven't heard so many components like you guys must have, but still. Right now I have a Rowland Model 2 next to it (without the BPS), and while the Rowland seems to have more refined mids, it somehow does not have the same bass as the 27.5
I realize there are serious diy projects going on here, very interesting. Re-engineering the 20.6, wow. But I will focus on the 27.5 now and have a look later...
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Ok, some learnings.
Working amp, inputs shortenend.
LS connected, LS Out.
LS disonnected -> slightly less noise.
LS connected again, lamps turned off in room.
same, but LS disconnected
only minus speaker cable disconnected -> oszillation
(no clue why I get this oszillation if only pos speaker cable connected. Makes no sense, does it?)
Note how the noise in the noisy 27.5 in the earlier pictures looks different (uneven).
BR
Working amp, inputs shortenend.
LS connected, LS Out.
LS disonnected -> slightly less noise.
LS connected again, lamps turned off in room.
same, but LS disconnected
only minus speaker cable disconnected -> oszillation
(no clue why I get this oszillation if only pos speaker cable connected. Makes no sense, does it?)
Note how the noise in the noisy 27.5 in the earlier pictures looks different (uneven).
BR
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This is no oscillation but your LS with only one lead connected acts as a long antena picking up 50Hz radiation from the mains.
Have you already replaced both 680uf caps on the noisy 27.5 or only one?
It’s indeed a pitty that you don’t have a FFT, because now you see a lot of signal outside your hearing range,
Don’t you happen to have a soundcard in your PC that can record the signal as a .wav file and sent it to me ?
Hans.
P.s. Is the heatsink of your noisy 27.7 properly connected to the chassis, it should measure zero ohm between them, very important !
Have you already replaced both 680uf caps on the noisy 27.5 or only one?
It’s indeed a pitty that you don’t have a FFT, because now you see a lot of signal outside your hearing range,
Don’t you happen to have a soundcard in your PC that can record the signal as a .wav file and sent it to me ?
Hans.
P.s. Is the heatsink of your noisy 27.7 properly connected to the chassis, it should measure zero ohm between them, very important !
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Antenna -> ok, makes sense. Thanks.
Only one replaced yet, with a temp. 570uF. 680uF incoming. Should probably get a 1000uF as m.l. recommended, the sprague really measures 800uF.
Sure, I'm glad to make you a .wav, but might take some time. Have to check if my PC sound in is good enough.
Will check the chassis! (the screwed-on transistor covers don't matter, right?)
BR
Only one replaced yet, with a temp. 570uF. 680uF incoming. Should probably get a 1000uF as m.l. recommended, the sprague really measures 800uF.
Sure, I'm glad to make you a .wav, but might take some time. Have to check if my PC sound in is good enough.
Will check the chassis! (the screwed-on transistor covers don't matter, right?)
BR
Question: if both inputs (+-) are shortened against gnd (black cable), can I be absolutely certain that the OL-3 board is not interfering? Or should I remove P18, P18, P19 from AC-8.5 to check this?
When the heatsink is not tightly electrically connected to the chassis gnd, a capacitive coupling between the transistors on the heatsink will take place leading to strange and unwanted interference.
Hans