Symasym and RF noise (?)... Ehm... Need help... - diyAudio
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Old 22nd October 2011, 12:18 AM   #1
Porfido is offline Porfido  Italy
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Default Symasym and RF noise (?)... Ehm... Need help...

Hi everybody,
first of all, as usual, sorry for my bad english...
Some days ago I finished building a pair of symasym 5.3 monoblocks (no input caps) and I'm really pleased about the sound but of course there is a little problem:
with no signal in the amps are totally quiet, but with signal in there is a little noise...
Using them with 86db speakers is quite impossible to hear (even with head near tweeter) but using my Pi speakers (95db) I can hear something also when my ear is 30cm far from speaker....
That's not a big problem, but I really don't like the idea!
Being sincere, I'm not sure it's exactly RF noise: it doesn't seem the typical hum, but it's quite similar to the RF noise I had on some gainclones I built....
In that case, I solved the problem putting a 330pf cap between the imputs...
Do you think I should try in this case also?
I already put two ground loop breakers in these amps and the cases are of course earthed....
Let me put some images:
Click the image to open in full size.
By giacintoa at 2011-10-21
Click the image to open in full size.
By giacintoa at 2011-10-21

Thank you so much for advices!
Regards!
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Old 22nd October 2011, 08:11 AM   #2
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Check if the amps are oscillating and getting excessively hot. Also check the DC offset on the outputs, if its more than around 100mV, you might wanna use the input caps after all, up to around 50mV is considered ok but less is ofc better, especially with high eff speakers as the cones get displaced more from center with increased distortion as a result.
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Old 22nd October 2011, 09:57 AM   #3
AndrewT is offline AndrewT  Scotland
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Measure the output Vac and Vdc with the input shorted, with the input open, with the input connected to source but source to off, and again with source switched to on.

The pattern of results should give a clue to what might be happening.
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Old 22nd October 2011, 12:22 PM   #4
Porfido is offline Porfido  Italy
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Hi guys, thank you very much for your replies!
The heatsinks are not getting very hot, even after hours of playing....
Here are some measurements:

input open: -4,9mv dc
input in (dac/pre off) : +45mv dc
input in (dac/pre on) : +59mv dc
input in (dac and buffer on): +78mv dc and 1,1mv ac
input shorted : +31mv dc

Mmmmmmmmh....
What can you guess from this?
Thank you very much, guys!
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Old 22nd October 2011, 03:08 PM   #5
AndrewT is offline AndrewT  Scotland
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The amplifier is DC coupled.
The input/source connection is changing the DC bias around the amp input stage.

I never recommend DC coupling for beginners. Too much can go wrong.
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Old 22nd October 2011, 03:24 PM   #6
Porfido is offline Porfido  Italy
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Hi Andrew!
Are you suggesting me to put the cap back, then?
Thank you!
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Old 22nd October 2011, 03:27 PM   #7
AndrewT is offline AndrewT  Scotland
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are you saying it was AC coupled with DC blocking caps in both the input and in the NFB route?
And you then changed it to DC coupled !!!!
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Old 22nd October 2011, 03:34 PM   #8
Porfido is offline Porfido  Italy
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Quote:
Originally Posted by AndrewT View Post
are you saying it was AC coupled with DC blocking caps in both the input and in the NFB route?
And you then changed it to DC coupled !!!!
Ehm....
Uhm....
Well....
Yes!


Understood... I'm heating the iron....
Thank you so much, Andrew!
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Old 22nd October 2011, 04:27 PM   #9
AndrewT is offline AndrewT  Scotland
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When you have it back to AC coupled.
Go back through and obtain all 8 test measurements.
Measure the Rail currents. Add in a 1R0 resistor if there is nothing there to measure Vdrop.
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Old 22nd October 2011, 04:27 PM   #10
Porfido is offline Porfido  Italy
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Soldered a 10uf petp cap on the input...
Click the image to open in full size.
By giacintoa at 2011-10-22
Well... Ehm... Dead silent amp and 2,1mv dc offset with source on....
I guess I should think twice before modifying something I don't know....

THANK YOU! AND THANK YOU AGAIN!
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