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:
By giacintoa at 2011-10-21
By giacintoa at 2011-10-21
Thank you so much for advices!
Regards!
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:
An externally hosted image should be here but it was not working when we last tested it.
By giacintoa at 2011-10-21
An externally hosted image should be here but it was not working when we last tested it.
By giacintoa at 2011-10-21
Thank you so much for advices!
Regards!
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.
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.
The pattern of results should give a clue to what might be happening.
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!
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!
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.
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.
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 !!!!
And you then changed it to DC coupled !!!!
Ehm....
Uhm....
Well....
Yes!😱
😀
Understood... I'm heating the iron....😉
Thank you so much, Andrew!
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.
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.
Soldered a 10uf petp cap on the input...
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!🙂
An externally hosted image should be here but it was not working when we last tested it.
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!🙂
Whoa! Readed now, sorry Andrew! I will solder next block and then measure again! Thank you very much!
post10 pic.
Could you label the wires/cables in the pic.
I think there is something wrong with your Grounding and Returns.
Could you label the wires/cables in the pic.
I think there is something wrong with your Grounding and Returns.
Here it is! I hope it's possible to read!
An externally hosted image should be here but it was not working when we last tested it.
OK! Rebiased both blocks and measured:
around 4,6mv dc in every situation: no, input, input in and source off, input in and source on!
Is it ok?
Thank you!
around 4,6mv dc in every situation: no, input, input in and source off, input in and source on!
Is it ok?
Thank you!
4-5mV is very good.
But, hook up a scope if you have one and have a look at whats on the output and at what amplitude. Your RF noise may turn out to be just common noise which i'd say is normal.
But, hook up a scope if you have one and have a look at whats on the output and at what amplitude. Your RF noise may turn out to be just common noise which i'd say is normal.
Hi Tekko!
Unfortunately I don't have a scope but, believe me, anyway I couldn't use it, so...🙂
Now the hiss is so low that on 95db speakers I have to stick the ear to the tweeter to hear it, so I think should be ok...
Let's say that my class-T amps produce more noise...
Let's say that I hoped to avoid a 10uf cap on signal path (using a B1 buffer that already has a cap that size on the output) but it seems it was a really bad idea...😀
Anyway, I'm listening to this amp and I like it... Dynamic and musical... Seems also more...ehm... "fluid" compared to 3875 and 3886 gainclones, for example....
- Status
- Not open for further replies.