Dual/Stereo mnats 1176 hum [SOLVED]

[ferrochrome]

Looks like you're in a 50Hz area,no?
The peak at 50Hz and the lower level at 100Hz (octave) normally indicates that the hum comes from pre rectifier.Maybe we should have a deeper look to the ground wiring including audio,psu amd safety earth?

Udo.

P.S.:Maybe you can add your location to your avatar/member/name (pardon,I don't know the correct word),that often makes it easier for the other members.

Hi Udo, yes I'm in Norway, so we have 230V 50hz mains going on here. Thanks for mentioning, I've updated the avatar stuff now

Pre rectifier hum:  I would think 50hz would be all over tha ac mains and the filter caps in the psu are there to filter the ripple after recitfication.

AC inlet earth is connected directly to chassi - I've never tried to disconnect that.
Ive tried a lot of different grounding points both for psu, 1176 audio grounds, and for stereolink cards now, that is including star ground at mains inlet earth and also star groiund at pin1. Are there any other options for grounding?

 

[okgb]

Strip back to the minimum , get that working and add from there
it only takes one connection , but you may have several giving you problems
is it out of a rack so not grounding with other gear as well ?

 

[ferrochrome]

Strip back to the minimum , get that working and add from there
it only takes one connection , but you may have several giving you problems
is it out of a rack so not grounding with other gear as well ?

It's not racked or connected to any other equipment except from the obvious mains earth. I tried connecting it in another mains socket as well. I'll try disconnecting one channel completely from the circuit.

 

[keefaz]

Did you scope the PSU voltage at PSU PCB OUT points with no other pcb connected to it, just to confirm the PSU works ?

 

[ferrochrome]

Did you scope the PSU voltage at PSU PCB OUT points with no other pcb connected to it, just to confirm the PSU works ?

Yes I did that right now. The ripple has a somewhat smaller amplitude when psu ground, +30 and -10 are not connected to anything. But it's there. Amplitude is approx 3 divisions instead of 4 with volts/div set to 0.2 V.

I also tried disconnecting the right channel (right stereolink card, 1176 + & - & ground.) Left channel still humming like before. I used a star ground for left channel.

 

[ferrochrome]

Maybe I'm wrong but I found it hard to believe that there is anything wrong except for the psu. Is there anything I can do to try to filter out the ripple on the +30 rail. Adding or replacing a filter cap maybe, I have several values laying around.

 

[keefaz]

What are the voltage rating of the capacitors you used for C1 and C2 ?

 

[ferrochrome]

What are the voltage rating of the capacitors you used for C1 and C2 ?

Those are 50V 1000uF. I have some 50V 2200uF laying around, don't know what they would do though..

 

[keefaz]

Could you measure the voltage at regulator input (the one for the 30V rail, LM317) with a volt meter ?

 

[ferrochrome]

Could you measure the voltage at regulator input (the one for the 30V rail, LM317) with a volt meter ?

Sure! DC Voltage at LM317 in is 32.10V with all pcb's connected and 30.00V at the rail.

 

[abechap024]

Could you measure the voltage at regulator input (the one for the 30V rail, LM317) with a volt meter ?

Sure! DC Voltage at LM317 in is 32.10V with all pcb's connected and 30.00V at the rail.

Seems like your hum is coming from the positive voltage rail. Maybe a bad LM317? Possibly 32v isn't enough headroom to regulate. (might be fine though) but trim down the regulator to 28V and just see if the hum disappears. You can actually just do this with your scope.

 

[keefaz]

You can actually just do this with your scope.

Sure, but it is easier to read DC voltage in big display of a digital voltmeter

Anyway 2.1V DC a drop out voltage seems low for the lm317, according to datasheet it requires minimum 3V DC

 

[ferrochrome]

THANKS(!!!!) for mentioning that, I didn't know that the lm317 needed to regulate more than 3V!!

I connected a voltmeter and scope in parallell to the 30v rail and saw the ripple gradually shrink untill it dissappeared when I got the voltage down to 28,2. That's just incredible thanks to you all for helping me out with this!!

Hopefully it will manage to function properly with the lower rail Voltage. I'll post back when I have the result!

 

[abechap024]

Great news! Hopefully that fixes it. Probably should still operate just fine - slightly decreased headroom, could possibly be cooler if your into overdriving things...

 

[okgb]

I think that's a rule of thumb , the regulator's always need to regulate Down , [ by at least 3 v ? ]
so Bravo abe for coming up with the right idea [ and an easier one ] Have fun

 

[ferrochrome]

Yeah thanks abechap and keefaz and all posters here! the compressors work perfectly and hum free now! maybe a calibration would be in place after lowering the rail voltage. I leart a lot about grounding and psu's from all of this as well, so thanks again!!!

 

[abechap024]

I could see were Keefaz was going with his questions Glad you got it worked out, always happy news when something is fixed.

 

[keefaz]

Nice to see it's solved, ferrochrome  8)

You knew the problem came from PSU, no ? (you posted PSU shematic two times !)

Maybe you could use a LDO regulator (low drop out) like LM1085 (adj version) in place of LM317, if you want to get 30V DC with your set up

 

[ferrochrome]

Nice to see it's solved, ferrochrome  8)

You knew the problem came from PSU, no ? (you posted PSU shematic two times !)

Maybe you could use a LDO regulator (low drop out) like LM1085 (adj version) in place of LM317, if you want to get 30V DC with your set up

Yes I was on to something but couldn't confirm my idea because of a lack of knowledge regarding ripple in psu's. Now I know more!

I'll see how it works in different tracking/mix applications on 28V before replacing with another regulator. Lower headroom in a 1176 might be a cool thing like abe said. If I decide to replace with a lm1085 would I need to modify the circuit otherwise or just replace regulator and adjust to 30V?

 

[abechap024]

. If I decide to replace with a lm1085 would I need to modify the circuit otherwise or just replace regulator and adjust to 30V?

http://www.ti.com/lit/ds/symlink/lm1085.pdf
They just drop right in.

Have fun tracking