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-   -   humm introduced to opamp? (http://www.diyaudio.com/forums/chip-amps/91077-humm-introduced-opamp.html)

h_a 26th November 2006 03:25 PM

Hi folks!

I've a question which is for you more experienced guys for sure easy to answer!

I changed the filter elcos of my phono board to larger ones - 2200muF instead of 470muF.

Of course space is limited, so I layed them flat down, unfortunately only some 1-2 millimeters away from an opamp (which is regulating the amplifiying opamp).

Problem is, if I crank the volume (no music playing) to the max I can hear humm in the (very low) noise quite clearly - unfortunately I don't know if this was already there before :xeye:

I've no possibilty to change them back right now, so I would be happy if someone can tell me if this is reasonable (humm introduced by cap) or not!

Thanks a lot! Hannes

PS: the 2200muF cap is filtering 20V dc-voltage; the board draws less than 100mA, opamp is an opa134.

turnA_zpoint 26th November 2006 04:29 PM

Hello.
I dont understand whether it is a correct answer, but to reference.
Originally,When it uses a big condenser for the input, it is a method to sometimes do.
It may be good to wrap the condenser with a wire for coherence and ground the line.

Minion 26th November 2006 10:04 PM

Um, You shouldn"t be running the OPA134 off of a +/- 20v supply as it has a Max Supply Volyage of +/-18v (If running off of a Dual Supply) ....I" don"t know if that is your problem but I thought I would point it out.....

I"m sure 20v would be ok with a Single Supply but I don"t know how stable the OPA134 is on a Single supply as the datasheet doesn"t mention anything about single supply operation.....

If the 2200uF caps you are useing are lower quality than the originals like for instance they have a Much higher Leakage current then the others than you might get more noise....

You might also try bypassing the caps with a 0.1uF to 1.0uF Poly or Metal film cap to see if that helps.....

Good luck

h_a 27th November 2006 01:42 PM

Thanks a lot for your replies!

@turnA_zpoint: I'm very sorry, but I don't understand, could you please explain a bit more?

@Minion: thanks for your advice, but the opa134 is driven by +-18V voltage regulators. I'm talking about the filter elco after the rectifying diodes!

Thanks also for the tipp with the leakage current, I'm going to check that, however I'm using hiqh quality elna caps.

With kind regards, Hannes

turnA_zpoint 27th November 2006 04:22 PM

2 Attachment(s)
Hi H_a
Sorry I'm not good at English.

So I Upload photo.
though only hlalf of parts I made, It is like this.
Originally, this means for input capacitor.
The wire is to ground line ,
so it is shield for capacitor.

h_a 29th November 2006 06:36 AM

@ turnA_zpoint: thanks a lot! I understand now what you mean! Interesting - I never saw that before.

With kind regards, Hannes

peranders 29th November 2006 06:56 AM

What was the reason in the first place to switch the caps? Was it to "improve" the sound or was it of some other reason?

If the PS is supplying only two OPA134 470 uF is quite sufficient. I'll guess your problem is capactive coupling. Do you have high resistor values in your design?

AndrewT 29th November 2006 07:30 AM

Hi,
supply voltage of 20V with some ripple and a regulator set to 18V.
Could there be a drop out problem?

Nordic 29th November 2006 07:38 AM

Hey Peranders, I don't think 1000uf is a bad choice... with my headphone amps useing those chips, there is a clear improvement just going from 470uf to 1000uf, and those were drawing only about 20mA.

lineup 29th November 2006 07:41 AM

Quote:

Originally posted by h_a

@Minion: thanks for your advice, but the opa134 is driven by +-18V voltage regulators.
.

Can you draw us the circuit or post a link to the schematic you use?
If we know more we can answer better.

Check that you have everything soldered properly.
And no possible 'extra contacts' via some solder.
Did you layout PCB yourself?


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