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Dougie085 28th March 2008 03:41 AM

Question about noise?
 
Friend found a guide that says that replacing the input capacitor with a 1k ohm resistor can fix a lot of noise issues? First question is this true? Second question is, is Cz in this circuit the input capacitor?

http://audiosector.com/images/lm3875_se_pcb.gif

seventenths 28th March 2008 04:06 AM

Cz and Rz form the zobel filter on the output.
As to noise reduction, I would ask, what noise?


cheers,
7/10

Dougie085 28th March 2008 04:16 AM

It's like a hum it doesn't change volume or anything. I've tried Peters grounding idea before with no luck. It's the only noise there. My friend has another GC amp thats from chipamp.com and he said they put a 1k ohm resistor in place of the input cap and said it fixed their hum problems.

gfiandy 29th March 2008 12:11 AM

As far as I can see this amplifer has no input capacitor it apears to be directly coupled.

Are you running both the amplifers from one transformer. If you are it is likely that is is this that is causing your hum. You could reduce it by making the wires from the PCB to the amps as short as possible. But the best way to solve it would be to use seperate transformers for the two amplifers.

Regards,
Andrew

jaycee 29th March 2008 12:39 AM

You could also probably cure this by using one power supply and rectifiers, but for best results all grounds would be returned to the psu, the signal ground via a 10 ohm resistor. Unfortunately it doesnt look like isolating signal ground from power ground is possible on the Audiosector boards.

My self build Gainclone uses one psu + transformer in this manner, and there's no hum at all.

Dougie085 29th March 2008 12:45 AM

It only uses 1 rectifier board and one transformer.

xiphmont 29th March 2008 12:59 AM

Re: Question about noise?
 
Quote:

Originally posted by Dougie085
Friend found a guide that says that replacing the input capacitor with a 1k ohm resistor can fix a lot of noise issues? First question is this true? Second question is, is Cz in this circuit the input capacitor?



It's true that film caps can and will pick up capacitively coupled noise on a high-impedence input. You can either eliminate the capacitor (this has liabilities) or shield the cap by enclosing it in foil/aluminum tape/can and grounding that shielding to the chassis.

But you haven't shown any evidence you actually *have* an input capacitor :-) There's none on that board. Did you add one inline with the input?

Dougie085 29th March 2008 01:04 AM

No I didn't I wasn't sure I had one. I'm just trying to figure out how to kill the noise. I have the audiosector integrated amp chassis. It's not setup for integrated use its just the chassis. The wires are relatively short. Let me post some pictures.

http://i24.photobucket.com/albums/c3...5/P1000865.jpg
http://i24.photobucket.com/albums/c3...5/P1000866.jpg

This are older images the binding posts are actually isolated from the chassis now and what not.

xiphmont 29th March 2008 01:15 AM

Quote:

Originally posted by Dougie085
No I didn't I wasn't sure I had one. I'm just trying to figure out how to kill the noise. I have the audiosector integrated amp chassis. It's not setup for integrated use its just the chassis. The wires are relatively short. Let me post some pictures.

This are older images the binding posts are actually isolated from the chassis now and what not.

I hope they're isolated :-)

Is the hum still there with the inputs shorted? Is it the speaker humming or the transformer itself? Only when it's plugged into the source? I don't see any obvious wiring errors. Please describe the problem in more detail.

Using more than one amp off the same PSU/tranny does *not* cause hum problems. It can cause power demand and cross-channel coupling problems, but it doesn't cause hum (all by itself).

edit: Oh, actually I do have a question from the picture... where is your chassis ground?

Monty

Dougie085 29th March 2008 01:28 AM

When the inputs are shorted it hums. It doesn't home when nothing is connected only when something is connected....umm like I said it doesn't get louder as volume goes up. What else would you like to know?


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