Irritating buzz... where is it comming from???

[Devius]

Hello

I just finished building a pre and a power amp. The problem is that upon connecting the two there is a buzz. I've done all the usual procedures to eliminate any earth loop. The strange thing is that the buzz only shows when the signal cable from the pre-amp is connected to the power amp. There is a phones output on the pre and it's completely silent. Also, when the two are not connected, the power amp is also extremely quiet. One last detail is that the buzz is only present when there is soemthing connected to the input, so if one channel is connected but the other one is not, the connected one produces buzz but the other doesn't. If this was an earth loop the buzz should be always present right? Any ideas??

The amps are based on the LM3886

[carlosfm]

Hi Devius,

Welcome to this forum, another Portuguese.

A pic of your power amp would help.
And tell us how you did the grounds.
I feel that the problem is the power amp.

[budwiser]

I have the same problem when connecting my VA versa preamp to my GC. When they are not connected, my GC is perfectly quiet. Could it be the grounding in my pream? When I connect a cd player with a line out and volume built into the cd player I don't get the buzzing sound. One thing though, the buzzing is really low level, you have to put your ear very close to the speaker to hear it. It does not increase in sound when you turn the volume up either.

[MWP]

The common and VERY annoying ground loop problem.

Ive been trying to fix this problem in my various amps for quite a long time.
Never have been able to remove it completly.

Hence why i have just finished building my new DAC/Preamp/Amps all on one PCB.
Total silence

[Franz G]

A few checks:

- are the input connectors isolated in the box?

- (How) is the ground of the amp connected to the box?

- does the ground from the inputs follow the way: input ==> signal ground ==> power ground?

- do you have a cable tv or -radio connected? If yes, remove the antenna cable for test. If it helps, you must insert a small device in the antenna cable to break the ground loop (in german: Mantelstromfilter, it is a simple DC-blocker for the ground).

- are all devices (even this one before the pre) connected to the same mains connector?

- are all devices grounded?

If all fails, then you could open the ground at the cables just before the pre and the amp and use ONE separate ground connection between (all) devices (like a ground rail). Be careful, not to build a loop and to have at least one device (pre?) connected to earth via mains.

For my experience, the chipamps based on LMchips are very resistive against this style of hum, because the high PSRR. I have a configuration with my PC, TV-cable (a classical ground loop), where every commercial amp produces hum, but not the chipclones!

Franz

P.S.
all above assuming, your pre is O.K.!

[moving_electron]

There could be a problem with your preamp.

I recently made changes to my active crossover and preamp that caused the combination to oscillate. When connected to the BrianGT gainclone there was a strong buzz. When I got rid of the oscillation, the buzz went away.

This does not make sense to me because the oscillation was likely very high frequency, certainly above audible. I know this because it was unhearable on another amp that is just 2 prong (double insulated) without an earth ground. That amp did heat up noticably though when connected to the crossover.

But when connected to the Gainclone there was a strong buzz until I fixed the oscillation. It doesn't make a lot of sense to me that a high frequency large input signal would result in a lowish buzz but that is what happened. Perhaps it was the chip protection circuit kicking in, in some way? But normal music sounded Ok through it, even though there was the constant level buzz overlayed on it.

Can anyone suggest how a strong high frequency signal from oscillation could induce a buzz in the amp?

[cjd]

I had two issues when I first put my GC together causing buzz. . . One is that I had switched signal and ground wiring the inputs. This mostly gave me trouble if I only had one channel attached. The other is that I discovered one of my cables was in bad shape. I don't know what was wrong with it - not shorted out, but there seemed to be some signal bleed.

I'm assuming most folks know better than to make the first mistake I made (heck, *I* knew better and I did it anyhow).

C

[MWP]

Originally posted by Franz G
For my experience, the chipamps based on LMchips are very resistive against this style of hum, because the high PSRR. I have a configuration with my PC, TV-cable (a classical ground loop), where every commercial amp produces hum, but not the chipclones!

PSRR doesnt have anything to do with ground loops.

Ground loops are a problem with noisy earths between equiptment creating noise on analog inputs.

Not noise on power supply lines which is where PSRR is a factor.

[Devius]

Well, I have no digital camera so there is no way I can photograph the amp... but it wouldn't do any good since it is now quite a mess.
So, here is the schematic of the present situation. The amp's power cable is connected to the pre-amp where a single switch turns on or off both. So there is only one connection to the mais. There is no more noise present when another appliance is connected to the pre-amp's input. The noise is independent of volume setting. The phones outpu on the pre is completely silent so I'm guessing the problem is with the power amp. Even still I've did some changes in the grounding of the pre-amp and there were differences although there was never buzz or hum from the phones output.
I have no idea of what's going on... I didn't use a zobel network at the amp's output. maybe there is instability since the buzz only appears when something mettalic touches the input pins of the input connector. The amp has no metal casing and the only shielding are the heatsinks.
The input ground used to be connected to the power supply caps ground but it was more noisy.

[Franz G]

When you have the same noise at the inverted and noninverted input, then PSRR is a factor.

Depends on the setup of ground wiring, imho.

Your posting, MWP, does it help Devius to solve his problem?

Franz