Question about noise?

Dougie085

Member
2005-11-19 11:49 pm
Ohio
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?

[IMGDEAD]http://audiosector.com/images/lm3875_se_pcb.gif[/IMGDEAD]
 

Dougie085

Member
2005-11-19 11:49 pm
Ohio
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.
 
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
 
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 said:
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

Member
2005-11-19 11:49 pm
Ohio
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.

P1000865.jpg

P1000866.jpg


This are older images the binding posts are actually isolated from the chassis now and what not.
 
Dougie085 said:
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
 
I've got a couple of wild guesses for you, and that's exactly what they are. Here goes.

There's no dampening on the input. If a potentiometer isn't used, then a 22k or 50k resistor (between hot and ground of input--make a load at input) could be helpful. That would be one resistor per each channel, if a dual gang stereo potentiometer is absent. Load at input--try this first.

The rectifier board is darn far from the amplifier, and the cables for left and right are uneven lengths. Uneven lengths for stereo layout is a pet peeve of mine, but the too long part is a problem. I think the power supply can't overcome that cable length, and thus perhaps the large caps onboard the amplifier boards are performing smoothing rather than having any reserve, and thus leaving the amplifier without that resource makes. . . rectifier buzzzzz
Well, that's just a guess.

Do you have a ground loop breaker? If not, then your source device may be preferring your amplifier's ground to its own power ground (buzzzz).
So, do you have a buzz when using a battery powered source? If not, then consider a ground loop breaker.

Also useful against ground loops, perhaps 8R in series with the input signal ground? When I tried that, it didn't help much, but it made a lovely bass booster, for no reason I could think of.
 
It still hums (no buzz here) even with battery sources. We just ordered some decent hookup wire tonight and some larger guage. I'm going to try and do a quad star ground. The wire thats in there now is some reprocessed stuff from radio shack because its what I had. So when I get that wire I'll rewire the amp with the star ground scheme. I don't really know how to get the rectifier board any closer. Peter Daniels sells these amps in this chassis and doesn't have any issues. So I don't think its the rectifier board being far away.
 
All the AC power wiring.
Start at the power cord connector (by the way where is the chassis ground ?)
Twist and dress the power wires against the chassis side wall.
Pull all the excess into the front compartment.
Rotate the transformer so that the wires are as near the front as possible.
No power wires should be anywhere near the amp PC boards.
Dress the audio input wires against the rear chassis wall then stright forward to the PC board connectors.
Probably do the same for the speaker wires.

Did I say twist all the power wires and get them far from the PC boards?
 
Speedskater said:
(by the way where is the chassis ground ?)


ditto. What is your ground pin of your power cord connected to?

I have the exact same boards. I use two transformers, and no case at all - all the parts are mounted on a wood board. I have absolutely no noise at all, which is pretty amazing.

Make sure CHG is connected to the ground of your power cord.