Amplifier Noise/Buzzing problem

Thread Starter

hilhan79

Joined Aug 22, 2020
9
Hi everyone,
I need some expert help. Problem is that whenever I connect RCA to any device the buzzing, noise starts coming from speakers. There is less noise when only one input is attached. If there is no connection, little bit.

I disconnected the input ground and negative path with a 10 ohm resistor. But there was no change. TL082 replacement to OPA2134; no changing.

What solution do you think I should find?

Thank you in advance for your help
 

Attachments

sghioto

Joined Dec 31, 2017
1,446
Noticed that C15 on the schematic is shown incorrectly at the output of the LM7915 negative regulator. +side should be ground. Is that how it is connected on the PCB?
SG
 
Last edited:

MrChips

Joined Oct 2, 2009
21,842
No. The other way around.
C15 is labeled incorrectly both in the schematic and the pcb.
C11 is ok.

Assembled pcb as in photos posted by TS looks correct.
 

Hymie

Joined Mar 30, 2018
859
From your post, clearly most of the noise is on the RCA inputs or due to their connection – possibly causing ground loop issues.

I have reduced such noise by resistive loading of the RCA inputs. I created a neat solution by enclosing the resistive potentiometers (5kΩ) in a plastic box fitted with RCA panel sockets. To avoid short circuit of the inputs I included 100Ω resistors, although audio line outputs should be able to withstand a continuous short circuit load.

In operation, adjust the potentiometers to give optimum hum reduction.

Before wasting time building a neat solution you could lash-up a crude connection on one channel with a preset pot.

The above solution will work where the noise hum has a high impedance source – whereas the line out signal should be able to drive into a resistance of a few hundred ohms.
 

MrChips

Joined Oct 2, 2009
21,842
Thanks for answer MrChips. I bought it as a boutique product.
I don't know what that means.
I am just curious.
In post #3 you said " I fixed it on the pcb and mounted it ".
What does that mean? Can you tell us more?
It was bought already assembled? You removed C15 and turned it around? What made you do that?
 

Thread Starter

hilhan79

Joined Aug 22, 2020
9
Yes, i turned capacitor pole, because must be lm7915 pin negative voltage. Circuit and PCB wrong. I shortened the signal line and only got gnd from a single pcb and i brought it closer the rca sockets. The noise is less now, but there is some hum sound.better than before.
 

MrChips

Joined Oct 2, 2009
21,842
You have two boards. Are they both showing the same symptom?
When the board was assembled and tested, C15 was installed backwards?
I don't know what that would do to C15. Try replacing C15 with new capacitors.
 

Thread Starter

hilhan79

Joined Aug 22, 2020
9
You have two boards. Are they both showing the same symptom?
When the board was assembled and tested, C15 was installed backwards?
I don't know what that would do to C15. Try replacing C15 with new capacitors.
No problem when using only one any RCA socket input. Or it is ok if either of them disable gnd grounding.
 

MrChips

Joined Oct 2, 2009
21,842
Here are things that can cause hum.

1) Too much AC line ripple on the power supply rails.
2) Pickup from the AC mains transformer.
3) Ground loop.
4) Poor shielding on signal cable
5) Bad ground connection.

From what you describe, the hum could be coming from the source of the RCA cable.
Try testing using a battery powered music source such as a portable radio or mp3 player.
 

Audioguru again

Joined Oct 21, 2019
1,908
Do you have good quality shielded audio input cables?
Years ago a Radio Shack product came with shielded audio cables with RCA plugs on the ends. They produced lots of hum because the shield made poor shielding, it was just a few strands of wire. The Dollar Store sold the same cheap Chinese cables.
 
Top