Help! ground hum

[Green77]

Hi!

I have build a PP KT88. Works greate but there is one problem.

When i have the rca cabels connected there is a small hum in the speakers (ground loop?), but's not a big problem. The hum is not hearable from 10 feet from the speakers.

The problem is when there is no rca cabel connected to the rca jacks there is a loud ground hum. The input jacks are grounded and the cable are shielded..
Anyone know what to do?

Thanks // Daniel

[bob91343]

It depends on the kind of hum. There are basically three kinds.
1. Power supply noise, due to poor filtering. At double the power line frequency, a sort of slightly raucous sound.
2. Power line leakage, a smooth clean hum that is at the power line frequency.
3. Pickup, also known as open grid hum, which is really raucous and often includes local radio station modulation. It has high pitch overtones that dominate.

Knowing what kind you have will help track it down. First, if you still hear it with the cables disconnected, try shorting the input jacks. If you still hear it, the problem is in the amplifier. However, if this amplifier is new, I presume it always had the hum, so it may be the way it was assembled.

[Loren42]

Yeah, short the inputs and see if it gets quiet.

If it does, then you might have a look at how the front end wires and jacks are shielded.

Also, what is the input impedance? I lowered mine to 10 kΩ to help that.

If shorting the input does not help, then you probably have a ground loop in the amp.

Generally, ground loop hum gets louder when you hook up other equipment.

[Green77]

I have shorted the input jacks and the ground hum disappeared..
But i can't see anything wrong with the cables from the input jack and to the first resistor..

[Loren42]

The cable from the input jack to the input stage of the amp, is that a shielded cable?

If so, which end is grounded (input jack, input stage, both)?

Is the RCA jack isolated or insulated from the chassis?

[Green77]

That's a shielded cable. And the shield is connected at ground on the input jacks. Not on the other side.
The RCA jacks are isolated from the chassie.

Thanks //Daniel

[AndrewT]

Can we assume this is a two channel amplifier?
What route does the grounding link take to get from the input sockets to the main Audio Ground?
Is there a separate Signal Ground?
Where is the main Audio Ground?

[Loren42]

Originally Posted by Green77

That's a shielded cable. And the shield is connected at ground on the input jacks. Not on the other side.
The RCA jacks are isolated from the chassie.

Thanks //Daniel

Bingo!

That's the problem. Think about what you have when the input jack is unplugged!

You have a center conductor running all the way to the isolated jack, but where is the shield connected?

It is floating. So, it does not act as a shield. You have a large AC antenna for the input circuit.

Ground the shield at the input side of the circuit and try it again. I will bet it will be quiet as a mouse.

Once you confirm that you can lift it at the jack and it should be good to go.

[Green77]

Thanks!

I will try this later. I have to pop some test music first!

[AndrewT]

Originally Posted by Loren42

Bingo!

Can I offer an alternative?
Grounding the shields at the input sockets only is perfectly valid.
The input pins at the amplifier compare the voltage at the two pins and amplifies the difference.
If the ground pins are connected to the sheilds then there are two routes to the signal ground. Two routes must form a loop. This loop will pick up interference and that in turn generates a voltage around the loop.
The two hot input pins will read the voltage differences between the two shields and at least one of these input pins will get that interference signal amplified.