Volume pot - Wiring

[Brian]

Another question......

My amp has an Alps Blue volume pot, but I think it could be any pot really.

The input to the volume pot has red and white plus earth shield, these come from the RCA input sockets obviously and are all connected, red to red, white to white, earth to earth.

The output points from the volume pot to the amplifier input circuit has red and white wires plus earth connected at the pot, but there is no earth connected in the amp', just the red and white wires. I understand that this is normal, but why?

What is the point of connecting the earth at the volume pot output terminals if there is nothing connected at the other end?

Hope someone can help.

Tks

Brian

[petercom]

You have answered it yourself, unknowingly.

The earth from the rear panel is to shield the volume pot and cables, nothing more. In a star earth system all earths are taken to a single point, usually on the chassis. If you trace the circuit through you will see that the main circuit has an earth to which everything is referenced.

If you were to connect the earth on the volume pot screen to the signal leads going to the amp you would create a 'ground loop' (chassis to volume pot, volume pot to amp board, amp board back to chassis) which would induce hum.

There is an alternative method which is to design the layout of the amplifier so that ground follows signal. This is normally seen in the complicated PCBs used in SS amps.

[KennyK]

Peter ( or anyone else for that matter), could you confirm whether the W(A)D HD83 HeadphoneII kit that I'm in the midst of building should be wired up in star earth configuration (i.e, connecting the ground from the phono sockets directly to the earth post, like the PreII) or whether the ground from the phono sockets should go to the 3rd pair of tags on the volume pot , or even the ground signal input on the board? the instructions are a little vague on this.

[Brian]

Thanks a lot for the reply, Peter. What I'm really saying though is why bother connecting anything to the earth point on the volume pot output if the other end is going nowhere? Why not just connect red and white leads to the pot output, and connect the other end of those to the red and white input in the amp' circuit and forget all about the earth from the pot?

[John Caswell]

Hi Brian,
This is quite normal, and is usually done to reduce/remove earth loops/hum etc.
The reason that the screen is left in place right up to the amp input is to reduce extraneous hum/unwanted signal pickup on the lead. It is common practice in professional audio circles to earth the "send" end only.

John aka Dr john

[Brian]

Originally Posted by John Caswell

Hi Brian,
This is quite normal, and is usually done to reduce/remove earth loops/hum etc.
The reason that the screen is left in place right up to the amp input is to reduce extraneous hum/unwanted signal pickup on the lead. It is common practice in professional audio circles to earth the "send" end only.

John aka Dr john

John

Ok, cheers. I've been thinking about re-wiring the volume pot with the teflon insultated silver wire I have, obviously these are single wires so I wondered about the earth.

So I could use silver wire from rca red & white sockets to volume pot red & white inputs, plus run some kind of earth wire from rca to volume pot earth input and that's the input bit done.

To do the output of the pot I can use silver wire from the pot output red & white to the circuit board, if I leave the volume pot earth output disconnected all I'm risking is some hum? I've noticed that the volume pot earth points for both input and output are joined wiith a wire.

Brian

[John Caswell]

Hi Brian,
Yes for the input wiring that is correct, and you can use the same silver wire for the earth wiring as well, and I suggest that you twist the the earth wire around the signal wire to provide some screening.
You then do the same for the vol control output wires with the earthy wire twisted around the signal wire, except you do not connect the earth at the amplifier input end.

John aka Dr John

[Brian]

John

Thanks for the replies. I know I can be hard work, so I appreciate the patience.

I'll do this once the new schottky diodes have settled down. I think I know already what they're doing, but I like to do one thing at a time so that I can get a grip on the effect of each change I make.

Brian

[Greg.]

Hi Brian,

As you are just re-wiring surely all you need to do is copy what you have already. As you are going to this trouble, it would be well worth incorporating the pseudo shunt mod found in the FAQ section.
See http://www.world-designs.co.uk/forum...read.php?t=198

Best wishes,

Greg

NB, Edit, posted before I read John's latest post.

[John Caswell]

Hi Brian,
Not a problem, that is what we are here for.
Very sensible to do one change at a time, it is what I always recommend.

John