Simple passive preamp. Neen some help in grounding.

[FaTTy]

Hi

I started to build passive preamp for my JLH monoblocks. It have only two functions :
- 4 way input selector on relays
- volume control on ALPS RK27 stereo pot
Simple at the first look, but not as is... So, i have some troubles with proper grounding. We have three ground lines, two signal and one power :
- left channel gnd
- right channel gnd
- relays feeding PSU's gnd
I think, that ground scheme sould be the same as on the attached pic. Is it a good way?

Thanks!

PS
Sorry if my english too hard to understand

[TerryO]

I have a question: If you are constructing a passive preamp, what is the PSU gnd for? A true "passive" is TTBOMK little more than source selection switches and an attenuator. The use of relays and a PS suggests that you have, in reality an "active" preamp that is amplifying, or modifying in some way, the source signal.

Best Regards,
TerryO

[FaTTy]

Oh, i fogot about pcb. Here it is, see attach. Any comments?

TerryO
Maybe you right. Let it be active I thinked that passive volume regulator automatically makes my preamp passive...

[FaTTy]

Help needed... How to make proper grounding?
What can you say about my pcb?

[Gordy]

From the information that you have posted it seems as if you have a good understanding of the requirements.

[Gordy]

From the information that you have posted it seems as if you have a good understanding of the requirements.

[AndrewT]

Hi Fatty,
check all your sources that may get plugged into your pre-amp.
If any have the ground on the left channel connected directly to the ground on the right channel, then you will have a ground loop by connecting the L+R grounds together inside the pre-amp.
A loop will almost certainly cause hum on the affected inputs.

[MikeBettinger]

Originally posted by AndrewT check all your sources that may get plugged into your pre-amp.
If any have the ground on the left channel connected directly to the ground on the right channel, then you will have a ground loop by connecting the L+R grounds together inside the pre-amp.
A loop will almost certainly cause hum on the affected inputs.

This I don't understand. Most every source component I've been around has a common ground on its output. It's mandatory based on the powersupply grounding. The drawing posted is (in my obviously skewed view of the world) correct. I would be facinated by any description that justifies separating the grounds at the input and output connections of a preamp.

Any system comprised of multiple powered signal sources (and loads for them) needs a common reference point. The input/output connections at the preamp (in my experience) is it.

I don't normally feeled compelled to speak up anymore, but this post is likely to confuse many a DIYer without a challenge. They'll be chasing stability problems that are near unsolvable based on this.

Nothing personal, just a different point of view.

Mike.

[AndrewT]

I would keep the pre-amp left and right completely separate.

[Gordy]

Yes, I see what you mean Andrew. It is possible to separate Left and Right, and only bring them together at a link to the safety / chassis ground. It is also possible to switch the input source grounds too, so that the only source ground connected is the one of the source selected. However that may be getting too complicated for FaTTy.

What we can say is that although the design as presented is not optimal, it should work reasonably well.


Mike / Andrew... I think the forum needs a reference "how to ground your pre-amp" paper. Maybe when I get a spare week... unless you know of a ready-made paper / web link?