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?

 

[Pathmark]

White paper or some agreed common practices would be great. I have one hell of a hum right now in my xbosoz. I am sure it is my own fault but not sure what is standard practice.

Unfortunately, I am using only rca not balanced therefore it may complicate the issue with this preamp.

Pathmark

 

[FaTTy]

Thanks for posting!

Gordy
I don't to be an experiensed in construction of audio devices, but already built two amplifiers. Since the last year this my hobby So i have basic knowledges and desire to learn more.
---
Any signal source have a common ground for both channels as i know. So if there are more then one type of ground in the scheme, we should to make a common ground point. Also I know that making a ground filling of small signal traces area helps to minimize hum and noises (impotant for quality preamps). Chassis needs to grounding - reduces hum level too.
On the other hand, uniting the grounds we receive a loop that produces hum... This starts to worry me.
In my amps (stereo) i was bring R and L grounds together as close as possible to input terminals and throwed a wire to the "star" point on the PSU board.
Ground loop arrived here, but hum was in low level.
Preamp is more sensitive device. Hum/noise reduction more impotant...

Maybe good screened signal cables helps to minimize
hum that produced by gnd loop?

This my thoughts about this. I ask to forgive if my words too chaotically.
---
AndrewT
"I would keep the pre-amp left and right completely separate"
And you don't connect them at all?

Gordy
"It is possible to separate Left and Right, and only bring them together at a link to the safety / chassis ground"
I think that going this way results a loop, but crosstalk betwen L and R reduces a bit.

PS
Several peoples says me that proper grounding is like some kind of art. I believe this now

 

[MikeBettinger]

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

You're kidding , right?

I would still be fascinated at the logic behind this. And I'm not being a jerk. I'm truly curious.

Mike.

 

[Gordy]

I suspect that Andrew meant doing it as if they were dual-mono, as would be the case with twin / separate left and right channel power amps, with the grounds joining only at the mains (or in this case, at the chassis).




Here are some pages for anyone wishing to read a little on the matter:

http://www.tcaas.btinternet.co.uk/jlhearthing.htm
http://sound.westhost.com/earthing.htm
http://www.gbaudio.co.uk/data/ground.htm
http://rane.com/note151.html

(links checked and live at time of posting)

 

[mpmarino]

with the grounds joining only at the mains (or in this case, at the chassis).

I can see reason to mains ground the chassis - but why take the audio ground to the mains ground? Looks like potential for a larger ground loop, No?

Mike and Andrew, I see both of your points.

Mike- yep your right in that if it's common at the source, why not common at the pre?

but, I think Andrew is thinking that 2 wires (the 2 rca cables) commonly grounded at each end IS a loop. Whether this can be a problem I dunno.

I'm getting ready to do some wiring on my active pre so the thread is timely. I think I will err on the side of caution and keep them seperate.

Edit: but yes- they do ultimately come together in a non dual mono amp -What to do?

 

[MikeBettinger]

I suspect that Andrew meant doing it as if they were dual-mono, as would be the case with twin / separate left and right channel power amps, with the grounds joining only at the mains (or in this case, at the chassis).

But that's not what he said or was implied by the concern about whether or not the source components had floating or common grounds; and dual mono as far as what? Ask yourself, what function does the ground to the output connector serve? It's not part of the operating current loops, not really part of the signal path loops, other than as a load for the output back to the supply, It is a signal reference for the input stage... (this is the solid state site, correct?).

I stand by my observation that the output and input grounds in a preamp need to all be at the same potential and referenced directly back to whatever central ground floats your boat. This ground is the central point in the system hooking all source and loads and thier various power systems together. Anything else allows a plethora of noises to become part of your signal path.

 

[Gordy]

I stand by my observation that the output and input grounds in a preamp need to all be at the same potential and referenced directly back to whatever central ground floats your boat.

And nobody disagreed with you.

 

[mpmarino]

I suspect that Andrew meant doing it as if they were dual-mono, as would be the case with twin / separate left and right channel power amps, with the grounds joining only at the mains (or in this case, at the chassis).

I think there are very few that tie audio ground to chassis(mains) ground anywhere ~ sometimes isolated with a resistor or thermistor but rarely direct. Therefore, in a dual mono setup, if grounds were kept separate thru the pre - the only place they would come together would be at the source (assuming the source has common ground, which I do).

I'm not trying to be argumentative, just trying to get to the bottom of it...

 

[Gordy]

I think there are very few that tie audio ground to chassis(mains) ground anywhere ~ sometimes isolated with a resistor or thermistor but rarely direct.

Nobody said tie them directly. Experience suggests that they should be linked such that there is a dc path, with a low value resistor or small inductance being suitable for the link.

 

[analog_sa]

I would definitely follow AndrewT's advice and have the passive pre as dual mono. If the source or power amp have a common ground this would eliminate potential ground loops in the pre.

There is also no reason whatsoever to contaminate signal ground with the relay ground. Just keep them separate.