Phono Preamp Grounding Behaves like an Antenna...

Hi,

Following on from this thread, I am now in the fortunate position of not having Radio Moscow superimposed over records when I play them (5 different values of PSU bypass caps were used in conjunction with coaxial-shielded power cable).

The preamp *seems* to be quiet, as far as phono preamps go, but…it’s nowhere near as quiet as the single stereo preamps I’ve made in terms of 100Hz ripple (UK mains is 50Hz)…

I realise that having 3x as many preamps in a case means there’s a lot more going on in terms of ground currents… Grounding protocol as follows (the preamps are in a 1u case – the noise floor is exactly the same whether I use an external lab PSU, or dedicated PSU):

Power (5-pin XLR) – pin 1 is Ground. 2 is +Ve and 3 is –Ve

Pin 1 has a thick (approx 2.5mm) copper bar welded to it which runs across the length of the chassis for distribution.

ALL Grounds return to the copper bar.

I have noticed that the hum is least noticeable the further I mount the CARTRIDGE Ground leads towards the socket… The nearer they get to the power GND returns, the louder the hum…

The hum seems to get quieter if you touch the case… This happens whether I common MAINS EARTH with the STAR GND in the PSU or not – this doesn’t make a difference, nor does it make a difference whether the 1u case is connected to the GND bar…

I’ve tried about 10 different GND distribution schemes so far, but I keep coming back to the classic STAR config, with everything returning to the bar. If I return the Cart GND (via an unused lead in PSU cable) to the PSU’s STAR point (cap centre point + CT), it gets much noisier… The cart seems to prefer getting a GND away from the rest of the pack. I’ve made many phono preamps over the years, but I’ve never had trouble such as this…

The hum is the same whether a turntable is connected, or whether I insert shorting plugs. Hum forms a perfect sine on ‘scope. Hum gets worst as I approach preamp and less when I touch it. I’ve tried turning off all lighting etc in workshop – no difference.

The preamp compares favourably to commercial units I have, but I am very worried it could misbehave out on the road in EMI-hostile environments… The inductive hum that gets louder as you approach the 1u case is very odd… If you’re across the room, the hum is virtually inaudible – below the hiss floor – it only gets loud if you stand by preamp, and quieter if you touch it…


Any comments gratefully received.

Thanks,
Justin

From a quick read it sounds like what you are doing is sometime derisively called "magic". You're searching for some "magic" ground point that is quiet under all conditions. Unfortunately magic design is often disturbed by other random influences (like full moons, etc).

Looking at this logically, from what I think is your situation. You have 3 preamps in one chassis with one power supply.

OK, common power supply, grounded to common chassis. Chassis should be grounded to earth/mains ground so if your buddy touches it, nothing happens. If anything hum in area should go down since he is now grounded too. The thickness of any ground bus doesn't matter, ground is still only 0V at one point.

Three preamps in common chassis with common power supply, will obviously share a common PS ground. To keep three phono preamps happy means you will need to properly reference preamp input circuitry to the turntable tone arms, and perhaps preamp outputs to output jacks if they are grounded to common chassis.

Since RIAA EQ involves some 60dB of boost at LF, to keep hum levels low you need to make sure signal and associated ground at gain stage is properly referenced to signal/ground at tone arm. Conventional consumer preamps telescope this ground from the preamp out to the turntable/tone arm. If you have a proper balanced or differential stage in front of your preamp I would tie your cartridge/preamp low to your chassis ground through something like a 100 ohm resistor in parallel with a .1 uF capacitor

While it is common practice in consumer phono setups to use 1 conductor shielded, In this case I would suggest using 2 conductor shielded, so this telescoped individual preamp ground (or differential input low), is inside a shielded cable with the shield tied to chassis ground, at least until it reaches the turntable.

The tone arm wiring should be isolated from the turntable ground (I hope). At the turntable end, terminate the 2 conductor shielded to appropriate plugs and float the chassis ground at that end. Just like in consumer gear, I would use separate ground wire to ground turntable chassis to common preamp chassis. If three turntables are grounded together the one ground wire should be adequate.

Complications to check for include A) are turntables already grounded to mains power (if yes, try to plug preamp power supply into same mains outlet and grab common chassis ground there). B) Confirm tone arm/cart wiring is floating. I have seen carts that tie one channel ground to shell. This might cause loops.

I hope this makes sense and is helpful.

JR

I can't help repeating myself: if you post the schematic of the preamplifier it might be much easier to give sensitive advice.

BTW, please avoide starting new threads with the same topic--makes the structure of this forum pretty inscrutable and is a pain for those wanting to contribute.

Samuel

Hi John,

The chassis is grounded to mains ground – at the connector and only there - that’s why I thought it so weird that the hum changed when you touched the case.

I always use twin-core cable with telescoping for unbalanced runs and have done for years now.

I have now managed to get rid of the hum… I couldn’t believe my luck, but it seems that the grounding scheme is so sensitive that just an added inch of cable makes all the difference… If I star-ground everything right at where GND comes in on the jack, with – literally – less than 1” of cable, everything is very quiet. The issues start when I run longer cables… The copper bar doesn’t help in this regard and picks up noise the further you are down it, despite its thickness.

What I find amazing is how sensitive the hum is to extra GND resistance. I don’t have the gear to measure it, but we must be talking about pico-Ohms here…

I can run the preamps to the connector with only an inch-or-two of cable on the bench, but not in reality… The current plan is to move the power connector to a point that’s equidistant from all the preamps, giving each preamp an identical impedance to Ground…

I can’t help but wonder if this finicky grounding protocol is due to a shortcoming elsewhere… The next stage will be to get a multi-layer PCB designed with a dedicated Ground-plane – if that doesn’t help then it will be back to the drawing board for this design… It sounds nice though…

Thanks again for your help :guinness:


Justin

Still sounds like magic.

If you use proper differential input and output design the signal ground can arbitrarily be connected almost anywhere. There will be a minor impact from coupling between shields but that is a secondary effect.

Large sensitivity to ground selection suggests to me the primary input signal is being corrupted and/or inadequately referenced to/from.

JR

Is there a television station near you? The times I've seen weird hum that was sensitive to body proximity and ultra-sensitive to grounding, it's been RFI from the 60Hz component of a TV signal, most often UHF.

Peace,
Paul

Thanks again, guys.

I was working on the preamp rack again last night...

This doesn't make sense to me:

As stated before, if I Star-Ground everything right at the power connector, there's no problem. However, if I move the star more than an inch-or-so from the socket, all manner of noises start...

I physically can't Ground 6 preamps within an inch of the connector - unless I redo the PCB and make them all SMD...

This is the bit that doesn't make sense: My power lead from the PSU is a quad-core / coax-braided type. I'm using the same cable for internal power / GND distribution... What's the difference between taking a feed from the socket and from the lead? The only difference is that the socket is shorted to Chassis GND...Hmmm....

When I added a few inches of cable to connect the Preamp furthest away from the connector, the hum and RFI came back with a vengeance... as if no progress had been made at all...

Whatever way I look at it, I have now established that the GND terminations from the op-amp / preamp board are acting like an antenna - the longer they are, the more inductive hum / RFI you get.

The chap who gave me the drawing for the preamp has sent me an email:

To me, the problem with remote PSU is that it disturbs the concept of grounding
in a unit.

I would have it all in the same case with a real earth-Ov-Chassis policy with
the usual metal screening for PSU section and the transformer, it would be cheaper than
two boxes anyway...

Click to expand...

I can honestly say that this is - by far - the most time-consuming and frustrating problem I have ever encountered in 2 decades of building gear... I've racked many preamps and never seen an issue like this.

Paul - there are no TV stations near me - to my knowledge anyway!

Any comments appreciated!


Justin

You need to approach this systematically and properly reference all signal paths to local or remote grounds as appropriate. All the different ground points have different voltage potentials so you must account for that in your signal interfaces.

JR

Hi John,

I'm now 99% sure that this issue is not due to grounding practice... I have tried so many schemes now (I have implemented all the suggestions here - to be honest, most were implemented as standard)...

Late last night, I measured a whole volt of DC coming from a cap that comes off the feedback resistor on the back of the differential pair to ground...

My theory is that the grounds can handle 2 preamps worth of DC, but the reason things get noisy when I power up all the preamps is because they are all "biasing" the ground wiring with DC.

I can get away with remote grounds for all returns, except for the aforementioned cap - it has to go right to the power connector...

DC is flowing around the grounds and effectively turning them into one big antenna - thus picking up any airborne interference such as mains harmonics and RFI???

This observation was made late last night and I haven't had a chance to prove it... I was shocked to see 1v DC coming from a series 1,000u / 25v cap... Very odd...

I tried a 100 Ohm wire-wound resistor between the cap and GND. The higher the value, the higher the noise, i.e. the less it was able to sink to ground, thus polluting the feedback circuit...

Considering I have the same problem whether I use the Lab PSU or dedicated unit - and - I have tried every grounding scheme known to man... This explanation also accounts for why I can get 2 channels to work quietly, but more sharing the ground and it gets noisy - particularly if the GND leads are longer.

Thanks again.

Justin

[quote author="thermionic"]DC is flowing around the grounds and effectively turning them into one big antenna - thus picking up any airborne interference such as mains harmonics and RFI???[/quote]
The first thing that came to mind when I read this was precious bodily fluids. This is just a different flavor of the magic that John mentioned earlier.

DC flowing through ground wires does not increase their susceptibility to EMI/RFI, other than possibly by saturating ferrite cores placed around said cables. AC flowing through ground wires of imperfectly balanced systems may get coupled onto signal lines, but that's another story.

JD purity of essence B.

Hi JD,

On reflection, the DC-through-GND idea came to me at 1am last night and I'm not so sure it's a vaild concept.

As I said before, I've made and racked countless projects before, and no problem has ever stopped me from going forward for more than a couple of days. This is ten-times more annoying than any problem I've encountered before.

I'm off to buy some batteries...we'll see what happens then.

Justin

Hi,

I just powered the preamps up from a pair of batteries...

The behaviour was identical to that of when it was hooked to a mains-powered PSU - identical.

The GND is behaving like a giant antenna. It seems most affected when you attach extra cabling to GND other channels - even when you don't connect the cable to channel, it acts as a GND aerial.

The scenario: a pair of channels work ok, with one channel totally quiet, and one with slight hum. You attach GND lead to power socket, hum and noise increases, and so on...

I went back and tried some channels from the original 2-ch versions of the preamp (totally on Vero stripboard) - they have the same problem... It's acceptable for stereo, but the hum and noise increase incrementally with every piece of copper you attach to GND...totally unviable for 6 channels in a 1u rack...

If anyone has advice I would be eternally grateful - I just can't find a way to crack this one.


Justin

Handle the signal differentially. Ground is a garbage dump for sundry noise currents, don't make it part of your signal.

But I suggested this before so it's illogical for me to keep repeating myself and expect a different result.

JR

[quote author="JohnRoberts"]Handle the signal differentially. Ground is a garbage dump for sundry noise currents, don't make it part of your signal.

But I suggested this before so it's illogical for me to keep repeating myself and expect a different result.

JR[/quote]

Hi John,

That's the one part of your advice I haven't tried.

Are you suggesting that I should make the preamp fully differential, so the GND from the cart becomes what is in effect, the "cold"?

The preamp has an unbalanced input, which drives a differential pair. The output is single-ended.

How do they keep noise at bay in commercial designs? The frustrating thing for me is that if my project were purely for stereo, I'd be ok. It's only when more than 2 channels share a GND everything goes wrong.

Thanks,

Justin

This has been by far the most confusing project I've ever taken on, and I hope the group can excuse any obtuseness on my behalf - I'm finding it very difficult to see the wood from the trees here.

This schematic is virtually identical to the op-amp upon which my preamp is based: http://www.dself.dsl.pipex.com/ampins/discrete/dsctamp2.gif

The only differences are that there is a current-mirror beneath the differential pair, and the base of Q3 feeds the RIAA filter, which loops back via a 1K resistor to the collector of the current mirror transistor underneath Q2 and also to the collector of Q4. Aside from that, and the obvious gain-resistor values, that is 99% the circuit I am using.

My head is really hurting...

Thanks,
Justin

You need to consider where the gain is applied in your system. The voltage differences between grounds are usually minor and ignored in most consumer gear. However, if ground potential differences are allowed to corrupt the signal integrity between the cartridge and the RIAA eq stage, this ground noise can get amplified near 1000x.

So your primary focus needs to be on protecting the signal between turntable and preamp. The ground connection of the gain leg in the RIAA stage should have a clean shot to the cartridge - pin.

Once you get a clean RIAA equalized signal at that stage you need a second differential amp stage to reference that signal to your output ground if you choose to connect output ground to chassis ground. If your following stage has differential input capability you could send this RIAA stage signal and ground reference forward with 3-circuit wiring.

Perhaps reread my earlier posts for some specific suggestions. The use of a common PS/chassis suggests to me 3 different turntable cartridge grounds, hard bonded at individual RIAA stages but floated from chassis/ps ground through a compliance (to prevent loops).

If it doesn't kill you it will make you stronger...

JR

Keeping things powered on batteries, I’ve noticed that the hum only occurs when either the input is shorted to input GND, connected to a cart, or when the output is connected to the amp. If input and output are left open-circuit, all you see is the expected noise, not sinusoidal ripple.

There doesn’t seem to be much difference whether powered via batteries or PSU.

The hum is louder the further the preamp card is from the power inlet.

Whenever you connect another device to input or output, you progressively get hum.


John,

You’ve given me a lot to digest. My laptop is a good way from the workshop, so I am printing your posts off and taking them into the workshop.

Thanks again.


Justin

This is hardly going to win the AES Science Prize, but what about using a "virtual ground", or "rail splitter" chip?

http://docs-europe.electrocomponents.com/webdocs/00b8/0900766b800b853b.pdf

The idea is to run it from a single-rail +30v supply, and give each preamp circuit its own "virtual ground".

Can anyone see a drawback?


Thanks,
Justin

:?:

Solution to a different problem...

I won't repeat myself,
I won't repeat myself,

oops.

JR

[quote author="JohnRoberts"]:?:

Solution to a different problem...

I won't repeat myself,
I won't repeat myself,

oops.

JR[/quote]

I can appreciate that the splitter chip is for an entirely different genre of issue, but would it work?

Providing the splitter IC is run well below its limits and plenty of rail-decoupling is used, why wouldn't it work?

The problem here isn't for running a couple of preamps on the same GND rail, but for 3+ preamps. If one or two run quietly on a dedicated GND, why can't a "virtual GND" IC be used?


Thanks.

Justin