Minimising RFI in a Multi-Channel Phono Preamp Rack

[thermionic]

Hi,

Over the last few months, I’ve built several 2-channel (stereo) phono preamps with outboard PSUs, to an associate’s design. The front-end of the op-amp is differential and CMRR appears to be very good - I’ve compared FFT plots of the preamp to many commercial units and have yet to find a unit that gets near in terms of hum or hiss.

So far, so good...

I put 6 (3 x stereo) preamps into a 1U case for a DJ friend… Considering the case will go into racks with all manner of PA gear, i.e. amps / PSUs etc, I am paranoid about ground loops. I test the unit thoroughly in the workshop prior to letting him have it – all looks good and I notice no issues.

I wasn't there, but my friend says that, on setting up at the club, there is no ground hum…but there is a serious hiss issue that increases the more turntables you connect, suggesting each turntable is acting like an antenna…

Note that I've implemented lots of PSU bypassing for each preamp (it's 6 preamps, full mono layout).

I’ve been building power amps for 20 years and have always found it beneficial to isolate Case Ground (Earth) from Signal Ground. However, with Phono preamps, I’ve never been able to get decent hum performance without letting the Sig GND meet Chassis GND at one point, i.e. usually a star point on the PSU, where the transformer CT and mid-point of the reservoir caps meet.

So, I’ve done a grand job keeping hum at bay, but RFI is rampant. My friend has said that he notices “noises” when he touches the rack case (I checked for this in the workshop, but I think I need to change my test amp as it has a 2-core lead with no mains earth – Denon – new amp btw).

One technique I could (should…) have employed is to put a 0.1u cap between where Signal GND and Chassis GND meet on the PSU (note that all my Phono amps use outboard PSUs, based on the 317/337 combo – nothing fancy). Another way of achieving a similar effect I’ve noticed on commercial mixers is to put the caps between the Sig GND and Case on each jack at the jack-field.

I 'scoped the PSU under load before I let the rack out and I couldn't see anything at 10x magnification / 1mV p/division.

I think that I could have used less cable for the Sig GND returns inside the case, although it is a video-grade EMC case.

The fact that a noise comes from the speaker when my friend touches the case is a giveaway that the case is acting like an antenna. My main concern here is to find a grounding scheme that gets rid of the RFI, but doesn’t incur a hum penalty…

Anyone got any tips? The problem I have is emulating a PA setup in my workshop… I would never have let a unit out if I thought this could happen and left the unit running for 2 days prior…I feel deeply embarrassed to be honest…

Thanks in advance.

Justin

 

[JohnRoberts]

I've done phono pre's with differential input before. One published in Popular Electronics magazine back in 1980. Some guy from Texas, actually had a patent for the concept of balanced input phono preamps, which seems ludicrous to me since old transformer input tube pres were balanced but I digress...his patent is now expired anyhow.

Yes indeed the phono cartridge wiring is sometimes not very compatible with balancing. In some cases one ground (-) lug on the cart is grounded to the case. Most turntables break out a ground wire to connect the tone arm/turntable ground to known clean ground.

I can imagine difficulty if several turntable are grounded to each other in a rack along with other grounded gear. All chassis need to be grounded to a solid earth ground. Likewise the turntables need to be well grounded.

I would be a little nervous about too much compliance between power ground and signal ground. I once had a recording console's PS screaming at 960kHz because of a local AM tower.

Good luck.. RF hybrid grounding (caps between grounds at multiple points) could be your friend.

JR

 

[Larrchild]

yep, if you could somehow get the rf to go into the outer case without the case creating an audio ground loop, you will have it sussed.
I think John's rf bypasses everywhere plan is a good one.

For instance..as the rca phono input crosses the threshold of the case, an rf bypass from rca gnd to case gnd may do just that. Doesn't affect ground loops and routes the rf on phono shield to a good rf gnd, the case.

It might work well.

 

[thermionic]

Many thanks to John and Larry – much appreciated.

I will be venturing into the workshop later today to try and tackle this issue… Current plan as follows (to refresh your memories, it’s a 2-box (PSU / Amp electronics) design):

Float Signal (CT / Cap-midpoint) Ground and keep it isolated from case ground in both PSU case and Preamp case. Route Sig GND via a single core of power cable.

Link PSU Case GND (Mains) and Preamp Case GND via Screen on cable.

Add 3-position switch in PSU between Case GND and Sig GND, where CT Star is located – ranges are: Floating from CT / Joined at Zero Ohms / Joined via series 0.1u Cap

Add 3-position switch in Preamp between Case GND and Sig GND at pin-1 on Power connector. Again, this switch alternates between Floating from Sig GND / Joined at Zero Ohms / Joined via series 0.1u Cap

Fit Tonearm grounding posts to both Preamp and PSU, giving 2 options. Could possibly add switch to give Sig GND or Case GND options.

Minimise any extraneous cabling pertaining to Sig GND.

0.1u Caps are added between case and all Input Jack GNDs

Add small-value (10n-ish) caps in parallel with original bypass electrolytics to op-amp power inputs.

Ferrite beads on all input cabling

BTW – One small thing I didn’t mention… The op-amp being used is supposedly only 3dB down at 300K… Although the input-coupling cap and RIAA network will introduce their own roll-off, I do wonder if using an op-amp with such extended bandwidth isn’t helping. Having said that, the Millennia gear goes to 300K on paper and it has even more gain…

If anyone can think of a technique I’ve missed out I’d be grateful to know. I realise the twin GND switches will be confusing, but it shouldn’t take too long to evaluate each condition using the ears once preamp is in rack.

Thanks again,

Justin

 

[Samuel Groner]

A schematic would be helpful.

Samuel

 

[thermionic]

Hi,

Just like the query I posted here a few weeks back when I suspected grounding practice to be the culprit (it wasn’t…), it now seems I’ve jumped the gun again… I could well be an OCGPD sufferer (Obsessive Compulsive Grounding Paranoia Disorder).

It turns out that the case is providing a fair amount of shielding; when you lift out a preamp card and power it alone via the bench test supply, the RFI (and now, inductive hum) issue is exacerbated by an order of magnitude compared to when it’s seated in the case….

My suspicions are:

The caps for the RIAA are rated at high voltages, i.e. 4-600v. In my experience, using high-voltage caps for anything below line-level is a recipe for RFI / Inductive hum-type issues. I’m now building a version with industry-standard polypropylene (like the ones you see in Neumann mics) WIMA caps rated for more modest voltages.

I also wonder if the op-amp has an inherent susceptibility to noise in the power rails… Having said that, the noise seems the same, whether powered from the Lab supply (an uber-regulated “Weir” unit) or the LM317/337-based dedicated supply.

The op-amp has a supposed bandwidth of 3dB down @ 300K…that’s got to make its implementation trickier than an LM741…

Samuel,

I’d love to print a schematic, but I’ve tried so many topologies of grounding / power-distribution now, I’d have to print about 30 schematics…

Could the caps could be the issue?

Thanks in advance for any comments.


Justin

 

[JohnRoberts]

I'm not familiar with any association between breakdown voltage and capacitor sensitivity to interference, but I do recall with axial film caps, like the polystyrenes I liked for RIAA EQ, there was an outer wrap to the foil that would self shield the inner foil when connected to a low impedance circuit point.

Regarding PS, besides making sure it is stable, some decent HF cap decoupling should be adequate.

JR

 

[Samuel Groner]

I'd love to print a schematic, but I've tried so many topologies of grounding / power-distribution now, I'd have to print about 30 schematics...

Just a schematic of the electronics, not the grounding.

Samuel

 

[SonsOfThunder]

>>>an outer wrap to the foil that would self shield the inner foil when connected to a low impedance circuit point.

Just to clarify, this would mean reversing the capacitor might reduce noise...unless you can figure out how to identify which end was the outer wrap, you'll just have to experiment.

I have heard about this before, but I can't remember if we were able to resolve the problem with this solution...

 

[JohnRoberts]

With the old polystyrenes there was a band marking the end with the outer wrap, and you could actually see it. I suspect you could determine empirically with a scope probe and hum source (like an old school soldering iron). Alternately ground one end of the cap while probing the other in a noisy environment.

JR

 

[SonsOfThunder]

Cool idea there, JR. Gonna have to try that one Monday at work and see if that'll do the trick for identifying that...

Peace!