Best way to shield a preamp

[fryingpan]

The amp I'm about to build will be housed in a metal enclosure (1mm "iron", according to the seller; keep in mind that in Italian, "iron" is often used as a shorthand for metal). Apparently to better shield the preamp from EMI/RFI, it would be appropriate to house its PCB (and the input jacks at least) in its own dedicated shield, but how does one go about it, in order to minimise the chances of ground loops? My idea was to find a metal box, remove the lid and install that within the enclosure with L-shaped holders, maybe with a plastic or so sheet underneath so that it does not have direct contact with the enclosure, and link one side of it to the surrounding metal (with a wire or so), so that it does not form a closed path but is still continuous with the enclosure. Am I going in the right direction?

[iainpunk]

when it comes to ground loops, its only important to negate when its connected to the circuit at multiple points, if you keep to a good star ground, you can have ground loops on another branch of the star without problems. keeping those from connecting might require isolated jacks, and not connecting PCB's to the case is also a good idea.
basically as long as you keep the 'branches' of the star ground not interconnected, you should be alright.

cheers

[anotherjim]

Is it AC mains powered with an internal PSU?

[fryingpan]

Is it AC mains powered with an internal PSU?

Yes, it will be powered by a 24-0-24 transformer which will be rectified, then I'll get +/-15V after a voltage divider (I'm considering whether to use regulators, Rod Elliott doesn't use any in his guitar amp power supply but I'm using not only opamps but also a few FETs).

[anotherjim]

OK. The AC safety earth wire (yellow/green) will be terminated to the case right by the IEC socket. Ideally, I use a crimp connection to a nut & bolt with shake-proof washers. Nothing else should share that termination point.
The amplifier DC ground star point connection to the case is as far away from the AC as practical. A soldered wire to a crimp, nut & bolt connection on the case is probably best. I like to place it near the input jacks. The actual star point will be the DC PSU 0v at the reservoir capacitors.

[fryingpan]

OK. The AC safety earth wire (yellow/green) will be terminated to the case right by the IEC socket. Ideally, I use a crimp connection to a nut & bolt with shake-proof washers. Nothing else should share that termination point.
The amplifier DC ground star point connection to the case is as far away from the AC as practical. A soldered wire to a crimp, nut & bolt connection on the case is probably best. I like to place it near the input jacks. The actual star point will be the DC PSU 0v at the reservoir capacitors.

Of course, that makes sense, but what about the dedicated shield? Should it be continuous with the case (ie. screwed onto the case, with the PCB inside it and its ground connection outside the shield of course, so for instance near the input jacks) or should it only be partially continuous (ie. one side screwed to the case, the others insulated from it) to achieve the best possible grounding?

[Bunkey]

Is it AC mains powered with an internal PSU?

Yes, it will be powered by a 24-0-24 transformer which will be rectified, then I'll get +/-15V after a voltage divider (I'm considering whether to use regulators, Rod Elliott doesn't use any in his guitar amp power supply but I'm using not only opamps but also a few FETs).

EDIT: Completely missed the 'pre-amp' in the title, sorry!
Regulators should be fine if that's what you want to do.

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The output from a voltage divider isn't a great way to power a load btw - typically they're just used to provide a reference voltage.

Regulators would be better - but still; using that 24-0-24 transformer, you're going to have +/- 30-something-volt DC rails coming from the rectifier and dropping half of that across a pair of regulators at power amp sort of currents is pretty intense.

You might be better with a 12-0-12 transformer that's going to output the required +-15v DC rails once rectified - There are plenty of amps which use FET inputs and the like without a regulated supply. If you do need a bit more stability in the earlier stages, splitting the pre and power sections by a simple zener regulator and large capacitor decouples the rails quite nicely. I've had some really good results with this approach recently.

Just a couple of suggestions there anyway.


What sort of amp are you putting together?

[Bunkey]

OK. The AC safety earth wire (yellow/green) will be terminated to the case right by the IEC socket. Ideally, I use a crimp connection to a nut & bolt with shake-proof washers. Nothing else should share that termination point.
The amplifier DC ground star point connection to the case is as far away from the AC as practical. A soldered wire to a crimp, nut & bolt connection on the case is probably best. I like to place it near the input jacks. The actual star point will be the DC PSU 0v at the reservoir capacitors.

Of course, that makes sense, but what about the dedicated shield? Should it be continuous with the case (ie. screwed onto the case, with the PCB inside it and its ground connection outside the shield of course, so for instance near the input jacks) or should it only be partially continuous (ie. one side screwed to the case, the others insulated from it) to achieve the best possible grounding?

If you make the inner shield continuous and flush with the metal outer case, all you're really doing is making the case thicker. To have a proper 'box within a box', it would be entirely floating within the outer case and connected to it via a single point.

TBH the only use and benefit I can see from having the pre-amp enclosed in its own double-skinned shield like this would be to shield the pre-amp from EMI being generated by other sections of the amplifier within that outer case. If you're concerned about the transformer coupling with the pre-amp then a simple dividing wall between them would be just as effective.

You can also put a 100n capacitor between tranformer secondary taps to help supress its contribution to EMI. That's what C2 is doing here:

[fryingpan]

I'm going to use his 60W/100W power amp with a preamp I designed (actually, I'm done with the preamp itself but I'm adding reverb and vibrato, plus some control signals for switches and/or relays). I want the amp to be as quiet as possible, because I'd use it for recording too and the rehearsal room I'm in is quite noisy), so that's why I'm going through the trouble of making shields and stuff (by the way, Rod himself suggests using shields for both the preamp and the input jacks; I guess the power amp doesn't need much shielding due to the signal levels). I might also add a line filter before the transformer to filter out line noise (*all* the amps I use in my rehearsal room whine). I would just go with post-rectifier filtering but I assume all the other amps (a Fender HRD, a GK 800rb, a Peavey Bandit, a Solton Chorus 100, the PA monitors) do that already, and they suffer from the issue, so I want to be extra safe.

[Bunkey]

Ah I see ...and that is why you're using a 24-0-24 transformer for +/- 35v rails.

Well in relation to my first post, let me share the power supply from the A&R A60.
It too has the +/- 35v power rails and regulated +/- 15v pre-amp supply. It's a very stable amplifier.




The regulator setup is pretty textbook, obviously you would include 10uF or 100uF decoupling on the rails after this and as close to your pre-amp as convenient (they're not shown here).
The bits highlighted in blue make up part of the amplifiers mute circuit and aren't relevant in this case.

It's a good simple supply and what I'd recommend.

A lot of these classic amplifiers have wooden enclosures - the A60 in particular just has a sheet of foil glued above the phono pre-amp and rear panel inputs, grounded at one side via the wooden cover's chassis screws; the main pre-amp is located just behind the metal faceplate.
With a big toroidal in a rather slim case there's no interference within the amp itself and it's all very quiet even on the phono input, which has enough gain to amplify the MM cartridge input signal of <5mv.

I think this minimalist approach to shielding in a non-conductive enclosure is maybe what Rod Elliott refers to when he says the pre-amp and inputs should be shielded.
If your whole amplifier is contained in a metal enclosure then the pre-amp and inputs - as well as the rest of the amp - is already shielded sufficiently from the outside. Again notice the 100n between secondaries on this supply.

I've also spent time with 90's Marantz PM- amplifiers enclosed in a stamped metal case that feature cheaper brick transformers in the PSU (more EMI-emmissive than toroidals) - the Marantz are arranged with a large heatsink acting as a dividing wall between the PSU and the rest of the amp. Whether this is strictly necessary I don't know but it would be all that's required as far as internal shielding within a metal enclosure.

[Rob Strand]

Well in relation to my first post, let me share the power supply from the A&R A60.

FWIW, the ground wire shown on the transformer on that circuit usually means the transformer has a Faraday shield
which is grounded.  (That's different to grounding the transformer chassis, which is normally done and assumed.)

(The transistors are used to indicate the unit has powered up.)

[fryingpan]

To that end, I could either get a barebones toroidal or an "encapsulated" one. Does the latter feature an actual Faraday cage? Otherwise, are there any standard cages for toroidals? Forgive me for the basic questions, but this is the kind of stuff you never really learn in electronics classes.

[bluebunny]

Great advice from Jim.

The AC safety earth wire (yellow/green) will be terminated to the case right by the IEC socket.

Remember to make this wire longer than the live and neutral, so that when/if someone yanks the mains lead from your preamp (and in this case the IEC socket isn't properly secured [seen it happen...]), then the earth is the last thing to be ripped away and disconnected.

[anotherjim]

I would advise against those snap-in AC sockets, switches & fuse holders. They only work properly in the exact size cutout in a particular thickness of chassis metal. A new plug fitted in a new socket can be an extremely tight fit.

The toroidal transformer should quieter. Lower magnetic radiation & mechanical/acoustic hum. You have to ensure that the top of the fixing bolt cannot ever contact the chassis via the lid. In that event, you have a short-circuit single turn winding via the bolt and a potentially massive current! If you think an accident could bend the lid down or the chassis up enough for the bolt to meet the lid, stick some insulation to it. An accident of this sort is highly likely for rack-mounted cases.

[Rob Strand]

To that end, I could either get a barebones toroidal or an "encapsulated" one. Does the latter feature an actual Faraday cage? Otherwise, are there any standard cages for toroidals? Forgive me for the basic questions, but this is the kind of stuff you never really learn in electronics classes.

For audio equipment with a mains ground it's much less trouble sourcing DIY friendly parts without a Faraday shield.    The Elliot sound pic Bunkey posted earlier will go a long way (the schematic with the diode + cap + resistor on the ground).   It's not common at all to find off the shelf transformers with Faraday shields.  It's a luxury for companies that get custom transformers made.   (Some medical products can only pass the regulations if they use a Faraday shield on the transformer.)

[fryingpan]

What about these?

https://uk.rs-online.com/web/p/toroidal-transformers/2238853/

https://uk.rs-online.com/web/p/toroidal-transformers/1234028/

(According to Rod Elliot, his power amp should be paired with anything over 200VA, he says that over 250VA would be overkill, but then when he talks about the same power amp used for bass, he recommends 300VA, and my amp would be used for both bass - recording - and guitar - anything really).

[Rob Strand]

What about these?

https://uk.rs-online.com/web/p/toroidal-transformers/2238853/

https://uk.rs-online.com/web/p/toroidal-transformers/1234028/

(According to Rod Elliot, his power amp should be paired with anything over 200VA, he says that over 250VA would be overkill, but then when he talks about the same power amp used for bass, he recommends 300VA, and my amp would be used for both bass - recording - and guitar - anything really).

I couldn't find where he talks about the bass amp.   

You could easily get away with 300VA for a two channel version for HiFi.

The 200VA min / 250VA max is a good recommendation for a one channel 100W amp.

Either of those transformers would work.

Unfortunately there's a lot of finer point for rating transformers on power amps.  Some of it is splitting hairs, some of it is designing the power supply for a special purpose or case.   One end of he scale would be a HiFi amp where you avoid clipping the amp (ignoring the two channel aspect) the other would be a test amp for a lab which is driving a square wave into a test load day after day.  The correct transformer VA can vary by a factor of two.   That rating would come from not cooking the transformer in normal use.   When it comes scraping a few watts here and there for audio that's more about supply voltages,  the decision point can be fairly arbitrary.

[ FYI,

I'm trying to avoid getting bogged down in silly finer points but I'll give you this to think about.

The output power of an amp is determined by the supply voltage.  A transformer produces the rated output voltage at full load (into a resistor).  When the transformer is unloaded the voltage is higher that the label voltage.   The voltage drop under load is the regulation.   When the actual load is less than the transformer rating the output voltage will be between the two voltage limits.  Large transformers tend to have better regulation than smaller.    At a 200VA load the output of a 200VA transformer will drop to the rated voltage.   However a 200VA load on a 300VA will not drop down to the rated voltage it will be a bit higher.   For a 100VA load it's not so clear what transformer will have a higher voltage.  The output of both transformers will drop under load but the 200VA starts off higher.   More often than not the 300VA will still win but not by much and not all transformers are alike.

RS 300VA:  regulation 7.4%
RS 225VA:  regulation 9.7%

but some older ILP brand toroids showed,
ILP 300VA:   regulation 6%
ILP 160VA:  regulation  8%
(guess a 225VA in that series would have 7% regulation)

The older ILP transformers were designed to run cooler and had lower resistance windings.
The new RS transformers might use higher temperature rating wires which allows them to run hotter but the side
effect is poorer regulation.  The regulation on the old ILP "225VA" is like the RS 300VA.

Here's another company's graph, (more like ILP),

]

[fryingpan]

Yeah, I know that larger transformers are not necessarily better and you should look at specs first and foremost. I can't find the passage where he states that for bass you should need a bigger transformer but anyway I guess it would make sense, as so much more energy is concentrated in the bass region (which requires more power), therefore the ability to provide the required voltage at full load should basically cause less compression and distortion in the power amp. I picked those two transformers because they are encapsulated really (I can't work out whether the case is plastic, therefore a safety device - not bad to have - or whether it is a metal shield), anyway anything else would do really. The former is a Nuvotem Talema device, which should be a quality product.

[jatalahd]

Lots of good info in this thread. I have been doing my first ever power amp build from scratch and, well, I have made all the possible rookie mistakes that can be made. Thanks to this thread, I manged to solve the noise issues I was having, mainly the input of the preamp was extremely sensitive to pick up any possible noise. The solution was to connect the green/yellow safety earth wire as adviced by anotherjim and to use the Rod Elliot schematic Bunkey posted for the earth connections. When I powered the amp on after making these additions to my connections, I could not tell if the amp was actually on or not - so silent!

My setup: I am using a cheap toroidal transformer (INDEL 100VA, 12 - 0 - 12) to get +- 18 volt unregulated DC voltage. I am using that unregulated DC also for the preamp. I have the preamp, and power amplifier both built using only discrete transistor stages, actually it is a copy/clone of my old solid state bass amplifier. The amp delivers about 10 to 15 watts (20 watts peak) power to 8 ohm speaker and drives very nicely a relatively small 7" or 8" speaker cone. I am using a wooden case for the amp, but the front and back sides have 1mm thich, 10x20cm aluminium plates, where the back plate connects to the safety earth and the front plate to the star ground via input jack. Seems to work very well.

So with this I am supporting what Rob S. wrote before:

For audio equipment with a mains ground it's much less trouble sourcing DIY friendly parts without a Faraday shield.  The Elliot sound pic Bunkey posted earlier will go a long way (the schematic with the diode + cap + resistor on the ground). 

[Rob Strand]

The former is a Nuvotem Talema device, which should be a quality product.

It's seems so.   I'm not sure where the motivation for potting come about.    It was almost never done on large toroids in the early days.   Potting is good to reduce buzz but not many toroids I've seen do that anyway.   It does help mounting a bit.  I have seen a few instances in production where transformer wires were broken during mounting.

Lots of good info in this thread. I have been doing my first ever power amp build from scratch and, well, I have made all the possible rookie mistakes that can be made. Thanks to this thread, I manged to solve the noise issues I was having, mainly the input of the preamp was extremely sensitive to pick up any possible noise. The solution was to connect the green/yellow safety earth wire as adviced by anotherjim and to use the Rod Elliot schematic Bunkey posted for the earth connections. When I powered the amp on after making these additions to my connections, I could not tell if the amp was actually on or not - so silent!

I had to solve a number of hum issues when I was a kid, not just on my equipment either.    I converged on the scheme Rod Elliot shows.  It works well.   The only thing to add is perhaps some small caps from the isolated audio sockets to ground to fend-off RF.   I've found it to work well when connecting up multiple pieces of equipment which are mains powered as well.