Based on Sangrams comments about the voltage of the bridge, if you were to take the loop breaker in post 16 and up the voltage of the bridge to 600V then you should be ok for either 120V or 240V operation.
A suitable bridge is this one https://au.element14.com/multicomp/gbpc3506/diode-bridge-rect-1-ph-600v-module/dp/2674970
For more info on this see the earth loop breaker section on Rod Elliot's page here Earthing (Grounding) Your Hi-Fi - Tricks and Techniques He uses a resistor rather than the thermistor but either is fine.
Tony.
A suitable bridge is this one https://au.element14.com/multicomp/gbpc3506/diode-bridge-rect-1-ph-600v-module/dp/2674970
For more info on this see the earth loop breaker section on Rod Elliot's page here Earthing (Grounding) Your Hi-Fi - Tricks and Techniques He uses a resistor rather than the thermistor but either is fine.
Tony.
The 'correct' solution is to directly connect circuit ground to chassis using a thick, short wire and a failsafe bolt. I am unaware of any other solution except a double-insulated transformer and the chassis connected to circuit ground, which is how many manufacturers do it. Others connect it directly.
My Benchmark DAC2 (as an example of a directly earthed system ground) is configured with a small switching supply directly bolted to the chassis, and every exposed connector is connected to an unpainted metal surface. It achieves excellent S/N ratio so a direct connection to earth is not problematic when done properly.
The 'Loop breakers' - either CL60+rectifier bridge or the Rod Elliot solution - are workarounds to be able to use regular transformers and exposed metal parts at circuit potential. But for either of them, the bridge is mandatory to ensure safe and repeatable operation.
hi - Thank you for your help. Yes, i have everything now but the discussion has made me quite nervous about doing the project. On one hand, the build guides are instructional and really could be followed by a layman by myself. On the other hand, it is clear that many have religiously followed it without realizing the potential dangers. Even if the bridge rectifier etc helps, would i trust my life in a circuit that relies on a thermistor to get hot to before i get killed? i don't know enough about the schema to know the real risks and i really need a person to explain to me in plain English - if a live wire touches the chassis with a person in contact, would it be safe?
It is very kind of you to offer. I live in guildford so it's not too far to travel down to hampshire.
It is very kind of you to offer. I live in guildford so it's not too far to travel down to hampshire.
Drive down and all will become clear. There are always many posters with their take and explanations on what their preference is (on lots of topics) but you cant go far wrong with what Mr Pass puts to paper. As you learn the fog lifts and becomes a mist which you can see through....Your only an hours away. this is a hobby, enjoy don't get stressed. The first job I do on a build is connect the mains earth to the chassis. All the comments above may all be relevant but very confusing to a budding DIYer on an FW amp. I think you might be better of here than on the pink place anyway.
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Short answer: for the scheme in post #3, the answer is maybe (if it's the first time it happened) - for the ones using a bridge rectifier, the answer is yes.
I've built many of Nelson's circuits and he is to my mind, one of the finest audio designers that ever lived, and by a long way the most generous to our little community.
That still doesn't make post #3 safe.
Lets have a go...
Think of some commercial item like an old VCR or a DVD player that is all metal construction and that uses a normal old school transformer like this amp.
Is it grounded? The answer for most such items would be no.
How then is it 'safe'? The answer to that is the manufacturer has 100% total control over the way it is built and critically the parts they specify to be used.
That means that there is no conceivable way for anything live to touch the chassis, and that is ensured by design such as shielding of live terminals and having recognised spacings between any holes in the cabinet and anything live. The design is repeatable and all examples are identically built.
The second thing is that critical parts such as the mains transformer will have been specified to comply with 'double insulated' standards meaning the manufacturer of the transformer guarantees that the part can not fail in such a way that anything live can transfer to any part of the transformer frame, mountings or secondary winding.
Such equipment qualifies to operate safely with no mains ground.
In the commercial product if the transformer were substituted for an off the shelf item that had not passed the required standards then technically the whole product would be non compliant. A failure of the replacement transformer could now not be guaranteed not to allow mains potential to couple through somewhere whereas the manufactured specified one can be guaranteed not to do so.
Our amplifiers are very similar in concept except we have no control over what parts are used and how each builder constructs their amplifier.
So the first safeguard now is to correctly ground the casework.
That 100% covers you from getting a shock should anything live contact the case, however even here there are twists and turns. If the ground connection is made to the baseplate (for example) then are the top and sides etc in perfect electrical contact with that base?
They most likely are, but if the case is painted and or anodised then there is a question mark and it could not be guaranteed.
Is that nit picking? Well it would not allow a product to pass safety tests, the answer would be to ensure each exposed panel was grounded by whatever means were appropriate.
And this is where your original question comes in... what if something bad happened... and could the thermistor protect me.
Can you see now that if that 'something bad' were a wire falling off something and touching your circuitry, or if the transformer used developed a problem that allowed live to couple through to the secondary side then the thermistor is totally inadequte as a safeguard. Only a direct low impedance connection will suffice.
So what of the bridge rectifier solution? Is it acceptable?
My own opinion is that I personally would be happy with such an approach on anything I built, however I wouldn't like to say if it would pass any legal type approval or meet the definition of a low impedance earth return.
Hope that helps 🙂 and remember that many many people build there own stuff without any issues. The fact you are asking questions is good and probably means you will implement a lot of these thoughts and ideas better that most.
FYI the build guide for the F5 still has the original PS schematic. I'm going to go ahead and implement the Elliott Sound Products design for my F5 and BA-3 using 400V bridges, 5W 10 ohm resistors, and .1UF safety caps.
The MOSFET failure and the PS DC rails' current to ground is a real possibility, especially when DIY-ers are mounting the keratherm insulators incorrectly (using the plastic bit, instead of the keratherm), or when they forget to smooth the heatsink surface before mounting the MOSFETs.
The statement about NTC's exploding is an interesting one, especially when we consider the fact that CL-60's are inserted in series with the mains primary winding(s), as used in all of the FW clone builds here on DIY forums... but, if it explodes, the root cause could be fixed and a new NTC inserted, so I do not see any issue here. Anyway, if the CL-60 is exposed to such currents that could cause it to explode, the mains fuse would blow first.
The CL-60 inserted between PS common and chassis? I will re-iterate: the fault current that could flow through that CL-60 would cause its resistance to drop to around 0.0-something ohms, and the weakest link, elsewhere, would give up first... a PCB track, or a fusible resistor. This type of fault happens in other consumer electronics, and I do not deem this as life-threatening occurrence at all. The bottom line is.... the fusible resistors (or fuses) should be used at the DC side to ensure prevention of fire hazard. There's no electrocution hazard here!!
The only life-threatening hazard is the mains voltage potential; however, as long as the mains IEC connector has its ground pin connected to chassis, the risk associated with this hazard is mitigated towards an RCD trip, and therefore, no life-threating scenario is guaranteed.
....so, the real question is: Is your household wiring safe? Is your mains / earthing done properly? Is your RCD being tested routinely -> to ensure its safe operation/protection?
Interesting, so with an F5 and the original PS design, do you know what value fuse would be appropriate and where on the secondary side?
Exterme_Boky, I think Mooly already gave the example where it can become unsafe.
Primary to secondary short in transformer. Fuse blows, breakers trip, CL60 fails open circuit.
Fuse replaced, breakers reset amp powered on. speaker earth and rca's now at mains potential.
Yes if you have earth leakage detectors in the house wiring, and you touch one of these live points this should result in the power being cut very quickly (that is the point of them). What if you don't have RCD's? I'm sure many places in the world don't have them. Here in Australia any house older than a certain age (built before 1990) won't necessarily have them. RCDs: The electrical safety switches that a third of homes go without - Allianz Australia
Also if you add the bridge rectifier the chances of it failing from a surge in current (the one I linked to can handle a 400A peak) is extremely small. It will blow the fuse every time it is replaced until the fault is rectified. So the bottom line is that adding a bridge rectifier that costs around $3.50 AU (a very small cost in the total cost of the amp) is an additional safefty feature that safegaurds against an unlikely fault condition. Don't you think that it is worth the extra $3.50 for peace of mind?
Tony.
Primary to secondary short in transformer. Fuse blows, breakers trip, CL60 fails open circuit.
Fuse replaced, breakers reset amp powered on. speaker earth and rca's now at mains potential.
Yes if you have earth leakage detectors in the house wiring, and you touch one of these live points this should result in the power being cut very quickly (that is the point of them). What if you don't have RCD's? I'm sure many places in the world don't have them. Here in Australia any house older than a certain age (built before 1990) won't necessarily have them. RCDs: The electrical safety switches that a third of homes go without - Allianz Australia
Also if you add the bridge rectifier the chances of it failing from a surge in current (the one I linked to can handle a 400A peak) is extremely small. It will blow the fuse every time it is replaced until the fault is rectified. So the bottom line is that adding a bridge rectifier that costs around $3.50 AU (a very small cost in the total cost of the amp) is an additional safefty feature that safegaurds against an unlikely fault condition. Don't you think that it is worth the extra $3.50 for peace of mind?
Tony.
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1. If you do not have hum/buzz issues, connect the PS common straight to chassis.
2. Are those drain/source Panasonic (0.47ohm/5W ???) resistors fusible? If they are, there's nothing else to worry about. If they are not..... start thinking about fuses on a DC side. This is ONLY to be absolutely sure you're not going to be facing a potential fire hazard. But, fusible resistors and fuses on DC side are terrible for sound quality🙂. the choice is yours.
The most important thing is to connect the mains 110V AC IEC connector ground pin to chassis and to ensure your house wiring is safe.
As far as I know here in the US, ground fault interrupters are used only near bathrooms/kitchens/basements and outdoors.
Not sure about the resistors, but I'll earth ground my PS common and see what kind of noise I get.
sorry gents but i need your help again.
1) I think reichelt electronics has sent me a wrong transformer. The transformer is this one Security Check. If you look at the datasheet, it shows a different transformer! The picture is the one i actually have. There are only 6 leads, 2 thin blacks and red/green/blue and yellow. I can only guess that one black for the primary and one for the secondary?
2) in the build guide, the thermistor for the primary is CL-12 (only 1 for 240v). Can't find this anywhere on RS Pro or Farnell. is there a substitute?
3) In the build guide, it says to dress the purple lead. I presume this is the copper plate used in case shorting in the primary. Why is this not connected to the chassis earth?
Sorry again for these questions, but i want to understand it before building.
Thank you
Philip
1) I think reichelt electronics has sent me a wrong transformer. The transformer is this one Security Check. If you look at the datasheet, it shows a different transformer! The picture is the one i actually have. There are only 6 leads, 2 thin blacks and red/green/blue and yellow. I can only guess that one black for the primary and one for the secondary?
2) in the build guide, the thermistor for the primary is CL-12 (only 1 for 240v). Can't find this anywhere on RS Pro or Farnell. is there a substitute?
3) In the build guide, it says to dress the purple lead. I presume this is the copper plate used in case shorting in the primary. Why is this not connected to the chassis earth?
Sorry again for these questions, but i want to understand it before building.
Thank you
Philip
What does the label on the transformer say? According to the webpage the specifications are as follows:
Performance
500 VA
Operating voltage
230 V
Output
2x 18 VAC
Current
2x 13.8
As for the leads, they are described in the label, and it looks like it is in German:
black schwarz
yellow Gelb
red rot
blue Blau
green Grün
Post a picture of the label of your transformer if you need further help.
Performance
500 VA
Operating voltage
230 V
Output
2x 18 VAC
Current
2x 13.8
As for the leads, they are described in the label, and it looks like it is in German:
black schwarz
yellow Gelb
red rot
blue Blau
green Grün
Post a picture of the label of your transformer if you need further help.
1) This is a single primary transformer, Black is the primary for 240v. The 4 others are the 2 secondaries. (red/yellow and blue/green)
2) Because it is a single primary you cant follow the build guide in putting the thermistor between the primaries. This acts as a soft start, I'm not sure how best to set up a single primary at 240v, perhaps consider a soft start board.
3) the Purple is a shield, not all transformers have a connection for that, yours doesn't appear to.
NTC is dumb - it doesn't know where it's put
in series with primary , irrelevant from which side you look at primary
so put it in phase lead , or neutral lead or in between series-ed primary halves , in case that you have halves
in series with primary , irrelevant from which side you look at primary
so put it in phase lead , or neutral lead or in between series-ed primary halves , in case that you have halves