Hi all,
I have been agonising over building an offline regulated linear PS for a preamp. AC Mains are nothing to mess with, right? So I have become obsessed with questions such as whether to use a PTC or regular fuses or both? Where to place XY Caps? What about NTC to manage inrush? Then there are transients to worry about - TVS or MOV? Not to mention a switch, snubber and on and on.
I decided eventually & sensibly, I think to get an AC/AC wall transformer and work from there (all this was because I need two rails +-12v and was hoping to use a CT transformer I have).
I also happened to have an old Sony STR 6046a on my bench and I was aware it used a similar +- V setup. so I lift the lid and have a look at the schematics and I see... a switch, a fuse, Xformer, diode bridge, filter cap, done. But what about that destructive inrush current (two 6800uf elna filtering caps rated at 50V)? What about transients? What about safety hardware!? I have described the input to the power board which takes +-27V, so 54V total. As I was scratching my head I decided to test the ESR on the big caps and they came back excellent. All were original components and at this stage of the circuitry, all were working to tolerance - we are talking 40 something year old parts here!
So this is a long winded way of asking -
1. is inrush current really something to worry about in lower power devices
(like a preamp or headphone amp etc)?
2. Are all the modern precautions simply a way of protecting lower quality modern parts?
3. Is it really irresponsible to DIY build something offline paying the same indifference to safety regulations and standards as the vintage power gear does? (I am aware laws, components etc were very different 40yrs ago and mean no disrespect to the vintage gear)
Thankyou all!
I have been agonising over building an offline regulated linear PS for a preamp. AC Mains are nothing to mess with, right? So I have become obsessed with questions such as whether to use a PTC or regular fuses or both? Where to place XY Caps? What about NTC to manage inrush? Then there are transients to worry about - TVS or MOV? Not to mention a switch, snubber and on and on.
I decided eventually & sensibly, I think to get an AC/AC wall transformer and work from there (all this was because I need two rails +-12v and was hoping to use a CT transformer I have).
I also happened to have an old Sony STR 6046a on my bench and I was aware it used a similar +- V setup. so I lift the lid and have a look at the schematics and I see... a switch, a fuse, Xformer, diode bridge, filter cap, done. But what about that destructive inrush current (two 6800uf elna filtering caps rated at 50V)? What about transients? What about safety hardware!? I have described the input to the power board which takes +-27V, so 54V total. As I was scratching my head I decided to test the ESR on the big caps and they came back excellent. All were original components and at this stage of the circuitry, all were working to tolerance - we are talking 40 something year old parts here!
So this is a long winded way of asking -
1. is inrush current really something to worry about in lower power devices
(like a preamp or headphone amp etc)?
2. Are all the modern precautions simply a way of protecting lower quality modern parts?
3. Is it really irresponsible to DIY build something offline paying the same indifference to safety regulations and standards as the vintage power gear does? (I am aware laws, components etc were very different 40yrs ago and mean no disrespect to the vintage gear)
Thankyou all!
Many people are highly sensitive about this subject but being "old school " I have never used current limiting in my life especially in a small preamp , there again I used oversize transformers and high quality parts .
I know its the done thing now but all I have used is mains noise suppression circuits ,its all about flux and its better to switch on from cold and SMPS are certainly not exempt from this.
There is talk of fitting a thermistor but I have yet to find a noiseless one and there are countless circuits/components for reducing this online so I am in a minority yet I have never blown even a fuse in my equipment on switch-on.
Your third point is you would not be breaking regulations relating to mains supply in the UK other than The Standards Institute insist on an earthed chassis or "double insulation " but if you live in California I can bet they have a law for it --re- their legislation latest on plastic causing cancer and other "cancer inducing products" which have to be prominently displayed when selling certain items.
For those critical viewers here is the gov.uk safety regulations referring to domestic electrical equipment -
https://assets.publishing.service.g...rical-equipment-regulations-2016-guidance.pdf
May I add that Amazon/eBay et al send TWO pin European equipment to the UK --totally illegal !
Why ? because their main market is the EU .
I know its the done thing now but all I have used is mains noise suppression circuits ,its all about flux and its better to switch on from cold and SMPS are certainly not exempt from this.
There is talk of fitting a thermistor but I have yet to find a noiseless one and there are countless circuits/components for reducing this online so I am in a minority yet I have never blown even a fuse in my equipment on switch-on.
Your third point is you would not be breaking regulations relating to mains supply in the UK other than The Standards Institute insist on an earthed chassis or "double insulation " but if you live in California I can bet they have a law for it --re- their legislation latest on plastic causing cancer and other "cancer inducing products" which have to be prominently displayed when selling certain items.
For those critical viewers here is the gov.uk safety regulations referring to domestic electrical equipment -
https://assets.publishing.service.g...rical-equipment-regulations-2016-guidance.pdf
May I add that Amazon/eBay et al send TWO pin European equipment to the UK --totally illegal !
Why ? because their main market is the EU .
Thanks for that Duncan. Funny I thought there was a raft of insistent regulations around offline sources (also in UK) but there you are. Thanks for the link too.
When you see the minimalist approach in older gear it really makes you wonder if
a) It should have blown up/burst into flames/killed someone by now
OR
b) It's totally fine.
I remain highly wary of mains AC so I'll avoid it for now.
When you see the minimalist approach in older gear it really makes you wonder if
a) It should have blown up/burst into flames/killed someone by now
OR
b) It's totally fine.
I remain highly wary of mains AC so I'll avoid it for now.
Transformers are (a) fairly robust and (b) inherently band-limiting in nature. As such, they rarely need more than fairly minimal protection (maybe a spark gap). Inrush current generally isn't much of an issue in smaller supplies, though it may accelerate the aging of integrated thermal fuses in the long run (and those buggers may be buried deep inside), as has happened to the odd soft-start resistor over the years.
Switch-mode supplies need far more attention even just due the lack of the transformer alone - there is nothing to "brake" input current surges or high-bandwidth voltage spikes. They also emit plenty of noise so line filtering (both common mode and differential mode) is essential for any hope of passing EMC regulations.
What I would recommend:
* the usual mains fuse, appropriately sized
* secondary-side fuses (maybe polyfuse)
* X capacitor across mains power switch or snubber
* capacitors across each rectifier diode (for EMI suppression / avoiding RF backfeed)
Small transformers up to about 20 VA should not have problematic amounts of mains leakage and can thus be used to implement an IEC Class II appliance ("double insulated") with appropriate care in primary-side construction (adequate insulation and gaps / spacing). Traditional hi-fi gear with higher power demands will use transformers with a shield winding to minimize mains leakage.
Staying with IEC Class II construction but using a 3-pin power connection gives you additional flexibility, as remaining mains leakage can be addressed by a small (low nF) Y2 capacitor between device ground and protective earth without unduly increasing the risk of ground loop issues.
Pro audio gear generally is IEC Class I (ground referenced), which makes mains-sode construction a bit easier, but I would not recommend it unless you audio connections are all balanced, in which case this type of construction is in fact preferred.
I once acquired someone else's abandoned DIY headphone amplifier project (built after one published on the web) years ago, perhaps a decade. The complaint was that it was blowing the fuse. This was an all-perfboard project using uninsulated wire to make the connections - and since there was a board-mounted transformer, this included the primary side. One of these ran very close to the board edge which slotted into the (earthed) metal housing, and you can probably imagine why it was blowing the fuse now... (It's cases like these when you're grateful that DIY projects often are IEC Class I like this one was.) A bit of bending of the offending wire actually made it work, but since poor construction didn't end there and the circuit itself was flawed as well, it ended up in the scrap pile where it resides to this day. That was sketchy. It doesn't generally get remotely near as problematic these days.
If you go by the rule that "no single insulation failure must make the enclosure to go live" and observe at least 1/4" primary-secondary spacing (clearance / creepage) in primary-side construction, then mains voltage really is not that scary and problematic at all, and going Class II is quite feasible. You'll be using heatshrink on wire connections, double-insulated mains wiring, and appropriate distances (plus possibly slots to increase creepage distance) on PC boards.
Switch-mode supplies need far more attention even just due the lack of the transformer alone - there is nothing to "brake" input current surges or high-bandwidth voltage spikes. They also emit plenty of noise so line filtering (both common mode and differential mode) is essential for any hope of passing EMC regulations.
What I would recommend:
* the usual mains fuse, appropriately sized
* secondary-side fuses (maybe polyfuse)
* X capacitor across mains power switch or snubber
* capacitors across each rectifier diode (for EMI suppression / avoiding RF backfeed)
Small transformers up to about 20 VA should not have problematic amounts of mains leakage and can thus be used to implement an IEC Class II appliance ("double insulated") with appropriate care in primary-side construction (adequate insulation and gaps / spacing). Traditional hi-fi gear with higher power demands will use transformers with a shield winding to minimize mains leakage.
Staying with IEC Class II construction but using a 3-pin power connection gives you additional flexibility, as remaining mains leakage can be addressed by a small (low nF) Y2 capacitor between device ground and protective earth without unduly increasing the risk of ground loop issues.
Pro audio gear generally is IEC Class I (ground referenced), which makes mains-sode construction a bit easier, but I would not recommend it unless you audio connections are all balanced, in which case this type of construction is in fact preferred.
I once acquired someone else's abandoned DIY headphone amplifier project (built after one published on the web) years ago, perhaps a decade. The complaint was that it was blowing the fuse. This was an all-perfboard project using uninsulated wire to make the connections - and since there was a board-mounted transformer, this included the primary side. One of these ran very close to the board edge which slotted into the (earthed) metal housing, and you can probably imagine why it was blowing the fuse now... (It's cases like these when you're grateful that DIY projects often are IEC Class I like this one was.) A bit of bending of the offending wire actually made it work, but since poor construction didn't end there and the circuit itself was flawed as well, it ended up in the scrap pile where it resides to this day. That was sketchy. It doesn't generally get remotely near as problematic these days.
If you go by the rule that "no single insulation failure must make the enclosure to go live" and observe at least 1/4" primary-secondary spacing (clearance / creepage) in primary-side construction, then mains voltage really is not that scary and problematic at all, and going Class II is quite feasible. You'll be using heatshrink on wire connections, double-insulated mains wiring, and appropriate distances (plus possibly slots to increase creepage distance) on PC boards.
Thanks Herr Grossklass. I have read your post and taken some time to think about the detyails. Where the device is class II is chassis grounding not an issue? If device has a 'floating' ground what is the normal procedure?
Thanks for all your help.
Edit: Just read that Class II relies on double insulation. This can obviously be physical insulation, but would the DC circuit being isolated from mains AC also count as a kind of insulation?
Thanks for all your help.
Edit: Just read that Class II relies on double insulation. This can obviously be physical insulation, but would the DC circuit being isolated from mains AC also count as a kind of insulation?
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