GRAHAM99 wrote: ↑Thu Feb 02, 2023 8:59 am
Still can’t find a power supply!
Maybe I could buy a suitable transformer but it would need to be safely housed etc
That would serve but as you say, it would need housing and making safe. The other drawback is the 18V secondary (you are unlikely to find 17.5V as a stock transformer) this will cause the internal regulators to dissipate a little more heat but would probably be acceptable. You could insert resistors in each winding to drop the voltage half a volt. One Ohm would be a good first guess.
The output voltage will depend on the input voltage anyway. I doubt the Behringer PSU for the EU had a different transformer in to the UK version, just a different mains plug, so the AC output was likely to be higher than 17.5v (it’s not 17v) when used in the UK depending on the local mains voltage.
I doubt whether 18v as opposed to 17.5v would bother it, especially when allowable mains tolerances are added on.
Assuming that it's selected for 17.5V at a nominal 230v, then you would get 18.2V AC at 240V. from the Behringer PSU.
Wonks wrote: ↑Thu Feb 02, 2023 9:47 am
The output voltage will depend on the input voltage anyway. I doubt the Behringer PSU for the EU had a different transformer in to the UK version, just a different mains plug, so the AC output was likely to be higher than 17.5v (it’s not 17v) when used in the UK depending on the local mains voltage.
I doubt whether 18v as opposed to 17.5v would bother it, especially when allowable mains tolerances are added on.
Assuming that it's selected for 17.5V at a nominal 230v, then you would get 18.2V AC at 240V. from the Behringer PSU.
Well, one has to start with design centre values! The transformer I showed would in any case have better regulation than Behringers, might get away with 16V?
Equipment should be designed to work to specification within the 10% mains tolerance but I wonder how many do, especially at this end of the market? I mention the extra regulator heat because Behringer do not have a good record for power supplies in the past!
Without an actual PSU to test, we're just guessing here. I did have a UB1202 for a short while which I gave to a friend, and that uses the same PSU as the Xenyx. That's been working quite happily for ages. Unfortunately it's residing about 80 miles away from me now, so a 160 mile round trip just to measure some voltages is not on the cards.
36V CT or 18-0-18 will give you 25.5V unloaded instead of 24.75V for 17.5V in using a standard bridge rectifier. Now I know behringer cut corners but I'd assume using 25V caps would have been cutting it too fine so they will probably be 50V caps. So you should be fine.
ajay_m wrote: ↑Thu Feb 02, 2023 3:26 pm
36V CT or 18-0-18 will give you 25.5V unloaded instead of 24.75V for 17.5V in using a standard bridge rectifier. Now I know behringer cut corners but I'd assume using 25V caps would have been cutting it too fine so they will probably be 50V caps. So you should be fine.
Even if the supply caps are only 25V I would not sweat it. The only result of the very slight over voltage would be a tiny increase in leakage current. The enemy of electrolytics is heat, not so much a volt or two over rating. In any case, the caps would soon reform to the voltage.
Back in the bad old days of 'domestic' valve gear caps were often just rated for the 'working voltage' and not the peak that a fast heating rectifier slammed on them (got even worse with Selenium rects!) Nonetheless, capacitors lasted decades in most bits of kit, the exception being TVs when Silicon diodes arrived and caps blew about every 3 or 4 years.
ef37a wrote: ↑Thu Feb 02, 2023 3:41 pm
Back in the bad old days of 'domestic' valve gear caps were often just rated for the 'working voltage' and not the peak that a fast heating rectifier slammed on them (got even worse with Selenium rects!) Nonetheless, capacitors lasted decades in most bits of kit, the exception being TVs when Silicon diodes arrived and caps blew about every 3 or 4 years.
Dave.
I have grim memories of the weird multi cap assemblies - often festooned with an unholy array of add-on ones, with some of the original terminals cut off (S/C cap) while others were just hanging across dried out ones. Then there were the copper oxide, or selenium rectifiers bodged with a silicon one bypassing them (so just one tag retained as a support, sometimes with a 10W resistor in series in an attempt to limit the surge.
ef37a wrote: ↑Thu Feb 02, 2023 3:41 pm
Back in the bad old days of 'domestic' valve gear caps were often just rated for the 'working voltage' and not the peak that a fast heating rectifier slammed on them (got even worse with Selenium rects!) Nonetheless, capacitors lasted decades in most bits of kit, the exception being TVs when Silicon diodes arrived and caps blew about every 3 or 4 years.
Dave.
I have grim memories of the weird multi cap assemblies - often festooned with an unholy array of add-on ones, with some of the original terminals cut off (S/C cap) while others were just hanging across dried out ones. Then there were the copper oxide, or selenium rectifiers bodged with a silicon one bypassing them (so just one tag retained as a support, sometimes with a 10W resistor in series in an attempt to limit the surge.
Yes, there were a lot of 'cowboys' about then Will. The old B&W TVs were a special case. The main caps were rated at 275V when peak mains is nearer 340V, the cheap buggers discovered that if you stress an electrolytic several tens of volts over its rating it draws a much greater leakage current and acts as a 'free' surge limiter. The caps did not take kindly to this and failed after just a few years. Then you could get an O/C heater chain so the set never drew any HT current...BIG bang and a big mess.
Well that transformer will 'do' but surely, if you have to buy a new one buy better specc? The mouser traff has a slightly lower VA rating than mine and nominally a volt more output and mine runs quite warm. A toroid will be more efficient, especially since it will be run constantly at close to VA limit. You are not subject to the cost restraints of production.
CPC & Rapid have a reasonable selection of 18-0-18V transformers.
The Rapid site is a bit more searchable/filterable. They also do enclosures [some plug top style].
Extra care is required mounting torroidals to make sure the bolt through the middle doesnt become a 1 turn coil !
N i g e l wrote: ↑Fri Feb 03, 2023 12:12 pmExtra care is required mounting torroidals to make sure the bolt through the middle doesnt become a 1 turn coil !
Sorry, Nigel, for the ignorant amongst us, i.e. me, can you elaborate on that please?
The fixing bolt passes through the centre of the toroid and needs to be insulated from forming a complete loop with the chassis, i.e. a secondary winding with one turn. If it does a very large current would flow through the bolt and the chassis. Have a look at those instructibles on making a spot welder with the transformer out of a broken microwave, the secondary is 3 turns and the current it can produce welds metal.
