Preamble: I'm doing some work renovating a 1980s-vintage, very lovely solid state, discrete component Meridian MCA pre-amp. They originally shipped with a crappy little single input and output IE transformer that rattled about in a plastic box external to the pre-amp itself. It put out an AC voltage through a twin-core umbilical into an unscreened socket marked with a notional 19V AC requirement. I have one of the 2 original transformers (the other is for another piece from the same stable - the MFM Radio module) which doesn't specify the VA value: there is just a faded label marked '225-115-0'. The only other thing that may or may not help is '35 x 3/4S' (3 over 4) engraved on the plastic back cover. A quick, gash, test on my home supply (nearer 240V than 230) gave me an unloaded secondary output of 22V - 3V more than the notional loaded figure.
There is a lot of discussion about suitable VA values for Power Amplifier transformers, including talk about extra headroom etc, but very little guidance about pre-amplifiers, which draw significantly fewer Amps in their normal use. Nor whether there is a need for headroom above and beyond the notional maxima of - say - linear regulator chips. The MCA pre-amp PSU has 78/79XX TO-220-3 regulators on its main rails, stepping down to +/- 15v via TO-92-3 devices further downstream so my instinct was to go for something that could go up to the full 1A capacity of the TO-220 components. But would this be overkill?
Question(s): Is there a need for a higher current-handling figure than that suggested by the schematic, when this may result in the transformer not being fully loaded to its nominal output? Is there a 'typical' total amperage figure/range for a domestic preamp that I could safely use? Instinctively I think we're talking tens of milliamps here. I'd value some guidance about whether there's a rule of thumb along the lines of 'known amperage x 1.25' or something... and, as I'll still be using a transformer in a separate housing outside the component case, are there any considerations about the best type to use, about screening the umbilical, filter components integrated with it, etc. Assuming that Meridian would have not gone overboard with the original spec, can I deduce the VA value of the original transformer from its loaded v unloaded voltages, and use that purchasing a new one?
There is a lot of discussion about suitable VA values for Power Amplifier transformers, including talk about extra headroom etc, but very little guidance about pre-amplifiers, which draw significantly fewer Amps in their normal use. Nor whether there is a need for headroom above and beyond the notional maxima of - say - linear regulator chips. The MCA pre-amp PSU has 78/79XX TO-220-3 regulators on its main rails, stepping down to +/- 15v via TO-92-3 devices further downstream so my instinct was to go for something that could go up to the full 1A capacity of the TO-220 components. But would this be overkill?
Question(s): Is there a need for a higher current-handling figure than that suggested by the schematic, when this may result in the transformer not being fully loaded to its nominal output? Is there a 'typical' total amperage figure/range for a domestic preamp that I could safely use? Instinctively I think we're talking tens of milliamps here. I'd value some guidance about whether there's a rule of thumb along the lines of 'known amperage x 1.25' or something... and, as I'll still be using a transformer in a separate housing outside the component case, are there any considerations about the best type to use, about screening the umbilical, filter components integrated with it, etc. Assuming that Meridian would have not gone overboard with the original spec, can I deduce the VA value of the original transformer from its loaded v unloaded voltages, and use that purchasing a new one?
they must have gotten a good deal money wise so they did that....
i too have that tendency to overbuild my power transformers, bigger means good regulation and lower heat in exchange for overall efficiency...
engineering economics dictate reasonable sizing of transformers specially you will be buying them in the thousands or even more....
ths is where diy;ers can get crazy, b-with builds of just one, why not?
i too have that tendency to overbuild my power transformers, bigger means good regulation and lower heat in exchange for overall efficiency...
engineering economics dictate reasonable sizing of transformers specially you will be buying them in the thousands or even more....
ths is where diy;ers can get crazy, b-with builds of just one, why not?
I always spec my transformers with at least 50% more current than needed. They run cooler and the voltage won’t sag under the worst conditions that will be seen. It’s only a little more expensive and you won’t wish you had more current capability from the power supply. It’s also forward thinking as it will handle any additional circuitry you may add in the future. I’ve never been upset that I had too big of a transformer when it comes to current.
copper losses are lower this way, with lower Losses comes cooler working temps...
I always maximize copper wire section/diameter.
For me it is a sin to leave empty window space.
Yes, copper is expensive, but from measurement, dissipation loss is way higher in copper than in iron , as in under heavy load copper winding heats up more than iron core, so it always pays to lower DCR.
Do not call it an "expense", call it an investment.
For me it is a sin to leave empty window space.
Yes, copper is expensive, but from measurement, dissipation loss is way higher in copper than in iron , as in under heavy load copper winding heats up more than iron core, so it always pays to lower DCR.
Do not call it an "expense", call it an investment.