I'm seeking some advice as to what rating a stepdown XFMR (VA, voltage) would require to be a candidate for stepping 12VAC up to 25-26VAC/CT (i.e. 50VAC series) when reverse-fed (i.e. primary/sec swapped). This is to create a bipolar supply (317/337 regulation) of ± 27-30V, with 60mA draw on each rail, to power a graphic EQ with discrete op amps. I'd like to apply Elvee's D-Noizator mod as well.
I have tried this with a 6VA 48V (24-CT-24) split bobbin XFMR (Triad Magnetics FS48-125-C2), and the results were abysmal. Flipping the XFMR and feeding the series secondary as a primary yielded 2 X 23.3VAC, or 32V rectified, (no load). These rails collapse to <14V with even a 1K load across them. Obviously this XFMR is woefully underrated for what I'm trying to do. The 12VAC supply was a 10A rated supply; the 12VAC supply did not sag, nor did it have any DC on it.
I now understand that XFMRs are not inherently bidirectional, and have extra windings to account for regulation. So it seems one must up the VA rating to antitipate lossy operation when reverse-feeding, and plan for the loss of voltage due to regulation compensation, the question is by how much? Are split bobbins notoriously bad for this? I've read toroids might offer better performance in this regard (?)
A copmpany engineer suggested a 7VA toroid would hold up to my demands, but I'm not so sure.
This is for a guitar effects pedal with discrete op amps that run at 25-30V. Connecting to mains isn't an option for me (and effects pedals typically have wall adapters anyway), and the emissions testing required for a SMPS is also prohibitive at this stage (I may make these units for commercial sale at some point). The plan is to utilise wall wart 12VAC adapters. There are other effects pedals that flip prim/sec sides to step up voltages in this manner (e.g. for tube plate voltages), which is where I hatched the idea.
I'm going to have to buy a bunch of different XFMRs to try out, but any advice on ballpark XFMR ratings (and what I need to consider generally) would help me greatly in saving on getting redundant parts.
TL;DR: Seeking advice on mimum XFMR specs for reverse feeding as a stepup (12VAC into secondary, now acting as primary) to obtain bipolar supply of ± 27-30V, 60mA draw per rail.
Thank you very much.
I have tried this with a 6VA 48V (24-CT-24) split bobbin XFMR (Triad Magnetics FS48-125-C2), and the results were abysmal. Flipping the XFMR and feeding the series secondary as a primary yielded 2 X 23.3VAC, or 32V rectified, (no load). These rails collapse to <14V with even a 1K load across them. Obviously this XFMR is woefully underrated for what I'm trying to do. The 12VAC supply was a 10A rated supply; the 12VAC supply did not sag, nor did it have any DC on it.
I now understand that XFMRs are not inherently bidirectional, and have extra windings to account for regulation. So it seems one must up the VA rating to antitipate lossy operation when reverse-feeding, and plan for the loss of voltage due to regulation compensation, the question is by how much? Are split bobbins notoriously bad for this? I've read toroids might offer better performance in this regard (?)
A copmpany engineer suggested a 7VA toroid would hold up to my demands, but I'm not so sure.
This is for a guitar effects pedal with discrete op amps that run at 25-30V. Connecting to mains isn't an option for me (and effects pedals typically have wall adapters anyway), and the emissions testing required for a SMPS is also prohibitive at this stage (I may make these units for commercial sale at some point). The plan is to utilise wall wart 12VAC adapters. There are other effects pedals that flip prim/sec sides to step up voltages in this manner (e.g. for tube plate voltages), which is where I hatched the idea.
I'm going to have to buy a bunch of different XFMRs to try out, but any advice on ballpark XFMR ratings (and what I need to consider generally) would help me greatly in saving on getting redundant parts.
TL;DR: Seeking advice on mimum XFMR specs for reverse feeding as a stepup (12VAC into secondary, now acting as primary) to obtain bipolar supply of ± 27-30V, 60mA draw per rail.
Thank you very much.