building ac/ac power supply 7.5-0-7.5

[Seattle2k]

I'm trying to bring an old synthesizer back to life. (Sequential Circuits Multi-Trak) I purchased it years ago, without the power supply, thinking it would be easy to find. Well.....it was, but procrastination on my part means the supplier no longer has any of them in stock today.

So, time to build one myself. I've built a regulated AC/AC power supply in the past, using a circuit diagram, but this time, I'm working from scratch, so some assistance would be great.

A reputable Sequential Circuits rebuilder, winecountrysequential.com, gave me the following specs for the power transformer:
Input: 120VAC, 60Hz, 25W
Output: 15 VAC, 1162mA; with Center Tap (C.T.)
5-pin DIN/Pin Outs: Pins #1 & #5 = AC, Pin #3 = C.T.

I understand 15vac w/ CT is the same as 7.5-0-7.5. Correct?
From this, I believe that each half would supply 581mA.

Assuming I can find a 7.5-0-7.5 transformer, I should be on a good start. I'm curious if I should be concerned about adding a filtering or regulation stage after the transformer. Would it make sense to start with a 9-0-9 tranformer, then add voltage regulators or zener diodes, to drop it down to +/- 7.5? (If I recall, I believe you can run a Zener in reverse direction, to take advantage of it's voltage-limiting properties, right?)


I may be way off base, so any help/suggestions are greatly appreciated.


Interesting thing to note is that on the back of the synthesizer, the power jack shows "15VAC @ 1.25A". So, the synth indicated 1.25A, but the transformer specs I got from WineCountrySequential state 1.16A

Thanks in advance,

Cameron Miller

 

[Diver300]

7.5 - 0 - 7.5 is the same as 15 V with centre tap. However, each half would need to be rated at the full current.

It's highly unlikely that you need to regulate the voltage. The rectification will be done inside the synthesiser, and the regulation will be done after that. You just need to buy a 7.5 - 0 - 7.5 transformer rated at 18 VA or more. The voltage will only change a bit when the transformer is loaded.

I've had a quick look at Newark.com and I can't find any 7.5 - 0 - 7.5 transformers of the power rating you need. You might be best to get a 15 - 0 - 15 transformer, but wire it for a 230 V supply. With the 120 V supply, you will get near to 7.5 - 0 - 7.5

That will need a 50 VA transformer.

Something like this would do fine.

MULTICOMP|MCTA050/15|TOROIDAL TRANSFORMER | Newark.com

The data sheet:-

https://www.farnell.com/datasheets/79607.pdf

says that the voltage is 15 V when fully loaded and 16.4 V when unloaded. If you are running at half voltage, you will get 7.5 V loaded and 8.2 V unloaded, or there about.

Regulating AC is very difficult. You have the right idea about zeners, but you need two in reverse-series with AC, and it will clip the peaks of the AC while not affecting it the rest of the time.