DIY Armaggedon with a twist

Posted by jorisdallaire (A) on March 25, 2015 at 11:33:02

Hello,

I can't believe the price Naim asks for what is basically an isolation tranny. If the issue is line noise reduction then the following project might give very good results. It combines concepts from a DIY line conditionner/filter and buck transformer wiring. Now I am just a beginner and no EE; so please do let me know if there are any mistakes and if you think it's worth pursuing.

The example I'll use is for 120V; Make changes accordingly for your local voltage. I begin with a 400VA transfo having two primaries and two secondaries: 120+120:20+20. The secondary voltage can be anything, I just had two of these one sitting here; however, you'll see later that it's more cost-effective if something in the 20 to 60V range is used.

First, let's tackle the line noise issue: see fig. 1. Wire the two primaries together, and the two secondaries together. Hooking the primaries to 120V, line noise becomes common mode at the secondary and mostly cancels. Now a second trans, ideally identical to the first, is connected backwards to the first one, but having coils in parallel. It's output gives back 120V to the motor, but with noise filtered.

The point of having two primaries in series is to stay with the same ratio once the dual secondaries are in series. If you have two secondaries with half the voltage of the second transfo's secondary you can get away without a dual primary. Same goes if you have a CT secondary, 20-0-20 for example, however in that case I'm not sure the cancellation will be optimal (CTs being rarely truly in the center of the winding).

The dual primary also provides a center tap that is grounded to the case. My trans also has a shield that is grounded there as well. The seondaries's center tap can also be grounded on the case with the others, but a better solution is to leave it as an isolated ground. Any turntable/sound system ground could also be connected there, as it is clean. Use orange-coded AC receptacles and connect your TT there; of course nothing other than TT's synchronous motors should be conected to it.

Now the fun begins with fig 2. Adding some capacitance (C1) on the secondaries connection further cleans the AC. I'd use film caps there - you now understand why it is best not to have too high secondary voltages.

By connecting the second transfo's secondaries in series and adding a switch that shunts one of them, you can get two voltages: 120V for startup when one is shunt, and 60V when the two secondaries are in circuit, for less vibrations during playback.

For the the last finishing touch, use a delay circuit that puts the second winding in circuit after a few seconds, and a dual-colored status led.

Makes any sense?

All comments welcome.
Joris

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Topic - DIY Armaggedon with a twist - jorisdallaire 11:33:02 03/25/15 (0)

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