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In Reply to: RE: Transformers offer some filtering, but not as much as chokes. posted by Al Sekela on July 03, 2007 at 14:22:41
That is the same (or 1/2)as the John Risch digital isolation filter, using back to back Rat Shack step down tranny's. He uses an RC circuit in-between them. X rated caps are used and some have gone upwards of 10uf.
I've got a stash of 12/6V secondary Signal's in the surplus closet.
As for "the vicinity of the audio system", do you imply it should be plugged into the same circuit/power strip or across that specific device?
I'd be curious how both options compare.
What size VA transformer do you suggest,as well as the uf?
Follow Ups:
I've got a typical smaller house but with four dedicated circuits to the audio system. Any of the circuits, dedicated, or original, can ring electrically. There is usually a vacant outlet on each circuit where a damping device improves the performance of the audio system. You have to find the right outlets by trial and error: with standing waves, some outlets will be closer to nodal peaks than others. With my dedicated circuits, the only outlets available are the ones in the duplexes not employed by the audio equipment, so this is where I plug in my devices for these circuits.
Damping devices may be R-C filters, where the capacitors are X- or Y-rated; loaded transformers where the capacitors are protected from spikes by the bandwidth and core properties of the transformer, or Hammond chokes.
My comment about damping devices near the audio system relates to the physical presence of a large magnetized object exposed to the sound. If the object can vibrate acoustically, it will interact with the audio signal and AC. You don't have to be quite as particular about mounting damping devices in other parts of your house, but use care if a motor is nearby.
For my filters, I use the biggest, ugliest E-I core transformers I can find. My 1 KVA transformer does an excellent job of damping the dedicated circuit for my CD player. I use R-C networks on the circuits for the power amps. At some point I will try Hammond chokes in place of these.
Different resistors and capacitors have different sonic properties when used in R-C networks, even on the secondaries of transformers. There is a maximum value of C that will work. Start with the resistor you want to use, and limit the 60 Hz AC power to half the rating of the resistor. Calculate the current required to dissipate this power. Estimate the capacitor size needed to give this current if connected across the transformer secondary. This is technically incorrect, as it ignores the phase shift between capacitor and resistor voltage, but will be close enough in practice. Do not use more than this amount of capacitance. You may want to bypass a big capacitor, especially if the transformer has wide bandwidth.
I have a little experience with both transformer devices, thanks to Al's help, but cannot offer anything on the comparison to the chokes.
The JR digital iso tranny, as I understand it, uses the characteristics of the EI cores to handle noise (not what they're designed for but a nice benefit) coupled with the cap/resistor array tweak. The caps between the trannies on the secondaries apparently can carry loads more safely becaust the voltage is way down. A cap, cap-resistor, resistor array in a set of laddered values permits the filtering in the same way it does in a DIY parallel filtering scheme, which you've probably read about here. All of this works very well in my system for digital.
For analog, again with Al's tutelage, I made the transformer/R-C device, because I had one spare tranny from the iso tranny project to play with. Mine aren't RS, BTW, they're surplus MCI and of higher secondary voltage than the RS trannies originally recommended by JR. This is physically "half the JR digital iso tranny" as you say, but I have a hunch that Al is going to tell you that electronically it's not at all the same animal. It too works very well in my system to reduce noise; just flipping that switch on and off proves it almost instantly to even non-audiophiles.
This latter device in my system was the result of what's lying around, not what's best in values for the job. Al did it right, but then, he's the EE here.
I keep getting told by the choke gurus here that the choke is definitely a cut above the transformer tweak. Al suggested to me that it might well be. Have yet to get to testing that, though, so nothing to offer there.
Both tweaks are working cumulatively with various other R-C parallel filters I use all over my audio system. They do work well together. I recall that the chokes might not work with other devices so well, but that's for others to say.
The issue with the digital iso tranny, as I found, is when you put the pair of PT's back to front (secondary > RC> primary) the regulation goes backwards, leaving you with around 100V, depending on the load. Even with Signal PT's. The solution is to put at least an 80VA before the pair. BUT...it gets expensive. Might as well use a similar sized PT in BP mode.
Excellent feedback on Al's 1/2 of JR's digital filter. I do plan to try it.
Al believes the choke tweak works well. BUT....Ya really shouldn't use a plug. It needs a switch and snubber (cap or mov). For power out and plugged in, a cap across the switch will keep the circuit closed and gently discharge the choke.
If the plug is pulled say for comparative purposes, the potential across the choke has nowhere to go, but opening the windings or worse! I'll revisit it and add the DPDT relay with RC across for de-energizing.
Thanks for the update.
W
You wrote: "...when you put the pair of PT's back to front (secondary > RC> primary) the regulation goes backwards..."
I'm not at all sure what you're doing here. I mentioned the JR iso tranny because it's attached between the secondaries, so that you have 110 going in one end and coming out the other, with filtration at lower voltage in between.
As to the transformer-R-C tweak of Al's, check his past posts for how to calculate values of the R-Cs for the particular transformer. He told me that the little tranny I was using was not going to be as powerful a filter as the very large one he's using or any larger values. But it's a noticeable improvement in my system just the same and the stuff was lying around anyway. I just attached a snipped PC and plug to it. When I get around to testing a choke, whichever works better I'll wire directly into my DIY power strip on a short leash.
Working on other projects now, so no time for that.
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