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In Reply to: RE: V = L (dI/dt) posted by wheezer on July 02, 2007 at 11:12:13
Or even a big MOV with adequate voltage rating.
I don't recall reading about your choke failure. FWIW, Hammond has the nick-name "Buzz and Burn," concerning the quality level of the transformers. Perhaps you were unlucky and got a defective unit.
My loaded transformers (similar purpose) are fed through 1-ampere panel-mount magnetic circuit breakers. Closing the breakers requires several tries to get them to remain closed. At least they get to absorb the arcing abuse instead of the plug contacts.
Follow Ups:
Yes, an adequate MOV should work as well.
I didn't post as I know what happened, especially after I saw the arc. Been there/done that with caps. I just wasn't sure as to a viable solution with a DC choke on AC.
I was meaning to ask you about using a small transformer in lieu of the choke, which you referenced in that thread. I've got a good selection of Signal PT's in the surplus closet. I would prefer to use the correct device for the job, if you don't mind offering suggestions.
Mitch
The Hammond choke tweak works, I believe, because the large inductance together with the parasitic coil capacitance makes it a wide-band capacitor. I have a 193H, a smaller 5-henry unit in the same Hammond series as the 193L, and it has a maximum impedance versus frequency at 5 KHz. This is likely due to the self-resonance, as typical transformer cores are useful up to 100 KHz in some cases.
Transformers have much less stray inductance than the 5-henry chokes, so they function as transformers over a wider frequency range. They would not offer filtering in themselves over their working frequency range, but you can load the secondary with an R-C network that will present a resistive impedance to the AC line over most of the transformer's working range. The upper frequency limit, whether due to primary leakage inductance and stray capacitance, or core magnetic domain switching speed limits, is a threshold above which the transformer shunts noise on the AC line regardless of what you attach to the secondary.
My DIY transformer shunt filters are designed to present 120 ohms to the AC line. Impedance transforms as the square of the turns ratio, so a 120-to-12-volt transformer would need 1.2 ohms load on the secondary to appear as a 120-ohm load at the primary, for example. The problem then is that a large capacitor is needed to keep the 60 Hz power out of the resistor, but allow the device to load the AC line over most of the audio band. It is better to use transformers with lower turns ratios.
Different transformers have different levels of performance as filters. I'm lucky to have a local store with a surplus room, and got a 1 KVA 14-volt transformer for cheap. A choke of similar size would cost a fortune to buy new. If the transformer filter is used in the vicinity of the audio system, it is important that it be mounted firmly and not have parts that can vibrate, such as end-bells. Shaking steel in the presence of strong magnetic fields is not a good idea.
Dear Al,
I had made two transformers for AC line noise filtering :
> One is a new 300VA EI transformer with a secondary of 6VAC X 2. I use the full 12VAC for the secondary voltage. A 0.33 Ohm resistor + multiple caps of total value of 90uF is connected across the secondary
> The other is an old 100VA EI transformer with a secondary of 18VAC X 2. I only use 18VAC for the secondary voltage. A 0.30 Ohm resistor + a 0.50 Ohm resistor + multiple caps of total value of 90uF is connected across the secondary
I noticed improvement in the resolution, bass and treble after adding the first transformer. More improvement was found after adding the second one
I would to ask the followings:
1. You mentioned that 'It is better to use transformers with lower turns ratios'. What is the reason behind ?
2. If you use a 14VAC secondary for a 120VAC primary, does it mean that I should use 24VAC or even 36VAC secondary if my primary is 220VAC ?
3. Also the temperature of my 300VA transformer is around 10 degree Celsius above the room temperature after switched on for one hour. Is this normal ?
4. Would it be better if connect one resistor for each small cap in the secondary of the transformer ?
Sorry for asking so much questions to you. But I like to learn more from you.
Thanks.
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?
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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