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PostPosted: May 4th, 2020, 2:27 pm 
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I was going to stay out of further comments to this post but I can't help myself. My fundamental problem with this approach is the use of an isolation transformer for noise suppression in an audio system. I do use isolation transformers but for the purpose of safety when I am working inside electronic equipment. If I'm coming off the isolated side and I accidentally come in contact with one of the AC leads and ground I won't get killed. Also when using test equipment like an oscilloscope, the shield side of the probe which connects to the shell of the BNC connector is not hot in relation to earth ground.

An isolation transformer can provide some noise immunity because it has leakage inductance which limits the bandwidth of high frequency signals it will pass from primary to secondary. Also if the secondary is truly isolated from utility power ground loops are minimized. However, once you ground any of the secondary to earth ground you have a potential ground loop path from the "isolated" side to utility ground and neutral.

In my opinion, there are two main things that can provide noise rejection in an audio setup. The first deals with ground loops and is to make sure that all the AC power to all your audio devices, assuming they are 120 volts, comes off the same breaker that is dedicated only to the audio system. The noise that can creep into the system when you work off both sides of the split phase coming into your house is eliminated. Second, for RF and other transient noise carried on the lines, a commercial grade noise filter provides a lot of rejection far more than you will ever get from an isolation transformer. Then the most important ground is the one that runs from the ground connection on the filter and a good system ground. For a long time my go to filter has been one from Corcom. Here is a link:

https://www.newark.com/corcom-te-connec ... dp/52K3304

It has multiple stages of line to line and line to ground filtering. This particular model is rated at 20 amps. There are smaller filters but they should have all the filtration mechanisms as this one. Most smaller ones don't. Here is a circuit diagram:

The advantage of this is that your AC supply is not limited by the capacity of the transformer which could limit performance and you have the safety (assuming your house wiring is to code) of working with standard utility power and approved grounding mechanisms.

In reality, all of these are band aids and there is no substitute for good power supply design in your equipment. However, unless you are building all your own stuff, what I have mentioned, in my not so humble opinion is the best way to go. Ok, now you can start throwing bricks.


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PostPosted: May 4th, 2020, 3:45 pm 
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Jim G wrote:
I haven't compared a regenerator yet.


Easy enough to build one. An r-pi, a large SS power amp, and an output transformer is all it takes....

Roscoe

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PostPosted: May 4th, 2020, 4:44 pm 
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I use an isolation transformer as the first line of defense and those nice CORCOM filters. When there was Ham-Fest I was able to snag many of them form 3-amps. to 40-amps. for pennies on the dollar.

The best money ever spent.

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PostPosted: May 4th, 2020, 5:08 pm 
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Roscoe Primrose wrote:
Jim G wrote:
I haven't compared a regenerator yet.


Easy enough to build one. An r-pi, a large SS power amp, and an output transformer is all it takes....

Roscoe


Or one of the large Crown amplifiers like the CDI400 that will put out 2KW at 140 volts and you don't need any output transformer to limit the output power. You would wire it in the bridge mono mode. All you need is a signal generator to provide 60 HZ, (50HZ if you want to test 50HZ equipment). If you can find one of the older Macro Reference or Studio Reference amps they will also swing enough voltage to run your equipment. I have run my shop vac off my Macro Reference. The harmonic distortion of the CDI4000 is .05%, better than you will get with any raw utility input voltage.

https://www.crownaudio.com/en/product_d ... b6172ca1e8

If you want to roll your own sine wave generator you can make a very low distortion sine wave generator using the LT1007 Op Amp. The schematic shown has values for 1KHz but the formula is there for any frequency. Note the distortion and noise. There are also circuits out there for crystal controlled oscillators.


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PostPosted: May 4th, 2020, 5:21 pm 
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Location: Baltimore MD
I’m my build I put the mains filter before the iso, Only because it made since to me.
Is this the right place?


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PostPosted: May 4th, 2020, 6:38 pm 
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The Corcom type filter would work more ideally in a a balanced power supply. On the assumption that noise is picked up equally on both lines equally, which is a fairly good assumption, then the noise on each line would be out of phase and theoretically cancel at the ground point in the center of the filter. However, since the neutral is tied to ground, this won't happen. Otherwise, the diverted noise current would enter the ground.

If you use a balanced power isolation transformer, I would put the Corcom on the secondary to take advantage of the balance. The filter could also be more protected from surges by the transformer, especially good if it contains MOVs as surge protectors, since they are only good for a limited number of hits. (The transformer would reduce the surges first.)

However, I don't want to promote balanced power here, even though I am, because of the details needed to carry it out. I don't want to be responsible for anyone's safety for not understanding how to implement it. A transformer big enough to power an amp is very heavy, and costly. The cost/benefit ratio is high, whether or not it is balanced. They have to be specifically wound for balance. It's not just about connecting a center tap to ground. That being said, if noise is minimized as much as possible in the rest of the equipment, a balanced transformer can be a way to minimize it more. I use a balanced transformer with 106db sensitive horns, and cannot hear any noise whatsoever, unless I stick my head into the horn!


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PostPosted: May 4th, 2020, 7:52 pm 
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[quote="Pooge"]The Corcom type filter would work more ideally in a a balanced power supply. On the assumption that noise is picked up equally on both lines equally, which is a fairly good assumption, then the noise on each line would be out of phase and theoretically cancel at the ground point in the center of the filter. However, since the neutral is tied to ground, this won't happen. Otherwise, the diverted noise current would enter the ground.

Your statement that on a balanced feed the noise would be equal on both lines and out of phase is only true if the transformer is passing a noise from the primary that is also equal and out of phase. If the noise on the primary is being received from an external source that is radiating into the lines depending on the frequency and the layout of the feed lines it would most likely be in phase on both lines. That is where balanced circuits especially with transformers reject common mode signals, that is signals that are in phase so that in phase noise would not make it through the transformer, ignoring capacitive coupling for this discussion. If they were out of phase, that is differential, the common mode rejection (CMRR) would not reject them because they are not common mode and they would pass to the secondary.

The out of phase voltages that would appear across the center tapped secondary would not truly cancel at the center tap. The voltage at the center tap is the middle of what ever total voltage appears across the secondary. Even if you ground that point the voltage relationship between the leads does not change and no current will flow to ground since the secondary is isolated from ground except at the point where you have made the connection. There is no circular path for any current to return to either lead of the secondary, again ignoring stray capacitance. All you are doing is referencing the center tap to a ground but that does not change the secondary lead voltages with respect to each other. If you connect a load across the whole secondary it would see the whole noise voltage on the secondary. If you connect from one lead to the center tap it would see half of the total noise. On the transformer, you cannot connect the two leads to each other or you would have a direct short. In that case the out of phase voltages would cancel along with the production of a lot of smoke.

If you look at the circuit diagram of the Corcom filter, there are only two lines plus a ground. They do not differentiate between a hot and neutral line. In fact, when the Corcom is used at 240 volts on a traditional split phase, ie balanced center neutral house feed both inputs are hot. The key to the figure is that both lines have inductors and capacitors in combinations that shunt a lot of the noise from either lead and either polarity to ground regardless where it comes from.


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PostPosted: May 5th, 2020, 1:06 pm 
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I've another thought regarding this. Paul's isolation transformer has two primary windings. It can be connected to a 240 volt circuit. This would eliminate noise pickup on the neutral leg of the circuit, which carries the imbalance and harmonics of all sorts of noisy critters in the house.

Of course, this would mean installing a dedicated circuit to the stereo room, and I've no idea how difficult that might be in Paul's house.

No changes to grounding, whichever route you choose: ungrounded (to house service), grounded via circuit ground or grounded via an insulated conductor to the service ground.

This is a journey of discovery, and who knows what sounds best: low noise, open dynamics, etc.? I know I don't. But that's the fun of this, finding out what works best.

Just a thought. HAVE FUN!

Stuart


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PostPosted: May 6th, 2020, 9:28 am 
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Location: Baltimore MD
I do have a dedicated line for my sound room.
To do this I would have to mount the iso near the breaker box put in a 240 volt breaker


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PostPosted: May 6th, 2020, 11:15 am 
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Location: Fairfax Station, VA
tomp wrote:
Pooge wrote:
The Corcom type filter would work more ideally in a a balanced power supply. On the assumption that noise is picked up equally on both lines equally, which is a fairly good assumption, then the noise on each line would be out of phase and theoretically cancel at the ground point in the center of the filter. However, since the neutral is tied to ground, this won't happen. Otherwise, the diverted noise current would enter the ground.

Your statement that on a balanced feed the noise would be equal on both lines and out of phase is only true if the transformer is passing a noise from the primary that is also equal and out of phase. If the noise on the primary is being received from an external source that is radiating into the lines depending on the frequency and the layout of the feed lines it would most likely be in phase on both lines. That is where balanced circuits especially with transformers reject common mode signals, that is signals that are in phase so that in phase noise would not make it through the transformer, ignoring capacitive coupling for this discussion. If they were out of phase, that is differential, the common mode rejection (CMRR) would not reject them because they are not common mode and they would pass to the secondary.


True. Transformers are an excellent common mode filter. And they are better for a broader range of frequencies.


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