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Author Topic: Distance between electrical and audio cables  (Read 21119 times)

Mike Sokol

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Re: Girding loins in iron
« Reply #40 on: January 19, 2015, 05:20:15 pm »

I have a small handfull of Rapco-Horizon ISOBlox, and a couple of WW podDI boxes. Do you have any knowledge or opinion of the robustness of the transformers in the ISOBlox vs the WW ISO-1 device?

I don't know enough yet to make a definite call, but I've got enough for an educated guess. I found the transformer chart for the audio transformer in the Rapco box. It lists the transformer having under 1% distortion from 20Hz to 60KHz at +2dBM. See below.

I've asked Al Keltz at Whirlwind about the transformer in their ISO-1, and they ran a preliminary test for distortion at low frequencies and high signal level. They measured just under 0.5% distortion at 20Hz and with something around +16 or +18dBM level. Don't shoot me if I can't remember the exact numbers since this was just a verbal confirmation with their test engineer.

Now I do know that a transformer's ability to maintian low distortion at low frequencies is largely dependent on the amount of metal in its core. Too little core material and it will saturate/distort at frequencies below 100 Hz. And that's why crappy 70-volt transformers sound so bad with bass frequencies. I'm guessing that the transformer in the Rapco box has to be pretty small to fit in that little case, so it very well could distort at bass frequencies and high signal levels. So maybe at +8 level on a subwoofer it could have significantly more than the 1% distortion listed for +2dBM level. But we need someone at Rapco to give us actual numbers so we can make a proper judgement. Anyone have a Rapco/Horizon connection we can ask?
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Frank DeWitt

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Re: Girding loins in iron
« Reply #41 on: January 19, 2015, 08:14:20 pm »

One of the many things I like about Jensen transformers is that they publish the test results.  Here is there DI box transformer.
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-db-e.pdf

Here is a line input one made for low frequency use
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-112p-2hpc.pdf
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Mike Sokol

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Re: Girding loins in iron
« Reply #42 on: January 19, 2015, 09:29:02 pm »

One of the many things I like about Jensen transformers is that they publish the test results.  Here is there DI box transformer.
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-db-e.pdf

Here is a line input one made for low frequency use
http://www.jensen-transformers.com/wp-content/uploads/2014/08/jt-112p-2hpc.pdf

Remember, we need to consider price as well as performance. If something's not affordable by a large enough base of users, then it won't be available where it's needed. While the Jensen iso-boxes are great performers, they cost at least four times as much as the Rapco units, and twice as much as the WW ISO-1 boxes. For high end installs I would agree that Jensen transformers can be the best choice. But for lots of mid and low-end installations, just getting rid of the hum will remarkably improve the listening experience for the audience, as well as help the musicians who have to deal with humming/buzzing monitors.

I've used lots of Ebtech and WW isolation transformers in hundreds of pro-sound situations to mitigate hum, and never felt I was corrupting the signal. However, I also used to work at radio stations where the transmitter was located within a few hundred feet of the studio, and can remember that Jensen and Lundahl transformers were the only ones that could survive that kind of RF punishment. So for some demanding situations the extra money is worth it. While most of the time you may be able to use a more affordable technology.
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Stephen Swaffer

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Re: Distance between electrical and audio cables
« Reply #43 on: January 19, 2015, 11:38:33 pm »


You've built what Whirlwind makes and calls a podDI. Does exactly what you describe, plus includes a built-in ground lift switch. Some laptops will couple their internal switching supplies to the audio ground and make exactly the noise you describe, while others will be perfectly quiet. I always try to put a WW pcDI or podDI in any computer feed to a mixing board. There are numerous other brands/models that do basically the same thing. But the one thing they all have in common is a way to combine the stereo headphone outputs of a computer or media player, plus an audio isolation transformer, plus a switch for a pin-1 shield lift. There's just no good way to predict if you're going to have this "Hash" problem from a particular laptop, so I typically put an audio isolation transformer in the line and don't worry about it.

The fault may not be the laptop-we have run into situations where same laptop/different power supply made a difference with this issue.  IMO a good USB interface is the best solution here-but not as universal as an iso transformer.
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Steve Swaffer

Steve M Smith

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Re: Distance between electrical and audio cables
« Reply #44 on: January 20, 2015, 01:55:49 am »

To connect headphone outputs to XLR, a small mains transformer works well.  I have used something like a 10VA 240v to 24v transformer in the past.

Connect left and right to the 240v winding with a couple of series resistors (I think I used 47 ohm) and connect the 24v winding to XLR 2 and 3.

It's a bit bulky and I wouldn't use it on line or instrument level signals but it works fine for headphone level outputs.

The fault may not be the laptop-we have run into situations where same laptop/different power supply made a difference with this issue.  IMO a good USB interface is the best solution here-but not as universal as an iso transformer.

It is more likely to be the power supply.  The pulsing of the noise being the comparator switching the oscillator on and off to maintain the output voltage.

In my case I had a USB interface but still needed to cut the ground.



Steve.
« Last Edit: January 20, 2015, 01:58:46 am by Steve M Smith »
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John Roberts {JR}

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Re: jensen xformers
« Reply #45 on: January 20, 2015, 10:03:21 am »

Remember, we need to consider price as well as performance. If something's not affordable by a large enough base of users, then it won't be available where it's needed. While the Jensen iso-boxes are great performers, they cost at least four times as much as the Rapco units, and twice as much as the WW ISO-1 boxes. For high end installs I would agree that Jensen transformers can be the best choice. But for lots of mid and low-end installations, just getting rid of the hum will remarkably improve the listening experience for the audience, as well as help the musicians who have to deal with humming/buzzing monitors.

I've used lots of Ebtech and WW isolation transformers in hundreds of pro-sound situations to mitigate hum, and never felt I was corrupting the signal. However, I also used to work at radio stations where the transmitter was located within a few hundred feet of the studio, and can remember that Jensen and Lundahl transformers were the only ones that could survive that kind of RF punishment. So for some demanding situations the extra money is worth it. While most of the time you may be able to use a more affordable technology.
Not sure what this means for pricing of Jensen transformers but they were purchased by Radial, which seems like a good thing for Radial.

JR
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Al Keltz

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Re: Girding loins in iron
« Reply #46 on: January 20, 2015, 12:06:25 pm »

Here's what we came up with:

THD @ 20 Hz, +19 dBu = 2.7%
THD @ 50 Hz, +19 dBu = 0.3%


THD @ 20 Hz, +4 dBu = 0.62%
THD @ 50 Hz, +4 dBu = 0.3%


Source impedance = 150 Ohms
Load impedance = 20 kOhms

- Al
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Frank DeWitt

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Re: jensen xformers
« Reply #47 on: January 20, 2015, 12:28:43 pm »

Not sure what this means for pricing of Jensen transformers but they were purchased by Radial, which seems like a good thing for Radial.

JR

Radial also owns Eclipse.


I have been watching the Jensen web site since Radial bought them.  The only significant change is that Jensen was selling there own single channel DI and a wall mounted DI.  Both are gone.  That makes sense.

They do still sell transformers to the public (me)  and I am grateful that they do.
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Mark Cadwallader

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Re: Girding loins in iron
« Reply #48 on: January 20, 2015, 01:45:50 pm »

Here's what we came up with:

THD @ 20 Hz, +19 dBu = 2.7%
THD @ 50 Hz, +19 dBu = 0.3%


THD @ 20 Hz, +4 dBu = 0.62%
THD @ 50 Hz, +4 dBu = 0.3%


Source impedance = 150 Ohms
Load impedance = 20 kOhms

- Al

Thank you for the information, Al. Mark C.
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Chris Clark

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Re: Distance between electrical and audio cables
« Reply #49 on: January 20, 2015, 02:50:30 pm »

The fault may not be the laptop-we have run into situations where same laptop/different power supply made a difference with this issue.  IMO a good USB interface is the best solution here-but not as universal as an iso transformer.

I have to agree with this sentiment. In every instance possible over the last few years I've been pushing for the USB audio interface (in sticking with the WW theme, http://whirlwindusa.com/catalog/black-boxes-effects-and-dis/specialty-interface-solutions/pc-usb ) more and more as a proper solution to computer usage. It solves a number of issues, not only isolating that power supply/hard drive hash, it reduces the possibility of having the laptop volume too high and overdriving the DI, and it also gets rid of the huge point of failure involving the horribly unreliable 1/8" connector. Additionally, most are designed as a standard audio interface that the computer (whether Windows or Mac) sees almost immediately without driver intervention (unless you go back to XP, which I should hope no one is anymore).
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Re: Distance between electrical and audio cables
« Reply #49 on: January 20, 2015, 02:50:30 pm »


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