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Old 25th August 2005, 07:06 PM   #1
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Default RCA into XLR interconnect Idea

OK, Let's say we have an amp with balanced and single ended inputs, and a switch/jumper to ground the neg pin when using the single ended RCA input.

And you have a pre with only RCA outs.

I have a hair brained idea that if I made interconnects out of high quality microphone cable with nice suple rubber sheath with braided shield and 2 twisted conductors.

Now let's solder up the XLR as it shoud be on one end and the pos wire to the center of the RCA and the neg and shield to the outer conductor on the other.

And let's leave the neg open at the amp

My thinking is you will gain most of the advantages of ballanced cable because anything picked up by the pos, will also be picked up by the neg and cancled by the amp. But will the sheild then inject noise to the neg, I don't know. If so, maybe lift it and just put singe end into the balanced. What effects would that gain.

Any thoughts?
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Old 26th August 2005, 04:01 AM   #2
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Default Re: RCA into XLR interconnect Idea

Quote:
Originally posted by Brian Donaldson
OK, Let's say we have an amp with balanced and single ended inputs, and a switch/jumper to ground the neg pin when using the single ended RCA input.

And you have a pre with only RCA outs.

I have a hair brained idea that if I made interconnects out of high quality microphone cable with nice suple rubber sheath with braided shield and 2 twisted conductors.

Now let's solder up the XLR as it shoud be on one end and the pos wire to the center of the RCA and the neg and shield to the outer conductor on the other.

And let's leave the neg open at the amp
That last part I don't get. Leave the negative open at the amplifier? That sounds to me like you're saying you just end up connecting the positive to the amplifier with no other connection.

Quote:
My thinking is you will gain most of the advantages of ballanced cable because anything picked up by the pos, will also be picked up by the neg and cancled by the amp. But will the sheild then inject noise to the neg, I don't know. If so, maybe lift it and just put singe end into the balanced. What effects would that gain.
Bottom line is that you're still driving the balanced input from an unbalanced source. Even if the cable is symmetrical, the imbalance of the unbalanced source is going to screw up any common-mode rejection capabilities of your balanced input.

Quote:
Any thoughts?
Sure. You could use an input transformer.

se
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Old 26th August 2005, 04:07 AM   #3
Enzo is offline Enzo  United States
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There is no free lunch. Unless you stick a transformer or equuivalent circuit at the RCA end, that cable will not be balanced regardless of the number of conductors. Balance is a description of the circuit, not a description of the wires.

The standard mic cord is often called a "balanced cord" but that is not accurate. It is only a balanced line if the feed and and terminal end are both balanced circuits.

No balanced advantage can be had then.

If the preamp has only unbalanced outputs, then just connect them to the unbalanced inputs on the amplifier. You won't gain anything by fudging around trying to connect balanced and unbalanced together.
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Old 26th August 2005, 02:07 PM   #4
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"And let's leave the neg open at the amp"

I meant not ground the neg at the amp with the jumper or switch.

In the end, you would still be driving a balanced in with an unbalances output; I'm aware of that. But you would be using the neg to cancle noise pickup by injecting ground + niose in the neg and hot + noise into the pos. I thought that should cancle 90% of the noise picked up in the cable run.

I had a similar idea with bi-wireing speakers so you put the speaker cable in the neg feedback loop but haven't yet tried it because of fears of instability with all of the extra inductance and capacitance.
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Old 26th August 2005, 03:35 PM   #5
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Quote:
Originally posted by Brian Donaldson
I meant not ground the neg at the amp with the jumper or switch.
Ok, I see what you're saying now.

Quote:
In the end, you would still be driving a balanced in with an unbalances output; I'm aware of that. But you would be using the neg to cancle noise pickup by injecting ground + niose in the neg and hot + noise into the pos. I thought that should cancle 90% of the noise picked up in the cable run.
That's not quite how it works.

For the balanced input to achieve its best common-mode noise rejection, it requires that the source is balanced as well. Even the slightest imbalance at the source can dramatically reduce the input's common-mode rejection.

So in this case you're not really gaining any advantage to speak of.

Transformers are different in that they have an exceedingly high common-mode input impedance so they're not anywhere near as sensitive to source impedance imbalances which is why a transformer can give you better common-mode noise rejection driven from an unbalanced source than an electronically balanced input can give you when driven from a balanced source.

Quote:
I had a similar idea with bi-wireing speakers so you put the speaker cable in the neg feedback loop but haven't yet tried it because of fears of instability with all of the extra inductance and capacitance.
I think your fears are warranted.

se
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Old 26th August 2005, 08:31 PM   #6
Clarkcr is offline Clarkcr  United States
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I guess what Brian is getting at is that if you have 100 meters of high quality microphone cable that has a twisted pair in the center and a braided shield on the outside.....how can this be best wired to run UNBALANCED from the source to the amp.

It's not a question of turning unbalanced to balanced....only I can do that on my third martini. It's a question of, how best to use said cable bought at a discount to hook up an unbalanced source. In addition, can improvements be made in the transmission of said source material as to cancel strange noise that lurks in my attic.

So, if you got the cable....how do you solder it up at the amp?


Chris

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Old 26th August 2005, 10:08 PM   #7
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Quote:
Originally posted by Clarkcr
I guess what Brian is getting at is that if you have 100 meters of high quality microphone cable that has a twisted pair in the center and a braided shield on the outside.....how can this be best wired to run UNBALANCED from the source to the amp.

It's not a question of turning unbalanced to balanced....only I can do that on my third martini. It's a question of, how best to use said cable bought at a discount to hook up an unbalanced source. In addition, can improvements be made in the transmission of said source material as to cancel strange noise that lurks in my attic.

So, if you got the cable....how do you solder it up at the amp?
If that's the question, then the center pin of the RCA goes to pin 2 on the RCA and the ground lead goes to pin 1 on the RCA and the shield is connected to ground at the source end.

se
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Old 26th August 2005, 10:35 PM   #8
sam9 is offline sam9  United States
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The way to transition RCA (unbalanced) to XLR/TRS (balanced is well documented). There should be a FAQ on this as it gets asked on a regular basis on virtually every audiophile forum.

See A-
http://www.rane.com/note151.html

See B- (contains a lot of additional stuff but pertinant digrams are near the end)
http://home.comcast.net/~marcwhitney...g_whitlock.pdf

I used the Rane article to take Radio Shack XLR cables, cut in half and replace each bare end with an RCA so that I could connect a Behringer EQ between my preamp and amp. Works fine. Next new amp I build, I'll use a banced a line reciever and XLR jacks and dispence with the RCAs.
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Old 28th August 2005, 01:16 PM   #9
Clarkcr is offline Clarkcr  United States
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I have been looking at the Behringer eq.....is that a true balanced out for the amp?

Are you happy with it?

c
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Old 28th August 2005, 04:11 PM   #10
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Quote:
Originally posted by Clarkcr
I have been looking at the Behringer eq.....is that a true balanced out for the amp?
What do you mean by "true balanced out"?

se
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