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Old 31st May 2012, 09:29 PM   #11
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You'll find a discussion of this circuit in Douglas Self's Small Signal Audio Design book - he designed it, the whole 840W in fact. Essentially the point was not only matching, but even exceeding unbalanced input noise specs since people usually (if mistakenly) expect balanced inputs to perform better. The mysterious "terrapin module" must have pretty good current drive capabilities.
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Old 1st June 2012, 01:21 AM   #12
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Default An Apology

My sincerest apology to all readers. I completely spoke out of turn, my comments based entirely upon misinformation instead of first hand knowledge regarding the Cambridge W840 power amplifier and E840 preamp.

Very truly,

Rich
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Old 1st June 2012, 02:47 AM   #13
Beranga is offline Beranga  Mexico
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Originally Posted by sgrossklass View Post
You'll find a discussion of this circuit in Douglas Self's Small Signal Audio Design book - he designed it, the whole 840W in fact. Essentially the point was not only matching, but even exceeding unbalanced input noise specs since people usually (if mistakenly) expect balanced inputs to perform better. The mysterious "terrapin module" must have pretty good current drive capabilities.
Excellent advise!!! That book contains everything I wanted to know about the amplifier. Thank you.
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Old 1st June 2012, 02:49 AM   #14
Beranga is offline Beranga  Mexico
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Originally Posted by RBorkowski View Post
My sincerest apology to all readers. I completely spoke out of turn, my comments based entirely upon misinformation instead of first hand knowledge regarding the Cambridge W840 power amplifier and E840 preamp.

Very truly,

Rich
No need to apologize, just clearing things up
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Old 2nd June 2012, 08:03 AM   #15
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Originally Posted by Beranga View Post
Thank you Dirk, I will have a look at that paper.
I have to clarify some things.
If the shield of the interconnect cable is connected to the signal earth you inject all interference into the audio path. So in that situation there is no benefit in going balanced. Unfortunately this is the case in 99% of all situations. So the first step of improvement in noise rejection is connecting the shield directly to the chassis so there is a separate path for the interference to go. This interference will always be there because of the voltage differences between the connected devices.

Second step in reducing interference is making sure the impedances of the positive and negative audio cables are the same across the audio frequency band. Best way to do this is using trannies. But you can also bootstrapp the input so differences in impedance are not important any more, the That 1200 series is developed for this purpose.

For a device to be AES48 compatible (What means that the device is immune to outside interference), you do not need it to be fully balanced. Just the input and output need to be balanced. The PDF has some good drawings that shows how a device should be wired.


But why anyone other than professional recording businesses or telephone companies would want balanced connections is a mystery to me.
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Old 2nd June 2012, 09:21 AM   #16
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Originally Posted by Dirk95100 View Post
I have to clarify some things.
If the shield of the interconnect cable is connected to the signal earth you inject all interference into the audio path. So in that situation there is no benefit in going balanced. Unfortunately this is the case in 99% of all situations. So the first step of improvement in noise rejection is connecting the shield directly to the chassis so there is a separate path for the interference to go. This interference will always be there because of the voltage differences between the connected devices.

Second step in reducing interference is making sure the impedances of the positive and negative audio cables are the same across the audio frequency band. Best way to do this is using trannies. But you can also bootstrapp the input so differences in impedance are not important any more, the That 1200 series is developed for this purpose.

For a device to be AES48 compatible (What means that the device is immune to outside interference), you do not need it to be fully balanced. Just the input and output need to be balanced. The PDF has some good drawings that shows how a device should be wired.


But why anyone other than professional recording businesses or telephone companies would want balanced connections is a mystery to me.
Hi Dirk,

I agree with you that it is good practice to connect the shields of balanced audio cables according to the AES48 standard, but I think you are exaggerating a bit when you say that balanced connections are useless when you connect the ground to the internal signal ground. Sure, stray currents through the internal signal ground will cause interfering voltages that the circuit can't reject when it is internally single-ended, but at least you are still insensitive to the voltages across the shields of the cables.

Balanced connections are useful when you have large ground loops, like when you have three studios that are all connected to each other, or when you have a home audio set that is grounded to the mains safety ground and to the cable RTV ground. In this latter case you can usually just as well break the ground loop by blocking capacitors or a tiny transformer in the cable connection.

Best regards,
Marcel
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Old 2nd June 2012, 03:55 PM   #17
Beranga is offline Beranga  Mexico
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Originally Posted by Dirk95100 View Post
No.

The shield is connected to signal earth and therefore all external interference/noise is directly injected into the audio path.
What you want is a separate path for the interference/noise as is described in the AES48 standard. Iaw shield to the chassis!
aes48-2005-f
Dirk:

Sorry for the newbie question, but why do you say the shield is connected to signal earth? What I see in the diagram is pin number one of the balanced input connected to chassis ground.
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Old 3rd June 2012, 12:53 PM   #18
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Originally Posted by Beranga View Post
Dirk:

Sorry for the newbie question, but why do you say the shield is connected to signal earth? What I see in the diagram is pin number one of the balanced input connected to chassis ground.
Your correct, I misread the schematic and made the wrong assumption that 99% of all equipment is not AES48 compatible.
But then you have to make sure all the rest of your equipment is also AES48 compatible or else a very loud noise will be heard.
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Old 3rd June 2012, 06:28 PM   #19
Beranga is offline Beranga  Mexico
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Originally Posted by Dirk95100 View Post
Your correct, I misread the schematic and made the wrong assumption that 99% of all equipment is not AES48 compatible.
But then you have to make sure all the rest of your equipment is also AES48 compatible or else a very loud noise will be heard.
Very interesting read, the AES paper. Thanks again.
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Old 6th June 2012, 06:32 AM   #20
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Originally Posted by Beranga View Post
Very interesting read, the AES paper. Thanks again.
The shield to chassis earth is key to reducing noise.

Then you must make sure CMRR is low to. This is not so eazy to do and puts great demands on interconnect dimensions and tolerances. Here you can find info on how to get the best CMRR: http://www.jensen-transformers.com/an/ingenaes.pdf
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