Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Baguette on May 30, 2014, 05:30:35 am
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Hello,
I must add a XLR balanced line out to a 100W tube amp (4x6L6GC).
And do not know exactly how I could do it.
I guess I would have to use a voltage divider from the speaker tap, and from then wire a transformer to the XLR socket.
Would it work?
Which values should I use for the voltage divider and transformer specs?
Thanks and all the very best.
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I haven't done it yet, but this is what I'm looking to do with this transformer from weber
(https://taweber.powweb.com/store/balun.jpg)
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You can "fake" a balanced line out with electronics, usually solid-state because of the low-impedance implied on the other end of the balanced line.
However, the easiest way is with a transformer as Bergera has shown. Many companies have the 1:1 transformer, like the Hammond 109T (10k:10K) or 109H (600:600) (http://www.hammondmfg.com/107.htm). These have PC Board pins, but are about 1.3" x 0.9", so you could mount on on some perfboard along with a p.c.-mount jack and p.c.-mount level control for a compact package that only needs the panel-mounting jack and control to hold it in place.
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Having played around with balanced line out I can tell you that the transformer is important for the sound you get.
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Please explain Labb. I think your experience would really help here.
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rIf you want good sound quality you need a good audio transformer. Any transformer of the proper ratio will give you a high impedance input, but not all will sound good. Cheap transformers add distortion at low frequencies and also tend to have poor high frequency response. I have tried the telephone line transformers from Radio Shack with out much luck. The ones from Weber are pretty good. Best I have used are Jensen JT-11-FL.Take a look at Jensen application ASO61 www.jensen-transformers.com/apps_sc.html
Hope this is helpful. I might add that you are looking for a voltage divider that will give you the correct out put voltage around 0.75 volt. R1 for a 100 watt amp probably needs to be about 33K if you use 600ohm
transformer.
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I should think a 100-Watt would carry most venues without help.
If you use a transformer, you probably want a good one.
It may be easier if you aim at Mike level instead of Line level. (Most mix-boards will take either.) Much lower level in the transformer, so less bass-strain.
It is NOT always essential to drive a balanced input (board) from a balanced output. If the input is properly balanced, it will accept an un-balanced source. If the line is at Line level, not super-long or exposed to bad signals, and the line impedance is under a few hundred ohms, this works fine most of the time. 1.5K + 100r voltage-divider off the 100W amp's speaker, then a 1K pot across the 100r for fine-trim.
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Hey guys, thanks so much for the replies.
I could still use a bit of help.
Here is the transformer can buy :
http://www.tube-town.net/ttstore/Transformer/Matching-Transformer/Audio-transformer-1-1-2-1-for-line-signals-LTR-110::3813.html (http://www.tube-town.net/ttstore/Transformer/Matching-Transformer/Audio-transformer-1-1-2-1-for-line-signals-LTR-110::3813.html)
It looks good (wide bandwidth) and moderately priced.
How should I wire this transformer to get a balanced line out?
As Labb suggests, would a 33k/1k voltage divider give me a good line signal?
Thanks again!
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Hey guys, thanks so much for the replies.
I could still use a bit of help.
Here is the transformer can buy :
http://www.tube-town.net/ttstore/Transformer/Matching-Transformer/Audio-transformer-1-1-2-1-for-line-signals-LTR-110::3813.html (http://www.tube-town.net/ttstore/Transformer/Matching-Transformer/Audio-transformer-1-1-2-1-for-line-signals-LTR-110::3813.html)
It looks good (wide bandwidth) and moderately priced.
How should I wire this transformer to get a balanced line out?
As Labb suggests, would a 33k/1k voltage divider give me a good line signal?
Thanks again!
The schematic Bergera posted shows you how to wire it using the centre-tapped secondary. You don't need to use the CT on the primary. Leave the other (300R) secondary unconnected.
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Here's another thought ...........................
http://hughes-and-kettner.com/products/redbox-5/ (http://hughes-and-kettner.com/products/redbox-5/)
With respect, Tubenit
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Maybe this is useful: http://www.jensen-transformers.com/as/as068.pdf (http://www.jensen-transformers.com/as/as068.pdf)
EDIT: This might be better: http://www.jensen-transformers.com/as/as061.pdf (http://www.jensen-transformers.com/as/as061.pdf)
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How should I wire this transformer to get a balanced line out?
The schematic Bergera posted shows you how to wire it using the centre-tapped secondary. You don't need to use the CT on the primary.
The secondary center-tap is not used either.
The ground symbol in the secondary on Bergera's schematic is for the shield of the connecting cable, and should be connected to the chassis of the sending device. It is actually might be better for a balanced signal to have no connection to ground (meaning at the secondary center-tap) because then you're assured of equal impedances to ground from either signal-carrying line.
It's also probably wise for the piece of gear receiving the balanced signal to have a properly-implemented ground lift switch, in case grounding the shield to the chassis of both pieces of equipment causes a ground loop hum (because each have a 3rd prong and connect circuit signal ground directly to chassis ground).
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Do not center-tap the secondary (tape-up that wire).
The CT is useful in some very special cases not found in normal performance. In early days of radio when the studio might be directly under the transmitter, grounded CT was the only way to divert 50,000 Watts of radio waves out of microphone circuits. It is also useful in special telephonic connections (but not grounded).
In ordinary studio, club, and stadium work, an un-grounded secondary is most likely to work with "any" working XLR/TRS input. If the XLR is actually unbalanced, it will be wired to ground the stray lead. If the CT were also grounded, this would short-out the secondary; floating works correctly.
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http://music-electronics-forum.com/t25684/ (http://music-electronics-forum.com/t25684/)
Pretty good discussion of slave line out that fits this