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Fredde79 14th February 2013 06:33 AM

Splitting and attenuating a balanced signal
 
Hi!

I'm building a tube preamp for bass, and I'm planning to build in a line trafo to get a balanced output for my power amp. However, I would also like to be able to send the signal at mic level to the stage mixer.

So my question is, will this work? My thinking is, the first resistors of the H-pad should isolate the attenuating resistor from the higher level signal and keep it intact.

(The resistor values in the H-pad are missing because I still don't know how much attenuation I'll need.)

http://subsite.tilaa.eu/share/projects/DI_out.png

turk 182 14th February 2013 11:52 AM

fredde79
you're not listing anything about the transformer which in most cases would achieve the signal reduction your looking for but if you feel further padding is needed the values of resistors is easy to determine.by the way your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.

Fredde79 14th February 2013 12:46 PM

Quote:

Originally Posted by turk 182 (Post 3369496)
fredde79
you're not listing anything about the transformer which in most cases would achieve the signal reduction your looking for but if you feel further padding is needed the values of resistors is easy to determine.by the way your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.

Thanks for your reply!

The transformer is a Hammond 108N 10K/600R. It produces a suitable mic-level when fed with a passive guitar signal (I've tested this), but the preamp will have a higher output so I'm thinking a -20dB attenuation (or thereabouts) would be a good thing.

The pad should be ok for a balanced signal, but I may be wrong of course. You could probably pulldown to ground as well, but as the signals in a balanced line have opposite phases, a resistor between them should have the same effect.

But my main question (whether the pad is correct or not) remains, will the splitting of the signal have some negative effects? DI-boxes usually have a switch for attenuation, but I've never seen a schematic for one that produces two balanced signals at the same time (which is what I'm trying to achieve).

One way would be to buy another transformer, but they are expensive...

/Fredde

JMFahey 14th February 2013 01:13 PM

Quote:

your schematic for the "pad" is, i think wrong, the resistor joining pins 2,3 should go to ground as in two t pads (one for pin2 one for pin3) six resistors total no interconnection between pins.
No, it's not wrong, it's a *real* transformer balanced circuit.
Any reference to ground will defeat its purpose.

To fredde79.
You chose the wrong Jensen transformer.
If you want to drive a power amp straight from a preamp, you need a 1:1 line out transformer, or signal will be **way** too weak.
As is, it will supply a mic level signal.
Any further attenuation is counter productive.

Fredde79 14th February 2013 02:12 PM

Quote:

Originally Posted by JMFahey (Post 3369585)
No, it's not wrong, it's a *real* transformer balanced circuit.
Any reference to ground will defeat its purpose.

To fredde79.
You chose the wrong Jensen transformer.
If you want to drive a power amp straight from a preamp, you need a 1:1 line out transformer, or signal will be **way** too weak.
As is, it will supply a mic level signal.
Any further attenuation is counter productive.

Thanks! Aah ok I see. So I'll have to buy another one then... Will my schematic (with the split mic level attenuation) work in case I have a 1:1 trafo, or should I use this one for the mic signal?

(If it works, I could save the original trafo for a DI box or something else later.)

JMFahey 14th February 2013 04:46 PM

Quote:

Originally Posted by Fredde79 (Post 3369653)
Thanks! Aah ok I see. So I'll have to buy another one then... Will my schematic (with the split mic level attenuation) work in case I have a 1:1 trafo, or should I use this one for the mic signal?

(If it works, I could save the original trafo for a DI box or something else later.)

Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

Fredde79 14th February 2013 05:46 PM

Quote:

Originally Posted by JMFahey (Post 3369814)
Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

Yes, I might just do that, my only concern is the rather low input impedance of the unbalanced input of my class D power amp. That's why I wanted to use the balanced input instead. (But a FET-buffer is a lot cheaper than a trafo, should this become a problem.)

Cheers!

cbdb 14th February 2013 05:51 PM

If you are getting another transformer, use both, one for the power amp, one for the mic send.

cbdb 14th February 2013 05:54 PM

Quote:

Originally Posted by JMFahey (Post 3369814)
Personally, I'd drive the power amp straight from the preamp, *and* use the transformer as is, only as a mic level out.
The best of both worlds.

If this is for gigs you will be happy you have the transformers, as this will almost gaurantee no ground loops.

JMFahey 14th February 2013 08:59 PM

Quote:

my only concern is the rather low input impedance of the unbalanced input of my class D power amp. That's why I wanted to use the balanced input instead.
Are you sure?
*Normal* is
Unbalanced=higher impedance
Balanced=lower impedance.
Please post specs of your amplifier.

EDIT: and what preamp are you using which can't drive a relatively low impedance?


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