Splitting balanced lines.

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eddperks

New member
Joined
May 1, 2013
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3
Hello one and all.

My first post here, but i've been a long time reader of the forum. Without you guys I couldn't have built my Gyraf G9 Mic Pre!

Okay, so I'm really pushing my electronics ability hard recently and I'm undertaking a new project. Basically I have designed a studio monitor controller that works in a similar way to the Crane Song Avocet, but i've tweaked it so that is ideal for our studio and has some features we need. The design runs a completely passive audio path, using a relay based attenuator (with precision metal-film resistors). This attenuator runs fully balanced from input to output.

The problem is I need to put some live-room outs and headphone outs that sum with a talkback microphones signal. This requires them to have their own respective volume controls. I need to split off the passive balanced lines into these amplifiers, and I'm wondering the best way to do this without harming the passive lines transparency? If I take a feed through an op amp buffer (essentially drawing 0 current) will this have a negative effect on the passive nature of the main signal path?

Any thoughts or advice is greatly appreciated, and thanks in advance!

Edd.

 
> essentially drawing 0 current

How big is "essentially 0"? 1 microAmp? 1 nanoAmp? 1 femtoAmp?

For general work, it is adequate to have the tap suck <1/10th of the primary load.

This may cause 1dB signal drop; and if the tap input is very non-linear maybe 10% distortion (though normally far less).

You are being fussy. <1/100 is a reasonable goal. <1/1,000 may be possible.

We can proportion current as inverse of impedance. You want input impedance 100 or 1,000 times higher than the circuit being tapped.

The basic concept is called (in audio) a Differential Input Unbalanced Output. (In radio they just say "balun"; but their techniques don't work at audio.)

A specific plan is an Instrumentation Amplifier. Specifically made to read signal voltage withOUT loading the signal.

There is a one-opamp "differential amplifier". You don't want this. The input impedance wants to be medium. If you raise all the resistors you raise the noise voltage.

There is a 3-opamp IA. This is what you want. For this use, the input pair should run at unity gain. The output device ideally (in modern systems) runs at gain of 0.5 (because with all +/-15V supplies, a balanced line "can" peak to twice the voltage of an unbalanced signal). However unless your circuit runs absurdly high levels, gain of 1 is probably fine.

Input devices can be good-old TL072. Naked input impedance is "essentially infinite" and pretty linear. Bias resistor can be 1Meg or 10Meg without problem.

(This application does have high common-mode voltage on the input devices, and TL072 is not the absolute best CM distortion. It don't suck neither, but someone may propose a spiffier device.)

Output device may as well be 5532. It's clean, and could drive a dozen 10K pots to your various loads.
 
Okay thanks, It good to get some reassurance on theory. When I said draw essentially 0 current I was referring to the ideal 'golden rules' of op-amp operation, where a very high input impedance would draw negligible currents. So it seems using instrumentation amplifiers as a unity gain buffer is the way to go to keep the passive line clear.

so just to clarify you think a TL072 or similar input buffer stage driving a 5532 into the separate stages would be a good design start?

Thanks again for the help.
 
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