diy +4dbv balanced outputs

[argonrepublic]

I have an older mixer that I wish had balanced direct outs. They are unbalanced. Is it quite a task to modify it so be true +4dbv balanced outs?

[AndrewT]

Hi,
balanced may be interfaced with commercial/PA. For this you should consider a higher output maybe +20dbV on each pole.
Why do you need balanced. High interference rejection? Long interconnects?
You could use a pair of opamps for an unbalanced to balanced conversion but they need good current ability because commercial balanced can have loads down to 600ohms.
Look up D. Self and ESP for balanced info. I'm sure there are many other sites that could be recommended.

[dhaen]

Since you are where you are (N. America) I think you might mean +4dBu?
+4dBu refers to a common reference signal level for interconnects. It is the unloaded voltage equivalent to +4dBm which is equal to 1mw into 600 ohms. The RMS voltage is approximately 1.23v.
This connection system normally assumes that there will be at least 20 dB headroom for signal excursion. The upshot of this is that the power supplies to your driving opamps will need to be at least + & - 15 volts, + & - 18v preferably.
If this is just for your own interconnect, you can of course choose your own levels. (In the UK we tend to use 0dBu, except in sound studio's that have unmodified imported equipment.)
Have a look at the DRV134 chip. It's got everything you need in it. Just add a couple of decoupling caps and power supply. There's an SSM...equivalent too.

[argonrepublic]

Thanks guys. It is starting to look a little promissing. I want +4db outs and balanced connectors to achieve highest signal to noise ratio. Going directly to A/D recording interface. I am glad one of you mentioned +/-18v because I have an external mixer power supply that has +/-18v and I wondered if the purpose of that circuit was to get 0 180 degrees output. I guess it is. This stuff is so difficult for me. I am just a struggling musician trying to make things work well while keeping costs down. You mention "interconnects". I thought I was step ahead by using 3 conductor XLR cables. Now you have opened the flood gates. I am basically using out of the box spdif, wclock, ac power and XLR cables. I am open to suggestions but please don't fry my brain with information. I am still trying to undersatnd dbv, dbu and dbf. BTW: can i measure outputs with voltmeter while playing a sine wave at 0db from software?
Thanks

[AndrewT]

Hi,
yes, you can measure the output voltage with a multimeter. However the accurate frequency range is very narrow with most DMMs.
0dbV=1Vrms
0dbu=0.7747Vrms
+2.22dbu=0dbV
so 10Vrms =+20dbV =+22.22dbu
+-15Vrails will just about supply a max output from most opamps of about 10Vrms (+20dbV)
+-18Vrails will definately supply +20dbV and with less distortion.
Check your opamp's maximum supply voltage.

[argonrepublic]

I stidied the op amps and think i am beginning to grasp it. Next question: Cab I use my +/-18v to supply all of my op amps providing they are within specified votage? Can I use all of my existing tone controls and faders? It seems thats the mixer weould be unbalanced internally. Is that how they are designed?
While studying the DRV134 I researched some input ICs as well. I could probably end up with quite a nice board If I follow advice.
Thanks

[dhaen]

Most mixers are unbalanced internally. It's only the inputs and outputs that need to be balanced, to keep the sig/noise good.

I wouldn't alter the supply voltage to existing opamps in the mixer. Change as little as possible. If you have +-18v available for the balanced driver stage that's great.
Yes, you can use the same supply for input and output stages that you build.

Improvements using fancy interconnects will be minimal and expensive. I'd keep to bog-standard ones until you're famous

You have mail....

[inatightspot]

Hi:

I have used the Burr-Brown (now TI) opa2604 dual op amp at its maximum supply voltage of +/- 24V for years now without any problems. You may consider using this component if you find that you need more headroom (+32 dbu). Although, in my experience, signals at or around line level, 1 or 2 vrms, would have no audible benefit from an increase in supply voltage above the +/- 18V that you are considering. Considering common mode noise, find a good, common ground point for all of your equipment. You'll be surprised at how well even average equipment is capable of performing.
Check www.Rane.com for a passive balanced output scheme.


Inatightspot