Mic preamp summing signals..

[Sallivan]

Hi all!!
I'm new on this forum!
I would like to document myself on the wonderful world of the audio pro.
I found this article:

http://sound.westhost.com/project66.htm

The article says that the two signal are summed after the first stage whit an operational amplifier, so the system changes from balanced to unbalanced.
This operation was to adapt the signal to the next  stage which will meet in a channel of a mixer, like eq, comp., ecc...
But if I want that the signal remains balanced, I'll have to use a balancer configuration after the summing op amp.
In this way the path of the signal changes from balanced to unbalanced...and again balanced.
I wanted to know if the stage of the sum can be avoided, so that the signal can always be balanced.
Could I sent the two signals, after the R10 and R11, in a two different op amp, one for the positive and one for the negative?
In this way I can avoid the stage of the sum? Or the sum is necessary in this circuit?
I hope I explained myself well.
Really thank's!

 

[Sallivan]

This is the circuit whit the two IC .

 

[johnR]

I wanted to know if the stage of the sum can be avoided, so that the signal can always be balanced.

No. The circuit relies on the single op amp to provide common mode rejection. It's a differential amplifier, so it amplifies the difference not the sum.

If you need a balanced output you need to add extra balancing components. You could keep it simple by making it "impedance balanced", using the original circuit with the addition of an impedance balancing resistor of 50 to 100 ohms from the cold pin of the output connector (pin 3 on XLR) to signal ground. The op amp output would go through another 50 to 100 ohm resistor to the output connector hot pin (pin 2 on XLR). The output connector ground pin (XLR pin 1) would go to chassis ground.

http://www.epanorama.net/blog/2010/02/23/unbalanced-to-impedance-balanced/

 

[PRR]

Your plan has common-mode (line humm) rejection of only 6dB.

That is hardly better than an UN-balanced mike line.

The Westhost plan's 2nd stage improves this to 20dB-40dB. Big improvement.

The classic way to get a balanced output is to take the U1 pin 6 signal as one side of balanced, and add a unity gain inverter to get the other side for true balanced.

There are other ways. Some may be better.

There "IS" a way to stay balanced all the way. Duplicate the 2nd stage but swap the inputs. If you are dressing-up this simple plan, you should also use 1% matched parts in the 2nd stage(s) and a beefier output amplifier. And in modern studios you MUST consider phantom blocking at the input.

There are other mike-amp plans.

 

[Sallivan]

Really thanks PRR for the nice solution!
I want to ask you: this circuit became a standard mic pres and is used inside the mixers over the years. Today many companies have built this circuit into a single op amp, like ssm2019 or better other IC. The Opamp technology is much more sophisticated.
There are a really differences between the IC and the discrete transistor circuit in terms of distortion, noise, ..?
If I want to built 8 channel of this mic preamp, what do you recommend to me, the ssm 2019 way or westhost way?

 

[PRR]

> what do you recommend to me, the ssm 2019 way or westhost way?

Use the new stuff. Westhost's plan was good-enough, but the new chips are better.

Or if building eight, first build one of each and compare? This _IS_ "DIY". That means listening for yourself, not taking opinions from strangers on the Web.

 

[Sallivan]

Thanks!

 

[Kingston]

Really thanks PRR for the nice solution!
I want to ask you: this circuit became a standard mic pres and is used inside the mixers over the years. Today many companies have built this circuit into a single op amp, like ssm2019 or better other IC. The Opamp technology is much more sophisticated.
There are a really differences between the IC and the discrete transistor circuit in terms of distortion, noise, ..?
If I want to built 8 channel of this mic preamp, what do you recommend to me, the ssm 2019 way or westhost way?

Closely related discussion: http://www.groupdiy.com/index.php?topic=40711.0