OT - Simple balanced mic preamp?

[FatMike]

Hi,

I'm planning to build a low noise microphone preamp that will provide (at least) eight amplified signals for my soundcard with 8 line inputs. Now I am looking for a simple schematic of an mic preamp that first converts a balanced mic signal into an unbalanced an then amplifies it.

I did already search the web and this forum for information, but nearly found anything useful, only one or two schematics but they way too complicated. As I am going to build at least eight of this balanced preamp circuits it would be good if it had only few parts.

I already thought about using that special SSM2019 mic preamp IC but this one is hard to get here in Germany and also quite expensive.


Does anybody know a simple circuit for that? Preferably opamp circuits.










Thanks,

Fat Mike

[Mike Burgundy]

An opamp in itself is already a balanced-to-unbalanced converter right there.- + and - inputs!
You do need a bit of housekeeping though. One half dual per channel is more than enough.
If you don't need phantom, it's easier - and the big caps can even be omitted if you use a dual supply, otherwise you'll need to isolate the DC levels that the opamp pins carry. The resistor to mass from the + input should go to Vcc if you don't use a dual supply. Actually, the two resistors to mass could also be omitted.
There's stuff like zeners protecting the inputs from spikes (parallel to the ttwo resistors from signal to ground) and stuff like that, but this should work:





Haven't tested it though, and I'm not completely sure about impedances, but it sould work just fine.
I'd stick in a 5532 for low noise. Have a go and see if this works.

[Outlaws]

Is that a fixed volume for the output?

[FatMike]

Hi,

thanks a lot for that cool schematic, it's just as easy as I need. I think I'm going to test it.
However, I wonder if this circuit also amplifies the signal (cause that's what I want, I need a "Gain" pot). Could you change the amplification by decreasing or increading that 33k from - to output (similar to a non-inverting opamp circuit)?


Fat Mike

[Mike Burgundy]

Well yah, but what's keeping you from adding a volume pot..
Actually I've been contemplating a simple yet complete mic pre.
What would that be?
-low parts count, keep it simple
-low noise, so BJT based. Shouldn't be a problem for mics (lo-Z)
-phantom power option
-spike protection (connecting mikes with phantom engaged, etc)
-radio interference protection?
-phase switch?
what else?

[Prive]

Just take a look at the INA217's datasheet in www.ti.com.

Saludos, Marcelo.

[Mike Burgundy]

No idea what thats costing, but I'm ordering samples!
Hey, our mixer is stuffed with SSM2017's... If these are good (which I assume they are) I can get a bag full, studio funded.