mic pre output biasing

A friend sent me this snippet, I believe its from an old Midas desk. Simple enough op amp arrangement, but I'm not sure I understand the purpose of the 12k resistors to +V?

Is it creating a higher output bias voltage since the desk is single supply?

dogears said:

A friend sent me this snippet, I believe its from an old Midas desk. Simple enough op amp arrangement, but I'm not sure I understand the purpose of the 12k resistors to +V?

Is it creating a higher output bias voltage since the desk is single supply?

Click to expand...

There are two 12k resistors to +v....

One creates a +V bias across the output electrolytic caps, the other creates a modest +V bias across the capacitor in the gain leg..you can figure out which is which.

I presume they expect that to make the (polar) electrolytic caps behave better..

JR

Makes sense... but why the 1k back across C4?

dogears said:

Makes sense... but why the 1k back across C4?

Click to expand...

To create the voltage divider that biases the capacitor with a DC voltage.

JR

Not trying to be dense, but in that case why not just omit the first capacitor altogether? 

And -- happy new year, and thank you for your patience with my basic questions.

dogears said:

Not trying to be dense, but in that case why not just omit the first capacitor altogether?

Click to expand...

This is pretty basic stuff....  but I'll play along for now. Perhaps find a book on basic electronics to read.
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Capacitors block DC but pass AC (a simplification).  Capacitors in series with the output of a circuit, prevents DC from corrupting the audio output.

Polar electrolytic capacitors have a preferred orientation wrt the DC they are blocking. The fat plate in the schematic representation indicates the positive end.

Putting two polar caps in series with both + leads connected together and that junction biased up to a DC voltage that is positive wrt the output insures the caps are seeing proper voltage across each while blocking DC from the audio output.

JR

Sorry I'm asking unclear questions. Yes, I have that much square.

Looking at the datasheet for the TDA1034N we actually get the input offset voltage and bias current. Vio max 4 mV, input bias current max 1.5 uA.

For this, in the two gain positions you get worse case offset ranging from -2  to  +46 mV. Typical would be -1 to +6 mV. 

So why not just use the single tantalum, oriented just the way it is, and flip  C3?  Is a 1.5V bias really going to matter?

Two electrolytics back-to-back with / without a bias voltage applied is supposed to reduce distortion with cheap electrolytic capacitors. Yes, you could just replace both with one tant. I think that's why it shows the tant option there. But today even the cheap electrolytics do not need this sort of jury-rigging. So this circuit is not really worth exploring for it's design. If you're trying to decide how to recap and with what, that is a different discussion.

Also this does not look like it's single-supply. Otherwise C3 would be reverse biased and the input transformer would require a cap.

squarewave said:

Two electrolytics back-to-back with / without a bias voltage applied is supposed to reduce distortion with cheap electrolytic capacitors. Yes, you could just replace both with one tant. I think that's why it shows the tant option there. But today even the cheap electrolytics do not need this sort of jury-rigging. So this circuit is not really worth exploring for it's design. If you're trying to decide how to recap and with what, that is a different discussion.

Click to expand...

Yeah, I just didn't get it. If I were recap I'd at least start by removing the biasing and leaving the tantalum. Or measuring the offset voltage to begin with.

Also this does not look like it's single-supply. Otherwise C3 would be reverse biased and the input transformer would require a cap.

Click to expand...

Ah you're right. they're just pulling from +V for the cap biasing.