MPP

[Joachim Gerhard]

Bal phono

Idea for a balanced passive Phonostage. Should work with Fets too.

 

[Mark Tillotson]

Idea for a balanced passive Phonostage.

If you feed the inputs both to one long-tailed-pair you'd have much higher common-mode impedance and fewer input devices contributing noise I think.

 

[Joachim Gerhard]

i do not really understand. Could you make a simple drawing of this ?

 

[as_audio]

Point here is a single diff pair is already balanced and the second is not required.

Of course the feedback scheme would have to be adapted accordingly.

What about CMRR in your circuit 11301 ?

 

[Joachim Gerhard]

I have not calculated it. For me this is a hybrid Intrumentation Amplifier. With discrete input transisors i see the main advantage in low voltage noise. As far as i can tell the better this input transisors are matched and thermo coupled, the better CMRR will be. Plus precission resistors .
An integrated INA does not get better then around 1nVqHz.
This one can be as low as 0,2nVqH with the right transistors and relatively high idle, say 20mA per pair.

 

[Joachim Gerhard]

I thgink this circuit can be direct coupled at the input even with BJTs.

 

[Mark Tillotson]

i do not really understand. Could you make a simple drawing of this ?

Something like this maybe?

The feedback goes to the emitters via a filter network (Z1 and Z2), then the outputs proceed to further balanced passive filtering and diff amp?

Then you have only one transistors' worth of noise for each input rather than two, I believe. Wasn't sure what loading you are going for.

 

[Joachim Gerhard]

I was thinking of an input loading of around 1kOhm. For low voltage noise Z2 should have a low value.
This circuit is interesting.

 

[Joachim Gerhard]

I developed the circuit a bit further to contain a RIAA.

 

[RCruz]

How do you set the gain Joachim ?

The need for two RIAA circuits bothers me a little.

It might need a bigger emitter resistor instead of the 3ohm

 

[Joachim Gerhard]

The emitter resistor sets the noise. I will first make a pre-pre and then we see…..l

 

[RCruz]

How is the gain set ?

 

[Joachim Gerhard]

I assume if we set Z1 100 OHM AND Z2 10 Ohm differential gain would be 100/10x 2 (+1 ? )

 

[Joachim Gerhard]

Passive RIAA

RIAA can also be setup this way :

 

[EUVL]

For balanced in-out, I find Scott's more interesting :
A simplified universal differential or single ended phono preamp


Patrick

 

[RCruz]

Thank you Joachim... Now it is clear.

In my sim the voltage reference in the opamps uninventing inputs must be set to -1v otherwise the circuit oscillates.

 

[as_audio]

RIAA can also be setup this way :

(Re post 11314) No.

.. In my sim the voltage reference in the opamps uninventing inputs must be set to -1v otherwise the circuit oscillates.

This is re post 11309. No, the circuit does not work with dc condition like this.

 

[RCruz]

Please elaborate... why does the circuit not work ?

It simulates quite well here:

 

[as_audio]

I do not have the opportunity to simulate but obviously the collector voltage should be higher than the emitter voltage.

 

[RCruz]

The emitters are not grounded ... they are connected to the opamps output only ... and the output of the opamps is always below -1.5 v.

But now you made me ponder the bjt minimum vce it must be carefully chosen