BBC Mic Preamp ?

[Fablab]

Hello, years ago I found this schematic I remember it should be the first stage of a BBC mic preamp.
There is someone who has realized it?

 

[Fablab]

Nobody knows it?

 

[ruffrecords]

It is a variant of the usual dc coupled pair with added emitter follower. Not sure why you have C5 connected to JP2 as if it is an input - C5 wold normally go to 0V via a gain setting resistor. Other than tat it is pretty standard. Vaguely reminiscent of the TG1234  gain block but only because it also uses a BCY70. Where did you find it?

Cheers

Ian

 

[Gert]

http://www.bbceng.info/ti/eqpt/

 

[Fablab]

Thank you Gert

 

[Fablab]

Hi Ian,
I don't remember where I  found it
It is interesting what you say about  the similarity with the EMI gain block
I've been looking for the EMI TG mic preamp circuit for a long time. Is it possible you have this schematic?
Ciao
Fabrizio

 

[ruffrecords]

I have not seen the TG mic pre schematic but I have seen some of the EQ ones and they use a similar circuit.

Cheers

Ian

 

[Fablab]

Meantime, "with a little help from my Photoshop" I have removed C5 as you suggested 
Ciao

 

[nille]

see also:  https://groupdiy.com/index.php?topic=4134.msg369040#msg369040

 

[ruffrecords]

Meantime, "with a little help from my Photoshop" I have removed C5 as you suggested 
Ciao

You still need C5 but it should be connected to 0V via a resistor which will set the gain.

Cheers

Ian

 

[Fablab]

In the complete schematic I have seen in the link posted by Nillie (thank you!) I have seen there is a gain switch
Do you think it is possible to remove it?

 

[ruffrecords]

In the complete schematic I have seen in the link posted by Nillie (thank you!) I have seen there is a gain switch
Do you think it is possible to remove it?

You need some resistance between C4 and 0V to set the gain. They have a switched resistor that gives two gains. You could have a fixed resistor that gives one gain or a pot to male it variable but you do need a resistor there.

Cheers

Ian

 

[Fablab]

So I can put a pot from C4 and 0V removing the other switch in the input and connecting the output of the  transformer to C3?
Ciao

 

[ruffrecords]

So I can put a pot from C4 and 0V removing the other switch in the input and connecting the output of the  transformer to C3?
Ciao

Yes.

Cheers

Ian

 

[Fablab]

Thank you Ian!
what's the right value for the gain pot?
Ciao
Fabrizio

 

[ruffrecords]

Thank you Ian!
what's the right value for the gain pot?
Ciao
Fabrizio

Depends on the range of gain you want. A 5K reverse LOG pot with a 150 ohm resistor in series will give a gain range from about 30dB to about 6dB (plus the gain of the input transformer).

Cheers

Ian

 

[Fablab]

Thenk you Ian!

 

[Strawtles]

It can  be useful for the headroom  to increase the power voltage to +28/-28 volt as in the EMI TG circuits?

 

[ruffrecords]

It can  be useful for the headroom  to increase the power voltage to +28/-28 volt as in the EMI TG circuits?

Unfortunately the transistors used in this circuit are not rated for such high voltages.

Cheers

Ian