East German transistor preamp help

[synthiaks]

Hello everybody!

Im trying to wrap my head around something but need some help. I have been looking at the schematic of the east German Microphone preamplifier MV810-2.

I'm currently only focusing on the first input card named "baustein A-10*

This is how I understand it and please contact me if im wrong:

Transistor 101 and 102  is in a common emitter topology and form a 2 stage transistor amplifier  transistor 103 is a emitter follower with positive feedback back to transistor stage 2 (transistor 102) through R116 which would control some amount of gain?

If this is about right what is wired to me is the Bias of transistor 101, or maybe the lack of such an arrangement. I would expect a voltage divider between ground and 20V. But the transistor seem to have the collector at 0 potential and emitter at 4,8V.

In a normal setup I would expect the Collector to be positive and the Emitter at 0 potential. My question now would be of this is another way of biasing the transistor. The Emitter being at 4,8V would with added voltage drop of 0,7V force the base to sit at 5.5VDC. the first stage could then swing about 10ptp....?

Or am I totally lost on this one?

I would love it if someone  could help me out

Regards

Sebastian

 

[synthiaks]

Schematic

 

[synthiaks]

Schematic2

 

[JohnRoberts]

Hello everybody!

Im trying to wrap my head around something but need some help. I have been looking at the schematic of the east German Microphone preamplifier MV810-2.

I'm currently only focusing on the first input card named "baustein A-10*

This is how I understand it and please contact me if im wrong:

Transistor 101 and 102  is in a common emitter topology and form a 2 stage transistor amplifier  transistor 103 is a emitter follower with positive feedback back to transistor stage 2 (transistor 102) through R116 which would control some amount of gain?

If this is about right what is wired to me is the Bias of transistor 101, or maybe the lack of such an arrangement. I would expect a voltage divider between ground and 20V. But the transistor seem to have the collector at 0 potential and emitter at 4,8V.

base of q101 is biased through the transformer winding which is biased up by a resistor divider  at roughly 40% of +V.

In a normal setup I would expect the Collector to be positive and the Emitter at 0 potential. My question now would be of this is another way of biasing the transistor. The Emitter being at 4,8V would with added voltage drop of 0,7V force the base to sit at 5.5VDC. the first stage could then swing about 10ptp....?

Or am I totally lost on this one?

I would love it if someone  could help me out

Regards

Sebastian

I can't open the second page of schematic but voltages in boxes may be AC volts.  Should be a legend somewhere on schematic explaining the notation.

JR

 

[synthiaks]

Thanks  for the. Reply John!


I uploaded the second part of the schematic again, the box is DC.

base of q101 is biased through the transformer winding which is biased up by a resistor divider  at roughly 40% of V+

I might be missing something, where is there a connection to V+ to the transformer? I only see what I interpret as an input attenuator on the input side of the transformer. But I can't figure where a resistor divider connected to V+ would be at?

S

 

[synthiaks]

The thing that confuses me is that C2 on the schematic block the DC to the Collector side of all the Transistors they are all tied to the circuit ground V+ is routed to the Emitter side through R118 and R111

 

[synthiaks]

Gain would be the switch "Sch 2" feeding AC back to the Emitter of T101 and changing the resistance of the Emitter?


Jeeez... I have so much to read up on

 

[DerEber]

The A-10 works with -20V not +20V.
So they blocked or filtered the connected -20V. Send them to the lower rail.
0V or +20V is connected via A3a

 

[synthiaks]

The A-10 works with -20V not +20V.
So they blocked or filtered the connected 20V. Send them to the lower rail and connected the upper rail to ground via the output transformer.

I just realized that.. they have as almost all the GDR equipment + ground reference. It confused me that the - wasn't in the schematic

The -V thing makes my head hurt

 

[synthiaks]

So this would put the t101 base at about 4.1V ( or -4.1 in a Western world)' So we could have about 8Vptp? From the first stage?

I still don't see a resistor divider to the input transformer on the schematic, am I blind? [Edit: R103, R104...?]

Thanks for the help!!!

S

 

[synthiaks]

I'm so sorry about my 2000 posts, sometimes I feel like I have an inner conversation with myself. .. I can understand that this is annoying to some of you anyway...

Thanks John!

I think I got it now. I have to study transformer theory a bit closer. R103 and R104 are the network you mentioned. Doing some calculations on that puts the Base voltage at 4.2V which is not far from my 4.1 ( I always think of Vbe as 0.7 and not 0.6) so the bias mystery is solved.

De AC decoupling  of the Emitter resistors are probably omitted while the positive feedback are much bigger than any potential negative AC feedback?