Rack and mod an RFZ V740

[Pusch3l]

Hello fellow DiYers,
so I got 2 of those old GDR modules. They're officially called "Übernahmeverstärker" and can handle Mic and Telephone level.
so my plan is to recap, rack and do some modifications.
But some things aren't really 100percent clear to me so maybe someone can shed some light into it.

First thing is: if I rewire the Input Transformer 1:3.2 (original is 3.2:1) i get more gain out of it (that's the plan) but I don't if I can leave the ceramic cap C2 or do I have to adjust it.

Second thing is R1/1 and R1/2 on St1 a and b. I think that are some kind of load resistors to adjust the input impedance like in Gyrafs SRPP stage from his pultec clone.
If I understand the manual correctly I just need the 6.8k for mic level.
I would then use them in parallel to the input for the right impedance.

Or do I miss something here?

Schematic attached

Greets
Sebastian

 

[jokeramik]

Hi Sebastian,

with rewiring you get 10 dB more gain.

But I think you don´t need this.

In normal working conditions the V740 have a imepdance of 6kOhm. If you rewire the transformer you change this to lower values and also change the connection conditions for the transformer. That´s not a good way. With a DAW you can add enough amplification by doubling the track or in others ways.

The 6,8 K resistor will be use parallel to the regulary input to receive 600 Ohms impedance for connection to the telephone line, which be must terminated with 600Ohms. Don´t connect these resistor to the input.

Another hint for using the V740

Please don´t use 24 Volts for powering. With this power supply voltage the first amplifier card don´t work in stable conditions an make HF or can oscillate. You can see this with an scope on point nr. 8 of the Zeibina-bar of the first amplifier card. Please use only 20 Volts power supply.

I developed an adapter for using these modules (V 740, MV 810, MV 810/2 and so on) -> Please look here: Tools

Best regards!

jokeramik