Telefunken V675 mod to mic pre

[Jitter]

Hi guys!

I´ve a pair of Telefunken V675 and I wanna mod to mic preamp.
Somebody was realized this mod?

I think that maybe is possible to leave R4,R6,R5 and instead a pot on this position (R5) to increase the gain.

It´s is possible?

 

[Bo Deadly]

Nope. That network looks like it's part of a negative impedance converter to compensate for the dc resistance of the transformer. Don't mess with that.

There's really only so much gain you can get out of this thing. If you push it too much, you'll probably start to just loose high frequencies. And it's kind of a nice circuit. Why not use it as-is?

If you really have to hack on it, you could probably get a little more gain out of it by adding a ~100uF capacitor and ~100 ohm resistor in series from the intersection of R9 and R11 to ground. Making the resistor smaller will increase gain.

One thing that might preseve bandwidth would be to make the capacitor you add smaller like 10uF. That will not increase the gain at low frequencies which is probably somewhat desirable if you're really pushing the gain anyway because limiting low frequencies also increases headroom.

 

[Jitter]

Some people says that the v675 can offer 38 to 60db of variable gain if it is modded.

 

[ruffrecords]

Some people says that the v675 can offer 38 to 60db of variable gain if it is modded.

Depends on the mods. As it stands, the last three transistors which form the output stage have unity gain. All the gain is in the first two transistors  and together they are unlikely to provide as much as 40dB of gain without serious sound compromises. What does the documentation say these are originally intended for?

Cheers

Ian

 

[Bo Deadly]

Actually that network in the upper left must also be controlling gain. It sure looks like positive feedback to me but I don't see any other feedback mechs so it must also be controlling gain. And that pot isn't drawn like a trimmer. Whatever it's for you if it has one knob on the front, it must be for gain.

 

[[silent:arts]]

What does the documentation say these are originally intended for?

Zero Ohm Summing Amp

 

[ruffrecords]

Actually that network in the upper left must also be controlling gain. It sure looks like positive feedback to me but I don't see any other feedback mechs so it must also be controlling gain. And that pot isn't drawn like a trimmer. Whatever it's for you if it has one knob on the front, it must be for gain.

What makes it negative is the way the feedback winding is wired at the input.

Cheers

Ian

 

[Bo Deadly]

What makes it negative is the way the feedback winding is wired at the input.

Ah, ok. So that winding is drawn upside down? I was wondering if that's what the "sw" and ws" meant.

 

[Pusch3l]

Ah, ok. So that winding is drawn upside down? I was wondering if that's what the "sw" and ws" meant.

That's just the color code of the wire..

sw means Schwarz (Black)
ws means Weiß (White)

And so on....
Its the same way the Volkswagen wiring diagrams work (that's where I learned it  ;D)

 

[Bo Deadly]

Or maybe I just don't understand magnetics. It looks to me like if there is a current flowing through the lower secondary through C1 to ground that can be described as downward looking at the drawing. That will serve to turn off T1 which will turn on T2 and sink current from the feedback network which is sourced through the other secondary from ground which is also downward looking at the drawing. If these currents are flowing in the same direction, the current drawn through the upper secondary will reinforce the magnetic field responsible for the current in the lower secondary which seems to me to be positive feedback. Right?? So the upper winding is drawn upside down? Or maybe it really is some kind of negative impedance converted that is unique to this type of zero ohm summing amp?

 

[Jitter]

I’m reading your replies and I see that the v675 haven’t posibilities to mod on a mic preamp.