Langevin 4102A Preamp

[smilan]

Hi,
I have two Langevin 4102A cards with Beyer TF407 transformers.
According to the manual (http://www.uneeda-audio.com/micbuilders/langevin_4000_pt2.pdf) the output impedance of this preamp is 200K which is quite unuseful for me.
Besides using the existing components and building a 4103 preamp with output impedance of 600 ohm, there's something else I can do with the original cards to use them with a modern studio gear?
My second question is does it really needs a DC supply of 42v to operate?

 

[chilidawg]

Hi,
I have two Langevin 4102A cards with Beyer TF407 transformers.
According to the manual (http://www.uneeda-audio.com/micbuilders/langevin_4000_pt2.pdf) the output impedance of this preamp is 200K which is quite unuseful for me.
Besides using the existing components and building a 4103 preamp with output impedance of 600 ohm, there's something else I can do with the original cards to use them with a modern studio gear?
My second question is does it really needs a DC supply of 42v to operate?

"The AM4102 pre-amplifier is designed for microphone pre-amplification into a system load of 10K ohms."

Modern studio gear's Hi-Z input section should be able to handle that.

4102 and 4103 has the same circuit, but with different resistor values and 4103 has an addition of a capacitor parallel with R4.

 

[smilan]

Thanks, I was looking on the 4101A and thought I'm looking on a 4102A...
What about the supply voltage? Does it really needs 42V?

 

[chilidawg]

Thanks, I was looking on the 4101A and thought I'm looking on a 4102A...
What about the supply voltage? Does it really needs 42V?

Yes, and if I were you, I would mod the 4102 to 4103.

 

[smilan]

Thanks, I'll definitely do that!

 

[smilan]

One more thing about the 4102 and 4103 preamps, There's a way to add a variable gain control to the circuit?
The feedback resistor between pins 4 and 5 is only adding gain to the fixed gain of this circuit. I would like to be able to reduce / increase the gain according to my source.
The schematic is here on page 4:
http://www.uneeda-audio.com/micbuilders/langevin_4000_pt2.pdf

 

[abbey road d enfer]

I believe there is something wrong with these schemo. The emitter resistor of the last stage is way too high, and cannot deliver any significant current to the load. I believe there is at least a factor 10.
I don't see why it would take a 2N3053 to operate at 0.2mA.

 

[smilan]

I believe there is something wrong with these schemo. The emitter resistor of the last stage is way too high, and cannot deliver any significant current to the load. I believe there is at least a factor 10.
I don't see why it would take a 2N3053 to operate at 0.2mA.

This resistor is 9.1K on both of my 4102 cards (not 91K).

 

[abbey road d enfer]

This resistor is 9.1K on both of my 4102 cards (not 91K).

Yes, that's what I thought.

 

[mjrippe]

One more thing about the 4102 and 4103 preamps, There's a way to add a variable gain control to the circuit?
The feedback resistor between pins 4 and 5 is only adding gain to the fixed gain of this circuit. I would like to be able to reduce / increase the gain according to my source.

Put a -20dB pad in front of the circuit, and a 100k pot across pins 4 & 5. 

Note that your output is unbalanced and will not drive a heavy load.  You can easily build the 4700 amplifier and add a 600:600 transformer if you need those features.

 

[smilan]

Put a -20dB pad in front of the circuit, and a 100k pot across pins 4 & 5. 

Note that your output is unbalanced and will not drive a heavy load.  You can easily build the 4700 amplifier and add a 600:600 transformer if you need those features.

Thanks, I'll definitely do so.
I would like to get an Edcor transformer with 600 Ω to 600 Ω (1:1 ratio).
Which series would be better for this application, the XSM ro the WSM?
https://www.edcorusa.com/openframematchers
And last question, when connecting an unbalanced signal to an output transformer with a center tap on the primary side , what would be the right way to wiring it?

 

[abbey road d enfer]

Thanks, I'll definitely do so.
I would like to get an Edcor transformer with 600 Ω to 600 Ω (1:1 ratio).

Do you want to use it to balance the output?
The output stage will struggle pushing a 600r xfmr.

And last question, when connecting an unbalanced signal to an output transformer with a center tap on the primary side , what would be the right way to wiring it?

If you want a 600:600, there is only one way of connecting the primary. Ignore the CT.

 

[smilan]

Do you want to use it to balance the output?
The output stage will struggle pushing a 600r xfmr.
If you want a 600:600, there is only one way of connecting the primary. Ignore the CT.

Thanks, mjrippe suggested me to build the 4700 amplifier and add a 600:600 transformer to the circuit.
According to the 4700 spec. it seems like a 600:600 transformer is correct, isn't it?

 

[abbey road d enfer]

Thanks, mjrippe suggested me to build the 4700 amplifier and add a 600:600 transformer to the circuit.
According to the 4700 spec. it seems like a 600:600 transformer is correct, isn't it?

AaH, OK for the 4700.

 

[smilan]

I've cloned the 4700 on a PCB today, so the only thing left to do is to choose an output transformer...
What info I should look at the transistors datasheets to be able to calculate the current consumption of the 4700 circuit?

 

[mjrippe]

I've cloned the 4700 on a PCB today, so the only thing left to do is to choose an output transformer...
What info I should look at the transistors datasheets to be able to calculate the current consumption of the 4700 circuit?

Very nice!  I have built mine on 2" x 2" perfboard.  The current draw is 36mA @ 42V according to the spec sheet.  They will work fine with up to 48VDC, which might take an mA or two less 

 

[realmseeker]

Interested to hear if anyone has used a different input transformer and what options might work with the 4700 circuit. I own a Langevin Am3 and although the Beyer’s work fine, I noticed the frequency response doesn’t have as much reach in the low end as the higher end TF434 transformer option Langevin offered at the time.

 

[CJ]

https://groupdiy.com/threads/langevin-transformers.74183/
Langevin transformer info just for the heck of it,