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Author Topic: Need help making Variable Pad for V72 input, for unbalanced line signal  (Read 5440 times)

LSA7

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I know this should be simple. But I'm a bit confused because I see there are 2 different specs involving the input transformer that, I believe results in a different input impedance depending which was implemented. One shows the transformer secondary loaded with 200K, and the other shows it loaded with 400K. Both my Siemens V72's have a 425K stock resistor which [based on an 800:8000 turn ratio yields and impedance ratio of 1:100 which should [I think] make the input impedance 4250 ohms rather than around 2000.

Anyway, I'd like to use the V72's for an Unbalanced Line Level signal [rated at 200 ohm] and would like to pad the input for that but with some adjustability. I haven't a clue what might work. I was thinking that something like this might work, but I'm just guessing Thanks.


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drafiny

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Re: Need help making Variable Pad for V72 input, for unbalanced line signal
« Reply #1 on: September 02, 2014, 12:04:11 am »

I try to abide by it. And can actually be done.
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goomeeru23

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yesser i got it.
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nokaw253123

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i thinking for this too. :)
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nong253123

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i need help to.same ploblem.
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wan253123

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its so busy ploblem.
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Uwe

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Which version of the V72 is in question here? The original NWDR-version used an input transformer CTV72/1 with 2 x 1:20 ratio (2 x 400 turns pri. and 4 x 2000 turns sec.) with a 400kΩ secondary termination resistor. The nearly identical first generation Siemens V72 uses a very similar input configuration, but without any terminating resistor for the input transformer secondary. Finally, there is the Maihak V72, which followed the 'Braunbuch' specifications with the CTV72/1 input transformer termination of 250kΩ. The V72 was followed by several versions of the V72a, all of them featuring slightly different input transformers and terminations. The series last 'official' implementation was the V72b by Siemens, which used a balanced push-pull circuit topology from input transformer (1 : 13.5) to output transformer (23.2 :1) and using two dual triode tubes instead of the two pentode tubes used in the previous versions.
In order to provide an unbalanced line input to the V72 or V72a I propose to disconnect the input transformer, making sure the grid of the first tube is returned to ground through the terminating resistor, and then providing the audio input through a suitable decoupling capacitor (22 nF to 100 nF). If necessary a variable resistor or fixed resistive pad could easily added to accommodate most signal sources.
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sungsum253123

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its have many variable also.

koknum2531

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Its good knowlage.for this help Topic.

meenoy25312396

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i hope with every anwsers.

isti2531

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good for some anwser.

jeedzy2531

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all this is about knowlage.
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