Disa vintage danish mic pre question.

[Studio Mollan]

Hi!
I have a question. I recently bought two vintage Disa preamps. The same style as V72, actually compatible with the same pinout, connector and have internal psu.
Mine are a fixed gain version that I intended to hook up with a pad switch for the input and possibly also output but I then saw this in the only available schematic for this pre. (See photos below)
And here's the questions: is this a circuit for variable gain and is it safe in your words to try to copy this to my pres? Maybe add a few steps to get more control of the gain.
See below for complete schematics, zoom of the area of interest and the schematic for my pres in the same area.
/
Emil

 

[Studio Mollan]

Zoom

 

[Studio Mollan]

My pres

 

[Gustav]

You may be able to build on the work Blue_Luke did here.

http://groupdiy.com/index.php?topic=32014.msg388802#msg388802

Gustav

 

[Studio Mollan]

You may be able to build on the work Blue_Luke did here.

http://groupdiy.com/index.php?topic=32014.msg388802#msg388802

Gustav

Alright!
That confirms it's a gain circuit. Wish the values for the gain switch resistors were in his schematics though... If anyone have input on this id be VERY happy!!
Mange tak Gustav!

/
Emil

 

[Gustav]

This neat spreadsheet from Harpo might come in handy.

(edit:sorry, found the wrong link - but its out there!)


Gustav

 

[Studio Mollan]

This neat spreadsheet from Harpo might come in handy.

(edit:sorry, found the wrong link - but its out there!)


Gustav

Hi!
The one I got was for a stepped gain switch for an API pre. Is that the spreadsheet you were aiming at? I really don't know the range of the resistors. The original schematics has a three pos switch witch 1. Complete short 2. 6,8k 3. 270k.
Seems to be quite a jump between the 2-3 steps.. If I know the top and bottom values I can always install a pot and find 12 values in equal dB steps by reading the output and measuring the pot on the specific gain steps. Not the fastest way but I guess it will work. Or?
Can you make out the range of the gain resistors by the resistors before and after the gain switch in the schematics?
/
E

 

[emrr]

At a glance looks similar to the McCurdy AU300. 

 

[abbey road d enfer]

The one I got was for a stepped gain switch for an API pre. Is that the spreadsheet you were aiming at?

A spreadsheet made for a very-high gain solid-state opamp is not applicable to a low-gain tube circuit. Everything is different.

I really don't know the range of the resistors. The original schematics has a three pos switch witch 1. Complete short 2. 6,8k 3. 270k.

So you know the range of resistors is gonna be between  and 270k, don't you. In fact, 270k is almost like an open-circuit there.

Seems to be quite a jump between the 2-3 steps..

What are these steps? 20dB? Not surprising.

  I can always install a pot and find 12 values in equal dB steps by reading the output and measuring the pot on the specific gain steps. Not the fastest way but I guess it will work.

As I mentioned in the "other" thread, you could calculate the gain expression for the whole circuit and solve the equations for the gain values - that is feasible but would take ages and give quite uncertain results - , you could also sim the circuit, which would give probably more accurate results, provided the tube models were perferct, and you can do it by trial and error, which you will have to do anyway just to prove the design. I guess it would take less than an hour to work it out.

Can you make out the range of the gain resistors by the resistors before and after the gain switch in the schematics?

You have to take into account the open loop gain, the 2nd tube's output impedance, the impedance of the 1st tube's cathode node, and all the resistors that are flying around the cathode; everything counts.

 

[emrr]

you can do it by trial and error, which you will have to do anyway just to prove the design. I guess it would take less than an hour to work it out.

It's almost always the fastest method. 

 

[Studio Mollan]

Thank you guys!
I'll get myself a resistor set and will start fooling around with this ASAP.
Trial and error is the way I usually do it but I feel very assured I'll work it out having heard you thoughts about this.
Thanks again!
/
Emil