Lundahl LL1524 vs Harrison 32C

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2sounds

Member
Joined
Dec 7, 2021
Messages
5
Location
Belgium
Hi, Im in need of some advice to implement the Lundahl LL1524. As part of my restoration of many Harrison 32C channelstrips I want to use the original parts (the PC1040 plug on microphone card) to build a 4ch preamp, with REV, PAD, Phantom and the HPF circuit. I have also some (recup) Lundahls LL1524 I would like to add to make the line out transformer balanced. These types are suited for line out balanced drive as stated in the info from Lundahl. But not much examples to find of schematics. Is there a way I can use them passive? Or is the use of a mixed feedback circuit essential for proper function?
This is passive? :
Screenshot 2021-12-07 at 20.06.48.png
then this is the active balanced mixed feedback :
Screenshot 2021-12-07 at 20.11.19.png
This is an example of the transformer Line driver of the Harrison 32C : Push pull configuration with unbalanced mixed feedback drive?
Screenshot 2021-12-07 at 20.15.09.png
 
Hi, thanks for the feedback.
Im a newbie when it comes to these type of designs.
I have some skills now, after working on my Harrison-restoration project (see Flickr)

Rebuilding stereo Line driver from original parts of 2 channel strips I completely destroyed (to make extenders)

But in this 4ch mic preamp I would like to use the Lundahls!
Did some testing and seems I need some amplification after the mic amp :
Here is the Harrison Mic pre :
Screenshot 2021-11-29 at 22.50.26.png
I assume I will need the active balanced mixed feedback circuit?
This one :
Screenshot 2021-12-07 at 20.11.19.png
 
To bad, don't have any spares of that, but have a pick :
lots of Harris 4605-5 or NE5532P, TL 072CP
The 5534 and 5532 I need for spare replacements.

IMG_0735.JPG

I wold use the Harris HA4605-5 quad for the line driver, But now I need a schema with value's.
Any suggestions?



Here the test setup ;
IMG_0734.JPG
 
I would start with these values:
Rin1, Rin2, Rf1, Rf2, Rr 10kohm
R 3R9 or 4R7
Cin, Cr 2u2/63V MKP
Cf1, Cf2 22pF COG
 
Thanks!
I see that the Harrison design is using 100pF as feedback cap, with indeed 10K R.
The R value should come a close to the value of the primary winding? measured with my LCR45 I have 7R4
 
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