Adding 20 / 30 db Pad the Ampex 602 Mic Pre Input

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smilan

Well-known member
Joined
May 29, 2017
Messages
463
Hi,
I would like to add a three stages pad  (with 0 / -20 / -40 db attenuation) to the mic channel input.
I'm thinking to add this (right side)  pad design to the input of my 602:
BalPad_switched-copy.gif

How can I calculate the values of the four resistors of this pad circuit so it will work fine with  the mic input transformer (it's a Beyer TR BV 351 015 006 / 1:15 ratio / 200 Ohms)

Here's the pad circuit connected to the mic input transformer:
2z6vywx.jpg


 
here's mine.  You won't get 20 and 40 nicely out of the JLM approach I don't think. 
 

Attachments

  • Mic_line_rotary_pad s.jpg
    Mic_line_rotary_pad s.jpg
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I was looking on some other preamps designs and I saw that in some preamps the pad is located after the input transformer like here on the  LA2A (R1):
2entgf6.jpg

What are the advantages / disadvantages of placing the pad / gain pot before / after the input transformer?
 
the input transformer will overload.

a pad at higher impedance on the secondary will be more susceptible to high frequency losses and will need very short and tight leads between parts. 
 
Thanks Doug,
That make sense...
Can you please explain me how your ped design connects to the circuit?
The I+ (A) and I- (B) connects to legs 2 and 3 of the XLR input connector?
What goes to the Input transformer?
 
OK, I got it, in the picture pin 4 and pin 8 both marked "O-" so I've got confused...
So I'll connect pin 4 to O+ and pin 8 to O-.

Thanks for your help  :)
 
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