1272 hotrod mod (switch mod)

Hi all -

I'm working with the JLM 1272 hotrod mod (attached).

I'd like to use a 24-position switch rather than the 12-position switch in the example circuit (allowing the steps to be 2.5 dB rather than 5 dB). From what I understand, the "SW1a" portion is simply a series-string attenuator that reduces the unbalanced signal (post input transformer). "SW1b" changes a resistance value in the preamp circuit to allow for a gain boost.

Could someone explain the math used to determine these resistance values?

Thanks in advance for your comments!

Why?

Just install a 5K pot along w/5K6 resistor (Note 1,Ver C) and you'll have 0-5db adjustments in between the steps.

Mark

Blasrocks - thanks for your reply. You are absolutely right - the 5K pot would allow for trimming of 5dB.

I'd like to try the 24-step switch approach for a couple reasons -- first, to eliminate the need for any potentiometers in the signal chain. Second, I'm finding it incredibly difficult to find an affordable 12-position shorting switch with 2 decks that doesn't have continuos rotation. (Let me know if anyone has links for a supplier).

24-position switches seem to be popping up on eBay regularly now from Hong Kong (see pic). I've used this type before in a project and they seem to serve the purpose quite well. Plus, I'm really interested in the math used to calculated the resistance values for each section of the switch!

Understood,

Having a pot in the signal chain isn't really a big deal, it's just a variable resistor and even if you don't install a pot, you still need a 5K6 resistor in that spot. I think you'll find that having that 5K fader is useful for gain staging purposes or if you like to push your Neve's.

Elma has the switch, PCB Mount 08-2113 is the 2 deck, 08-3113 is the three deck as used in the 1290 and 1073's. Solder tags, 04-2133, 04-3133, not cheap.

http://www.elma.com/Admin/ProductionFiles1//ProductTypeFile/371/English/13%20Selector%20Switch%20Type%2004.pdf

Electroswitch also has some options

http://www.electro-nc.com/rotarysearch.php

3 deck, 12 position
http://www.electro-nc.com/rotaryswitchspec.php?id=C4D0312S-A
http://www.electro-nc.com/rotaryswitchspec.php?id=C1D0312S-A
http://www.electro-nc.com/rotaryswitchspec.php?id=D4C0312S

http://ca.mouser.com/ProductDetail/Electroswitch/C4D0312S-A/?qs=J%252bLxE1TJwwXiGCSwp7eXYQ%3d%3d

As far as the math goes. You could probably divide each step in half and get the desired result. Or wire up the switch, run some tones and start subbing resistors until you find the correct values.

A word of caution, once you get past 50db on a 1272 your beyond the intended Neve gain staging design. Proper implementation of additional gain would be with a 3 deck switch and an extra 283 gain stage, al 1290/1073.

Mark