High Power Strapping for Altec 9475a

[labcoat man]

Finally have my long benched altec 9475a mic pre working quietly.  I tried though last night to strap pins 5 and 6 for high power and 9db extra headroom as recommended in past discussions. Obviously this works well for most  but for some reason not happening here.

When input was strapped for 150 ohm impedance it didn't like at all , a very loud static /(feedback I guess) noise that I didn't leave running long enough to tempt fate.

Strapped for 600 ohm input it didn't seem to have any affect at all though I noticed a very slight decrease in the noise floor.

I'm sure there is a good reason that's beyond my understanding  but am I really meant to be connecting the primary of the input xformer to the secondary of the output in some way ? When I look at the schematic thats what strapping 5 and 6 seems to do. When strapped for 150ohm I'm connecting one leg of the actual input  in to this loop as well.

Here is a link to the schematic

www.studiomanuals.com/docs/altec/Altec-9475A-MicPre.pdf‎


Meanwhile gain seems pretty good  but I'd like to get the most out of it.
Any help appreciated!

thanks

Tim

 

[emrr]

The input does not connect to the output in any way. 

5-6 is a strap on the 9470, not the 9475.  Look at the right manual! 

9475 is already set in high power mode, you'd have to rewire on the circuit board to R14 to get low power mode. 

 

[labcoat man]

Thanks Doug for clearing that up, good to know!

I'd never seen the 9470 manual, I just read the advice on discussions relating to both as virtually the same unit hence the confusion. True , the 9475a manual doesn't mention it at all.

Cheers

Tim