Adding an output trim to a Fairchild 662 mic preamp

[petereugenefoley]

Hello all. I got some Fairchild 662 preamps complete with input and output transformers for a steal. I hooked em up to my bench supply and they sound very nice; however, I would like to add an output trim so you can crank the input. Attached is the schematic -- the only thing that was cutoff was a zener diode between the -18VDC and R21, a 1W 56r resistor. I have questions:

1. The input control is a little 150r trimpot. If I want a more accessible input pot, would I simply replace this with a similar value and mount it to the faceplate of the rack unit it will go in? How might I add a stepped attenuator instead of a pot?
2. Where would I attach an output trim pot, and what would be an appropriate value?

Thanks.

Edit: after looking at the schematic more closely, would it be more correct to say the 150r trimpot is actually the output trim and I would in fact need to add an input pot?

 

[emrr]

The range of the one existing pot is total volume control. Doubt there's anything useful out of an output attenuator, headroom is already low here. An input pad is usually required with ancient amps like this, with the balance between pad and trim being the primary mechanism. If R8 was a larger value pot, there'd be more feedback and gain could be lower. You'll have to play designer and measure results. R9 down to Zero increases gain, but it may be garbage.

 

[petereugenefoley]

The range of the one existing pot is total volume control. Doubt there's anything useful out of an output attenuator, headroom is already low here. An input pad is usually required with ancient amps like this, with the balance between pad and trim being the primary mechanism. If R8 was a larger value pot, there'd be more feedback and gain could be lower. You'll have to play designer and measure results. R9 down to Zero increases gain, but it may be garbage.

I tried a 1k, 10k and 100k pot for R8 and the lower value ones sounded way better. The higher values were distorting it like crazy.

how would I replace R8 with say a stepped attenuator rather than a pot?

 

[emrr]

Try R8 higher values as a rheostat rather than a pot, sweeper to the high side. 100K way too high. Useful might be 500.

There are stepped pot calcs on the site here somewhere, and elsewhere on line. It's honestly easier to measure gain steps with the pot and calculate nearest resistor values for a string, then you don't miss some effect within the circuit.

 

[mattrad]

I've used this simple output attenuator on a couple builds, has always worked fine. Found it in some old thread around here somewhere. Maybe has some drawbacks i'm not aware of. Here ya go: