RCA BC-2B preamp attenuation

[mrtnasty]

Hi, can I add a potentiometer to this circuit as a volume control, or do I need a 600 ohm attenuator at the end? Thanks

 

[dmp]

Since it has the feedback path with r3/c2, you don't want to use r4 as a pot for a level control, unless you remove the feedback, which would change the character of the preamp. So you could do a input pad /output pad

 

[emrr]

You can vary the NFB. I’ve posted about it.

 

[mrtnasty]

You can vary the NFB. I’ve posted about it.

Thanks, is this the post you're referring to?

https://groupdiy.com/threads/rca-bc-2b-tube-console-restorations.53576/#post-694136
You've got volume pot and a 3 negative feedback switch, right? I didn't quite understand the discussion on the 150 / 300 ohm attenuators, they're on the output? I actually some of those I could use. I am just looking for the simplest most useful solution to add a level control.

 

[mrtnasty]

Actually the pictures of the pots I was referring to was here:

https://realgearonline.com/thread/7553/finished-another-rack-consolette-modules
Or are those actually small attenuators? I couldn't see where they are connected from the photos.

 

[emrr]

No volume pots, just pad switches and rotary negative feedback control

 

[mrtnasty]

No volume pots, just pad switches and rotary negative feedback control

Oh I thought those switches were for the negative feedback and rotary control was for the pad. Where are the pad switches attached to the circuit? I couldn't see from the above threads. Thanks.

 

[emrr]

Pad is always before everything

 

[mrtnasty]

Pad is always before everything

What kind of pad would you recommend? 150/150?

 

[mrtnasty]

10 db u pad (with R1=510 Ohms, R2=470 Ohms, R3=510 Ohms) on input? Then vary the feedback resistor a bit with a pot (or different resistor values built into a rotary switch) a bit above and a bit below the stock value, and the reason is that the response stays pretty flat when this resistor is varied not too much? With the two mods you have a wide array of possible attenuation without significantly affecting the circuit?