Balanced Input Attenuator

[1wildthing]

Hi there, I'm working on a new build and I hope in your advice. I would like to add to the circuit an input level Potentiometer in order to attenuate the incoming signal.
As a balanced input is concerned, I would like to know if I can get the job done with a simple 10k pot as shown in the image or I need something more complex.
It's going to be a dual channel build so may I would consider other issues?

 

[ruffrecords]

It will certainly work but it has a couple of problems. With the pot fully up it just adds a 10K additional load which should not be a problem. However, at the 50% point, the mic pre this feeds will see a source impedance of about 2K5 which is too high for most mic pres. It will be lower at other pot positions but it will vary a lot. The other problem is a single pot loses symmetry so the CMRR will suffer and a mic input is where you need good CMRR. You can overcome most of these problems by using a dual 1K pot instead of the 10K and take the output from the two wipers but the problem then becomes you have lowered the input impedance by the series 2K of the pots that will always be there.

There are no easy single/dual pot solutions to this problem AFAIK. The simplest workable solution is a simple switched 20dB pad at the input.

Cheers

Ian

 

[1wildthing]

It will certainly work but it has a couple of problems. With the pot fully up it just adds a 10K additional load which should not be a problem. However, at the 50% point, the mic pre this feeds will see a source impedance of about 2K5 which is too high for most mic pres. It will be lower at other pot positions but it will vary a lot. The other problem is a single pot loses symmetry so the CMRR will suffer and a mic input is where you need good CMRR. You can overcome most of these problems by using a dual 1K pot instead of the 10K and take the output from the two wipers but the problem then becomes you have lowered the input impedance by the series 2K of the pots that will always be there.

There are no easy single/dual pot solutions to this problem AFAIK. The simplest workable solution is a simple switched 20dB pad at the input.

Cheers

Ian

Thanks a lot Ian, actually I ‘m building an Eq not a Mic pre but may be the pad solution is the better one anyway.
Thanks again.
C.

 

[ruffrecords]

Thanks a lot Ian, actually I ‘m building an Eq not a Mic pre but may be the pad solution is the better one anyway.
Thanks again.
C.

Ooops, basic boo boo there on my part. I blame Covid. If your input impedance is about 10K then the 10K pot will be fine.Most of my EQs these days start with a 10K:10K transformer followed by a 10K pot.

Cheers

IAn