simple active fader box

could someone please look this over for me and see if anything sticks out as in error?

the concept is a simple 8 channel fader box with db25 input for a 500 series rack offering 20 db of gain and balanced transformer output summing. this is pieces cut up from esp's active mixer pages. I'm not sure if I need another buffer for the fader or not or if the insert section has enough resistance to ground.   

https://www.dropbox.com/s/wfbmgys92gw0ljs/whole%20circuit%20with%20inserts.jpg?dl=0

I had a quick look at your schematic. Comments as follows:

1. DB25 is usually balanced inputs. Your inputs are unbalanced - is that what you intended?
2. I would add a fixed resistor in series with VR1A to limit the maximum gain
3. I am not sure of the purpose of R106. It just throws away gain and worsens the S/N ratio
4. Your pan circuit only has a very small dip at the centre point. You need to increase R5 and R6 to about 10K.
5. R1 on the + input of the mix amplifier just adds noise. It might help balance bias current but for audio it should be decoupled.
6. I am not sure of the purpoase of R5 and R1 in the output amps. As configured they just throw away 10dB of gain.

Cheers

IAn

Fred Forssell has something very similar

http://www.forsselltech.com/media/attachments/8chsum_1.pdf

-Casey

For the panpots, I suggest using a linear taper, dual section pot with "pull up" resistors from the wiper to the signal input terminal on each section.

Bri

thanks for that. I made the modifications you recommended. I was piecing bits from a full mixer section from Elliott Sound Products site  and I think some of those resistors were used for other things like prefade listen and things. I thought I removed most of them but guess not. Yes for now I have the mixer unbalanced in and balanced out.   

I was hoping to avoid 8 dual pots but might have to go that route.

doulos35 said:

I was hoping to avoid 8 dual pots but might have to go that route.

Click to expand...

Dual pots have the advantage that they have little insertion loss so the noise performance will be better. However, in a line level mixer, the noise floor is more likely to be determined by the S/N of the source so in most cases it will not be an issue.

Cheers

Ian

thanks for the explanation Ian

Just ordered a pair of jensen JT-123s will see how this circuit preforms .