Help with active mixer design using LND150

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A theoretical situation: I want to mix the output of two preamps ahead of the DCCF with as little interaction and loss as possible.

The usual mixing resistor approach leads to some interaction and a bit of signal loss due to the increase attenuation. This is usually ignored for the sake of simplicity and no-one notices the difference. Been like that for 80 years and not bothered anyone.

However, to meet my goal I'd need to use some sort of active mixer.

Attached is my attempt based on a virtual earth mixer, but I'm using an LND150 so I don't need to use another valve or come up with a low voltage supply / attenuation etc for a low voltage opamp.

I'm modelling the two 'channels' on the left (a plexi and cascaded front end) by points A and B. These inputs are then mixed (all the bits in the dotted box) and the output taken at point C.

Can anyone see any issues with this? Improvements?

Also attached is a frequency sweep to show the difference between point B & C - slight roll-off at the top and bottom.

How would inserting this before the DCCF impact on things like blocking distortion or grid current? The 100n coupling caps surely would change the behaviour of the following stages?

For the sake of a few parts and a fairly small PCB just wondering why this isn't implemented widely...
 

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A theoretical situation: I want to mix the output of two preamps ahead of the DCCF with as little interaction and loss as possible.

For the sake of a few parts and a fairly small PCB just wondering why this isn't implemented widely...
For one thing the cost of a 275 v supply. I can get 30 v wall transformer for $2 or 12 v for $1 at the local Salvation Army Resale. 500 v 0.1 uf caps are over $1 too. And try to get COG dielectric! NA
I don't know what you're paying for your fet, but 2n7000 for 50 v is $.35. Blown up PCAT supplies are full of free ones.
I tried a nfet gate follower like this on my H100 organ to replace an EF86 vacuum tube, with 200 v supply, and immediately the gate protection zener on the nfet came unsoldered & the pull up resistor blew up the fet. End of project.
What is a DCCF. Some sort of digital appliance?
 
What interaction? The only drawback is a small loss of level, and an increase in output impedance, niether of which should be a problem for your aplication. I would imagine the FET introduces more noise and distortion than a resistive divider. Easy enugh to compare the 2 circuits if you build the FET one.
 
What interaction?
Here's the variation simulated in a full amp at the mixing point when varying the gain knobs on the 'unused' channel. So only about 4dB.

I notice the slight drop when simulating when sine waves distort through the amp. I also notice the difference between two amp builds - one is a single preamp, the other is a combination of two preamps. The former has more distortion on tap, the latter cleaner. However, there are many other factors at play so might not be the best of tests.
The only drawback is a small loss of level, and an increase in output impedance, niether of which should be a problem for your aplication. I would imagine the FET introduces more noise and distortion than a resistive divider. Easy enugh to compare the 2 circuits if you build the FET one.
That's a good reason - perhaps this should remain a theoretical excercise!
 

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