Simple front-end tube mixer for DSD

[Tubetec]

Hi Richard and welcome to the forum , its small compared to GDIY , but the atmosphere is very friendly and helpfull .

I like this idea of an ultra simple single stage mic mixer with line out direct to stereo A/D .
I know Ian has been working on a passive low level/lo-z mic mixer which might make a good starting point for this project .

[rjuly]

Many thanks for the welcome Tubetec! That sounds interesting - I would love to learn more about that project.
- Richard

[Tubetec]

Heres a link to Ians three mic passive mixer , although there may be another thread because I remember contributing to the discussion about a bridged T attenuation network at some point .
https://groupdiy.com/threads/three-inpu ... xer.88384/

This is the other post I was refering to ,
https://groupdiy.com/threads/sescom-ses ... st-1158661

and this one here ,
https://groupdiy.com/threads/emi-rs144.88291/

[ruffrecords]

The three tube circuit is basically a simple op amp. The -ve input is the cathode of the 12AX7 (the +ve input is its grid) and the output is the cathode of the top half of the 6922.

So you can see there is a 47K resistor connected from the output to the -ve input. So, following standard op amp theory you can set the gain with a resistor R from the -ve input to ground and the gain would be (47K + R)/R .There is already another 47K resistor from the 12AX7 cathode to ground so the gain with no additional resistor is (47K + 47K)/47K = 2 or 6dB. Since the amplifier normally drives a 2:1 step down transformer (6dB loss) the overall system gain would be 0dB.

Again, following standard op amp theory, adding another resistor in parallel with the 47K from the 12AX7 cathode to ground will allow you to increase the gain. So with a 430 ohm resistor the gain would be 111 times or 40.9dB. In practice, because the open loop gain of the op amp is relatively low you actually obtain 40dB.

This is why the lowest gain is obtained with the highest value of R and vice versa.

Cheers

Ian

[ruffrecords]

Many thanks for the welcome Tubetec! That sounds interesting - I would love to learn more about that project.
- Richard

I am currently working on the Mark 2 version. More details when it is done.

The Mark 1 is currently being put to good use as a drum kit sub-mixer in a studio in Portland.

Cheers

Ian

[Tubetec]

Would that be Portland ,Dorset, UK or Portland , Oregon, USA ?

Vortexion at one point way back, probably late 40's early 50's, made a three channel lo-z low level mixer , you passively sub mixed your three microphones then fed the result into a 30/50 ohm step up to grid transformer ,followed by either an EF40 or later an EF86 with feedback from output to grid via the lower end of the transformer secondary .

[ruffrecords]

That would be Portland Oregon. The little passive mixer is to complement the four channel tube mixer I delivered last year to the same customer.

Cheers

Ian