Mixing stereo down to mono with a dual-triode

[thorpej]

For Christmas, I got my son a 2-tube regenerative radio kit and we've been having a lot of fun putting it together (slowly over the course of many evenings ). His birthday is coming up, and I thought it would be fun to build a little transmitter to go with it so he can listen to something other than baseball on it. The idea would be to let him transmit music / audiobooks from his iPod to his radio. AES makes a nice little kit that fits the bill, which is probably what I'll pick up.

What I'd like to do is add a stage in front of the transmitter to properly mix down the stereo from the iPod into mono for the AM transmitter. I am thinking that I can just do this by running a dual-triode (12AU7 is probably what I'll use) in parallel, i.e. sharing a common plate load resistor, and then feeding one grid with the L channel and the other with the R channel. I guess I would calculate the plate resistor as usual, but then divide the resistance by 2 because I'd have 2x the current flowing.

I want to make sure I have a little gain, just to match the input sensitivity of the transmitter (the AES kit uses a 12SA7, but I haven't calculated the operating point from the schematic just yet, so I don't know how much gain I'm likely to need).

 

[Joe Roberts]

You can build a summer with three equal value resistors, one in each leg of a Y, then append a simple mono gain stage after that if needed.

 

[gadget73]

There is actually a schematic for just this sort of thing in the back of the RCA manuals. Depending on the year, it usually runs a 6SN7 or 12AU7. I don't recall the gain offhand, but there is some. Its a dirt simple circuit really, just two triode amps with the two output coupling caps going to a common point.

 

[Retrovert]

Tube mixer:

www.tubecad.com/april_may2001/page22.html
The simplest tube mixer is made up of several series input resistors feeding one common grid. The resistors further the isolation between inputs, i.e. serve to limit the back-feeding of input signals. The input signals are summed at the grid and amplifier at the plate. Unfortunately, this circuit is too simple. It fails to truly isolate the inputs; it provides unwanted gain; and it adds excessive distortion.

The discussion continues with a very elegant solution.