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Michael1 13th December 2011 06:05 PM

Analog Summing
Hey Guys,
I'd like to build a analog summing amplifier. Perfect would be with tubes. I want to get the typical tube sound, if it's possible.

So I want to know if it's possible to built a summer like the Neumann 475 or LAWO DV975 by myself.

I've searched for some "open source" projects like SymAsym which is about building a mono power amplifier but theres nothing like this about building a summing amplifier.

I studie electrical Engineering so I know what the summer does but not how to built it. :D

Thanks for your help :D

kevinkr 14th December 2011 12:25 AM

:cop: Of course it is, and I feel this belongs in the tube forum so I am going to move it there.

DavesNotHere 14th December 2011 08:11 AM

do you want to balance control so you can have an adjustable ratio or straight 50/50.

take a look at the 2 channel mixer (summer) from this rca manual:

Two-Channel Audio Mixer Schematic
Electronic Vacuum Tube Circuit

AndreasS 14th December 2011 04:42 PM


Originally Posted by Michael1 (
... Perfect would be with tubes.

Hello Michael,

pls. look here:

Tube summing amp

Regards Andreas

ChrisA 14th December 2011 05:20 PM


Originally Posted by Michael1 (
Hey Guys,
I'd like to build a analog summing amplifier. Perfect would be with tubes. I want to get the typical tube sound, if it's possible.

There is a range of solutions from simple and cheap to complex and not cheap.

The most simple tube mixer is just a simple passive resistor mixer followed by a simple gain stage to make up for the los in the mixer.

A better but more complex design uses one triode for each input channel. Then the outputs are summed as above. Having one tube per input means the inputs don't interact nearly as much. If you want a "tube sound" this is what you want as it has some gain.

To this you can add improvements. (1) the input triode can be followed by a cathode follower. Now you can better drive the resistors (2) you can add a line driver to the resistor mixing network. (3) mix tube types using 12au7 for the followers and 12ax7 for the common cathode gain stages. (4) regulated DC heaters. (5) transformers in inputs and output to handle balanced interconnects for pro-audio use. (6) mu-metal shields on the transformers and shielded coax hookup wires in the signal path.

It depends on if this is a simple home use box or if you are using this for recording how many of the improvements you want. The passive mixer folloed by a gain stag might be good enough and it only takes one tube for a stereo mixer. So decide on how this will be used and what you need.

One thing. that "tube sound" requires gain. a unitiy gain buffer does not give you much.

Chris Hornbeck 14th December 2011 05:31 PM

How many channels do you need to sum? This question is critical because it determines what kind of summing junction you'll need to make. Power levels escalate rapidly with number of sources because of noise factors.

All good fortune,

Michael1 14th December 2011 06:19 PM

Thank you first for the links and schematics.
I need only 16 mono channels.

I found this PDF file about the LAWO DV975.

I don't know if it's a good way to recreate this?!?!

Is anyone here who has experience with such a schematic? Is it possible to apply this part "step by step"

Frank Berry 14th December 2011 07:14 PM

Are these microphone levels or line levels?

Michael1 14th December 2011 07:22 PM

Line Levels....

Chris Hornbeck 14th December 2011 11:18 PM

Paul Stamler published an excellent overview of mixer design requirements in _Audio Amateur_ in about 1995, that I haven't seen bettered - might be worth finding.

Link doesn't work, so I can't see your reference, but 16 channels is enough to require some serious thought to dynamic range in the summing amp. Its output must drive a feedback resistor 1/16 as large as the source resistors (more or less) and they must be kept to a reasonably small size because they *all* add noise to the output.


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