4 X 2 X 1 Tube Mixer build complete (pics) Schematics added

[lassoharp]

This project started at the end of Jan '10.  I'd been wanting to build an all tube mixer for some time and I wanted it to meet two major criteria:

(1) The individual preamps should be fully functional as separate units for studio use - apart from the mixer and that the mixer could accept any line level source of a specified impedance

(2) The mixer should have 1 auxiliary send/ret for reverb or compression and 2 outputs for main and mon for PA use

So, here's what I came up with for the mixer and output amp:


http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/mixeramp_front.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/mixeramp_rear.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/TubeMixer2007.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/TM_BackPanel.jpg


The mixer uses 2 coils to mix/sum the 4 line level inputs from the preamps- split resistively into 2 pairs of 4 (one main, one aux)  into 2 single outputs.  Each output from the coils are fed to the grids of a twin triode (6SN7) which mixes via a common plate load.  The 6SN7 also comprises part of the output amp - the other half being a 6C5.  The mono output is then split using a repeat coil and attenuators to give a main and monitor feed.

So what you have are coils handling the passive mixing of the main and aux lines each going 4 to 1.  The 6SN7 then mixes those 2 to a single mono output.

The most tedious part of this build was making proper test jigs for each part of the mixer to insure everything would work as planned.  Polarity switches on the line ins are a must.

The twin triode mixing circuit  can be found in the RDH4 as well as the basic outline and relevant calculations of coil mixing.  I was attracted to the twin triode method because of it's economy compared to other methods, and of course the sound - after testing it out with an echo unit it was pretty much a done deal.  From readings it looked to be potentially problematic but I've had no problems with it - works like a charm.

If anyone's interested in the actual schematics and diagrams I can post them.  It will take several days to get them scanned.

The preamps are really nothing new - 4 broadcast console preamps from Gates, ABC/AWA, and Western Electric built essentially verbatim with a few minor mods for lower current draw/output level.

Here's some pics:

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/TubeMixer2001.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/TM_preamps3.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/TM_Rack_Front_copy.jpg

http://i409.photobucket.com/albums/pp179/lassoharp/Tube%20Mixer/preamps_inside1.jpg

It works and sounds great. I would not recommend building 4 preamps on a single chassis though. Interaction can be an issue.  One of the nice things about the mixer section is that any 600ohm source can be used - it isn't tied to the set of preamps I built for it.

Special thanks to Doug(emrr) for some good troubleshooting tips on the power supply and his overall knowledge of vintage tube circuits - ditto for PRR. Also my thanks go out to every contributor on the forum for their generosity, knowledge and inspiration (inspiration, as Einstein's quote goes often dwarfs knowledge).  This build was probably 80% inspiration - 20% knowledge.   ;D

enjoy

 

[emrr]

Wow, you did knock yerself out!  Can't wait to check it out. 

 

[Tubemooley]

That looks really nice. Excellent craftsmanship. What is the LS-141 doing? Is that the repeat coil doing the main/monitor split? I have 4 of those bad boys just waiting to get some use. If you can scan your schematics, I would love to see them. Beautiful work. DW.

 

[ioaudio]

great stuff. + extra bonus points  ;D for original thinking.

-max

 

[riggler]

I fainted. Awesome build, please be cool and post some sound clips!! Congrats!

 

[mrclunk]

sweet man! 
Thanks for the tip on the mix circuit in RDH4 too, sounds like it might be perfect for a multi channel eq we've been working on.

 

[lassoharp]

That looks really nice. Excellent craftsmanship. What is the LS-141 doing? Is that the repeat coil doing the main/monitor split? I have 4 of those bad boys just waiting to get some use. If you can scan your schematics, I would love to see them. Beautiful work. DW.

The LS-141 sums/mixes the four line inputs.  There's an H-283/A-20 that does the same thing for the aux sends. 
For splitting I have that wired basically like you would a mic splitter.  The splitting job doesn't require the centertaps on each coil like the mixing does.  Probably most any 600:600 type would do the splitting job.

sweet man! 
Thanks for the tip on the mix circuit in RDH4 too, sounds like it might be perfect for a multi channel eq we've been working on.

Supposedly you can use a number of 6SN7s or any lo/med mu twin triode in tandem for several channels of mixing.  I only needed two and it was nice to be able to get that job done with a single tube.

I will try and get some sound clips together though it may take a while.  Schematics should be up in a day or two.

 

[Mark Slocombe]

Looking at the inside of the preamps chassis reminds me of looking in the bottom of my O L D Silvertone / Danelectro 15 watt tube bass amp, all point to point with terminal strips wherever they were needed.

Nice job. Looking forward to seeing the schematic.

 

[pucho812]

so I get to keep it right? that is sexy... congrads

 

[lassoharp]

Looking at the inside of the preamps chassis reminds me of looking in the bottom of my O L D Silvertone / Danelectro 15 watt tube bass amp, all point to point with terminal strips wherever they were needed.

Nice job. Looking forward to seeing the schematic.

The grounding bus bar is a well sanded down clothes hanger      I was surprised at how well it took/flowed solder without additional flux.

 

[lassoharp]

Here's a set of schematics - (hopefully I've corrected the image link/sizing problem) - let me know if you trouble viewing these.

Passive Coil Mixer Network:

http://i409.photobucket.com/albums/pp179/lassoharp/Coil_mixer_schematic.jpg

Mixer & Output Amp:

http://i409.photobucket.com/albums/pp179/lassoharp/Mixer__OutputAmp.jpg

Block Diagram:

http://i409.photobucket.com/albums/pp179/lassoharp/Block_Diagram.jpg

 

[Tubemooley]

Schweet. Excellent schematics. On your grounding bar..... I would make sure you tin the entire piece of wire. Otherwise that's bare steel and it will rust up quickly. Soldering mild steel is a no-brainer with a little flux and enough heat. Excellent use of household items. DW.

 

[lassoharp]

Schweet. Excellent schematics. On your grounding bar..... I would make sure you tin the entire piece of wire. Otherwise that's bare steel and it will rust up quickly. Soldering mild steel is a no-brainer with a little flux and enough heat. Excellent use of household items. DW.

Good point - I'm off to tin the remainders.  Thanks.

 

[stitch-o]

wow...

I'm soaking all this up!

 

[zebra50]

... Inspirational stuff! We're impressed.

Now go and record stuff!

 

[lassoharp]

Thanks!


Record stuff? - hardly ever have time for that!

 

[grantlack]

If anyone's interested in the actual schematics and diagrams I can post them.  It will take several days to get them scanned.

yes, yes, and... um... yes.  the pres look phenomenal  .  what sort of gain are you seeing out of them?

 

[lassoharp]

If anyone's interested in the actual schematics and diagrams I can post them.  It will take several days to get them scanned.

yes, yes, and... um... yes.  the pres look phenomenal  .  what sort of gain are you seeing out of them?

Thanks,
All of the mixer schematics are still posted above. The pres are quasi-clones of some broadcast console pres. None of them use the original iron.  Officially there's 2 channels of Gates SA-70s (~35-40db), 1 channel of ABC/AWA type "F" program amp (~55db) and one RCA BA-2 ( ~40db).  These are the actual gain values I'm getting - the original units had higher values.  The mixer output amp gives maybe 25db or so.  You could easily get 65-70+db using one of the pres and the miixer amp to tape.  The 'FX' loop is handy for doing parallel compression. Nice thing about the mixer is that it's a separate unit so any 4 preamps or other line sources can be used with it.