# All tube 4 channel mic pre



## Elliott Studio (Mar 15, 2008)

Here's a project I did about 10 years ago on commission for a recording studio. It was designed to bypass the mic pre's on their Pro-Tools interface (Digidesign?) and get tubes in the signal path at the front end. It's basically 4 discreet preamps sharing one box with 48v phantom, -18dB input pad, line-in on channels 1 & 2 and VU meters. It turned out real nice and they loved it!

  

The circuit is simple: A Jensen input transformer gives 20dB of gain from the outset, which feeds a common cathode amplifier with another 20dB of gain. The volume control is nested between this gain stage and the output buffer, another common cathode amplifier with 20dB of gain. This feeds a 10K to 600 ohm transformer to knock the gain back down for a total of about 45dB and provide a balanced low impedance drive. 

The unit is powered from an external regulated supply to keep the noise down. 

The casework I had done at Buckeye Shapeform. They will do one-off projects to your spec's; I sent them AutoCad drawings. Now there are others offering the same service. I've used Protocase on another project with good results, too.

If anyone is interested in this project I will gladly supply schematics and drawings.


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## jonathanm (Mar 24, 2010)

Nice looking box of tricks


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## Elliott Studio (Mar 15, 2008)

thanks!


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## Recordman (Apr 23, 2010)

That looks awesome man. Just wondering if i were to build this, how would i go about increasing the amount of gain it has? I'd be worried that 45dB of gain wouldnt be enough for some mics. Anyway, would you be willing to put up some info on this build?
Thanks


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## Elliott Studio (Mar 15, 2008)

Sorry, I mis-spoke about the gain of the first and second stages. :innocent: The transformer gain is 22dbm and the first stage gain is 28dB. The second stage (and line input) has a gain of 14dBm for a total of abougt 60dBm of undistorted gain. The volume control is between the first and second stages. And for those high-output mics, there's a pad right at the input so there's no overloading of the input preamp. It will take up to -16dBm on the input without clipping with _no pad_, and up to -2dBm with the pad engaged.

The line input is inverting, but you can reverse the phase on the output if you're going out balanced. The meters are calibrated so that +4dBm on the output = 0vu.

It's been a few years since I've looked at it. I built two, one for the studio, one for myself.. though I have no use for it!

I'll post the schematic and the autocad drawings when I have a few spare minutes. The schematics are mostly accurate, there's a few things I might change if I were to do it over, like use 12au7a's instead of 6922's.. The 6922's tend to be microphonic and the 12au7a's are a bit cheaper (I really like the JJ 12au7a's - I use them in my phono preamps). The power supply could be done differently, I designed it around parts that I had on hand at the time. I would change the filament voltage to 12V AC if the tubes were changed to the 12au7a's (less current and heat than 6.3V needed for the 6922's), and not use a voltage tripler for the B+. You just need 325V @ around 500ma regulated.


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## Recordman (Apr 23, 2010)

Hmmm seems a bit overwhelming. Although i appreciate the clarification on the Gain staging. I dont think im quite at the stage where i want to tackle this guy yet. I might chug away on some more pedals and maybe a pre kit or two before i go for this guy. However, maybe when the time comes we can hammer out these details.


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## Elliott Studio (Mar 15, 2008)

Great! Have fun whatever you do, as long as you do it with solder!


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## ejbragg (Dec 13, 2009)

Nice job on the mic pre box! It certainly LOOKS impressive enough! Wish I could hear it for myself.


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## Elliott Studio (Mar 15, 2008)

Thanks! This is the project that I founded my company on. I was going to make custom vacuum tube studio gear like mic pre's, acoustic instrument pre's, compressors, eq's and even had visions of an all-tube console.. but alas, no one was even interested in my mic pre because they all want boutique name brand bragging-rights stuff to adorn their studio's... :doh: and musicians never have money.:unbelievable:

So I ventured into my real passion, tube Hi-Fi gear..


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## Elliott Studio (Mar 15, 2008)

Go to the gallery to see the schematics for this. I had some confusion uploading so there are two sets of identical drawings up there..:doh:


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## PepAX7 (Mar 11, 2008)

Nice Job Kevin and thanks for posting the schemes!

Do you have a source for your transformers?

Pep


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## Elliott Studio (Mar 15, 2008)

Sure, they're Edcor wsm 10K/600 on the output sides, and you can use your favorite on the input. I used Jensens :spend: on the one pictured, on the one I kept I used surplus Shure input transformers. Just be sure that the impedance ratio is 150/10K or 150/15K.

The power transformer is a 120/120 isolation transformer rated at 1A. I used surplus but these are pretty common, and you could use an alternate PSU configuration that doesn't involve voltage multiplication. You just need around 325V at .3A regulated. And the filament transformer is a standard 6.3V 3A unit, also surplus but commonly available.


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## Elliott Studio (Mar 15, 2008)

Oh, by the way, that choke in the PSU is actually a Triad C-24X, 1hy @ 300ma. I need to correct the schematics but I can't figure out how to delete them from the gallery once they're up there. If anyone is interested in building this project contact me and I'll email you the full sized schematics and autocad drawings for the casework.


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