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Old 30th December 2008, 10:56 PM   #1
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Default Tube Direct Box - Got design?

I'm trying to find a vacuum tube impedance matching box design or even an old "broken" or just "used up" device.

These are often called a "direct box" by musicians so they can plug older guitars and mics with higher impedance (> 15k) in and get line level impedance matching out (~ 600 to 850 or so) to mixers and PA's. All of the ones I am finding are solid state types ...

The out of production models I have seen are like the Aquilar Db-900 or the Bellari ADB3B ... or similar from Uncle Albert and many other short line makers.

Criteria might be:
Unbalance input (5K to 15K or more)
Balanced output (600 to 1K)
Some have a single 12AX7 or equal
Most old designs are powered by "phantom power" of the balanced line or by batteries of 9 to 18 VDC.
(gains of +20/0/-20db and "ground lift" switches optional)

Got suggestions? Got a source for schemos?

This search is for as high a quality as possible.
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Old 30th December 2008, 11:22 PM   #2
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I believe you will find that a direct box converts the hi impedance or low impedance line level signal to a MIC level NOT a LINE level output. A direct box is meant to connect to a microphone input in a mixer.


There are many passive DA's on the market that could probably be adapted to your use.

check out

http://www.whirlwindusa.com/tech02.html
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Old 30th December 2008, 11:31 PM   #3
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" ... a direct box converts the hi impedance signal to a MIC level NOT a LINE level. ... "

... Oops = correct you are.

" ... There are many passive DAs ..."

" The Aguilar DB 900 Tube Direct Box is incredible for reproducing a "real" Dobro sound if you're using a pickup. A huge difference from what is available for use in most venues. Has all tube circuit design and high headroom. " - Cindy Cashdollar = http://www.cindycashdollar.com/favorites/
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Old 31st December 2008, 09:35 AM   #4
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This is not hard to DIY. If you don't need additional gain, and you can possibly move off phantom power, you could simply use a cathode follower to lower the impedance for you. If you needed some gain, you could look into a simple gain stage with a 15k:600 output transformer to accomplish both goals. If either is of interest, I can draw a schematic.
-Paul
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Old 31st December 2008, 12:12 PM   #5
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It is possible that the older devices you were thinking of were AC line cord powered also? The phantom circuit is only capable of roughly 7mA if its 48 VDC with 6.2K resistors limiting the 48 phantom supply. Now the tubes heater certainly cannot be powered off the phantom supply because the current needs will not be met. Won't a 12AX7's heater demands be around 150mA @12 volts? Certainly well past the Phantom power capability.

I would think a buffer stage driving a 15K-600ohm or 15K-150 ohm transformer would work. You don't need a lot of level if the output of the device is going to the mic input of the mixer.
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Old 31st December 2008, 12:23 PM   #6
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Check out the Groove Tubes Brick. That thing sounds great, will do what you want and is very reasonably priced.

Not phantom powered and pretty large though.
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Old 31st December 2008, 03:58 PM   #7
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audiowize: " ... If you don't need additional gain, and you can possibly move off phantom power, you could simply use a cathode follower to lower the impedance for you. ..."

Got a schematic? With or without the "matching" transformer? ... I have no problem with battery power, maybe a pair of 9 volt batteries or some such. Tube heaters of course use a lot of power to keep warm, but ... I don't have any problem with a plug-in-the wall power supply either = I just want the best quality possible and am willing to build a pretty "interesting" design to accomplish this. Intended use would be through a stage PA / mixing board or in the studio = all balanced input equipment.

The Groove Tube Brick IS a very interesting approach = it has two input types and balanced output. I'll check that one out for sure ... http://www.groovetubes.com/The_Brick_P2084.cfm (AC line powered)

burnedfingers: " ... It is possible that the older devices you were thinking of were AC line cord powered also? ... Won't a 12AX7's heater demands be around 150mA @12 volts? ... well past the Phantom power capability. ... "

Yes, probably correct = well past the battery capability too, unless we were to lug around a deep cycle truck battery or other.

AC Line Power as a part of the circuit should not be a problem ... whatever it takes to get that "tube sound" and a healthy noise floor / signal to noise ratio / quiet operation.

Burnedfingers: got a good schematic idea?
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Old 31st December 2008, 04:20 PM   #8
imix500 is offline imix500  United States
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Also check out Valvotronics. We use a bunch of their GainRyders and DI's at work. Good stuff.
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Old 2nd January 2009, 04:56 PM   #9
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Being a lazy DIYer, I have purchased a kit from PAIA.com, the "Stack in a Box" direct box with a single 12ax7 tube: http://www.paia.com/proddetail.asp?prod=9210K ( schemo: http://www.paia.com/prodimages/siabsch.pdf )

Anyway, I'll let you all know how this works out. If anyone has any good "mod" or improvement ideas to the circuit, please advise ...
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